Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 12
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program RESEARCH PROGRAM DEVELOPMENT Technology Areas The CMS and its working groups—the Materials Work Group and the Design Work Group—recommend a multiyear plan that introduces new, long-range performance concerns. Long-range research guidance comes from: the working groups' thrust-development sessions held in June 1994; SSC report recommendations; the annual joint meeting of the CMS and the SSSC; the SSC fall meeting; and the expertise of the working groups, whose members were selected for their broad experience in the areas of concern. The working groups' specific concerns, activities, and common interest in structural reliability are as follows. Design Work Group—extreme wave loads, higher-order forces, and responses; ice, groundings, and collisions; large-scale structural tests; operations-oriented monitoring systems; modeling errors in loads and responses; procedures for fatigue stress computations; design process improvement; producibility; and reliability-based design codes. Materials Work Group—new marine structural materials; fracture mechanics; fatigue (including corrosion fatigue); corrosion and its prevention; welding; inspection; and deep-ocean inspection and repair. To varying degrees, these specific activities and recommendations contribute knowledge and data needed for the SSC's overall objective of improving the structural reliability of vessels and other marine structures. The proposed multiyear plan addresses five technology areas that provide the underlying technical support for the thrust areas. The technology areas are materials criteria; loads and response; design methods; fabrication and maintenance techniques; and reliability. The research program that has been developed represents the collective judgement of the CMS. Individual projects were developed by the Design Work Group and the Materials Work Group at their meeting in June 1994 when the two groups met jointly and separately. Projects were prioritized as high, medium, or low priority. All these projects were discussed at the joint meeting of the SSSC and the CMS in September 1994. All these projects were then evaluated by the CMS at their September
OCR for page 13
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program 1994 meeting. Factors taken into account during discussions were the interest shown by the agencies for individual projects, the cost and time for accomplishment, technical feasibility, timeliness of the project with respect to ongoing work, and current needs. The projects were originally proposed by the work groups on the basis of technology areas. The CMS considered their own thrust areas and the national goals of the strategic plan of the SSC but maintained an independent attitude based upon their own professional experience. Based on these considerations, the CMS determined an initial prioritization by ballot vote. Further discussions were then held to consider the balance of the entire program, and revised priorities were assigned. At the November 1994 meeting of the CMS, project priorities were again discussed based upon the factors listed above. Final priorities for accomplishment were then assigned and are indicated by the assigned project numbers. Therefore, Project 96-1 represents the highest priority of the CMS. Relationships Among Strategic Plan, Technology Areas, and Thrust Areas The Table 1 relationships between CMS-recommended projects and thrust areas are further expanded in Table 2 and Figure 1. Table 2 relates the goals of the strategic plan to thrust areas of the CMS and to the technology areas of the multiyear research program. Table 2 identifies all technology area descriptions of this report in which the particular project is discussed. As Table 2 reflects, the research projects recommended for FY 1996–97 are heavily oriented toward the CMS's traditional emphasis on safety and integrity of structures with diverse technology areas required for performing these projects. Figure 1 outlines a multiyear research program. The CMS recommendations are organized on the basis of the four thrust areas (producibility/competitiveness, reliability, inspection/maintenance, and composites). They are further organized on the basis of their primary technology area. The proposed later-year projects (see Appendix A) are included for completeness, without indicating estimates of initiation dates. Again, the CMS recognizes that many recommended research projects are related to more than one strategic goal and objective. The following sections of this chapter describe the technology areas of the 5-year program plan development and the programmatic argument for the recommended FY 1995 projects. The project descriptions identify the objectives, benefits, SSC nationals goals, and SSC strategies that the individual projects address. In most cases, these are complementary, such as the fulfillment of a SSC strategy representing a benefit.
