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Materials Criteria Manne structures operate under vaned conditions, and they must meet many requirements. The need to design marine structures that are more efficient presents challenges with regard to the selection and use of matenals. These challenges are being met in two ways: improvements to established materials technology are being sought, and new concepts and materials are being introduced. Of special interest to the CMS is materials research that introduces new analytical approaches; reviews progress in other industnes that is transferable to the marine industry; increases productivity of marine structures; and, most importantly, increases life and integnty. The CMS has identified key areas of materials research as: · novel marine materials; · fracture; and · corrosion and corrosion fatigue. Novel Manne Materials Composites have been identified by the CMS as a thrust area upon which special attention should be focused and, as such, has been addressed in the preceding section. A major barrier to more extensive use of composites in marine structures is uncertainty over their performance in a fire. Recommended for FY 95 is a program to address fire and safety issues associated with nonmetallic materials, Project 95-14, "Assessment of Fire, Smoke and Toxicity Characteristics of Composite Materials Proposed for Manne Applications." In a parallel effort focused on structural monitoring and safety, a program centered on Smart structures'' is recommended Project 95M-C' ''Intelligent Composite Structure Development for Marine Applications." A program is also recommended to address long-term durability effects of composites in the marine environment, Project 95M-S, "Long Terser Durability of Polymer-Based Composites and Corrosion at Metal- Composite Interfaces." A parallel effort is recommended to develop the analytical tools and design methods needed to achieve the full benefits of using composite materials, Project 95M-T, "Analysis and Design Techno~o~r Development for Manne Composite Structures." Fracture The recent failures of bulk carriers, weld cracking of structural members of Trans- Alaska Pipeline Service trade tankers, and the large number of SSC projects related to reliability of marine structures are cause for the CMS to recommend a hands-on workshop addressing fracture assessments and the application of reliability methods to real design problems. Project SR 1362, "Symposium and Workshop on the Prevention of 21
Fracture in Ship Structure." is a two-day session designed to bring together experts in the field to rationally discuss causes and remedies for the rash of failures occurring in ships. The major fracture technology activities are (~) incorporate elastic-plastic fracture models in the durability assessment of marine structures, (2) include the effects of small defects and residual stresses in such models, (3) address durability criteria for double- hulled ships, (4) explore nondestructive techniques for evaluating durability of aging ship structures, and (5) develop a guide for damage tolerance assessment of marine structures. Evaluation of ship-structure reliability can be based on ductile fracture methodologies, which are now developed enough to be applied to complex structures. These methodologies are generally employed for laboratory test specimens with a simple geometry and loading mode. One ductile fracture method, Project 95-13, 'two Parameter Approach to Fracture Prediction in Ship Stnlctures," wild be explored analytically. Another elastic-plastic fracture assessment method, the Failure Assessment Diagram, is presented as part of Project 95-1O, "A Guide to Damage Tolerance Analysis of Marine Structures." Further evaluation is required regarding how these methods apply to some of the more complicated details found in ship structures. Project SR-1349, "Evaluation of Ductile Fracture Models for the Prediction of Fracture Behavior of Ship Structure Details," is considering various ductile fracture methodologies by testing complex model components. These experiments will serve as benchmark tests for evaluating the methodologies. However, many defects in structures are small and may not conform to the same toughness criteria or have the same constraint as for laboratory specimens. To address this problem, a future-year project, 95M-M, "Development of Ductile Fracture Assessment Techniques for Small Defects in Ship Structure Components," is proposed. It wall better quantify the fracture criteria associated with relatively small defects or cracks and concurrent gross section yielding. These projects wall help identify a more complete ductile fracture methodology for ship structure evaluation and future design considerations. Most fracture problems originate in the weidment area; hence, prevention of fracture of weldments has become an important issue for ensuring safety and reliability in ship structures. Project 95M-Q, "Evaluation of Residual Stress Effects on Weldment Fracture," proposes to analytically calibrate the fracture parameters in the presence of mismatched welds. Another project, which may be a likely joint industry effort, Project 95M-R' ttFracture Methodology for Strength Mismatched Weldments," proposes to expenmentally evaluate some new numenca] results that have calibrated fracture- toughness parameters for mismatched weldments both overmatched and undermatched. Each of these projects would greatly improve the safety and integrity assessment of ship structures. Three new projects, Project 95-8, "Fatigue and Fracture Criteria for Double- HulIed Ships," Project 95-10, "A Guide to Damage Tolerance Analysis of Manne Structures," and Project 95M-H, "In Situ Nondestructive Evaluation of Fatigue and Fracture Properties for Aging Ship Structures," address one or more of three ship- structure situations. The situations are (~) initial design, (2) in-serv~ce inspection and analysis of continued fitness for service, and (3) life-extension planning. 22
Corroswn art Corroswn Fatigue Prevention of corrosion of marine structures is important because of the increased cost for preservation and repair of corroded structure. Emerging technology presents the opportunity to reduce these costs and to increase reliability of marine structures. Corrosion assumes one or a combination of forms of electrochemical attack. These are categorized as unify, dissimilar metal (galvanic), pitting, crevice, intergranular, dealIoying, velocity-induced, and environmental cracking (stress-corrosion cracking, corrosion fatigue, and hydrogen embnttiement). Although mechanical fracture is typically an instantaneous failure, such separations can also occur over time as environmental cracking. When this happens, concurrent corrosion and stressing yield a premature failure, compared with situations where either factor acts separately. Which form of corrosion occurs depends on alloy composition, microstructure, and exposure conditions. Corrosion mitigation has histoncally involved design for corrosion resistance, materials selection, inhibitors, cathodic protection, coatings, and modiBed operating conditions. Unfortunately, attention often focuses on corrosion after the structure is In service. This precludes use of the first nvo techniques, which are often the most effective and efficient alternatives. Life-cycIe maintenance costs for commercial vessels could be significantly reduced if current corrosion-mitigation principles such as design for drainage, incorporation of a sprayed metal coating, and design for the extension of organic coating life were implemented in the design and fabrication stages. Project 95-3, entitled "Commercial Ship Design and Fabncation for Corrosion Control (Phase I)," addresses these issues and wall indicate the capability to improve competitiveness of the shipbuilding industry and result in cost savings to owners/operators. A follow-on Phase 2 wall develop specific recommendations for various ship classes. Many surface ship hulls are protected from corrosion by cathodic protection. However, cathodic protection systems are designed only for adequate average or overall protection. Consequently, locations of overprotection and underprotection Epically occur, with major effects on corrosion and corrosion fatigue. With regard to fatigue, structural marine materials are invanab~y exposed to cyclic or time-vanable stressing due to wind, wave, tide, service, and perhaps dynamic amplification. This stressing leads progressively to localized plastic deformation in the matenal and to damage accumulation, which may ultimately cause failure. Fatigue cracks typically begin at locations of geometrical irregulanty (stress concentration) such as those associated with structural details like weld toes. Metallurgical defectsinclusions, porosity, undercutting, or incomplete fusion, for example may also influence fatigue cracks but are generally secondary to stress concentration, unless the defect is relatively large. Once initiated, a fatigue crack propagates through the we]~-heat-affected zone until the crack size becomes critical; then tenninal (fast) fracture occurs. In addition to geometrical, metallurgical, and welding variables, fatigue damage accumulation is also influenced bv 1__ ~ ~ ~ ~ _ ~ a_ _~ _ ~ ~ .~ ~ 1~umg and environmental factors. because or the ranoom aspects of these parameters, fatigue data often exhibit large scatter? and many tests may be required to establish statistically significant information. 23
As emphasized above, cathodic protection is the most important active corrosion- mitigation technique for structural steels in submerged marine service. This corTosion- mitigation approach has typically been designed for the anticipated life of the structure. Because of increased emphasis upon life extension, structures are, after appropriate repairs or structural augmentations, being retained in service after the cathodic protection system has expired. Thus, retrofit or replacement of the corrosion-protection system is required. This is the subject of Project 95M-A, "Retrofit of Marine Cathodic Protection Systems." The Oil Pollution Act of 1990 mandates double hulls for all tankers that operate in U.S. terntonal waters. Corrosion in these ships can be a serious problem, especially when it occurs in compartments that can not be easily inspected after fabrication is completed. Double hulls add significant surface area, which increases the consequences of corrosion and the amount of inspection and repair necessary. Corrosion can also occur in the compartments where seawater is used as ballast and in flue-gas scrubber areas. Differences in military and commercial designs may require the implementation of more than one scheme for protecting these regions. These issues are the topic of SR 1366, "Corrosion Control of Inner-hul] Spaces." This program wall enhance vessel integrity and indirectly impact environmental protection. Much progress has been made in the area of design for the minimization of corrosion, and this knowledge is slowly being implemented. However, corrosion detection and corrosion rate determinations for existing structures remain a problem. One access-limited area where corrosion rate information would be especially useful is in interhul] spaces in new and existing ships. Project 95M-D, "Development of a Sensor for Corrosion Evaluation in Areas not Easily Accessed for Inspection," has been proposed to address development of a corrosion monitor for these areas. Design of marine structures against fatigue is typically based upon a series of stress range versus number of stress cycles (S-N curves) established by organizations such as the American Petroleum Institute and the American Welding Society. However, the high-cycle regime of these S-N curves, which is where much of the fatigue damage normally occurs, is based upon limited data arid extrapolation from relatively high-stress- range test results. Much of the data that do exist has been based upon specimens that were tested either in air or freely corroding in sea water or an aqueous substitute. Of particular concern are (1) lack of confident definition of the design-curve slope and (2) uncertainty regarding whether or not an endurance limit exists under conditions of cathodic protection. Project 95M-B, "High Cycle Fatigue of Welded, Cathodically Protected Structural Steel in Sea Water," would serve to resolve these issues so that design of marine structures can be performed more effectively with enhancement of stricture reliability and elimination of unnecessary conservatism. Most of the "time" associated with fatigue fracture is in initiation of a crack or propagation at ultra low growth rates (10~-10-' mm/cycle) in the near-threshold regime. Fatigue test data are difficult to acquire even under ambient exposure conditions' but the problem becomes even more formidable when concurrent corrosion is involved. This difficult occurs because the cyclic stressing rate under service conditions is often in the 0.05-0.5 Hz range, and it is generally recognized that frequency acceleration prolongs 24
