Marine Structures Research Recommendations: Recommendations for the Interagency Ship Structure Committee's FY 1995 and Later-Years Research Program (1995)

Table 7 lists the projects recommended for accomplishment in later years than FYs 1996–97. They are not listed in any priority order but are grouped by CMS thrust area. The projects are designated with a “ 96D-” prefix if they have been developed by the Design Work Group and with “96M-” if they have been developed by the Materials Work Group. If the project is continued from a project proposed in the 1994 report, the designation used in that report is appended to the title. In the individual project descriptions, the initials of the work group member who originated the project is given.

Objective Foster interest in the design of innovative marine structures through sponsorship of a student design contest in the United States and Canada.

Benefit Provide a significant educational experience for students in marine-structures engineering. Bring practitioners selected by the SSC to foster and reinforce student and faculty interest in naval architecture and marine engineering. Foster the development of innovative and producible marine structures. Publicize the activities and function of the SSC.

SSC National Goal Support the U.S. and Canadian maritime industry in shipbuilding, maintenance, and repair.

SSC Strategy Sponsoring university research in areas such as design tools development, producibility, production processes, reliability design, and damage-tolerant structures

Background During the last few years, conferences have speculated on the next generation of commercial and naval ships and on offshore platforms ranging from near-shore to deep-water platform designs. Relevant research is underway in the areas of composite materials, plastics adhesives, fabrication, maintenance, inspection, and many other areas, which will allow the development of novel hull designs.

All undergraduate engineering students are required to complete a capstone design course, and graduate programs are considering a similar requirement. Thus, there appears to be an opportunity for the SSC to foster interest in the design of innovative marine structures through the sponsorship of a design contest in the United States and Canada.

The design competition would require that the design be practical and optimized for producibility and maintainability. A single international committee would be selected from practitioners and academicians by the SSC to (1) oversee the competition and present brief lectures on key subjects to each of the student design teams, (2) serve as outside consultants to the students, and (3) evaluate and determine the winning project.

This contest would serve several functions. First, it would educate prospective U.S. and Canadian marine structural engineers by immersing them in “real-life” design problems. It would also bring a high level of design expertise together with the innovativeness of youth in order to work on the conceptual structural design of platforms and hulls. Third, it would serve as a conduit leading to a professional career in the shipbuilding and offshore communities for a limited number of gifted young people.

Recommendations Perform the following tasks:

• The SSC will name and fund the travel of a committee to oversee the design competition and publicize the contest and its results.

• Together with the SSC, the committee will decide whether the competition will focus on one mission statement with competing designs or whether it will allow a variety of problems with the attendant variation in designs.

• Together with the SSC, the committee will establish environmental design conditions and problem constraints.

• Together with the SSC, the committee will decide whether to have a single competition open to both graduate and undergraduate students or whether to hold two concurrent competitions, one open to graduate students and the other to undergraduate students.

• Make awards in the amount of $5,000 to a maximum of eight universities with strong programs in naval architecture or ocean engineering. The monies should be used to support the design project, prepare the final report, and provide funds for students to travel to the SSC to make a presentation. Matching funds from the university coffers would be encouraged. The basis for selection of these universities will be the breadth and depth of their naval architecture or ocean engineering programs and the technical merit and general viability of their initial design proposal. The initial brief proposal will outline the proposed design concept and technical approach and will provide pertinent information concerning the university's naval architecture or ocean engineering program (e.g. curriculum, faculty credentials, number of graduate/undergraduate students, etc). The ensuing design projects by the selected universities will span the fall and spring semesters of one academic year.

• Award a prize of $5,000 to the department and winning design team with the project that is considered to be the best on the basis of a written report and formal oral presentation. Award a second prize of $3,000. Details will be finalized by the committee and the SSC.

• Each school will:

  1. Select a faculty member to lead the initial proposal preparation and direct the ensuing design project.

  2. Develop a conceptual futuristic structural design based on the most up-to-date technology available and on the operational requirements for the structure. The design may reflect as-yet unproven features, but it must have a high probability of success of the necessary research and development if performed. The design team should outline the structural research and development required for the proposed concept.

• Produce a final report and give an oral presentation to a special meeting of the SSC in conjunction with a Society of Naval Architects and Marine Engineers meeting, perhaps as a special session in the regular meeting program.

Duration 200 labor hours over 2 years, with up to $48,000 in awards and prizes to schools. An additional $30,000 will be required to fund travel and per diem expenses of committee member visits to the competing universities for guest lectures and design consultation (four individual visits during the school year to each university).

Objective Develop human and organizational error (HOE) quality assurance and quality control approaches for construction and maintenance of ship structures.

Benefit Improved ship structure safety (strength), durability, and serviceability.

SSC National Goals

• Improve the safety and integrity of marine structures.

• Reduce marine environmental risks.

• Support the U.S. and Canadian marine industry in shipbuilding, maintenance, and repair.

SSC Strategy

• Development of better design tools and information systems

• Improved engineering analysis and evaluation

Background From 1993 to 1994, the Ship Structure Committee sponsored the project “The Role of Human Error in Design, Construction, and Reliability of Marine Structures” (SSC-378). This project explored the implications of HOE as it related to development of a probability-based load and resistance factor design guideline for ship structures. The study concluded that the management of HOE should be concentrated on improved quality assurance and quality control in the design of ship structures. Examples were developed to illustrate how this might be done in the design of ship structures for durability (fatigue resistance) and in the applications of finite element methods to the design of ship structures.