OCR for page 14
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program TABLE 2 Relationships Among the Strategic Plan, Thrust Areas, and Technology Areas in the Research Plan. Sheet 1. Research Projects STRATEGIC PLAN: NATIONAL GOALS THRUST AREA TECHNOLOGY AREAS 96-1 Evaluation of Effect of Construction Tolerances on Vessel Strength Maritime Industry Support Safety and Integrity Producibility/Competitiveness Design 96-2 A Predictive Methodology for the Evaluation of Residual Stress and Distortion in Double Hull Ship Structures Environmental Risk Reduction Safety and Integrity Producibility/Competitiveness Fabrication and Maintenance 96-3 Failure Definition for Structural Reliability Assessment Safety and Integrity Reliability Reliability 96-4 Probability-Based Design (Phase 5): Load and Resistance Factor Design (LRFD) Methods for Ship Structures Safety and Integrity Environmental Risk Reduction Reliability Reliability 96-5 In Situ Nondestructive Evaluation of Fatigue and Fracture Properties for Aging Ship Structures (95M-H) Maritime Industry Support Environmental Risk Reduction Safety and Integrity Inspection/Maintenance Materials Criteria 96-6 Methodology for Systematic Collection of Corrosion Data Using Ultrasonic Thickness Measurements of Ship Structures (95TC-B Revised) Safety and Integrity Inspection/Maintenance Fabrication and Maintenance 96-7 Workshop on Industry Standards for Integrated Ship Design Software Interfaces Maritime Industry Support Safety and Integrity Producibility/Competitiveness Design 96-8 Alternative Stiffening Systems for Double-Skin Tankers Maritime Industry Support Safety and Integrity Producibility/Competitiveness Design Fabrication and Maintenance 96-9 Rupture Resistance of Cargo Tanks of Double Hull Tankers to Low Energy Impacts (95-12) Environmental Risk Reduction Safety and Integrity Producibility/Competitiveness Design
OCR for page 15
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program 96-10 Fatigue and Fracture Criteria for Assessing Safety in Double-Hulled Ships (95-8) Environmental Risk Reduction Reliability Materials Criteria 96-11 Evaluation and Assesssment of Fillet Welding of Double Bottom Structure to Resist Pollution in Groundings Maritime Industry Support Safety and Integrity Producibility/Competitiveness Design 96-12 Environmental Risk Assessment Associated with the Use of Polymer Matrix Composites in Marine Environments (95-14) Environmental Risk Reduction Safety and Integrity Composites Materials Criteria 96-13 Durability of Polymer-Based Composites in Marine Environments (95M-S) Safety and Integrity Environmental Risk Reduction Composites Materials Criteria 96-14 Crack-Arrest Toughness of Steel Weldments Safety and Integrity Reliability Materials Criteria 96-15 Ship Bow Structural Guidance Maritime Industry Support Producibility/Competitiveness Design 96-16 Weldable Primers for Ship Construction (95M-V) Maritime Industry Support Producibility/Competitiveness Fabrication and Maintenance 96-17 Sea-Operational Profile for Structural Reliability Assessment Safety and Integrity Reliability Reliability 96-18 Condition Assessment and Optimal Maintenance of Existing Surface Coating System for Tankers Safety and Integrity Maritime Industry Support Inspection/Maintenance Fabrication and Maintenance 96-19 Development of a Sensor for Evaluating Corrosion in Areas Not Easily Accessed for Inspection (95M-D) Safety and Integrity Inspection/Maintenance Fabrication and Maintenance Materials Criteria 96-20 Experiments on Stiffened Panel Collapse and Estimation of Modeling Bias Safety and Integrity Maritime Industry Support Reliability Reliability
OCR for page 16
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program FIGURE 1 Recommendations of the Committee on Marine Structures for the Ship Structure Committee's research program. Sheet 1.
OCR for page 17
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program FIGURE 1 Recommendations of the Committee on Marine Structures for the Ship Structure Committee's research program. Sheet 2.
OCR for page 18
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program FIGURE 1 Recommendations of the Committee on Marine Structures for the Ship Structure Committee's research program. Sheet 3.
OCR for page 19
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program FIGURE 1 Recommendations of the Committee on Marine Structures for the Ship Structure Committee's research program. Sheet 4.