fatigue life and thereby yields data that indicates higher fatigue strength than occurs In service. Thus, according to standardized test procedures, it may take a minimum of several months to generate a single S-N curve data point or one crack growth rate versus stress intensity range (da-~K) curve. The project 'Threshold Corrosion Fatigue of Welded Manne Steels" (SSC-366) is now completed and has evaluated different approaches to developing realistic, near-threshold fatigue data more rapidly, under conditions relevant to offshore structures. As a part of this activity, a new test specimen and accompanying procedure have been proposed that reduce by a factor of about two the time required to develop data for near-threshold fatigue crack growth. Project 95M- N, "Threshold and Near-Threshold Corrosion Fatigue Testing of Manne Steels," proposes to further investigate this along with alternative approaches, in order to fully establish the technology of realistic determination of the rate of high-cycle corrosion fatigue crack growth. Loads and Response The development of sound and rational analysis procedures for marine structures requires accurate estimations of wave charactenstics, wave loads, and structural responses. In addition, a thorough understanding of reliability-based design methodologies is essential to incorporate the analysis results in practical design. Large waves frequently result in extremely high hydrodynamic impact forces on marine structures (e.g., the ship's forebody and the underdeck of a smal]-waterplane-area twin-hull [SWATH] ship or semisubmersible). This impact phenomenon is highly nonlinear. Advanced techniques are needed to quantify extreme wave kinematics and hydrodynamic impact forces so that a time-domain simulation technique can be developed for design applications. Project SR-1342, "Hydrodynamic Impact on Displacement Hulls," initiated in 1991, wall assess the state of the art in estimating forebody hydrodynamic impact loadings on ship structures and identify weaknesses in current technology. A great dead of research has been performed on the nonlinear aspects of loads and responses of marine structures. But experts around the world employ different approaches to solve many of these nonlinear problems, and a unified approach is generally missing. With this in mind, a 2-day symposium and workshop has been proposedProject 95-l, "Symposium/Workshop: Higher Order Prediction Methods for Hydrodynamic Loads and Response of Manne Structures"- by which these experts can be brought together to discuss the pros and cons of their methods and progress to date. Project 95-5, "Combined Load Effects for Design and Strength Assessment of Ship Structures" (94D-~), unit address the area of combining all loads in an assessment of extreme loads. In addition to the nonlinear loads, the responses experienced by a floating structure include several nonlineanties. The stability of a shallow draft structure is a subject of study under Project 95D-T, "Nonlinear Rolling of a Lightship Tanker and Other Shallow Draft Structures." Project 95D-S, "Consistent Stochastic Analysis 2s
Procedure for Design of Floating Manne Structures," is proposed to investigate a consistent stochastic methodology in integrating structural and hydrodynamic analysis. There is great interest in reducing the weight of hull structure for many vessels, such as high-speed combatants and SWATH ships. In recent years, there have been increasing activities in the design and building of twin-hull vessels. However, a comprehensive design guide is missing in this area. With this in mind, Project 95-7, Structural Design Guide for Twin Hull Vessels, is proposed. For these vessels, reliable estimates of local hydrodynamic impact pressures, such as slamming pressures and impact forces associated with breaking waves, are necessary in order to rationally develop hull structure design. The global hall girder load and local pressures and inertia loads are not properly combined in a systematic design analysis. Also of interest is refining forebody forms (both below and above the water line) to improve seakeeping characteristics and reduce slamming loads. SSC research associated with hydrodynamic impact loads would be a valuable contribution in these areas. Design Methods The need to maintain a pipeline of well-trained structural designers, versed In the pnnciples of structural integrity, requires support to the technical education community. Accordingly, the CMS recommends Project 95-16, ''Evaluation of Manne Structures Education in North Amenca" and Project 95D-F, "Design of Innovative Marine Stn~ctures." The thrust of these projects is to get more schools and students interested In stn~ctural design in naval architecture and marine eng~neenng and to provide an opportunity for several students to obtain grants for senior or graduate studies in the area of structural concepts for a practical marine structure and to present their Endings. Adequate Strength in Serv~ce Fatigue is a pnmaIy source of in-service structural damage in most types of ships and platforms. Thus, it remains a key research topic. The recently completed project Fatigue Design Procedures," (SSC-367), summarizes fatigue design methodologies for ship applications and reviews their relative meets. Details with similar endurance characteristics have been combined in the recently completed project, ~Reduction, Classification, and Application of S-N Curves for Ship Details, (SSC-3694. In addition, that in-se~v~ce fatigue often involves complex stressing was addressed by a recently completed project, Fatigue Performance under Multia~a] Loadings (SSC-356~. Ships are sometimes required to follow specific transport routes, which have unique effects on the ships' long-term structural behavior. Analysis of route-dependent fatigue failure is important for the design of such ships. Project 95D-Q, "A Study of the Effects of Transport Route Profiles Upon the Fatigue Failures of Ships," is proposed to study the effects of transport route profiles upon the fatigue failures of ships. 26
Ship-Hull Structural Design National and international concern for environmental pollution from tanker collisions and groundings led to passage of the Oil Pollution Act of 1990. In addition, the International Maritime Organisation has established requirements for double hulls on tankers. The law mandates double hulls, but the structural behavior of double hulls in collisions and groundings is not weld understood. The consequences of low-energy impact in double-hull tankers and the effect of the rupture of the outer hull on the inner hull is not known. In addition, the stiffening system for a double-hull design and its influence on low-energy impacts are also yet unknown. Since the International Maritime Organisation mandate and the Oil Pollution Act of 1990, the shipbuilding industry is producing double-hull tankers. However, the design of these ships is generally based on the concept of single-skin design. Project 93-3, ~Double-Hull Vessel Assessment Methods for Collision and Grounding Protection," proposed in FY 1993 Recommendations, was combined into a joint industry project under Project SR-1354, "Grounding Protection of Tankers." To expand this work to include the effect of collisions, the CMS recommends Project 95-12, "Rupture Resistance Cargo Tanks of Double Hull Tankers to Low-Energy Impact." Project 95-~l, "Alternative Stiffening Systems for Double Skin Tankers," should produce concepts that wall reduce the cost of these ships. To enhance the performance of ship structure, Project SR-1346, improved Ship Hull Structural Details Relative to Fatigues began in 1992, and Project SR-1350, Reexamination of Design Criteria for Stiffened Plate Panels" began in 1993. Generally, compensation for openings in primary structural members is made in accordance with guidelines based on past experience. Improperly placed and inadequately compensated openings can result in cracking and associated problems. Project SR-136S, "Compensation for Openings in Primary Structural Members of Ships," wall begin in 1994 to establish rational guidelines for such design features. Two other projects are proposed that wall improve the design of ship-hull structure. Project 95-9, "Hull Response Monitonng System," is proposed to provide the ship operator with real-time reliable data on hull girder stresses and external loads, such as ice loadings. This system watt also record data so as to provide real-time information necessary for reliable design. The project wall define and recommend a monitoring system. In order to facilitate hull-stress monitoring, a new project is proposed to evaluate a recent advance on strain gauging Project 95TC-C, "Fiber Optic Strain Gauge." Project 95D-A, "Comparative Design of Orthogonally Stiffened Plates for Production and Structural Integrity," will examine alternative structural arrangements to provide the structure which costs least among alternatives that provide equivalent strength. Use of Newer High-Performance Steels Thermo-mechanica] controlled process (TMCP) steels are becoming the material of choice for ship and marine structures because they possess a very attractive 27
combination of high strength, excellent weldability, and relatively low cost. Available data demonstrate that TMCP-welded joints can have better fracture resistance than conventional steed joints of the same static strength. To develop proper design criteria that wild facilitate more effective utilization of high-strength MAP steels, it is necessary to document available information on fatigue (S-N), crack growth, and fracture data. To do so, Project SR-135S, ~Optimized Design Parameters for Welded IMCP Steels,. has been initiated. A 65,000-psi ye-strength steed has been identified as an optimum material for use in ship structural applications where savings in plate and weld metal weight are an important consideration. The CMS has identified out-year Project 95M-E, "Performance and Optimized Weld Properties for 65,000-psi Yield Strength Steel Plate," in order to characterize the performance of this plate matenal. The project wall evaluate the potential to use existing 70-class welding consumables and stir] obtain 100 percent joint efficiency. The project wiD be able to build upon the results of the recently initiated Project SR-1343, "Optunized Weld Metal Properties for Ship Structures," which is a project to develop guidelines to improve productivity by optimizing weld metal properties for use with high-strength steels. Potential for increases in productivity is offered by incorporation of these new high-strength, high-performance steels into ship designs. Unfortunately, movement to higher design stresses to efficiently use these steels has resulted in the shortening of useful fatigue life. The CMS has therefore recommended Project 95-6, "Weld Detail Fatigue Life Improvement Techniques." This project evaluates techniques that have potential for improving the fatigue life of cntica] details. These procedures may be useful in original construction as weld as in a life-extension or repair procedure. Finite-EIement Modeling of Skip Structures One of the most difficult aspects of the application of finite element analysis in the design and evaluation of ship structure is the development of appropriate models. Considerations in the assessment of mode] appropriateness would include reliability, size, and detail with respect to the purpose of the analysis and cost. Project SR-1364, "Guidelines for Finite-Element Computer Models," is intended to address the issue of appropriate models in the use of finite-element analysis. Fabrication and Maintenance Techniques The U.S. ship design and shipbuilding community is aware of the paramount need for improving fabrication or producibility and maintainability. It supports the use, whenever possible, of the concepts of zone construction, parts and systems standardization, parts and systems interchangeability, and use of international/commercial standards, whenever possible. Attention must be focused on productivity, env~rorunental 28