SSC-378 was a first step that was intended to develop approaches to explicitly integrate considerations of HOE into the design of ship structures. Similar attention needs to be focused on developing methods to explicitly integrate considerations of HOE into the construction and maintenance of ship structures through the development of HOE quality assurance and quality control approaches.

Recommendations Perform the following tasks:

• Conduct a literature survey, shipyard visits, visits to ship operators, and a visit to the American Bureau of Shipping to identify current quality assurance and quality control procedures relative to ship construction and maintenance.

• Analyze the current procedures relative to HOE considerations.

• Recommend specific changes in current procedures or additional steps needed to reduce the undesirable impacts of HOE.

Duration 1,400 labor hours over 1.5 years

Objective Define the actual nature and causes of local brittle zones in the heat-affected zones of low-alloy high-strength steel weldments.

Benefit Successful completion of this project will result in improved understanding of the nature of local brittle zones and their causes and will lead to criteria for weldable steels and welding processes that are more realistic for marine structures.

SSC National Goal Improve the safety and integrity of marine structures.

SSC Strategies

• Development of structures-related producibility technology

• Development of reliability design techniques to optimize material use

Background In recent years, the incidence of low fracture toughness in test-weld heat-affected zones (HAZs) in high-strength low-alloy steels has increased. Specifically, those low-toughness values are attributed to local brittle zones (LBZs) in the coarse-grained region of the HAZ adjacent to the fusion line. Recognizing the significance of LBZs, the offshore energy industry has established criteria for the preproduction qualification of steel plates to be welded in offshore structures. These criteria cite, among other things, (1) the amount and type of coarse-grained HAZ structure that must be included in the fatigue-crack-front of the fracture mechanics specimen taken from a test weldment and (2) the minimum acceptable elastic-plastic fracture toughness exhibited by such specimens.

Uncertainties are introduced in this process through the indefiniteness in the extent of LBZs and what their characteristics are, such as constituents, their volume fraction, grain-size presence or absence, or some combination of these parameters. At the present time, the offshore energy industry believes that LBZs are brittle martensite/austenite constituents that are likely to occur in the unaltered, intercritically, and subcritically reheated coarse-grained regions of the HAZ in a multipass weld. They also believe, collectively, that a low carbon equivalent is the important parameter controlling the occurrence of LBZs. Further, at least 15 percent of the fatigue-crack front of the fracture specimen (taken from the test weldment) must sample the three coarse-grained regions mentioned above for a specimen to be a valid assessment of HAZ toughness in the eyes of that industry. Conversely, at least one prominent investigator contends that HAZ-toughness is controlled only by the length of the intercritically reheated coarse-grained HAZ near the point of fracture initiation.

Recent work has questioned the influence of vanadium on the formation of brittle martensite/austenite constituents in the HAZ of high-strength, low-alloy steels. No significant role of nitrogen in HAZ toughness has been recognized for some time, but HAZ toughness is complicated by the presence and amount of titanium. Above a certain volume and size (possibly influenced by niobium), titanium nitrides may act as LBZs in the HAZ.

In all likelihood, all of the above phenomena may act at one time or another, or in concert, to reduce the HAZ toughness of high-strength, low-alloy weldments. Identification of LBZs should point to their sources—the chemical composition and thermal-mechanical processing of the steel and the welding parameters of the joining process. Thus, specifications for both high-strength, low-alloy steel and the concomitant joining processes should result that will reduce significantly, or eliminate, the incidence of low HAZ toughness in these welds.

Recommendations Perform the following tasks:

• Manufacture high-strength, low-alloy plates with a variety of compositions and using several thermal-mechanical processes (e.g., normalized, accelerated-cooled, direct quenched, or quenched and tempered).

• Manufacture high-strength, low-alloy weldments using optimum parameters at several heat inputs.

• Perform fracture toughness tests on specimens taken from the high-strength, low-alloy weldments.

• Through metallographic examination, identify the point of fracture initiation in each specimen and identify the LBZ(s) responsible for that initiation.

• Recommend criteria bounding the chemical composition of the steel, its thermal-mechanical processing parameters, and the welding parameters of the joining processes to reduce significantly, or eliminate, the incidence of low HAZ toughness.

Duration 2,000 labor hours over 2 years

Objective Reduce new-ship construction costs and improve reliability through the development of high-strength weld metals with yield strengths of 550 MPa to 690 MPa and above (80 ksi to 100 ksi) that combine this high strength with excellent toughness and the ability to be deposited without the application of preheat.

Benefit Development of this type of filler will allow the advantages of the new high-strength, low-carbon, low-alloy steels to be fully realized.

SSC National Goal Support the U.S. and Canadian maritime industry in shipbuilding, maintenance, and repair.

SSC Strategy Development of structures-related producibility technology, such as faster welding techniques

Background New high-toughness, high-strength steels are beginning to be available commercially. These materials could be used in designs that increase reliability, reduce ship weight, and possibly reduce costs. Innovative hull designs may be able to

significantly benefit from utilization of these steels. Previously, steels of this strength level were not generally used, in part because of their susceptibility to hydrogen-assisted cold cracking in the HAZ. The application of preheat is generally needed when conventional quenched and tempered high-strength steels are used. The new steels with low carbon and high toughness are less susceptible to this type cracking and are candidates for ship fabrication welding without preheat.