OCR for page 20
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program FIGURE 1 Recommendations of the Committee on Marine Structures for the Ship Structure Committee's research program. Sheet 5.
OCR for page 21
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program FIGURE 1 Recommendations of the Committee on Marine Structures for the Ship Structure Committee's research program. Sheet 6.
OCR for page 22
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program FIGURE 1 Recommendations of the Committee on Marine Structures for the Ship Structure Committee's research program. Sheet 7.
OCR for page 30
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program 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 will be applied unerringly. Some structural failures can be attributed 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 higher-strength steel, reduced corrosion allowances, or both. Physical uncertainty, the probability of modeling error, and statistical uncertainty can be quantified and explicitly included in a probability-based code or ad hoc assessment of reliability. Human error, a nonphysical factor, is difficult to quantify relative to its influence on design and is not ordinarily considered in formal design procedures. Human error is, however, an important component of uncertainty that is not yet 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 Marine Structures,” was initiated to assess the impact of human error on the safety of marine structures. It was a first step to reduce the risk of failures due to human error and to establish procedures and guidelines for considering the effects of human errors in design and the appropriate reformulation of structural design criteria. Similar attention should be focused on developing methods to integrate human and organizational error into ship structure construction and maintenance. Hence, the CMS proposes Project 96D-Q, “Integrate HOE Consideration into Ship Structure Construction and Maintenance.” The SSC has chosen the subject “quality and human/organizational errors in marine structures” for their joint SSC/Society of Naval and Marine Engineers symposium to be held in 1996. Fabrication and Maintenance Techniques Technology Area The U.S. ship design and shipbuilding community is aware of the paramount need for improving fabrication methods which will lead to greater producibility and maintainability. It supports the use, whenever possible, of the concepts of zone construction, parts and systems standardization, parts and systems interchangeability, and international/commercial standards. Attention must also be focused on environmental awareness and safety in decisions relating to new construction and repair and maintenance. Improvements in fabrication and maintenance directly benefit competitiveness in the global marketplace, safety and integrity of marine structures, and risks to the marine environment. These benefits, plus reduced costs, also accrue to owners/operators. Increasing productivity, however, requires an interdisciplinary approach involving concurrent engineering, where the design of the product occurs simultaneously with design of the production process and consideration of maintenance procedures. Projects based upon this approach should be emphasized. Keeping this in mind, the CMS has
OCR for page 31
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program initiated Project SR-1368, “Compensation for Openings in Primary Structural Members of Ships. ” This effort is intended to establish rational methodologies and guidelines for determining appropriate compensation for openings small and large in the primary structural members of ships. The benefit should include improvement in the efficiency, reliability, and producibility of openings. 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 simply, inspection is expensive. Therefore, there is a strong motivation to develop strategies for minimizing the cost of inspection without impacting the quality of 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 should be 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. Inspection for shipbuilding is predominantly performed visually, with varying levels of volumetric nondestructive evaluation required for different applications. Visual inspection for surface profile and flaws is qualitative in nature, and the meaning of results is often questioned. Consequently, the probability of detecting surface defects visually will be investigated in Project SR-1375, “Detection Probability Assessment of Visual Inspection of Ships.” This project is intended to provide guidance as to the effectiveness of visual inspection. Once the reliability of visual inspection to determine surface quality is quantified, the impact of surface quality on weld integrity would be addressed in proposed Project 96M-H, “Effect of Weld Surface Quality on Hull Structural Integrity.” Finally, there are instances in which it is necessary to augment visual inspection by ultrasonic testing or radiographic