awareness, and safety in decisions relating to new construction, and repair and maintenance. Increasing productivity requires an interdisciplinary approach involving concurrent enPineenn~. where the design of the Product occurs in conjunction with design of the ~ cam - - - -cat- -- - r - --- - · --- - -- - ---cry production process. ProJects based upon this approach Should be emphasized. Keeping this in mind, the CMS has initiated Project SR-136S, "Compensation for Openings in Primal Structural Members of Ships." This effort wild establish rational methodologies and guidelines for the determination of appropriate compensation of small and large openings in primary structural members of ships. The benefit wild be an improvement in the efficiency, reliability, and producibility of openings. Another example of the emphasis on improving producibility is the proposed Project 95D-A, "Comparative Design of Orthogonally Stiffened Plates for Production and Structural Inte~ritv." This effort will establish a rational methodolo~v for comparison of C7 , - , structurally equivalent alternative hull configurations and wall improve the shipyard- recommended structural designs. Part of the effort wall be establishment of data banks with information on standard structural elements and fabrication times. This wall be a step forward in providing the designer/engineer with a mechanism to estimate product costs on a basis of estimated work content, rather than assembly weight. There has been growing interest in the issue of maintaining integrity in aging marine structures. A large number of papers have been published on this topic in recent Years. Stated simDlY: inspection is expensive. Therefore. there is a strong motivation to ~ ~ ~ ~ A ' ~ , ~ , , . ~ . . . . ,, , '* . , · ·,, , · , · . ~ ~ ~ oevelop strategies tor mlulmlzmg the COSt Ot inspection without 1mpactmg the quality or the maintenance program. Project SR-1365, "Optimal Strategies for Inspection of Ships for Fatigue and/or Corrosion Damage," has been initiated to rationalize the inspection process. The result wall be an improvement in the system, with optimization of the cost factors to force a reduction of maintenance costs of ships and possibly an improvement in structural quality. There has been continuing concern with the problems resulting from corrosion of ship structures. As discussed earlier, in recognition of that concern, Project 95-3, "Commercial Ship Design and Fabrication for Corrosion Control," was recommended. This project wall identify corrosion control methodologies that wall improve life-cycle maintenance costs and enhance the safety and integrity of marine structures. A higher number of labor hours compared to foreign practice is required for design and fabrication of U.S.-buiIt ships. In an attempt to reduce labor hours in design and construction, a project has been proposed that will improve ship design efficiency and also support creation of data bases that can be utilized in production improvement. Project 95D-D, "Integrated Stiffening Systems for Double-Skin Tankers," wall evaluate and develop alternative stiffening systems for the doubJe-side and double-bottom structures of tanks for enhancing pollution prevention, fatigue strength, and producibility of the structure. This project concentrates on the double-hull design in an effort to provide U.S. designers and builders with important information necessary to optimize their ability to respond to an important new market (created by U.S. legislation) for tankers that wall be entering U.S. ports. 29
One of the new technologies that Will possibly impact ship design and fabrication in future years is the use of chemical adhesives as a mechanism for joining structural assemblies. In recognition of this fact, a project has been developed that wall evaluate the technology relative to its application in marine structure fastening. Project 95D-B, "Use of Adhesives for Structural Bonding of Marine Structures," wall provide a rational analysis of the areas where chemical adhesives might be utilized in marine applications and wall develop a manual for use bv the clesi~nerfenaineer in selecting the anuronnate material. ~ , ,= --= ~ rr--r- Improved fabrication technologies are necessary for enhancing shipyard productivity. These technologies cannot be implemented in isolation but rather require an interdisciplinary approach involving design, material delivery, cutting and forming operations, joining, assembly, inspection, and coating methods. As methods are developed to increase productivity, the structural integrity has to be maintained. Fabrication productivity can be enhanced by using welding methods that optimize the speed of the process. In response to this need, the CMS has developed Project 95-15, "High-Productiv~ty Welding Processes," to evaluate the latest techniques available from the worId's shipyards, including an assessment of the benefits, including increased reliability and structural soundness, of more-consistent quality welds produced by automated processes. As domestic shipyards focus on commercial new-ship construction as a consequence of the decline of military business, opportunities wall increase for utilization of extremely efficient higher-deposition processes that are produced by more automated and robotic processes. It is also essential that the toughness cr~tena for the heat-affected zone of very high heat input weldments be weld established for specific ship applications. Therefore, the CMS proposes Project 95M-O, "HAZ Toughness of High- Heat-Input Welds," to establish acceptable toughness criteria for these welds. Welding technology also needs to be improved to allow for more efficient welding of high-strength steels with greater confidence that the structural integrity is not --I compromised. The steed industry is introducing new high-strength, less crack-sensitive steels that offer lower fabrication costs (principally by reducing or eliminating weld preheat requirements). The first generation of these steels, the high-strength, low-alloy grades, has lower carbon and lower carbon equivalent, which provides less sensitivity to hydrogen-assisted cracking and improved weldability. These steels may ultimately be produced at lower alloy costs if the domestic steed industry builds TMCP facilities either under a Department of Defense Title IIT program,3 (which is currently in jeopardy for lack of funding), or with their own funds. The industry has also developed a new sense of awareness of the problem of weld-meta] hydrogen-assisted cracking and the strong effect that welding process parameters have on the properties of those welds. As a resllit, two Droiects. Project 93-2. "Methodolo~v for Shiovard Weld-Metal Hv~ro~en- Cracking Prediction," and Project 93-4, "New Method for Retaining Weld Properties Over the Range of Fabrication and Repair Conditions," are combined into the new Project SR-1357, "Retention of Weld Metal Properties and Hydrogen Cracking," that wild O ~ ~ ' ~ ' "7J ~ J ~ ~ 3 Commerce Business Daily, 22 April 1988: USA 9575~. 30
contribute to the capability of the shipbuliding industry to incorporate the new, more weldable, high-strength steels. An area that has been identified as one where enhanced producibility and capability for fabrication could be realized involves weld-over-pnmer technology. Primers are applied in the fabrication yard to provide temporary corrosion protection; however, it is generally necessary to remove these coatings prior to welding because of their adverse effects (porosity in particular), on weld-metal properties. Development of weld-over primer technology would eliminate Ending before welding, which would result in the saving of weld-surface preparation. An SP-7 project funded by the National Shipbuilding Research Program wall evaluate improved weldable pnmers. However, there is no standard method for paint manufacturers or shipyards to test these coatings to compare their weldability. Although weldable primers have been used by domestic shipyards, the industry needs methods of evaluating their acceptability based on weld properties and en~neenn~ analysis. The CMS proposes Proiect 95M-V. "Weldable Coatings for Shin ---my o -A -a --- ~ - r - - r ~ - -- - - - - - - ~ ~ --r Construction," to better facilitate use of primers that could be applied to stn~ctura] steed for the purpose of minimizing weld preparation costs. The SSC has funded several projects related to phenomena of structural deterioration, damage, and failure, as weld as to monitonng, inspection, maintenance, and repair of structures. However, the projects have not been formally grouped and summanzed for content and usability. The SSC has completed the project "Maintenance of Manne Structures: A State-of-the-Art Summary," (SSC-372) to meet this need. Inspection and Integrity Marine structural integrity during construction and seance is a continuing concern of the CMS, especially with exterlded service life of existing structures. The predictability of structural integrity has been imD rove d bv better understanding how specific defects ._ ~ ~ e. coninouTe To structural rapture. Approaches and methodologies of inspection for these defects are being improved. Nondestructive examination is becoming more quantitative with new analytical approaches to the assessment of structural integnty. The possibility of using imbedded sensors and interpretation analysis to monitor structural behavior also exists. The capability to identify a significant defect is a potentially useful engineering tool. Knowledge of defect shape, orientation, and distnbution can now be translated into a statement of integrity and residual service life through fracture mechanics. Visual inspection remains the most popular and used method to assess the status of ship hulls, but questions concerning accuracy, reliability, and repeatability remain. In this regard, Project 95-4, "Detection Probability Assessment of Visual Inspection of Ships," is proposed. Another project to standardize inspection procedure is proposed under Project 95TC-B, "Standardized Approach to Methodology and Analysis of Ultrasonic Inspection of Ship Structures." The CMS is promoting these new analytical techniques in the marine structures industry. The SSC has completed two relevant projects: Relationship Between 31
Inspection Findings and Fatigue Reliability (SSC-355) and Influence of Weld Porosity on the Integrity of Manne Strictures (SSC-334~. Innovative methodologies are needed to reduce the amount and frequency of nondestructive testing. The petroleum production industry will be relying increasingly upon remote inspection techniques for deepwater structures, that is, using remotely operated vehicles with nondestructive test packages. Standards for such testing need to be established. In addition, failure research needs to be consistent with advanced reliability theory. The CMS wild continue to recommend research that introduces new analytical approaches to the marine industry through research and technology transfer. A broad conceptual examination of redundancy, as it relates to residual and reserve strength, was undertaken in the project "Structural Redundancy for Discrete and Continuous Systems" (SSC-354~. Initially the project defined structural redundancy, residual strength, and reserve capacity relative to framed structures (offshore platforms), monocoque structures (ships), and other structures (e.g., guyed-tower mooring systems and tension-leg platforms). Cnteria defining structural stability for simple discrete systems were used to illustrate applicability to more complex systems. Repair of Marine Structures Routine maintenance and repair intervals for marine structures affect service life and operating costs. However, the lack of adequate repair techniques can reduce structural integrity, shorten service life, or require costly replacements. The scheduling of inspection and routine maintenance needs to be more analytically determined. This need is addressed in the reliability analysis program described above. Unfortunately it is often difficult to determine whether a proposed weld repair wall be an improvement. The decision of whether to repair or to operate Without further repairs should be made on a technical basis. Thus the CMS recommends Project 95-2 "Methodology to Establish the Adequacy of Weld Repairs." This project will provide guidelines that wall both improve safety and reduce costs of maintenance procedures. A major component of the structural maintenance process is inspection. Decisions on design and maintenance of ship structures hinge on the effectiveness and cost of inspection. Project SR-1355, YInspection of Marine Structures, was initiated to quantify inspection performance and address inspection cost. This program wall supplement Project SR-1340, Structural Maintenance for New and Existing Ships, which addresses structural maintenance and life extension for aging tankers. Another program objective involves development of new underwater repair methods for ship hulls, semisubmersibles, and fixed platforms. A combination of temporary wet welds and permanent d~ welds may ultimately be approved for permanent repairs, and, at a minimum, they could extend ship usefulness until a scheduled dry-docking. The recently completed project Permanent In-Service Repair Procedures for Underwater Damage to Ship Hulls, (SSC-370), reviewed present practices and recommends procedures for permanent in-serv~ce underwater repair of 32