The difficulty with welding these steels without preheat, however, is that matching weld metals are still susceptible to hydrogen-assisted cold cracking. The introduction of new thermomechanical processing techniques is allowing the steel makers to lower carbon and alloy content in these base materials. However, the current weld metals cannot take advantage of these techniques and generally require increased alloy content to match the strength and toughness levels of the base metal. These trends in alloy content have changed the relative transformation characteristics of the weld metal compared with those of the HAZ. Shifts in location of the austenite to ferrite transformation and the martensite start temperatures may allow less time for hydrogen movement into the HAZ. As a result, there may be a tendency for more hydrogen to remain in the weld metal and the hydrogen-assisted cold cracking problem can move from the HAZ to the weld metal.

Closely aligned with the development of new consumables must be the development of better weldability test procedures for weld metals. The current project SR-1357, “Retention of Weld Metal Properties and Hydrogen,” addresses this area. New consumables, with very low carbon bainitic microstructures, or cored wire with metal powder containing hydrogen-getters, hold promise for high-performance weld fillers. The very low carbon, bainitic weld metals also appear to have less sensitivity to the effect of heat input on properties than conventional weld metals. Application of lower-strength under-matching weld metals can be effective but is limited to specific applications.

Significant research toward the development of solid wire welding consumables in these strength ranges is underway or has been completed at the National Center for Excellence in Metals Technology, the Carderock Division of the Naval Surface Warfare Center, and the Colorado School of Mines,¹ as well as at other locations. It is proposed that this project concentrate on the tubular or flux-cored filler wire product, building on the results of the aforementioned research.

Recommendations Perform the following tasks:

Phase 1

• Review the results of studies sponsored by the National Shipbuilding Research Program and the U.S. Navy.

• Identify one or more commercial suppliers of weld consumables who could form a partnership to address the problem(s).

• Identify theoretical and empirical methodologies and appropriate weldability test procedures applicable to weld consumable development. This should be

performed in close alignment with Project SR-1357, “Retention of Weld Metal Properties and Prediction of Hydrogen Cracking.”

Phase 2

• In partnership with a commercial weld consumable supplier, identify candidate tubular or flux-cored product with potential capability of matching the strength and toughness properties of new high-performance steels when welded over a wide range of heat input without preheat. Good operability and good feedability for operation in all positions are required. Cost-sharing with a supplier should be explored.

• Produce and test candidate materials, and report results and recommendations.

• Recommend commercial specification and commercialization plan.

Duration Phase 1 1,500 labor hours over 1 year

Phase 2 4,000 labor hours over 2 years

Objective Investigate the available literature and data on the effect of weld quality as determined by visual inspection of the surface of the weld on weld properties and hull structural integrity. Where data are needed, recommend experimental work, and issue a comprehensive report that can be used by the industry to access the effect of the weld surface attributes on service performance.

Benefit The industry needs information relating weld surface quality to service performance to make rational decisions on the specifications for new commercial ships, as well as to make judgments on the shop floor when special situations arise. A review of the subject and development of the comprehensive report would avoid costly grinding and weld finishing of nonrelevant imperfections in weld surfaces. The application of rational criteria would preclude service failures resulting from acceptance based on inadequate specifications now in use.

SSC National Goal Improve the safety and integrity of marine structures.

SSC Strategy Improved engineering analysis and evaluation

Background As a result of problems with service failures from a recent generation of tankers, there has been a heightened concern for the effect of workmanship and weld-surface quality on hull integrity. Weld visual quality is measured by a number of attributes that are inspected on the job. The National Shipbuilding Research Program Welding Panel (SP-7) has developed a set of plastic replicas representing acceptable, marginal, and rejectable conditions for visual inspection attributes as described in current specifications for welds. Attributes, such as porosity, surface roughness, undercut, irregular contour, and re-entrant angle, are modeled representing conditions that the industry agreed were the limits for various categories of ship structure. As a result of

recent concerns over tanker failures, there has been renewed interest in tightening standards that have previously been considered acceptable for hull structure. The quality of an as-deposited weld can be controlled within certain limits by the technique and process used. However, finishing processes such as grinding must be used to achieve tighter surface-quality requirements. These weld finishing operations are costly to implement. It is possible that by attention to specific, as-deposited weld-quality attributes, weld performance could be significantly improved without imposing higher construction costs.

Recommendations Perform the following tasks:

• Conduct a literature search, and collect available data on the relationships between weld-surface quality and weld mechanical properties including fatigue performance. Provide a quantitative assessment of each surface attribute's impact on hull integrity. Use the different levels of acceptable conditions established by the National Shipbuilding Research Program SP-7 Panel.

• Prepare a report summarizing results of the investigation, and make recommendations for further experimental work that may be needed to relate weld-surface quality to service performance and hull integrity.

Duration 1,500 labor hours over 1 year

Objective Provide an easy-to-use source of information for welding properties and procedures for common shipbuilding materials. Identify areas where insufficient information is available.

Benefit Completion of this project would improve competitiveness by reducing time required for design and manufacturing decisions and improving consistency and quality of manufactured structures.

SSC National Goal Support the U.S. and Canadian maritime industry in shipbuilding, maintenance, and repair.

SSC Strategy Development of better design tools and information systems

Background When designing manufacturing and repair procedures for welding 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. A single source of information does not exist for many of the important materials used by the maritime industry. This project will develop an atlas of welding procedures and properties to be used for design and manufacturing of critical components. A designer could then refer to the atlas to assess whether suitable welding procedures existed for materials considered in new designs. Shipyard contractors

could also qualify procedures more quickly if existing service-proven procedures were identified.

Recommendations Perform the following tasks:

• Define commonly used materials, welding processes, and the type of information needed.

• Accumulate property and procedure information from literature searches and contacts with material suppliers and shipbuilders.

• Compile information into an easily used information base or atlas.

• Define areas where insufficient information is available and more work needs to be done.