testing. Ultrasonic testing is much less expensive to perform and should detect more damaging linear indications than radiographic testing. However, some concern is being directed toward ultrasonic testing inspection for commercial tankers as a result of failures in vessels inspected using this type of testing. Proposed Project 96M-I, “Ultrasonic/ Radiographic Inspection of Hull Structures,” is intended to clarify the advantages and disadvantages of each process and result in recommendations for the most reliable and cost-effective approach. Corrosion of marine structures can produce significant damage and result in extensive repair costs. The existence and extent of corrosion damage can often be difficult to determine nondestructively, particularly under paint and in hard to reach areas. In recognition of that concern, Project SR-1377, “Commercial Ship Design and Fabrication for Corrosion Control,” was initiated in 1993. This project is intended to identify corrosion-control methodologies that will improve life-cycle maintenance costs and enhance the safety and integrity of marine structures. Two proposed projects to evaluate alternative approaches to corrosion detection are proposed, Project 96-19, “Development of a Sensor for Evaluating Corrosion in Areas not Easily Accessed for Inspection, ” and Project 96M-J, “Development of Smart Coatings for Early Detection of Underfilm Corrosion. ” The first would examine the use of novel nondestructive sensors, and the second would evaluate paints that would change colors or otherwise indicate the presence of corrosion. Design and fabrication of U.S.–built ships require a higher number of labor hours than is customary in foreign practice. In an attempt to reduce labor hours in design and
OCR for page 32
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program construction, a project has been proposed to improve the efficiency of the ship design process by developing alternatives that are more structurally efficient and producible and that will be available to structural designers prior to beginning a specific ship design. Project 96-8, “Alternative Stiffening Systems for Double-Skin Tankers, ” proposes to evaluate and develop alternative stiffening systems for the double-side and double-bottom structures of tanks to enhance pollution prevention, fatigue strength, and producibility of the structure. This project supplements work being done at the Carderock Division of the Naval Surface Warfare Center and concentrates on the double-hull design in an effort to provide U.S. designers and builders with the information necessary to compete successfully in the important new market (created in part by U.S. legislation) for tankers entering U.S. ports in the future. The proposed Project 96-2, “A Predictive Methodology for the Evaluation of Residual Stress and Distortion in Double Hull Ship Structures,” should help in identifying means for minimizing the deleterious effects of residual stresses on ship structures and, thus, also add to the abilities of U.S. designers and builders of double-hull ships. Most existing tankers with combined cargo and ballast tanks experience serious pitting of their bottom plates. This threatens to add to environmental pollution. Project 96D-L, “Strategies for Control of Bottom Pitting Corrosion in Tankers and Evaluation of Optimum Inspection Frequency,” is intended to develop recommendations for an optimum control strategy for corrosion and optimum inspection intervals. For the control to be effective, accurate data on corrosion must be available, and a standard methodology for collection and analysis must be in hand; the latter is lacking. The proposed Project 96-6, “Methodology for Systematic Collection of Corrosion Data Using Ultrasonic Thickness Measurements of Ship Structures,” is intended to investigate this problem. Another area important to tanker maintenance is the tanker surface coating system. There is increased need to assess the integrity of coatings to ensure proper maintenance of ballast tanks and guard against pollution. The proposed Project 96-18, “Condition Assessment and Optimal Maintenance of Existing Surface Coating Systems for Tankers, ” is intended to address this problem and develop guidelines and methodology for coating maintenance. Weldability Weldability and welding clearly play a significant role in the economics of shipbuilding. Fabrication issues focus on development of improved weld consumables, weld process control, understanding of inspection requirements and impact of inspection results, and concurrent engineering approaches to shipbuilding. In the maintenance area, primary focus is on weld repair methods and adequacy of repairs. Successful incorporation of new steels and procedures requires a thorough understanding of their effects on weldability. Although much development has been performed over the past few years for welding high-strength steels (80–100 ksi) without pre-heat, there are still no commercially available tubular or flux-cored wire consumables.