structural damage. Project SR-1356, "Strength Assessment of Pitted Plate Panels," has been initiated and relates to this. Frequently weld repairs are required on matenals that are crack sensitive and require a postweld heat treatment. However, this is often impractical for these structures. A temper-bead technique is one method of performing sound repairs in crack-sensitive matenals without postweld heat treatment. Since there are no definitive guidelines for using this technique, the CMS is considering developing a project on recent developments, applications, and guidelines for temper-bead techniques. Development of the project is pending review of a joint industry project proposal from an independent source. Reliability Reliability technology is the application of probability and statistics to engineenug analysis and design. Incorporating modern reliability technology into decision making for marine structural engineering promises to improve quality and performance. The results wall ensure production of a structure having an improved balance of risk relative to that produced by current procedures. The implication is a more efficient, balanced design and possible structural weight savings compared with current design procedures. The ciesign of any structure depends on predicted loads and on the structure's calculated capacity to resist them. There is always some element of uncertainty In determining either. Engineenng design has compensated for these uncertainties by experience and subjective judgment. With reliability technology, these uncertainties can be considered quantitatively. The development and implementation of probability-based structural design procedures have been under way in other areas since the early 1960s. In addition to the existing design procedure that is based on the conceot of a working stress. a ~ ~ , probability-based load and resistance factor design procedure was issued by the American Institute for Steel Construction in 1986, with a second edition published in 1993. Further, the Amencan Petroleum Institute has recommended this technology for offshore structures with their publication RP2A, unrecommended Practice for Design, Fabrication, and Installation of Fixed Offshore Structures." The development of probability-based design codes in other areas appears to have stimulated important advances in structural design. In addition, the codes become a living document that can be revised to include new sources of information and to reflect additional statistical data on loads and load effects. It is a top-down approach that actively encourages the collection of better data. Final design decisions about materials, sizes, and arrangements should be based on expenence, regardless of the overall approach. The main advantage of a probabilistic approach as a design method is that it provides a mechanism for taking advantage of ah . . . ~ . ~ . . ... . . .. ~ .. . . . . . . relevant Information. ~ronaolllstlc methods allow engineers tO make Decisions cased on a quantitative descnption of uncertainty, in addition to reaching a consensus in structure] design based on experience and judgment. The process of developing reliability 33
technology for marine structures unifies the thrusts of many other active and recommended projects that had appeared to be unrelated. The advantages of new probabilistic design strategies are expected to produce a more balanced design and allow use of different safety levels (or safety factors) that depend on the predicted accuracy of venous loads and structural capabilities. Although existing working stress or code designs are based on satisfactory expenence, they typically provide the same safety factor for venous loading types, thereby resulting in a loss of structural efficiency. To kick off the program, the SSC cosponsored a symposium and funded a tutonal on structural reliability to inform the marine community of this new technology. Sponsored by the SSC and the Society of Naval Architects and Marine Engineers, the Manne Structural Reliability Symposium was held in Arlington' Virginia, in October ]987.4 It attracted exerts from around the world and prodded a fomm for assessing the state of the art in reliability methods. Project SR-1310, Application of Reliability Methods to Analysis and Design of Manne Structures (SSC-351), was a tutona] on applying reliability to marine structures. A one-week seminar, in which the draft document was presented to SSC participants and colleagues, was held in San Francisco in January 1988 and repeated In Washington, D.C., in October 1990. A multiyear research program is under way to apply reliability technology and develop probability-based design criteria for ship structures. It represents a major sustained effort that will make significant changes in structural design, improve the reliability of ship structures, and permit the results of research to be more easily incorporated into future designs. The program consists of the following projects recommended by the CMS. Probability-hased Design Approach for Ship Structures Phase 1: Demo,~stranon Project A demonstration project ~Probability-based Ship Design Procedures: A Demonstrations (SSC-368) was completed in 1993. It compares a hull girder designed by present conventional American Bureau of Shipping rules with a design using probability-based procedures, illustrating the applications of this approach and identifying its advantages and problems. The project report wall be useful for information, instruction, and future reference. Phase 2: I-oads acid Load Combi'~aiio~ls The Phase 2 project ~Probability-Based Ship Design: Loads and Load Combinations, (SSC-373), which defines ship design loads suitable for use in reliability analysis, was completed in 1993. This study includes statistical distributions of extreme wave loads, fatigue loads, and modeling errors. Load- combination issues that require funkier investigation are addressed In this project. 4 Ship Structure Committee and the Society of Naval Architects and Marine Engineers. 1987. Proceedings of the Manne Structural Reliability Symposium, October 5~, 1987. New York: Society of Naval Architects and Manne Engineers. 34
Modeling errors were addressed in the project, Uncertainties in Stress Analysis on Manne Structures. (SSC-363), which was completed in 1991. The project's materials counterpart is Project SR-133S, Uncertainty in Strength Models for Manne Structures.. Phase 3: Implementation The third phase of probability-based design approaches is Project SR-134S, ~Probability-Based Design: Implementation of Design Guidelines for Ships, which wild develop a more detailed probability-based design procedure for ships. Load models provided by the Phase 2 project wall be combined with strength formulations in a supporting project, Project SR-133S, Uncertainty in Strength Models for Manne Structures. (Although not critical to the success of Phase 3, Project SR-1338 wall enhance it.) This project wall develop design procedures based on reliability considerations similar to reliability-based design procedures used for other structural applications worldwide. The procedures wild include provisions for ultimate strength of hull Orders; design of stiffened panels; fatigue of details (typically connections); and buckling, which wall require further investigation for marine structures. Phase 4: Synthesis of the Reliability Thrust Area The fourth phase will provide a summary and synthesis of the various projects in the Reliability Thrust Area, including the complementary projects in design methods and load uncertainties. There have been several programs and several investigators, and there is now a need to put al] of the pieces together. The synthesis wild provide a summary of reliability technology for specific application to (~) design code development, (2) failure analysis, and (3) reliability assessment of existing designs. Project SR-1362, "Probability-Based Design Synthesis of the Reliability Thrust Area," has been initiated by the CMS. Phase 5: Novel HuR Forms and Environments The fifth phase wall address reliability-based design processes for novel structures. Project 95D-V, ~Probabili~ Based Design (Phase 5~: Novel Hull Forms and Environments, is proposed for Phase 5. The term ''novel" in this project applies to unconventional hull forms or structures subject to uncommon environments. The premise of the project is that in novel situations, first pnnciples must be applied because these situations cannot be based on extrapolation or interpolation of current practice or existing structures, as implied in the third phase. This project will determine whether the current data base, existing structural reliability literature, and practice contain the necessary elements to probabilistically assess the performance and safety of ship structures that have unusual forms car are subject to uncommon environments. Rel~abdity of Existing Ship Structures Knowledge of the risks associated with important failure modes, which have been tolerated by existing marine structures, would be useful in developing rational probability-based design cnteria. This information could be used on an ad hoc basis to review or revise present procedures or to develop an entire design-cnteria document. as
Moreover, such risk data may be useful in Staining requirements for maintenance or for retirement of older structures. In support of this effort, Project 95TC-A, "Post Yield Strength Of Structural Members," wall provide additional guidance to verify the load- carIying capacity of structural components. The first phase, Project SR-1344, Assessment of Reliability of Existing Ship Structures (Phase If, is now complete, and it will be useful for the Phase 3 project, Implementation of Design Guideiir~es for Shiest (SR-1343~. Phase 2 of Project SR- 1344 wall begin in 1994. Human Error and its impact on Design Estimates have shown that 75-90 percent of all structural failures are due to nonphysical factors: errors in calculations, poor judgement, incomplete professional understanding, inadequate design review, poor workmanship, improper inspection, and abuse by the operator. Human error is a departure from acceptable or desirable practice on the part on an individual that results in unacceptable or undesirable results. Human errors can also develop as a result of influences from organizations or groups of individuals. The third aspect of human errors regards the physical systems with which individuals and organizations interface. These systems can be prone to and encouraging of errors. Structural design procedures and codes do not recognize this aspect of the design process but assume that methods wall be applied unerringly. Some structural failures can be attnbuted to errors in the design procedures themselves, such as the failure to require fatigue analysis when higher operating stresses were permitted through the use of either higher-strength steed or reduced corrosion allowances, or both. Physical uncertainty, modeling error, and statistical uncertainty can be quantized and explicitly included in a probability-based code or ad hoc assessment of reliability. Human error, a nonphysical factor, is difficult to quantity relative to its influence on design and is not ordinanly considered in formal design procedures. It is also an important component of uncertainty that is not introduced explicitly in the probabilistic mathematics leading to probability-based code requirements. Project SR-1353, The Role of Human Error in the Design, Construction, and Reliability of Manne Structures, has been initiated to perform a state-of-the-art assessment of the impact of human error on the safety of marine structures. It would be a first step to reduce the risk of failures due to human error and establish procedures and guidelines to consider the effects of human errors in design and the formulation of structural design criteria. Development of Internafion~ Standards The SSC Strategic Plan recommends that SSC should expand its involvement in the international standards development arena. To aid in accomplishing this, two projects have been developed this yearProject 95D-K, "Metrication Impact Assessment 36