Duration 1,000 labor hours over 9 months

Objective Characterize the performance of domestically produced, high-performance, accelerated-cooled/direct-quenched (AC/DQ) steel-plate material.

Benefit Reduce the weight and cost of ship structure while maintaining reliability.

SSC National Goal Support the U.S. and Canadian maritime industry in shipbuilding, maintenance, and repair.

SSC Strategy Development of structures-related producibility technology

Background High-performance steels will be produced in the future by advanced processing equipment capable of accelerated cooling or direct quenching. These steels will have better properties from less costly chemistries. The currently produced high-strength steel grades, AH36, DH36, and similar grades, should be producible at 65-ksi minimum yield strength at little or no extra cost, and steels with a yields strength of 80-ksi to 100-ksi will be provided at less cost. A 65-ksi yield-strength steel structure is potentially producible for almost the same cost as a 50-ksi yield-strength steel structure, even without accounting for weight and weld-volume reductions that could occur with the use of higher strength steel.

Steel with a 65-ksi yield-strength can also be produced to offer improved weldability, formability, and toughness. Plate with a 65-ksi yield-strength should be able to be joined with slightly undermatched 70-class (70-ksi ultimate tensile strength) welding consumables and still obtain 100 percent joint efficiency based on transverse strength. Initial results from Project SR-1343, “Optimized Weld Metal Properties for Ship Structure,” on steels with a 100-ksi yield-strength indicates that weldment transverse strength is not affected by a slight yield-strength undermatch. If 70-class weld metals could be used to join a steel with a 65-ksi yield-strength steel, the shipbuilder could use

the same electrodes and production procedures now used to join 50-ksi and ordinary-strength steels. Fabrication and welding costs would then essentially be the same as for welding the lowest-strength steels.

The new processing methods will produce very tough structures that should exceed the toughness of E-level steels with no increase in cost. This high toughness will be useful for crack-arrest applications as well as for hull structure.

Weight savings can be realized where design permits use of thinner steel with a 65-ksi yield-strength in lieu of steel with a 50-ksi yield-strength. Cost savings could be realized from lower weld-metal volume.

Recommendations Perform the following tasks:

• Obtain representative steel plates with a 65-ksi yield-strength produced by accelerated cooling/direct quenching, and fabricate weldments from them using near worst-case conditions for strength and toughness. Worst case implies higher heat input and higher interpass temperatures and thinner plate.

• Evaluate static strength and toughness.

• Make recommendations on welding consumables considering integrity and design.

Duration 1,500 labor hours over 1 year

Objective Develop prototype probability-based design criteria for novel marine structures.

Benefit Results of this project would provide the basis for reliability assessment and probability-based design for novel marine structures.

SSC National Goals

• Improve the safety and integrity of marine structures.

• Support the U.S. and Canadian maritime industry in shipbuilding, maintenance, and repair.

SSC Strategies

• Development of reliability design techniques to optimize material use

• Development of better design tools

Background A 5-year research program has been initiated by the SSC to apply reliability technology to marine structures and to develop probability-based design criteria for ship structures. The program consists of a sequence of six projects leading to probability-based design for both ships and novel marine structures as well as to related reliability issues. This project is the sixth phase of this program. The first phase of the program

was a demonstration project (SSC-368). The second phase defined loads and load combinations (SSC-373). The third phase, implementation of design guidelines for ships, was an SSC FY 1992 project (SR-1345), and the fourth phase (SR-1362) summarized the present state of the art in reliability design. The fifth phase, recommended as Project 96-4, “Probability-Based Design (Phase 5): Load and Resistance Factor Design Method for Ship Structures,” is a four-phase project that will implement the load and resistance factor design methodology in ship designs.

The sixth-phase project, proposed for the out-years, will address reliability-based design processes for yet undefined novel structures. Novel structures are unconventional hull forms or structures subject to unusual loads. In recent years, innovative (sometimes large and costly) marine structures have been designed, built, and operated for offshore oil and gas development and marine transportation. The premise of this project is that novel situations must be addressed by applying first-principles of engineering because design of novel structures cannot be based on extrapolation or interpolation of the current practice used to design existing structures such as conventional vessels. This project will define the current database, existing body of structural reliability literature, and necessary elements to conduct a probabilistic assessment of performance and safety of marine structures having special features or subject to special environmental loads. The main thrust of the project will be to produce a generic prototype code for novel marine structures (i.e., a “roadmap” for development of a probability-based code).

Recommendations Perform the following tasks:

• Develop a strategy for constructing reliability-based design criteria for novel structures.

• Study the literature on structural reliability, particularly as it relates to special marine structures, including SSC-sponsored reliability projects.

• Study documentation of the American Institute of Steel Construction load and resistance factor design method, American Petroleum Institute load and resistance factor design method, the Canadian Standard Association 's proposed program for offshore structures, and other relevant literature on probabilistic design-code development, particularly that literature related to marine environments.

• Identify generic failure modes that would apply to all types of marine structures, and develop appropriate limit-state expressions to be used for general reference and default values.

• Identify generic loading models appropriate for special marine structures.

• Provide data summaries, particularly uncertainty levels, on stress and strength design factors, as appropriate, to be used for general reference and default values.

• Propose practical procedures for performing ad hoc reliability assessment for special marine structures.

• Provide a brief summary of the issues, methods, and alternatives associated with development of a probabilistic design code. Issues to be addressed include the format of the code (e.g., working stress versus partial safety factors) and

methods for establishing design criteria (e.g., calibration versus an absolute probability of failure level).