OCR for page 33
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program Project 96M-G, “Development of High Performance Tubular/Flux-Cored Weld Filler Metal, ” has been proposed to unite a developer to a commercialization source to develop these materials and ultimately to make them commercially available. As overall toughness increases with better steels, isolated low toughness attributable to local brittle zones becomes more of a problem. Recognizing the significance of local brittle zones, the offshore marine structures industry has established criteria for the preproduction qualification of steel plates to be welded. However, there is not complete agreement among researchers as to the mechanism(s) involved in the creation of these zones. The CMS recommends that Project 96M-B, “The Definition of Local Brittle Zones in the Heat-Affected Zones of Low Alloy High-Strength Steel Weldments,” be undertaken to define the nature and causes of local brittle zones. The results of this project would lead to improved understanding of the nature of the zones and their causes, resulting in more-realistic criteria for weldable steels and welding processes in marine structures. One area identified as promising for enhanced producibility involves weld-over-primer technology. It is generally necessary to remove primers prior to welding because of their adverse effects (porosity in particular) on weld-metal properties. Development of weld through primer technology would eliminate grinding before welding and would result in a major saving in weld-surface preparation. An SP-7 project funded by the National Shipbuilding Research Program will develop and evaluate improved weldable primers. However, there is no agreed-upon standard method for acceptance criteria to guide paint manufacturers or shipyards in testing these coatings and comparing their weldability. The industry needs such methods and criteria to evaluate their acceptability, based on weld properties and engineering analysis. The industry also needs to verify that the current porosity criterion for acceptance is sufficient for new paint systems. The CMS, therefore, proposes Project 96-16, “Weldable Primers for Ship Construction,” to better facilitate development and use of primers that can be applied to structural steel for the purpose of minimizing weld preparation costs. When developing procedures for manufacturing and repair of welded ships, it is necessary to predict or estimate the properties of welded alloys. Currently, information is pieced together from various sources, and, in many instances, insufficient information is available for a given application. One single source of information does not exist for many of the important materials used by the maritime industry. The CMS recommends Project 96M-N, “Atlas of Welding Properties and Procedures,” to develop an atlas to be used for design and manufacturing of critical components. The domestic steel industry will soon produce high performance TMCP steels produced with new accelerated cooling (AC) and/or direct quenching (DQ) methods. These steels will have higher strengths, better toughness, and lower costs than conventional 50-ksi steels. Weight and cost savings, as well as better toughness, could be realized by the substitution of 65-ksi TMCP steels, for example, for conventional 50-ksi steels in hull structural applications. Key to successful implementation, however, is verifying that current welding procedures, used for lower-strength steels, can also be used for these new steels. Thus, the CMS recommends Project 96M-Q, “Performance and Optimized Weld Metal Properties of Domestically Produced AC/DQ Steels,” to facilitate introduction of these steels in shipbuilding. The evaluation of consumables for 65-ksi
OCR for page 34
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program yield steel in this project would follow Project SR-1343 “Optimized Weld Metal Properties for Ship Structures,” which is establishing the viability of slightly undermatched consumables for 100-ksi-yield steels. Improved fabrication technologies of many kinds are necessary to enhance 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 domestic shipyards focus on commercial new-ship construction as a sequel to the decline of military business, opportunities will increase for utilizing more automated and robotic processes. One of these is extremely efficient higher-deposition welding processes. It is essential, however, that the toughness criteria for the heat-affected zone and weld metal of very high heat input weldments be well established for specific ship applications. Welding technology also needs to be improved so that more-productive welding of high-strength steels can be done without compromising structural integrity. The steel 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 steel industry builds TMCP facilities either under a Department of Defense Title III program (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-metal hydrogen-assisted cracking and the strong effect that welding process parameters have on the properties of those welds. As a result, Project SR-1357, “Retention of Weld Metal Properties and Hydrogen Cracking,” is contributing to the capability of the shipbuilding industry to incorporate the new, high-strength steels that are easier to weld. Economic and reliable methods for performing repair welding of ships ' structure are also major concerns. Project SR-1376, “Methodology to Establish the Adequacy of Weld Repairs,” will be initiated soon and is intended to provide guidance as to whether the repair should be performed. This will improve the safety and reduce the costs of maintenance procedures. Another proposal, Project 96 M-M, “Underwater Weld Repair Using Solid State Welding, ” would evaluate methods to repair corrosion and other damage. This research is intended to evaluate a novel method to perform wet-weld repairs with potential for better properties, minimum porosity, and lower operator skills than conventional wet-arc welding. Inspection and Integrity Marine structural integrity during construction and service is a continuing concern of the CMS, especially with the extended service life of existing structures. Predictability of structural integrity has been improved by improved understanding as to how specific defects contribute to structural failure. Approaches and methodologies of inspection for these defects are being further improved. Nondestructive examination is becoming more quantitative with new analytical approaches to the assessment of structural integrity. The