and Implementation Guide," and Project 95D-~ "Analysis/Companson of International versus U.S. Structural Standards." The first proposal recommends ways to convert design and construction In the U.S. ship industry to the worldwide measurement standardSystem Iniernaizonal. The second project will help the SSC to become more involved In the development of ~ntemationa] standards. Other international standards should also be considered, including the International Institute of Welding fatigue design S-N cumes (Eurocode 3), which may be adopted by the Intemationa] Standards Organisation. 37
38 RESEARCHRECOMMENDATIONS FOR FYs 1998-1999 98-$ Fracture Toughness of Marine Structural Steels Objective Develop a database for the fracture toughness of base-metal, weld- metal, and heat-affected zones of marine structural steels to be used in the damage tolerance analysis of marine structures. Benefit The assessment of structural reliability will be enhanced through the use of fracture-mechanics-based analysis that recognizes the reserves of elastic-plastic fracture toughness that exist in materials after crack initiation. Conservatism in design and material selection can be reduced through better knowledge of material properties. SSC National Goals Improve the safety and integrity of marine structures. Reduce marine environmental risks. SSC Strateeies Improved engineering analysis and evaluation Development of better design tools and information systems Prevention research including damage-tolerant structures Background Criteria for fracture-resistant design, fatigue-damage repair, and damage tolerance and structural integrity analyses require laboratory-generated elastic-plastic fracture-toughness data from fracture-mechanics-based tests, e.g., I-integral or crack-tip opening displacement (CTOD). Such data, gathered over a range of temperatures, thicknesses, chemistries, thermo-mechanical processing, and welding parameters for marine steels, will serve as input to the "Guide to Damage Tolerance Analysis of Marine Structures" developed in SSC project SR- 1374. A previous project, "Marine Structural Steel Toughness Data Bank," SSC- 352, focused on the development of a data bank that includes] the then available ~ i 990) values of Charpy V-notch impact energy, critical initiation I-integral (J~c), nil-ductility transition temperature KNOTT), and dynamic tear (DT) energies for 12 structural steels. However, these data may be of limited value for damage tolerance and structural integrity analyses. Such analyses require more general elastic-plastic fracture-toughness data, such as the I-integral (not limited to plane- strain fracture toughness) or CTOD. To achieve the full intent of the project, it will be necessary to define the available data and the extent of the additional work required before any testing should be undertaken. This will require a Phase r program to gather the existing data and identify gaps, followed by a Phase IT program of data generation if needed. Owing to the nature of testing required to obtain elastic-plastic toughness data, any Phase TI program will be a focused effort aimed at the most critical data gaps, rather than an attempt to cover the missing toughness properties of all or
FISCAL YEAR 1995 PROJECT' RECOMMENDATIONS Table 3 lists the projects proposed for the 1995 program in pnor~ty order based on the composite judgment of the CMS members. TABLE 3 Proposed EY 1995 Projects Number Project Title Page 95-1 Symposium/Workshop: Higher Order Prediction Methods for Hydrodynamic Loadings and Response of Marine Structures . . . . . . . . . . . . . . . . 95-2 MPthntinlnov try F.~t~hli~h the. Arle.n,,~rv of We.1~1 R f~nz~irc pea s 95-3 95-4 95-5 ~ A ~^ ''~~~~~~r~ v~ ~, ..... Con~n~ercial Ship Design and Fabrication for Corrosion Control (94-4) Detection Probability Assessment of Visual Inspection of Ships Combined Load Effects for Design and Strength Assessment of Ship Structures (94D-~) ............................. 95-6 Weld Detail Fatigue Life Improvement Techniques . . . 95-7 Structural Design Guide for Twin Hull Vessels Fatigue and Fracture Cntena for Double-Hulled Shins 40 ..... 41 42 43 · 95-8 ~ "~ - "11" ~ `~1~ ~11~1" ~1 ~JtJUUl~-llU11~" ~111~O ............... 95-9 Hull Response Monitonng System 95-10 A Guide to Damage Tolerance Analysis of Marine Structures 95-11 Alternative Stiffening Systems for Double-Skin Tankers (9~14) .. . . . .. . . . 95-12 Rupture Resistance of Cargo Tanks of Double Hull Tankers to Low Energy Imn~t~ · ~ 95-13 Two-Parameter Approach to Fracture Prediction in Ship Structures 95-14 Assessment of Fire, Smoke, and Toxicity Characteristics of Composite Matenals Proposed for Manne Applications (94M-O) . . .. .. . .. . . 95-15 High Productivity Welding Processes (94-13) . . . . . . . . . . . . . . . 95-16 Evaluation of Marine Structures Education in North America 44 46 47 48 49 51 52 54 56 57 58 59 39
95-1 Symposium/Workshop: Higher Order Prediction Methods for Hydrodynamic Loadings and Response of Marine Structures Objective Provide a means for planning necessary research on higher-order predictive methods. Benefit The symposium~orkshop will provide a forum for dissemination and for evaluating the latest predictive technology and establishing a basis for a rational agenda of near- and longer-te~m research needs. SSC National Goals · Improve the safety and integn~ of marine structures. · Support the U.S. maritime industry in shipbuilding, maintenance, and repair. SSC Strategies · Development of better design tools · Improved engineering analysis and evaluation Background Linear techniques have played a central role in the design of ship and marine structures. In recent years, second- and higher-order methods have been under developments 2 but a genera] understanding of these nonlinear methodologies and their importance to the design process is not generally appreciated. Furthermore, a clear presentation identifying the venous nonlineanties and the degree to which new methods can address them is not available. As a consequence, efforts to improve analyses along these lines are being made without a central focus or a prioritization of needs. The resulting Improvements in analysis and simulation technology can, therefore, in the absence of some type of timely initiative, be expected to be slow in dissemination of knowledge and In acceptance of them. In the absence of these improvements, developments and new construction can be expected to be of longer duration and more costly as a result of deficiencies in the predictions of design analyses. A two-day symposium/workshop is proposed to improve the design analysis methodology. During the first day, invited lecturers would present their work. On the second day, a panel discussion would summarize the state of current knowledge and examine topics for future research. Recommendations Perform the following tasks: · Plan and organize the symposiumAvorkshop, inviting international experts to give papers. ~ 1992 Carderock Division, Naval Surface Warfare Center Workshop report 2 Proceedings on Sixth International Computational Marine Hydromechanics Symposium, Iowa City, IA, 1993. 40
. Appoint a group of internationally recognized experts to read and critique invited papers, and prepare a list of topics to be discussed in the workshop portion (pane] discussion). · Hold the symposium and workshop, soliciting written discussion from the audience. · For distribution to the ship-structure community, prepare a written report on the symposiumEvorkshop that addresses current capan~t~es short and long term research needs, Duration 1,000 labor hours over ~ year gS-2 Methodology to Establish the Adequapy of Weld Repairs (94-~) , ongomg researc :1, Objective Experimentally evaluate the adequacy of weld repairs so that a decision whether to repair at all, to repair further, or to operate without such repairs can be made on a technical basis. This wall both improve safebr and reduce the costs of maintenance procedures. Benefit A methodology for assessing the operational strength of weld repairs would provide a technical basis for deciding whether or not weld repairs should be made on structural components with defects or damage. SSC National Goals · Improve the safety and integrity of marine structures. · Support the U.S. maritime industry in shipbuilding, maintenance, and repair. SSC Strategy Improved efficiency for repair technology Background Defects In ship structural components are often weld repaired. The decision to repair a defect is sometimes made without proper consideration of the long- term integrity of the structure after repair. En the absence of reliable methods for estimating weld-repair strength, for example, defects may be repaired because they are aesthetically displeasing or simply because the repair procedure can be performed. Sometimes the repaired structure may have a higher potential for failure than if the ong~nal defect were not weld repaired. Welds can cause metallurgical changes in the adjacent material, with resulting degraded properties. Repaired defects may also create new load paths or stress concentrations that are detrimental. It is often difficult to determine whether a proposed defect repair will be an improvement. A method for evaluating the adequacy of weld repairs is therefore needed and should consider both the mechanical and metallurgical aspects of the repair. 41
Recommendations Lee proposed program involves full-scale testing of specimens welded under conditions representative of actual shipyard repairs. Perform the following tasks: · Prepare specimens, both with and without weld repairs, that cover the range of repair procedures, defects/damage, and structural sizes actuary used or encountered. · Determine the fracture and fatigue behavior of the structures with unrepaired damage and those with weld repair under loads representative of those encountered in actual operation. · Based on the results of the preceding items, develop guidelines for establishing the adequacy of weld repairs as compared With the strength of unrepaired defects. · Coordinate these activities with similar ones at Defense Research Establishment Atlantic of Canada, American Welding Society, and National Shipbuilding Research Program. Duration 2,000 labor hours over 1.5 years 95-3 Commercial Ship Design and Fabrication for Corrosion Control (Phase I) (94-4) Objective Identity corrosion-control methodologies that, when incorporated into ship design and fabncation, can Improve life-cycle maintenance costs and enhance the safety and integrity of marine structures. Benefit Results of this project should improve the competitiveness of the shipbuilding industry and result In savings to owners/operators. Improved vessel safety and pollution avoidance should also be realized. SSC National Goal Improve the safety and integrity of marine structures. SSC Strategy Development of reliability design techniques to optimize material use Background Life-cycie maintenance represents a significant fraction of ship operation costs. Major contributors to this are corrosion damage correction and coating repair and replacement. Typically, commercial marine structures, including ships, are designed and constructed with consideration of corrosion that excludes use of particularly susceptible materials, provisions for drainage, and the fact that certain areas will be either painted or cathodicaBy protected or both. There are, however, numerous corrosion-inhibit~ng principles that, if considered in the structural design and fabrication stages, could extend the time between repairs and reduce the cost of repairs, thereby reducing life- cycle costs. Examples Include: (~) design for life extension of coating and (2) avoiding confined or inaccessible spaces. Regarding the first, coatings typically fad] first at detail edges, because the paint is thinnest there, and at welds, either for the same reason, 42
