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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
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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
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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 defects—inclusions, 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
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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
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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
proposed—Project 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
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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
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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
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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
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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.
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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
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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
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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.
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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.
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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.
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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.
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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
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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
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(including the cra-~-~rowth resistance or R-cu~ve) of the matenal—either 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.
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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
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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.
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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.
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· 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
Representative terms from entire chapter:
ship structures