• Produce a generic structural design code that could be employed for a wide variety of novel marine structures, including general recommendations for specifying the code format and default values for loading and strength variables.

Duration 2,000 labor hours over 1.5 years

Objective Define the statistical characteristics of strength properties of currently used steel in marine structures.

Benefits The results of this project will help to develop load and resistance factor design criteria and thus improve structural safety and integrity.

SSC National Goals

• Improve the safety and integrity of marine structures.

• Reduce marine environmental risks.

• Support the U.S. and Canadian marine industry in shipbuilding, maintenance, and repair.

SSC Strategy Development of reliability design techniques to optimize material use

Background The SSC sponsored a group of projects whose object was to develop load and resistance factor design criteria for ship structures. In order to achieve this objective, basic strength parameters need to be defined in statistical forms. The literature reviewed in the area reports information that is either incomplete or not current.

Recommendations Perform the following tasks:

• Review the literature on strength properties of steel currently used in marine structures.

• Contact various sources, including shipyards, steel producers, and users to quantify the statistical characteristics of yield strength, ultimate strength, modulus of elasticity, and Poisson ratio. The developed characteristics should be based on currently used steel.

• Provide the information in the form of descriptive statistics and histograms that have been normalized with respect to specified values.

Duration 1,500 labor hours over 1 year

Objective Define the statistical characteristics of geometric properties of currently used plates and structural shapes in marine structures.

Benefits The results of this project will help to develop load and resistance factor design criteria and, thus, improve structural safety and integrity.

SSC National Goals

• Improve the safety and integrity of marine structures.

• Reduce marine environmental risks.

• Support the U.S. and Canadian marine industry in shipbuilding, maintenance, and repair.

SSC Strategy Development of reliability design techniques to optimize material use

Background The SSC sponsored several projects whose goal was to develop load and resistance factor design criteria for ship structures. In order to achieve this objective, basic geometric parameters need to be defined in statistical forms. The literature reviewed in the area reports information that is either incomplete or not current.

Recommendations Perform the following tasks:

• Conduct a literature review to collect statistical data on the geometric properties of plates and structural shapes used by the marine industry.

• Contact various sources, including shipyards, steel producers, and users to quantify the statistical characteristics of plate thickness, stiffener geometry, spacing of stiffeners, geometry of transverse elements, and geometry of bulkheads. The developed characteristics should be based on currently used steel. Also, any special requirements or specifications that affect these statistics need to be reported.

• Provide the information in the form of descriptive statistics and histograms that have been normalized with respect to specified values.

Duration 1,500 labor hours over 1 year

Objective Review and develop correlations between easily conducted qualitative toughness tests (such as the Charpy V-notch impact test) and other quantitative tests for ductile fracture that are more precise and meet criteria for valid fracture-toughness testing.

Benefits Allow the efficient use of new, high-performance steels with superior weldability and toughness. Allow for easier flaw-tolerance analysis procedures, and possibly reduce the need for stringent nondestructive evaluation.

SSC National Goal Improve the safety and integrity of marine structures.

SSC Strategy Development of reliability design techniques to optimize material use

Background Charpy V-notch (CVN) impact energies are not directly usable in a quantitative flaw tolerance calculation but must be converted to some form of fracture toughness measurement, such as a critical stress intensity, K_IC, a critical J-integral, J_IC, or a critical crack-tip opening displacement, CTOD_C. New high-performance steels are expected to be attractive for ship structures because of their improved weldability and toughness. A significant benefit is expected to accrue in flaw tolerance with these new, high-performance steels because these steels will be operating in the upper-shelf toughness regime. Thus, brittle fracture can be completely eliminated in these steels. In order to benefit from this potential improved toughness, the specification requirements must ensure that the steel will have adequate toughness and be in the upper-shelf region at all expected operating temperatures.

Usable CVN–K_IC correlations exist in the transition temperature region. However, there do not appear to be adequate CVN–K_IC correlations in the upper-shelf region. Questions also exist about how to treat the discontinuity at a temperature near the beginning of the upper shelf of these correlations, especially if a temperature shift is introduced to account for the shift from static to dynamic fracture behavior.

The specification of toughness for steels has characteristically been accomplished by requiring a certain CVN impact energy at some specified temperature. CVN testing is the preferred method of toughness specification because of the ease of testing and low costs. Specification of new, high-performance steels should be based on a fitness-for-purpose scheme. That is, the toughness should be sufficient to resist fracture for the anticipated crack sizes and service stresses. If the toughness is to be specified in terms of CVN energy and if the failure must be ductile rather than brittle, then better upper-shelf correlations are needed.

If sufficient confidence can be developed, these improved correlations can also be utilized in a simplified flaw-tolerance analysis.

Recommendations Perform the following tasks:

Phase 1

• Survey the literature pertaining to ductile fracture (upper-shelf) correlations between CVN or other techniques and K_IC or other valid fracture toughness measures. Obtain as much literature data as possible in which both CVN and J_IC or CTOD_C have been measured for upper-shelf conditions, and enter these data into a database to support the development of correlations. Both CVN energy and lateral expansion data should be considered.

• Suggest improvements to those correlation procedures that have significant potential.

• Promulgate revised correlation procedures as necessary.

Phase 2

• Recommend and implement testing of a range of candidate steels to validate or improve the upper-shelf correlation procedures.

Duration Phase 1: 750 hours over 1 year

Phase 2: 1,500 hours over 1 year

Objective Recommend an optimum strategy for control of pitting corrosion in tankers, and develop a methodology for determining an optimum inspection interval that would minimize life-cycle inspection and maintenance costs and will meet safety and pollution-prevention requirements.

Benefit The results of this project would reduce maintenance costs, reduce environmental pollution risks, and compile and divulge lessons learned by the tanker industry on pitting prevention.

SSC National Goals

• Improve the safety and integrity of marine structures.

• Reduce marine environmental risks.

SSC Strategy Improved efficiency for repair technology

Background Tankers with combined cargo and ballast tanks usually present serious pitting problems in the bottom plate, which can result in environment pollution. This problem usually aggravates with a vessel's age due to increase in corrosion rate. Corrosion-control strategies include coating of the bottom plate, cathodic protection, periodic voyage inspection, and inspection during drydock and repair of shallow pits by filling with epoxy pit filler and repair of the deep pits by clad welding. Each strategy has an associated cost and risk of failure that depends on, among other factors, percentage of time in ballast, type of cargo, temperature of cargo, vessel age, and the location of pits on the vessel.

Several studies have been performed and extensive experience on effectiveness of methods for controlling pitting corrosion has been accumulated by tanker operators and owners; classification societies; and other institutions, such as the Tanker Structure Cooperative Forum. In addition, extensive inspection data (such as number of pits, proximity of pits, pit depth, and pit diameter) are usually collected by tanker owners as part of their maintenance and repair program. In order to reduce pollution risks and maintenance costs, there is a need to develop strategies for optimum control of pitting corrosion, inspection frequency, and repair criteria. Due to the random nature of the pitting phenomena, a probabilistic model should be developed to determine the optimum

inspection interval based on the acceptable level of failure risk. The model would be updated based on inspection results.

Recommendations Perform the following tasks:

• Carry out a literature survey in the areas of pitting corrosion prevention and probabilistic modeling of the pitting process.

• Survey operators, ship owners, and classification societies in order to compile information about the performance of pitting prevention systems, and experiments on test pitting-corrosion.

• Develop strategies for pitting-corrosion control that establish optimal consideration of pollution risks and maintenance costs.

• Develop a probabilistic methodology for estimating pitting occurrence and for determining optimum inspection intervals. The model should include the parameters affecting the problem. A methodology for updating the model based on inspection results (number of pits, pit depth, and diameter) should be developed.

• Promulgate pit repair criteria.

Duration 1,000 labor hours over 1 year

Objective Investigate the suitability of current volumetric nondestructive evaluation techniques for inspection of butt welds in hull structure.

Benefit Facilitate use of the most sensitive and least costly method for volumetric inspection of welds. This work would help preclude future weld failures as well as help control inspection costs.

SSC National Goal Improve the safety and integrity of marine structures.

SSC Strategy Improved structural inspection techniques

Background The ultrasonic testing method has been used extensively in Navy ship construction, and considerable documentation is available comparing ultrasonic methods with radiographic ones. Today the most critical hull structural welds in Navy ships are inspected with ultrasonic testing. However, radiography is still used in a number of structural and piping applications. Since ultrasonic testing does not have a permanent record of the process comparable to radiographic film, attention to control of the process through proper training, qualification, auditing, and quality control may be more critical. However, ultrasonic inspection is less than half the cost of radiography and does not cause the major disruption that radiography causes from its radiation. Ultrasonic methods are also more sensitive to planar-type discontinuities, which have more effect on structural integrity.

Recommendations Perform the following tasks:

• Summarize the results of Navy and other work that has been done to establish the relative capabilities of the ultrasonic and radiographic testing methods.

• Investigate the causes of tanker weld failures for association with any nondestructive evaluation technique.

• Develop recommendations, if any, for improvement to specifications and quality control of ultrasonic or radiographic inspection methods.

• Provide a comprehensive report summarizing results of the above.

Duration 1,000 labor hours over 1 year

Objective Develop smart coatings that are capable of detecting early stages of corrosion.

Benefit Smart coatings should greatly assist inspections, enhance reliability, and improve safety.

SSC National Goal Improve the safety and integrity of marine structures.

SSC Strategy Improved structural inspection techniques

Background Protective coatings are typically opaque, and early visual detection of degradation is difficult. In general, failure of protective coatings as revealed by rust-through or blistering is usually indicative of advanced substrate deterioration. Hence, early detection of underfilm corrosion could provide better opportunities to mitigate further degradation, reduce maintenance costs, and ensure continued reliability. There are various electrical and magnetic techniques that can be employed to evaluate coatings for underfilm corrosion. However, these techniques require the development and use of expensive portable equipment, and data interpretation is sometimes difficult. Therefore, development of smart coatings that are sensitive to subtle changes in the chemistry of the metal–coating interface and are capable of indicating early stages of corrosion through color changes or, perhaps, through fluorescence should aid the inspection process. Research and development efforts in this area might include (1) chemical formulation changes of existing coatings, (2) mixing of additives to existing coatings, (3) development of a new generation of coatings, or (4) altering the surface of the substance through various pretreatment steps.

Recommendations Perform the following tasks:

Phase 1

• Conduct surveys on existing technologies.

• Conduct surveys to identify the most common forms of coating failures on commercial ships. The surveys should include the location and the extent of failure, and the environment in which the coating was operating.

• Develop a smart coating system that is capable of indicating early stages of underfilm corrosion.

• Demonstrate the sensitivity of smart coatings to underfilm deterioration.

• Develop the scope for the Phase 2 follow-on project.

Phase 2 (follow-on project)

• Conduct the recommendations as defined in Phase 1 with special emphasis on field demonstrations.

• Develop a plan for establishing a cooperative program among research organizations, the coating industry, and the commercial shipbuilding industry to incorporate the use of smart coating systems.

Duration Phase 1: 3,000 labor hours over 2 years

Phase 2: 2,000 labor hours over 2 years

Objective Evaluate the use of underwater friction welding for permanent repair of damaged commercial ship hulls.

Benefits Solid-state underwater wet welding offers the potential for better properties, no porosity, and lower operator skill requirements when compared with conventional wet-arc welding. Successful use of this technique for permanent repairs could significantly reduce the need for unscheduled dry docking to perform permanent repairs.

SSC National Goal Support the U.S. and Canadian maritime industry in shipbuilding, maintenance, and repair.

SSC Strategy Improved efficiency for repair technology

Background Current underwater wet-weld repairs of damaged hulls, made by arc welding, are typically considered to be temporary due to inferior deposited weld metal and heat-affected zone properties as compared with dry welds. This sometimes leads to unscheduled or early dry docking to produce a more reliable permanent repair. Underwater friction welding trials have shown promise for repair of ship hulls. Feasibility of this process is currently being evaluated for U.S. Navy applications through the Navy Joining Center. This project would be a follow-on to the Navy project and would evaluate the process for commercial ship repair.

Recommendations Perform the following tasks:

• Define property requirements for commercial underwater repairs.

• Review results from the Navy Joining Center feasibility study.

• Perform weld trials and property determinations for commercial ship applications.

• Quantify costs and benefits, and recommend any additional necessary research and testing.

Duration 2,000 hours over 1 year

Objective Establish a design protocol and procedure for cathodic protection retrofit on aging marine structures.

Benefit This project will lead to efficient utilization and life extension of aged marine structures.

SSC National Goal Improve the safety and integrity of marine structures.

SSC Strategy Improved efficiency for repair technology

Background For the past several decades, the primary technique for protecting marine structures, such as tankers, offshore petroleum production structures, and pipelines from corrosion has been cathodic protection. This corrosion mitigation approach has typically been designed for the anticipated life of the structure. However, because of increased emphasis upon life extension, many of these structures receive appropriate repairs or structural augmentations and are retained in service after the cathodic protection system has expired. Thus, retrofit or replacement of the corrosion prevention system is required. Limited recent studies have indicated that for some types of structures adequate retrofit of the cathodic protection system may require only a fraction of the current that was needed initially. Understanding the conditions in which reduced cathodic protection is adequate would reduce costs and lower structure weight.

Recommendations Perform the following tasks:

Phase 1

• Comprehensively and critically review literature pertaining to design and practice of retrofit cathodic protection for structures and structural components in sea water.

• Make recommendations for a Phase 2 research/testing program as required based upon results from Phase 1.

Phase 2 Conduct the Phase 2 research testing plan.

Phase 3 Based upon the results from Phase 2, propose design criteria for retrofit cathodic protection of marine structures. These should address structures where the existing level of protection from cathodic protection ranges from adequately protected to complete loss of protection.

Duration

Phase 1: 500 labor hours over 6 months

Phase 2: 2,000 labor hours over 2 years

Phase 3: 500 labor hours over 6 months

Objective Support more-efficient use of material in marine structures by critically evaluating the appropriateness of new methods for fatigue-crack growth-rate determination in the threshold and near-threshold regimes in the presence of corrosion.

Benefits A test procedure would be established for developing fatigue data for marine steels under realistic test conditions more efficiently. This should contribute to enhanced vessel integrity and safety.

SSC National Goal Improve the safety and integrity of marine structures.

SSC Strategy Development of reliability design techniques to optimize material use

Background Fatigue and corrosion fatigue are generally recognized as important failure modes for marine structures because of the unsteady nature of service loadings. Such loading typically occurs at low frequencies (<0.3 Hz). This, in combination with the fact that the fatigue-crack growth-rates of interest are typically low (10^-5 mm/cycle to less than 10^-7 mm/cycle), means that testing periods on the order of several months or longer may be required to generate even a limited amount of data. In a recently completed SSC project, “Threshold Corrosion Fatigue of Welded Shipbuilding Steels” (SSC-366), a new technique for more expediently developing near-threshold fatigue-crack growth-rates and threshold stress intensity range (∆K_TH) was proposed. The technique uses painted, tapered, constant stress intensity (K) specimens. After load shedding to about K=10 MPa(m) ^½, a locally exposed portion of the specimen near the crack tip is hydrogen charged. Subsequent to achieving steady state, the charging is stopped, paint in front of the propagating crack is removed, and the crack is propagated into uncharged material. Once this occurs, a fatigue-crack growth-rate at low ∆K can be measured. This procedure is repeated at progressively lower ∆K values.

A second, more recently proposed procedure¹ uses tapered specimens with different K values. Their configuration is conducive to testing multiple specimens in series. Consequently, the amount of data acquired per unit test time is in direct proportion to the number of specimens being concurrently tested.

Recommendations Perform the following tasks:

• Critically review and evaluate each of the above two test procedures, including specimen types, along with others that might be identified, with regard to appropriateness for determining fatigue-crack growth-rate for marine applications. Combining or integrating procedures should also be considered.

• Conduct fatigue experiments in air and in sea water (or American Society for Testing and Materials substitute ocean water) on two marine steels (one structural and the second a high-strength, low-alloy steel or one obtained in a thermal-mechanical controlled process), both freely corroding and cathodically polarized. From these experiments, crack-growth curves for each steel under each of three environmental conditions for the range of 10^-4 to 10^-7 (or less) mm/cycle should be determined.

• Recommend a specimen type and test procedure for near-threshold fatigue-crack growth-rate and ∆K^TH determinations for test conditions relevant to marine service.

Duration 3,000 labor hours over 2 years

Objective Identify applications for composite materials in ship structures. These applications should have the potential to reduce weight, provide better damage tolerance, and optimize material use. Draft a plan to incorporate these materials into new and existing ship structures.

Benefit The results of this project will assist in efforts to replace conventional materials with composites for the purpose of enhancing performance, reliability, maintainability, and safety.

SSC National Goal Support the U.S. and Canadian maritime industry in shipbuilding, maintenance, and repair.

SSC Strategy Development of structures-related producibility technology

Background Composites are an attractive class of materials for marine applications. The properties of composites can be tailored to meet specific engineering needs; they offer high specific strength and stiffness. Composites offer the potential to reduce weight, provide better damage tolerance, and optimize material use. In order to fully realize the benefits of composites as applied to ship structural applications, it is necessary to identify those applications that have a high probability for incorporation into ship structure based on currently available technology. Therefore, a comprehensive study must be directed toward identifying those structures or components that could most readily be replaced by composites. To accomplish this, risk assessments should be

conducted in the areas of environmental compatibility, toxicity, and fire resistance. Manufacturing and joining problem areas must be identified. The project should identify a comprehensive plan to phase in the use of composites that will meet SSC goals and strategies. A similar project under the auspices of the Great Lakes Composites Consortium is available.¹ This information should be used in the development of an implementation plan for the use of composites in ship structures.

Recommendations Perform the following tasks:

• Conduct a survey on the use of composites in marine applications.

• Identify those structures and components made of conventional materials that could be replaced by composites.

• Establish the benefits of replacing conventional material with composites, and compare the benefits of using each of the materials.

• Perform risk assessments in terms of environmental concerns.

• Identify challenges to manufacturing, fabrication, and joining.

• Develop a comprehensive plan to phase-in the use of composites in those areas previously identified.

• Develop the scope of follow-on research needed to facilitate incorporation of composites into ship structures.

Duration 1,500 hours over 1 year

Objective Adapt and develop as required, current analysis and design techniques, methodologies, and practices to permit composite materials to become a practical and cost-effective option for the construction of ships and offshore-platform structural components.

Benefit The development of analytic capabilities specifically tailored to meet the requirements of marine composite structures will permit the benefits of composites to be introduced more rapidly into future marine-structure applications.

SSC National Goal Support the U.S. and Canadian maritime industry in shipbuilding, maintenance, and repair.

SSC Strategy

• Development of principles of design for production

• Prevention research, including damage-tolerant structures, structural monitoring, and human factors

Background Although there is currently some usage of composite materials in marine structures, there is immense potential for high-volume usage, not only with the more common fiber reinforcement of glass but also with the more advanced fibers, such as graphite and Kevlar^®. Marine structures such as offshore platforms, naval vessels (ships and submarines), and commercial marine vessels can benefit from increased usage of composite materials.

In order to evaluate the use of composite materials, it is necessary to develop analytical and design techniques specific to the needs of marine structures. The aerospace industry has worked with composite materials for over 20 years and has gained much knowledge and experience. Some of the techniques and analyses it has developed will be applicable to marine structures. However, other specific analytic capabilities must be developed and exercised in the study of marine structure applications. For example, structural optimization methods developed to address complex aerospace design problems need to be introduced into the design of composite structures for ships and offshore platforms. Certain projected applications will extend the performance requirements well beyond the design experience envelope applied in aerospace. Large, nonlinear strain response, for example, has been identified as characteristic of certain marine applications, and thick-walled, three-dimensional analysis methods need to be developed and exercised. Creep effects may also become important in designs tailored for low axial stiffness.

In the recommendations developed at the National Conference on the Use of Composite Materials in Load-Bearing Marine Structures (SR-1331), it was suggested that design and analysis techniques be developed and exercised to study complex structural issues. It was specifically suggested that a collaborative marine–aerospace–automotive industry effort be launched in order to assess the analysis/design issues in marine structural applications of composites. Such an effort should address such questions as: What methods are currently used by each industry? How is concurrent engineering being carried out? What tools and techniques are adaptable to marine needs? And, what segments of the marine industry can benefit from which tools and techniques? These studies are also likely to identify problems requiring unique, new analytical solutions as discussed above.

Recommendations Perform the following tasks:

Phase 1 Analysis Capability Assessment

• Identify the requirements and build an enhanced analytical capability with which to predict and understand the critical structural-design issues associated with ship and offshore-platform-type structures. The approach should involve several investigators assembled from the marine, aerospace, and automotive industries.

• Identify the design requirements and analytical needs for marine structures.

• Identify applicable analytical capabilities currently available in the aerospace and automotive industries.

• Identify analytical capabilities needing further development or not currently available.

• The range of analytical interests should include, but not be limited to, analytical capabilities for basic design, damage-tolerance analysis, detail design of joining methods, study of three-dimensional effects, prediction of failure and residual strength, structural optimization, study of high-strain designs, and creep and nonlinear effects, and similar analyses.

Phase 2 Advanced Analysis Development

• Modify or develop key analytical capabilities that require development as identified in Phase 1 above in order to meet specific marine-structure requirements.

Phase 3 Design Studies

• Exercise the analytical tools adapted from aerospace and automotive experience and developed in Phase 2 above to study critical design issues identified in Phase 1.

Duration Phase 1: 2,000 labor hours over 1 year

Phase 2: 2,000 labor hours over 1 year

Phase 3: 2,000 labor hours over 1 year