because substrate preparation (cleaning) was inadequate, or because of differing weldment and base-meta] material properties. Recommendations Perform the following tasks: · Review current ship design and fabrication practices within the context of established corrosion-control principles. · Review emerging new matenals for coatings and joining techniques with the potential for improved corrosion control. · Make specific detailed recommendations regarding applicability and practice of corrosion-prevention methodologies that would reduce life-cycle costs. Recommendations should be specific by design phase and include economic analyses that quantity savings. · Develop the scope of a Phase 2 follow-on effort to extend the above recommendations as necessary to include specific recommendations for venous ship classes. Duration 1,000 labor hours over 6 months 95-4 Detection Probability Assessment of Visual Inspection of Ships Objective Develop a probabilistic model, based on inspection data, that wall help eliminate uncertainties associated with the use of visual methods for the detection of fatigue cracks. Background Numerous probabilistic methods have been developed to make inspection results more meaningful when the inspection results are used in fatigue analysis. The information obtained from an inspection of a structure can be used to eliminate some uncertainties associated with several parameters in the fatigue mode] and to upgrade the reliability of life predictions. These methods have also been combined with cost analyses to define, among other issues, the inspection interval that will minimize long-term maintenance costs. The dependability of the inspection data is usually accounted for by using a probability-of-detection curve. This curve can depend on crack size, inspection method, and the inspector's expenence, among other things. In the case of ship structures, fatigue cracking often originates on continuous welds at points of initial welding defects and at those structural joints where high stress concentrations occur. Due to the large number and wide distribution of fatigue crack sites in a ship, visual inspection is commonly used to detect surface cracks. Large uncertainties are associated with visual Inspection, however, and to date the data available to quantity these uncertainties are limited. To exploit inspection results as a means of improving the fatigue reliability of ships, it is critical that realistic probability-of-detection curves be established for visual inspection. A broader program of research into ship inspection issues is underway in 43
Project SR-1355. The project proposed here deals with the specific problem of proper interpretation of visual inspection results. Benefit Successful performance of this project Vi-~ ~ advance ship inspection methodology and, thereby, improve the structural reliability of ships through improved fatigue assessment. It follows that environmental risk due to failures will be mitigated. SSC National Goals · Improve the safety and integrity of marine structures. · Reduce marine environmental risks. SSC Strategies · Structural reliability engineering · Improved structural inspection techniques Recommendations Perform the following tasks: · Review the literature and compile available statistical data for visual inspection and detection of cracks on ship structures, emphasizing welds and joints. Coordinate with research underway in the ongoing Project SR-1355. · Develop and apply procedures for obtaining statistical data on the detection of fatigue cracks by visual inspection. Inspection of welds and joints with different crack sizes, detected by different inspectors and at different locations of the ship, should be carried out. · Repeat the second task for the identical crack locations, using ultrasonic crack- detection methods. · Establish probability of detection curves by correlating the results of visual and ultrasonic inspections obtained In the second and third tasks. Duration 2,000 labor hours over 1.5 years 95-5 Combined Load Effects for Design and Strength Assessment of Ship Structures (94D-J) Objective Perform a systematic analysis to determine the most probable combined effects of the global (hulI-g~rder) and local (external and internal) loads for design and strength assessment of ship structures. Benefit The expected outcomes of this project will provide guidelines for developing load crated for structural designs and strength assessments. SSC National Goal Improve the safety and integrity of marine structure. SSC Strategy Improved engineenug analysis and evaluation 44
Background Traditionally, the strength criteria for design of the hull structure of ships were formulated with respect to a simplified dominant load for structural components in association with a specified stress limit. The effects of other loads present were not explicitly identified and considered. Strength assessment, if required, was mostly performed with an assumed quasi-static regular wave to generate the nominal-design- wave bending moments and the corresponding static-wave pressure heads. Because of the improper treatment for the local load effects, recurrence of damages in local structures has been experienced In recent years. In development of design cnteria, it is important to consider the wave-induced local loads in terms of hydrodynamic pressures, inertia forces, and added pressure heads (for liquid cargoes and ballast water). Presently, a linear ship-motion computer program, in association with long-term prediction schemes, can be used for predicting maximum values for each individual wave- induced loac] component at selected probability levels. It is, however, difficult and complex to determine the joint probability of occurrence for al] the load components under consideration, especially for nonlinear components that are predicted through · emp~ca~ means. In addition, a certain confidence level has been established for predicting the wave-induced longitudinal bending moments for fulI-form ships, but the same degree of confidence is not yet established for hydrodynamic pressures and the dynamic components of internal loads. Therefore, the probability level selected for venous influential load components in determination of combined load effects needs to be calibrated with service experience. The results of comparison with probability levels of satisfactory, as weld as damaged, structures would shed light on appropriate target probabilities. If the combined load effects would not yield the same trends with the service expenence, appropriate modifications would then be required. Furthermore, for combining the dynamic load components with static loads, the effects of the two types of loads on structural responses for the entire service life span are to be carefully weighted based on the probability of occurrence in development of load cr~tena for design and strength assessment. Recommendations Perform the foBow~ng tasks: · Develop methodology to take into account the long-term combined effects of the global (hull-g~rder) and local (external and internal) loads for design and strength assessment of ship structures. · Perform a pilot study for a selected ship type, considering a group of representative locations and structural components, to determine the most probable combined effects of the global and local loads, using long-te~m prediction techniques of time-domain simulations. For example, the external and internal loads may be first coupled based on a predetermined phase relationship. - ~ global and local loads investigated. ~ and then the combined effects of · Calibrate the selected probability levels and the determined combined ejects of the dynamic load components with service experience. Duration 2,000 labor hours over ~ year 4s
95-6 Weld Detail Fatigue Life Improvement Techniques Objective Review fatigue-life improvement techniques and procedures to provide design guidance relating to these techniques, including the feasibility of techniques and procedures. Benefit Provides modifications to improve fatigue life of poor weld details and allows the efficient utilization of new high-strength, high-performance steels with superior weldability. SSC National Goals · Improve the safety and integrity of marine structures. · Support the U.S. maritime industry in shipbuilding, maintenance, and repair. SSC Strategy Development of reliability design techniques to optimize material use Background Crack initiation at discontinuities in welded connection details are a primary cause of fatigue failures. Some ships are not directly designed for fatigue crack control, although there is some underlying consideration of fatigue strength in setting the allowable stress. Research using large-scale specimens has shown that fatigue life of large welded structures is not a function of base metal strength but rather is related to weld geometry. That is, the fatigue behavior of large welded structures is largely independent of base- materia] strength. Therefore if designs move to higher-strength steels, stress ranges will increase and useful life will be shortened. The movement to higher design stresses to efficiently utilize these higher-strength steels has resulted in a shortening of useful life. The distnbution of micro and macro discontinuities combined with residual stresses that are a function of the base material's yield strength come together In large structures and generate this strength-independent fatigue behavior. Procedures that can either reduce the severity of the discontinuity distribution or lower the level of residual tensile stress, or both, have the potential of improving the fatigue life of these large structures. Procedures and techniques such as contour grinding, peening, gas-tungsten- arc dressing, and the use of welding consumables that improve weld toe characteristics have been shown to be effective in this regard. These techniques could be utilized selectively in critical design details where the structure would otherwise have to be strengthened at a considerable increase In costs. These procedures should be evaluated to determine their potential to extend the life of existing details that have histories of poor fatigue performance. The feasibility and practicality of adding an additional step in the fabrication process should also be examined. The increased economies offered by better weldabilibr and greater performance of new high-performance steels must be weighed against the increased costs of these techniques. The possibility exists of automating these techniques and linking the equipment with the welding apparatus and thus incurring minimum additional time penalty. 46