OCR for page 35
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program possibility of using imbedded sensors and interpretation analysis to monitor in-service structural behavior also exists. The capability to identify a significant defect by such means is a potentially useful engineering tool, since knowledge of defect shape, orientation, and distribution can be translated into a statement of integrity and residual service life through fracture mechanics. For these reasons, and to evaluate the adequacy of inspection methods, Project 96M-I, “Ultrasonic/Radiographic Inspection of Hull Structure,” is proposed. Visual inspection remains the most popular and extensively used method for assessing the status of ship hulls, but questions concerning accuracy, reliability, and repeatability remain. To address these questions, Project SR-1375, “Detection Probability Assessment of Visual Inspection of Ships,” is being initiated. The CMS has promoted new analytical techniques for evaluating defective structure in the marine structures industry, by completing two relevant projects, “Relationship Between Inspection Findings and Fatigue Reliability ” (SSC-355) and “Influence of Weld Porosity on the Integrity of Marine Structures” (SSC-334). Innovative methodologies are needed to reduce the amount and frequency of nondestructive testing. One such method is addressed in Proposed Project 96M-J, “Development of Smart Coatings for Early Detection of Underfilm Corrosion.” The petroleum production industry will be relying increasingly on remote inspection techniques for deepwater structures, for example, by 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 will 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). Criteria 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. As regards weld repairs, it is, unfortunately, often difficult to determine whether a proposed repair will be an improvement. The decision whether to repair or to operate without specific repairs should be made on a technical basis. Project SR-1376, “Methodology to Establish the Adequacy of Weld Repairs,” will be initiated to provide guidelines that are intended to both improve safety and reduce costs of maintenance procedures.
OCR for page 36
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program 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, “Inspection of Marine Structures,” was initiated to quantify inspection performance and address inspection cost. This program will 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 dry 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 recommended procedures for permanent in-service underwater repair of structural damage. Project SR-1356, “Strength Assessment of Pitted Plate Panels,” has been initiated and relates to this. Frequently weld repairs are required on materials that are crack sensitive and require a postweld heat treatment. However, postweld treatment is often impractical for these structures. A temper-bead technique is one method of performing sound repairs in crack-sensitive materials without postweld heat treatment. Since there are no definitive guidelines for using this technique, the CMS is considering defining a project on recent developments, applications, and guidelines for temper-bead techniques. Establishment of this project, and the proposal of any specific new research in this area awaits completion of a joint industry project from an independent source. Reliability Technology Area Reliability technology is the application of probability and statistics to engineering analysis and design. Incorporating modern reliability technology into decision making for marine structural engineering promises to improve quality and performance. The results will ensure production of a structure that has an improved balance of risk relative to that produced by current procedures. The implication is a more efficient, balanced design and has possible structural weight savings compared with current design procedures. The design 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. Engineering 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 concept 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 American Petroleum Institute has recommended this technology for offshore structures
OCR for page 37
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program with their publication RP2A, “Recommended 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 experience, 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 all relevant information. Probabilistic methods allow engineers to make decisions based on a quantitative description of uncertainty, in addition to reaching a consensus in structural design based on experience and judgment. The process of developing reliability technology for marine structures unifies the thrusts of many other active and recommended projects that had appeared to be unrelated in earlier times. 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 various loads and structural capabilities. To kick off the program, the SSC cosponsored a symposium and funded a tutorial 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 Marine Structural Reliability Symposium was held in Arlington, Virginia, in October 1987.3 It attracted experts from around the world and provided a forum for assessing the state of the art in reliability methods. The report “Application of Reliability Methods to Analysis and Design of Marine Structures” (SSC-351) is a tutorial on applying reliability to marine structures. A 1-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. The program 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-Based Design Approach for Ship Structures Phase 1: Demonstration Project A demonstration project, “Probability-Based Ship Design Procedures: A Demonstration” (SSC-368), was completed in 1993. The study compares a hull girder designed by present conventional American Bureau of Shipping rules with a design that uses probability-based procedures, illustrating the applications of 3 Ship Structure Committee and the Society of Naval Architects and Marine Engineers. Proceedings of the Marine Structural Reliability Symposium, October 5-6, 1987. New York: Society of Naval Architects and Marine Engineers, 1987.