Recommendations Perform the following tasks: · Sunrey the literature pertaining to fatigue-life enhancement techniques. · Evaluate the feasibility and practicality of these techniques both as a life extension/repair procedure and a production procedure. · Suggest improvements to those procedures that have significant potential to provide reliable fatigue-life enhancement. · Recommend any additional testing of candidate procedures. Duration 1,500 labor hours over ~ year 95-7 Structural Design Guide for Twin Hull Vessels Objective Provide the designers of twin-hull vessel structure with a practical design guide reflecting the knowledge gained by the U.S. Navy and the American Bureau of Shipping on the subject in recent years. Benefit A structural design guide will result · , , , · . · - · · in more reliable, producible, and ma~mamaole ~w~n-nul1 snip structures. --ens will improve the international competitiveness of the U.S. ship design and construction industry and allow a more and wiser use of the twin-huB ship configuration. SSC National Goals · Improve the safety and integn~r of marine structures. · Support the U.S. marine industry in shipbuilding, maintenance, and repair. SSC Strategy Development of better design tools Background The term "twin-hull vessels" refers to conventional catamarans as well as small-waterplane-area twin-hull and other unusual hull forms. The advantages of twin hulls from the standpoints of initial stability, seakeeping, and high-speed resistance have resulted In a dramatic increase in their utilization in both commercial and military applications. Large numbers are being built today for cruise ship, passenger ferry, and research ship applications. The small-waterplane-area twin-hull form is also being used increasingly for smaller ship types. The structural design of twin-hull vessels, especially the prediction of hydrodynamic loads, is not weld understood to date. Design data, tools, and procedures are not readily available as is the case for conventional monohulis. Because of this, some naval architects tend to shy away Tom recommending an attractive twin-huff solution to their customer. Others will apply exaggerated margins to ensure safety, resulting in an excessively heavy, costly, and ultimately noncompetitive design solution. Others wild produce designs with inadequate hull strength to resist the extreme loads experienced in service. 47
In fact, the U.S. Navy has funded extensive research in hydrodynamic loads prediction and structural design for twin-hull vessels over the past two decades. Based on this research, useful computer programs have been developed and utilized in design applications. Furthermore, the U.S. Navy has designed and built several large, oceangoing twin-hull vessels in recent years, which have proven successful. These ships, built for the Military Sealift Command, have been certified by the American Bureau of Shipping. The bureau has shared in the fruits of the U.S. Navy research and development work and has also built upon that work by the development and refinement of its own design procedures and tools. The bureau performed extensive structural analyses of the recent twin-hull ships in the course of certifying the designs. Thus the U.S. Navy and the American Bureau of Shipping possess extensive knowledge, some of which could be shared with U.S. industry to enable it to produce better twin-hull ship designs and enhance the international competitiveness of U.S. ship design and construction firms. Recommendations Perform the following tasks: · Develop a structural-design guide for twin-hull vessels that: · reflects the knowledge gained by the U.S. Navy and the Amencan Bureau of Shipping in the subject area in recent years; · focuses on practical information of direct value to the twin-hull ship designer; · presents design procedures, data, and algorithms; and · emphasizes loads prediction and present methods appropriate for early stage design in the absence of mode] tests. years. · Prepare the guide In a format so that it can be expanded and updated in future Duration 2,000 labor hours over ~ year 95-S Fatigue and Fracture Criteria for Double-Hulled Ships Objective Develop fatigue and fracture criteria that can be used to assess the safety and reliability of double-hulled ships. Use these criteria as input to design. Benefit The development of fatigue and fracture criteria for double-hulled ships can be used to evaluate the effectiveness of the double-hull design for avoiding failure and can be used In the design process to build safer double-hulled tankers and ships. SSC National Goal Reduce marine environmental nsks. SSC Strategy Research of doubIe-hull vessel technology. 48
Background The double-hull design of tankers and ships is used to provide an extra margin of safetr against failure. The effectiveness of this arrangement cannot be quantitatively evaluated without the incorporation of fatigue and fracture criteria that can predict the process of failure In this double-hulled design. Presumably a crack would begin in one hull and propagate to failure, at which point the second hull would act to eliminate any further damage, and the ship would essentially remain intact. However, the rapid propagation of the crack in one hull could have the effect of initiating cracking in the second hull, thus reducing the effectiveness of the double-hull design. The fatigue and fracture methodology needed to address this problem is essentially developed. The recently completed Manufacturing Technology project) on double-hull failure modes wall provide technical input. The project would require the application of existing technology to the particular problem of a double-hull configuration. This approach could be incorporated into the initial design of double- hulied ships and tankers to ensure that the design gives an optimum margin of safety against failure. Recommendations Perform the following tasks: · Apply existing fatigue and fracture methodology to evaluate the failure potential of existing double-hulled ships. · Incorporate this approach into the design process for double hulled ships. Duration 1,500 labor hours over ~ year 95-9 Bull Response Monitoring System Objective Define a hull-response monitonug system capable of measuring, recording and storing hull girder stresses and their associated external loadings, and providing real-time information to the operators as cntical stress values are approached or exceeded. Benefit The project will result in improved structural integrity and operational efficiency under extreme or unusual external loadings. SSC National Goals · Improve the safety and integrity of marine structures. · Reduce marine environmental nsks. SSC Strategy Structural monitoring of vessels in service 1 Fisher, J.W., et. al., "Development of Advanced Double-Hull Concepts-Structural Failure Modes: Fatigues, Vol. 3a, TDL 91-01, Lehigh University, March 1993. 49
Background The operation of ships In severe conditions such as heavy weather or ice fields can lead to structural damage if design conditions are exceeded. However, because the actual stress being Imposed on the structure is unknown, there is little to base course and speed on other than expenence, which may not apply to new ship types or unusual conditions. There is a need for a real-time system to monitor hull stresses under operational conditions. Also, during the development of the new Canadian Arctic Shipping Pollution Prevention Regulations, it became apparent that for several classes of ice strengthened ships there is a need for onboard monitoring of stresses due to ice-hull interaction. It is also necessary to assess the ice environment encountered during arctic operations in order to correlate the loadings and structural responses experienced by reasonable levels of ice class structures. Me monitoring is required in conjunction with efforts to reduce to an acceptable level the risk of major damage and consequent pollution, etc., for arctic shipping. Project SR-1300, Phases ~ and II, dealt with the definition and evaluation of a generalized onboard response monitoring system, and Project SR-1314 dealt with the test and evaluation of a laser-based wave height sensor. While the interest in these systems was related to elevated sea-state conditions, the technology is more broadly applicable to operations In the marine environment, as is evident from more recent developments in the field. The Arctic environment and ice-huD interaction loads constitute a special case which has extreme aspects that make it uncertain how effective a generalized monitoring system would be in such an application. Thus, there is a need for further investigation to assess the particular nature of the Arctic environment, the ice-hull interaction loads and moriitonng systems, and the consequent requirements of a monitoring system that would be fully effective In the Arctic. Recommendafions Perform the following tasks: · Review results of the extensive recent research and define the nature and magnitude of expected external loads, responses, and the associated environment In terms appropriate for defining monitoring systems. · Assess the requirements of a response monitoring system to convey operationally useful information and record the data. · Review the current state of the art in onboard monitoring systems, hull iIlstnlmentation sensors, data recording devices, and information displays. · Evaluate the potential electiveness of a generalized system in meeting the identified requirements and any modifications needed to make it satisfactory for assessing the environment, ice loads, and responses. · Review and recommend requirements for bridge information display and mon~tonng systems operation. Duration 1,000 labor hours over ~ year so
95-10 A Guide to Damage Tolerance Analysis of Marine Structures Objective Develop an engineering guide for the analysis of the damage tolerance of marine structures that is suitable for (~) initial design, (2) fitness-for-purpose analysis during in-serv~ce inspection, and (3) life-extension analysis. Benefit Enhance the durability, reliability, and safety of marine structures through a more realistic definition of critical regions in marine structures. SSC National Goals · Improve the safety and integrity of marine structures. · Support the U.S. maritime industry in shipbuilding. maintenance. and repair. -- a -A fir ~ Do ~ ~ ~ r ~~- SSC Strategy Improved engineering analysis and evaluation Background The rational approach to structural design is based on a recognition of possible failure modes of a flawed member and determination of the margin of safety between the ultimate response of a member and its state at the time of analysis. Currently, techniques are available to estimate, quantitatively, the durability of a flawed structural member for a given service environment (load history, corrosive medium, and service temperature). First, in the absence of a flaw, the time to crack initiation in the given service environment is estimated. (Initiation is defined as the growth and coalescence of micro flaws to form a macro flaw amenable to continuum analysis.) Next, the crack-length versus time relation is estimated terminating at the achievement of cntica] crack length (to be estimated in the next step). The final phase in durability is the achievement of critical crack length and the concomitant exceedance of residual strength (i.e., plastic collapse, buckling, or either complete or arrested fracture). The influence of service environment is considered in all three phases of durability. The crack-initiation phase may be estimated by the S-N curve approach. For example, the U.K. Department of Energy fatigue criteria for offshore structures or the Neuber local strain mode] both accommodate variable amplitude load histories and a corrosive medium. The crack-growth phase may be modeled through linear elastic fracture mechanics that recognizes the effects of mean load, crack growth threshold, critical crack length, residual stress, and environment on the relation between crack Mining force and crack-growth rate. The determination of cntica] crack length is straightforward in the cases of plastic collapse. In contrast, the cntical crack length for fracture is somewhat complicated by the elastic-plastic response of the near-crack-tip material. Optimized modeling of the fracture of structural members requires the application of elastic-plastic fracture mechanics. However, such applications can require sophisticated analytical techniques that demand tune and resources not readily available to the engineer. One solution to this dilemma is the Failure Assessment Diagram, a semigraphical approach that requires only (~) the specific solution of the elastic crack driving force, (2) the mechanical properties of the material in the member, and (3) the elastic-plastic fracture toughness 51
(including the cra-~-~rowth resistance or R-cu~ve) of the matenaleither J-~ntegral or crack-tip operiin^ Placement. The Failure Assessment Diagram compares the ultimate response of the i: i member with its state at the tune of analysis and estimates the margin of safety. Recommendations Perform the following tasks: · Develop a guide for engineers to perform damage tolerance assessment of potentially or actually flawed members in marine structures. · Develop examples of damage tolerance assessment in (~) the initial design stage, (2) in-service inspection, and (3) life-extension planning. Include such examples in the guide. · Identify areas in, or requirements for, analysis of damage tolerance of marine structures that need further definition or exploration. · Consider existing criteria, such as the British Standards Institute ED 6493, in establishing new criteria. Duration 1,000 labor hours over 1 year 95-~1 Alternative Stiffening Systems for Double-Skin Tankers (94-14) Objective Explore and evaluate alternative stiffening systems for the double-sided and double-bottom structures of tankers to improve the maintainability and producibility of the structure and enhance its reliability and structural resistance to accidental loads.: Benefit The results of the project are expected to improve competitiveness through reduced costs and greater producibility and to enhance the safe operation of tankers with respect to pollution prevention and structural inte~ty. SSC National Goals · Improve the safety and integrity of marine structures. · Support the U.S. maritime industry in shipbuilding, maintenance, and repair. SSC Strategy Research of double-hull vessel technology Background Out of concern for environmental pollution, the Oil Pollution Act of 1990 has mandated double-s~n (double-hull) construction for tankers that operate in U.S. temtoria] waters, and the International Mantime Organisation subsequently adopted this and alternatives. Such double-barner protection does offer effective pollution protection In low-energy collisions and soft groundings. New analytical methods are being developed and evaluated for double-hull configurations, and others, with emphasis on ~ Note that this research concentrates on stiffening concepts, rather than double bottom hull proportions, as in Project 95-12. s2
energy-absorption capability and reduced outflow from breached tanks in Project SR- 1354, "Grounding Protection of Tankers." Although a broad-based program, Project SR-1354 is not emphasizing the conceptual design of stiffening members. Presently, the design and construction of double-skin tankers are primarily based on conventional ship framing and practices employed for single-skin tankers. Outer skin, sicle shell, and bottom plating are regarded as constituting prime hulI-g~rder components, which are conventionally stiffened with long~tudina~s together with support from transverse webs/floors. Within this system, the long~tudinals are usually supported by flat- bar stiffeners on the transverse web/floor through fillet welds to the longitudinal flange/face plates. Low-grade fillet weld connections at highly stressed points would significantly decrease the fatigue strength of the structure. Experience with single-skin tankers using higher-strength materials shows that such connections are the source of numerous cracks in longitudinals. These fatigue cracks can propagate into the shell plating if undetected and unrepaired. Current approaches to improving the fatigue strength of this type of connection include installing brackets and double collar plates to relocate the weakest welded spot to lower-stressed regions and to minimize local distortions. Such measures significantly increase the cost of welded joints. The inner skin (side lon~tudina] bulkhead and inner bottom) may also be regarded as a prime hull- g~rder component. Double-skin construction wall thus increase the frequency, and hence the importance, of such problems. Recommendations Perform the following tasks: · Review the results of extensive studies that have been camed out on structural resistance and sequence of damage with respect to the seaway in recent years. This should include close coordination with Project SR-1354, "Grounding Protection of Tankers," the British Mantime Technology computer program for computing damage, and related research under the shipbuilding program and the double-hull investigations at the Carderock Division of the Naval Surface Warfare Center. · Derive guidelines from past and current studies for ma~miz~ng the structural resistance to seaway loads. · Conduct a pilot study to examine alternative stiffening systems, both conventional and unconventional, for feasibility. · Evaluate feasible solutions, including factors of first and life-pycIe costs and producibility. and resistance to accidental loads. warranted. at, · Make recommendations as to remaining problem areas, and further research, if · Review such cost data as is available regarding production and maintenance of hull-stiffening arrangements. Duration 2,000 labor hours over ~ year s3
95-12 Rupture Resistance of Cargo Tanks of Double Bull Tankers to Low Energy Impacts Objective: Evaluate the effect of double-bottom design, including integrated inner and outer hull thicknesses, their separation, and their stiffening-system design' on the structural resistance of double-hull tankers to outer hull and cargo tank rupture under low energy impacts. Develop integrated bottom-hull design alternatives that m~nun~ze the risk of such rupture. Benefit: This project will result in improved safety, economics of operations, and integrity of tankers under low impact loads. Enhanced environmental protection wild also result. SSC National Goals · Improve the safety and integrity of marine structures. · Reduce marine environmental nsks. SSC Strategies · Research of double-hull vessel technology · Prevention research including damage-tolerant structures · Improved eng~neenng analysis and evaluation Background: To reduce the risk of environmental pollution due to grounding and collision, the Oil Pollution Act of 1990 and the International Mantime Organisation mandated that new tankers be either double hulled or, according to the organization, equivalent Tom an of] outflow standpoint. The double-hull design was selected as mandatory in the United States because of an assumption that the double-hull structure provides enhanced pollution protection for low-energy impacts. In other words, mandating double hulls assumes first, that the impact energy will be completely absorbed in the deformation of the double-hull structure leaving the inner hull intact when the outer hull is ruptured, and second, that the outer hull will not rupture more frequently than a single skin design under the same low-energy impacts. Such structural behavior is, of course, highly desirable, but to date the industry has little knowledge as to how effectively a particular double-hull design will stand up to a low- energy impact. It is not known whether either hub wait be able to deflect considerably before rupturing or if the double-huh stiffening members wall prematurely punch through the outer hull as well as the inner hull, which would result both in structural damage and of] spill. In addition, the influence of double side width and double bottom height on the risk of hull rupture for venous stiffening system configurations has not been investigated. ~ Note that this research concentrates on double bottom proportions and plate thickness rather than alternative stiffener concepts, as in Project 95-11. s4
Methods to evaluate the energy absorption capacity of tankers have been developed since 1970 and are currently being developed (such as in Project SR-1354 "Grounding Protection of Tankers"~. These methods, however, do not seem to address the problem of local structural interaction of internal members with hull shell structure and the sequence of failures In the case of low-energy impact. For instance, the analytical method developed by Germanischer Lloyd to evaluate plastic deformation energy In ship-ship collisions assumes that rupture of the cargo tank occurs when penetration depth equals double side width. The local behavior of the internal members (web frames and stnugers) that connect the inner to the outer hull is not taken into account. The possibility of premature failure of inner or outer hulls due to concentrated loads at the attachment points of stiffening system members is not accounted for. The design of double-hull tankers to date has been based largely on conventional s~ngle-hull design practices, without optimizing the structural design to reduce risks of damage to outer hulls and pollution due to inner-hull rupture. There is a need to evaluate the damage resistance of current double-hull designs and to develop double-hull structural arrangements and details that minimize the possibility of premature hull rupture under low-energy collision and grounding impacts. Reconnnendations Perform the forgoing tasks: · Review the results of previous studies and the associated test data with regard to the methodologies developed to predict structural response to low-energy impacts. Coordinate with those organizations that have related studies underway as weld as with Project SR-1354. · Using results Tom the first task, establish a methodology to evaluate the structural resistance and sequence of damage in tanker designs subjected to impact loads. The methodology should take into account the local interaction and deformation of internal-stifferung-system members and their post-failure behavior. Rather than attempting to determine the extent of damage, the main objective of this study is to understand the failure mechanism sequence and to identify structural features that can lead to premature failure of either inner or outer hull. · Validate the methodology of the second task, using both available test results and operational data. · Formulate impact scenarios defining the location and extent of impact. These scenarios should include grounding on a soft bottom, stranding on a rock, and side-shel] impact. · Formulate an impact load model. · Apply the developed methodology (i.e., scenarios and load models developed in the fourth and fifth tasks, to evaluate the damage resistance and sequence of failure of a particular double-hull tanker). · Carry out a parametric study of different design concepts for conventional double-hull structures (configuration of internal members and details, relative proportion of inner versus outer hull thickness, etc.) in order to develop designs that minimize the risk of ruptures. ss
· If necessary, recommend a Phase IT for additional mode] or full-scale tests where the first and thud tasks have defined important gaps in the data. Duration 2,000 labor hours over I.5 years 95-13 Two-parameter Approach to Fracture Prediction in Ship Structures Objective Explore the use of the two-parameter fracture-mechanics approach as it applies to ship structure components. Benefit The two-parameter fracture-mechanics approach allows the incorporation of planar constraint conditions into the fracture-prediction capability and allows a more accurate transfer of laboratory test results to the prediction of component fracture behavior. SSC National Goals · Improve the safety and integrity of marine structures. · Reduce marine environmental risks. SSC Strategy Structural reliability engineering Background A two-parameter approach to fracture mechanics has recently been developed to account for the effect of planar constraint conditions. This allows a more accurate use of laboratory test results for predicting the failure of structural components. Often the laboratory test specimen has a different planar constraint than the component for which the test result is being used. This can mean that the prediction of the fracture behavior in the component may not be as accurate as it could be if constraint conditions were accurately matched. The two-parameter approach uses the parameters K and T for linear elastic conditions and ~ and Q for elastic-plastic conditions. Recently this has been a subject of investigation for the nuclear industry, the British Navy, and others. Its use for ship structures has not been explored. It is an approach that, if properly used, could benefit the evaluation of structural integrity in ship structures. Recommendations Perform the following tasks: · Investigate the constraint of ship-structure components containing defects compared with those of laboratory fracture-toughness specimens. · Explore the use of the two-parameter fracture-mechanics approach to characterize fracture behavior under these different constraints. · Develop a predictive methodology to assess fracture potential in ship structural components that uses the two-parameter fracture-mechanics approach. Duration 1,500 labor hours over ~ year s6