OCR for page 38
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program this approach and identifying its advantages and problems. The project report will be useful for information, instruction, and future reference. Phase 2: Loads and Load Combinations 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 further investigation are addressed in this project. Modeling errors were addressed in the project “Uncertainties in Stress Analysis on Marine Structures” (SSC-363), which was completed in 1991. The project's materials counterpart, “Uncertainty in Strength Models for Marine Structures” (SSC-375), was completed in 1993. Phase 3: Implementation The third phase of probability-based design approaches is Project SR-1345, “Probability-Based Design: Implementation of Design Guidelines for Ships,” which will develop a more detailed probability-based design procedure for ships. Load models provided by the Phase 2 project will be combined with strength formulations from the supporting project, “Uncertainty in Strength Models for Marine Structures” (SSC-375). This Phase 2 project will develop design procedures based on reliability considerations similar to reliability-based design procedures used for other structural applications worldwide. The procedures will include provisions for ultimate strength of hull girders; design of stiffened panels; fatigue of details (typically connections); and buckling, which will require further investigation for marine structures. The first part of Project SR-1344, “Assessment of Reliability of Existing Ship Structures (Phase 1),” is now complete, and it will be useful for the Phase 3 reliability project. Phase 2 of Project SR-1344 began in 1994. 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 all of the pieces together. The synthesis will provide a summary of reliability technology for specific application to (1) 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: LRFD Design Practice Several SSC projects have introduced load and resistance factor design. It is time now to put this design procedure into practice. Proposed Project 96-4, “Probability-Based Design (Phase 5): Load and Resistance Factor Design (LRFD) Methods for Ship Structures, ” will include a rigorous and complete code calibration for the design of ship structure. The resulting load and resistance factor design criteria, including all failure modes, will be written in a code style that is suitable for the direct use of practicing engineers. This phase should have higher priority than the phase for novel hull-form design, but the two research projects may be performed concurrently.
OCR for page 39
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program Phase 6: Novel Hull Forms and Environments The sixth phase will address reliability-based design processes for novel structures. Project 96D-O, “Probability-Based Design (Phase 6): Novel Hull Forms and Environments,” is proposed for Phase 6. 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 principles must be applied, because these designs 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 or are subject to uncommon environments. Reliability of Existing Ship Structures Knowledge of the probabilistic characteristics of important failure modes would be useful in developing rational probability-based design criteria. This information could be used on an ad hoc basis to review or revise present procedures or to develop an entire design-criteria document. For successful implementation of a load and resistance factor design code, an estimation of modeling bias is required. In support of efforts to develop this information, the CMS recommends Project 96-20, “Experiments on Stiffened Panel Collapse and Estimation of Modeling Bias.” This research should provide, for example, the much needed uncertainty data on stiffened panel collapse. In order to develop a method for the kind of failure definition needed to calculate structural reliability, Project 96-3, “Failure Definition for Structural Reliability Assessment,” has been proposed. In support of this effort, Project SR-1380, “Post Yield Strength Of Structural Members,” is intended to provide additional means to verify the load-carrying capacity of structural components.
OCR for page 40
Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FYs 1996-'97 and Later-Years Research Program This page in the original is blank.
Representative terms from entire chapter: