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

Table 3 lists the projects proposed for the 1996–1997 program in priority order based on the composite judgment of the CMS members.

Objective Evaluate the suitability of typical shipyard fabrication standards for ship structure constructed of mild steel and high-tensile steels. The evaluation of these standards should be based on an analysis of the structural imperfections that are introduced during ship construction on the structural behavior of the ship. Evaluate the need for revising these standards.

Benefit This project will develop a methodology to determine the effect of construction imperfections on structural behavior and will provide a technical basis for evaluating the suitability of standard tolerances, especially for revising these standards in order to enhance structural behavior.

SSC National Goals

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

• Improve the safety and integrity of marine structures.

SSC Strategy

• Development of principles of design for production

• Prevention research including damage-tolerant structure

Background In general, the vessel designer assumes that the finished construction will represent accurately the configuration defined on the structural drawings, and not make allowance in the design process for certain deviations that may exist in the completed vessel. Yet it is impossible to eliminate all structural imperfections due to inherent errors in the automatic fabrication equipment and the human factors involved.

A number of international agencies and institutions have developed and published ship structural tolerance standards. One of the approaches in developing these standards consists of taking actual measurements of structural deviations, developing histograms of the measured deviations, and establishing the mean standard (range) and the maximum allowable value (tolerance) for each structural deviation. The report, “Survey of Structural Tolerances in the United States Commercial Shipbuilding Industry” (SSC-273), has applied this approach to establish structural tolerance limits representative of U.S. shipbuilding practice. The structural tolerances in general reflect an assessment of shipyard capabilities within a reasonable cost framework, U.S. shipyards' past experience, and the results of some basic research into material and welding properties. With the increase in use of high-tensile steels in vessels and consequent increase in the levels of stresses experienced by these vessels, more research work is needed to develop the technical basis for determining tolerances and to increase the understanding of the combined effect of deviations on stress levels at critical joints. It is important to determine the effect on strength of the structure when all or some percentage of the deviations are at maximum tolerance limit.

Recommendations Perform the following tasks:

• Review results of research regarding the effect of structural imperfections on structural strength.

• Review existing construction tolerance standards and the methodology used in their development.

• Investigate methods such as finite-element analysis techniques and testing procedures to determine the influence of imperfections on structural behavior.

• Develop a methodology or testing procedures to determine the combined effect on structural performance of various types of imperfections at different locations, especially in the strength-critical areas.

• For a selected tolerance standard, apply the methodology in order to determine suitability of the standard and, if necessary, to recommend revisions. The study should compare the effect of imperfections on structural performance, assuming various types and values of the imperfections (up to maximum tolerance), for mild-steel and various high-tensile steels.

Duration 2,000 labor hours over 2 years

Objective Develop a predictive methodology, including the effect of the use of block welding, for evaluating residual stress and distortion in ship structures. This methodology will be used to mitigate residual stress and distortion problems due to welding fabrication.

Benefit Completion of this project will improve the structural integrity and appearance of ships. The production cost will also be reduced through reduction of rework, (flame straightening, coping with poor fit ups, and overwelding).

SSC National Goals

• Reduce marine environmental risks.

• Improve the safety and integrity of marine structures.

SSC Strategy Development of principles of design for production

Background The structural integrity and appearance of section-stiffened panel fabrication as it is used in ship production is plagued by accumulation of the distortion that is caused by the residual stresses that occur during welding. This problem becomes more severe with the increasing use of thin sections and with double-hull fabrication. The absence of distortion measurements and control techniques during the fabrication processes leads to additional distortion and residual stress formation, which would not develop if control mechanisms were in place. These distortions produce significant cost increases due to the additional effort required to fit parts together, overweld big gaps,

and flame straighten the final structure. The application of control techniques provides an effective means of distortion reduction. However, effective use of control techniques requires knowledge of the mechanisms that produce distortion and the existence of accurate distortion prediction tools.

Recommendations Perform the following tasks:

• Assess the suitability and applicability of ongoing work in the determination of residual stress and distortion, including ongoing work at the Navy Joining Center.

• Identify potential applications of residual stress and distortion predictive capabilities on double-hull ship construction, and perform an economic assessment of their impact.

• Develop a predictive model that incorporates global structural interactions into the evaluation of residual stress and distortion produced by welding.

• Validate the model with experimental measurements.

• Develop guidelines for residual stress and distortion modeling in ship structure that account for joint configuration and the welding processes employed.

Duration 3,000 labor hours over 2 years

Objective Develop a method for defining failure, which is needed for structural reliability assessment of marine structures and suitable for structural reliability assessment. Establish definitions of failures for surface ship structures at several levels that include fatigue of details, fracture, and collapse of stiffened and unstiffened panels, grillages and hull girders. The analytical criteria (or failure definitions) should be compared with data developed from actual practice.

Benefit Failure definitions that are realistic will result in reliability estimates of ship structures with higher accuracy. Also, any resulting load and resistance factor design criteria from the reliability thrust area will be more realistic and practical.

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

SSC Strategy Development of reliability-based design criteria for ship structures

Background Classical structural-reliability assessment techniques are based on precise, clear-cut definitions of failure and nonfailure (survival) of a structure in meeting a set of strength, function, and serviceability criteria. These definitions are provided in the form of performance functions and limit state equations that convey the notion of an abrupt change from structural survival to structural failure. However, based on observations of the failure and survival of real structures, the transition from a survival state to a failure state, and from serviceability criteria to strength criteria are continuous and gradual rather than clear-cut and abrupt. In reality, an entire spectrum of damage (or failure

levels or grades) is observed during the transition to total collapse. In the process, serviceability criteria are gradually violated with monotonically increasing levels of violation and progressively lead into the strength criteria violation. An assessment of reliability of a structure is dependent on these failure definitions. Therefore, a method for failure definition is needed. Also, failure definitions need to be established for structural reliability assessment.

The failure definitions need to be suitable for structural reliability techniques that were previously developed and are currently being developed in ongoing SSC projects (including SR-1344 and SR-1345). The failure definitions can be at several structural levels, which include fatigue of details, fracture, and collapse of stiffened and unstiffened panels, grillages and hull girders. The failure definitions should be suitable for structural reliability analysis, and they should include several classes such as serviceability, partial failure, and complete failure. The expected outcomes of this project are both a method for defining failures in structural reliability analysis and failure definitions that are suitable for structural reliability analysis. These definitions can be developed using analytical techniques, field measurements, or both.

Recommendations Perform the following tasks:

• Develop a method for defining failure that is suitable for structural reliability analysis and design.

• Provide failure definitions for unstiffened panels, stiffened panels, hull girders, grillages, fatigue, and fracture that are suitable for reliability analysis and structural design.

• Assess the uncertainty in the developed failure definitions in the previous task.

• Illustrate the use of these definitions and their associated uncertainties in the reliability assessment of ship structures.

Duration 2,000 labor hours over 1 year

Objective: Develop load and resistance factor design methods for ship structures to replace the currently used design procedures for both commercial and naval applications. The following components of design should be included in this project: unstiffened panels, stiffened panels, hull girder, fatigue, and fracture. The load and resistance factor design methods should be presented in a code format suitable for direct use by engineers.

Benefit This project will result in load and resistance factor design methods for ship structures.

SSC National Goals

• Improve the safety and integrity of marine structures.

• Reduce marine environmental risks.

SSC Strategy Development of reliability-based design criteria for ship structures

Background A series of projects were developed by the SSC for the purpose of developing reliability-based design criteria for ship structures. In the first phase of Project SR-1344, “Assessment of Reliability of Ship Structures,” methods for reliability assessment were applied. Prototype load and resistance factor criteria for ship structures are being developed in Project SR-1345, “Probability-Based Design (Phase 3): Implementation of Design Guidelines for Ships.”

The current practice for designing commercial and naval ship structures needs to be revised according to the prototype load and resistance factor design format that is being developed in Project SR-1345. The revision should include all pertinent failure modes and should have a rigorous and complete code calibration for the design of ship structures. The resulting methods should be written in a code style that is suitable for the direct use of practicing engineers. Although all data for complete code development are not available now, and the development is ongoing, the code can be developed with the best information currently available. There will be a need for periodic updating of this code as more data become available.

Recommendations Perform the following tasks:

Phase 1 - Load and resistance factor design methods for unstiffened panels.

• Review SR-1344 and SR-1345 results and recommendations.

• Establish a procedure for code calibration.

• Perform code calibration of unstiffened panels for commercial and naval applications.

• Present the resulting load and resistance factor design methods for unstiffened panels in a code format suitable for the direct use of engineers. The methods should be concise and complete with appendices as supporting developmental materials.

Phase 2 - LRFD methods for stiffened panels.

• Review the results and recommendations of Phase I.

• Perform code calibration of stiffened panels for commercial and naval applications. The project should also include the development of reliability-based design criteria for gross panels. The code calibration method that is used in this phase should be similar to the one established in Phase I. Any deviations in the method should be justified and approved by the project technical committee.

• Present the resulting load and resistance factor design methods for stiffened panels in a code format suitable for the direct use of engineers. The methods should be concise and complete with appendices provided that contain supporting developmental materials. The code format used in this phase should be similar to the one developed in Phase I. Any deviations in format should be justified and approved by the project technical committee.

Phase 3 - Load and resistance factor design methods for hull-girder bending

• Review the results and recommendations of Phase I.

• Perform code calibration of hull-girder bending for commercial and naval applications. The code calibration method that is used in this phase should be similar to the one established in Phase I. Any deviations in the method should be justified and approved by the project technical committee.

• Present the resulting load and resistance factor design methods for hull-girder bending in a code format suitable for the direct use of engineers. The methods should be concise and complete with appendices provided that contain supporting developmental materials. The format used in this phase should be similar to the one developed in Phase I. Any deviations in format should be justified and approved by the project technical committee.

Phase 4 - Load and resistance factor design methods for fatigue and fracture

• Review the results and recommendations of Phase I.

• Perform code calibration of detail design for fatigue and fracture for both applications to both commercial and naval ships. The code calibration method that is used in this phase should be similar to the one established in Phase I. Any deviations in the method should be justified and approved by the project technical committee.

• Present the resulting load and resistance factor design for fatigue and fracture in a code format suitable for the direct use of engineers. The methods should be concise and complete with appendices provided that contain supporting developmental materials. The format used in this phase should be similar to the one developed in Phase I. Any deviations in format should be justified and approved by the project technical committee.

Phase I should be completed before starting the remaining phases. Phases 2, 3, and 4 can be performed in parallel.

Duration

Phase 1: 2,000 labor hours over 1 year

Phase 2: 2,000 labor hours over 1 year

Phase 3: 1,500 labor hours over 1 year

Phase 4: 2,000 labor hours over 1 year

Objective Develop an innovative method to nondestructively evaluate the fatigue and fracture properties of in-service ship structures so that a remaining life evaluation can be performed.

Benefit The ability to nondestructively evaluate the fatigue and fracture properties of ship structural materials would allow an evaluation of remaining life to ensure safety and reliability for continued service in aging ships.

SSC National Goals

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

• Reduce marine environmental risks.

• Improve the safety and integrity of marine structures.

SSC Strategy Structural monitoring of vessels in service

Background The evaluation of remaining life in structural components requires knowledge of the fatigue and fracture properties for these materials. Often information about these properties is not available for ships that have had a long service life. If these properties of the materials were measured before the vessel was fabricated, they may have changed due to material degradation encountered during service life. Many structures have a continued useful life after the original design life has expired. However, a thorough evaluation of the remaining life potential is necessary to ensure safe continued service. A method that could determine, or even make a reasonable estimate of, the fracture and fatigue properties from a nondestructive evaluation of materials in the ship structure would be invaluable. Such an evaluation would require an innovative development of new technology.

Recommendations Perform the following tasks:

• Review the literature to identify possible methods for nondestructive evaluation of fatigue and fracture properties.

• Develop or adapt a nondestructive method to evaluate the fatigue and fracture properties of ship steels that have had extensive service exposure.

• Show how these properties can be used to evaluate the remaining life of ship structural components.

Duration 2,500 labor hours over 2 years

Objective Develop a standardized methodology for the collection and analysis of ultrasonic thickness measurements on ship structures in order to accurately determine corrosion rate and to help develop an optimum structure maintenance program.

Benefit A standard methodology for ultrasonic thickness measurements will allow regulatory agencies and ship owners to determine corrosion rates and trends and to determine a long-term maintenance plan.

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

SSC Strategy Improved structural-inspection techniques

Background Presently ultrasonic inspections are performed periodically on ship structures, and extensive thickness measurement data are collected. These data are used to determine the steel condition at the time of the inspection and to develop repair plans based on certain methods for minimum plate thickness. Since ultrasonic thickness measurements are not usually taken at the same location during inspections and there is no systematic approach for the data collection (including the number of points and distribution of points per location), accurate determination of the corrosion rate is not possible. As a result, the data have limited application and are not suitable for predicting steel condition and developing long-term maintenance plans that would minimize costs and failure risks. With the increased surface area of ballast spaces in double-hull tankers and the potential for large maintenance costs associated with steel and coating repair, it has become very important to determine corrosion trends in order to optimize maintenance plans. Corrosion is random in nature and is influenced by various factors, such as type and usage of tank, location within the tank, existing corrosion control systems, and temperature. A probabilistic model for corrosion should then be developed that includes all these factors. Based on this model, a systematic approach for ultrasonic thickness measurements should be proposed. The inspection results would then be used to update the probabilistic model.

Recommendations Perform the following tasks:

• Carry out a literature survey on corrosion of tankers in order to determine parameters that influence corrosion rate. Confer with regulatory agencies, classification societies, and ship operators regarding information currently recorded and perceived needs.

• Develop a probabilistic model for corrosion rate that depends on parameters selected in the first task.

• Based on this model, develop a standard approach for collecting and recording ultrasonic-thickness measurement data. This approach should identify grid patterns and locations of readings for each structural member.

• Develop a methodology for updating the probabilistic model based on inspection data.

• Recommend a format for storing and presenting the data (graphical, pictorial, database) and related results such as corrosion rate, alarm and action limits, and the anticipated time to reach the limits.

Duration 1,500 labor hours over 1.5 years

Objective Evaluate the status of current software developments for ship design and determine if guidelines for common interfaces can be established that would enhance the exchange of all types of data needed for the initial ship design, fabrication, life-cycle maintenance, and future design modifications to ship structures.

Benefit The workshop will bring together key representatives of the shipbuilding industry, the offshore petroleum industry, military organizations, universities, standards organizations, and the software industry. They would be charged to establish a national and achievable agenda for solving near-term and longer-term problems and defining a priority list of research needs to facilitate the longer-term goals.

SSC National Goals

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

• Improve the safety and integrity of marine structures.

SSC Strategies

• Development of better design tools

• Improved engineering and design strategies

Background There is a significant and growing degree of fragmentation in the design process as naval architects and engineers utilize new software and data sets that have incompatible formats and levels of documentation. As a result, information is continually being redeveloped by different users and productivity suffers. To some extent, private companies, universities, and the military have separately addressed aspects of this problem that are essential to their own activities. However, a global strategy and a much broader vision are needed.

Designs evolve as they pass from initial concept to working drawings. Archival information on completed designs is becoming increasingly important as ship structures are modified during their service life for functions or activities never envisioned by the original designers.

There is a great deal of commonality between the problems faced in ship design and in other engineering disciplines, and conduct of this workshop would provide an opportunity for the SSC to take a timely leadership position in computer-aided design. Some progress is being made in this area. The initiative by the U.S. Navy^1,2,3 has started a standard for a digital ship model. The individuals involved in this development should be included in the expert panel for this workshop.

It would be unrealistic to think that one workshop could solve this very significant problem. Rather, the workshop is intended to produce a working document for providing a balanced and generic view of the process needed to solve this problem. In the near term, an appropriate software interface would be defined, so that much of the existing software could continue to be utilized. Once a standard was defined, newer software would automatically incorporate the standard interface. A second phase of research projects can be anticipated as a product of the workshop. This project could lend itself to a project funded under phase one of the U.S. government's program, Small Business Innovative Research (SBIR) Project, Phase I. The second SBIR phase could be the actual software development and the third phase would be marketing.

Recommendations Perform the following tasks:

• Appoint an organizing committee that will be responsible for workshop organization and preparation of the final report.

• Convene a 2 or 3-day workshop with keynote speakers and panel participants allowing some opportunity for open discussion.

• Prepare a written report that includes the keynote lectures, a summary of each panel session, and a list of recommendations on near-term and longer-term action items for review and distribution by the SSC.

Duration 2,000 labor hours over 1 year

Objective Explore and evaluate alternative stiffening systems of 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.¹

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 integrity.

SSC National Goals

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

• Improve the safety and integrity of marine structures.

SSC Strategy Research of double-hull vessel technology

Background The recently completed Project SR-1351, “Hull Structural Concepts for Improved Producibility,” addresses alternative structural systems for conventional and double-hull ship types. Out of concern for environmental pollution, the Oil Pollution Act of 1990 has mandated double-skin (double-hull) construction for tankers that operate in U.S. territorial waters, and the International Maritime Organisation subsequently adopted this and alternative concepts. Such double-barrier protection does offer effective pollution protection in low-energy collisions and soft groundings. New analytical methods are being developed and evaluated for double-hull and other configurations, with emphasis on energy-absorption capability and reduced outflow from breached tanks, in Project SR-1354, “Grounding Protection of Tankers” (Massachusetts Institute of Technology Joint Industry Project), and in the Advanced Double Hull Project by the Carderock Division of the Naval Surface Warfare Center. Although these are broad-based programs, they are 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, side shell, and bottom plating are regarded as prime hull-girder components, which are conventionally stiffened with longitudinals, together with support from transverse webs/floors. Within this system, the longitudinals 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 longitudinal bulkheads and inner bottom) may also be regarded as a prime hull-girder component. Double-skin construction will 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 carried 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 Maritime Technology computer programs for computing damage, the American Bureau of Shipping Dynamic Loading Analysis and Safehull computer programs for fatigue analysis, the related research under the shipbuilding program and the double-hull investigations at the Carderock Division of the Naval Surface Warfare Center, and the recommended Project 96-9, “Rupture Resistance of Cargo Tanks of Double Hull Tankers to Low Energy Impacts.”

• Derive guidelines from past and current studies for maximizing the structural resistance to seaway loads.

• Examine alternative stiffening systems, both conventional and unconventional, for feasibility, taking into account the results of the two pilot studies in this area, SR-1354 and Naval Surface Warfare Center double-hull projects.

• Evaluate feasible solutions, including factors of first and life-cycle costs, producibility, and resistance to accidental load.

• Make recommendations as to remaining problem areas and further research if warranted.

• Review such cost data as are available regarding production and maintenance of hull-stiffening arrangements. Findings from projects SR-1366, “Corrosion Control of Inner-Hull Spaces,” and SR-1377, “Commercial Ship Design and Fabrication for Corrosion Control,” should be taken into account.

Duration 2,000 labor hours over 1 year

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 minimize the risk of such rupture. ¹

Benefit This project will result in improved safety, economics of operations, and integrity of tankers under low-impact loads. Enhanced environmental protection will also result.

SSC National Goals

• Reduce marine environmental risks.

• Improve the safety and integrity of marine structures.

SSC Strategies

• Research of double-hull vessel technology

• Prevention research including damage-tolerant structures

• Improved engineering 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 Maritime Organisation mandated that new tankers be either double hulled or, according to the organization, provide equivalent prevention of oil outflow after a collision or grounding. 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 hull will be able to deflect considerably before rupturing or if the double-hull stiffening members will prematurely punch through the outer hull as well as the inner hull, which would result both in structural damage and oil spill. In addition, the influence of the width of the double side and the height of the double bottom on the risk of hull rupture for various stiffening system configurations has not been investigated systematically with the new analysis tools that are now being developed or are already available.

Methods to evaluate the energy absorption capacity of tankers have been developed since 1970 and are currently being developed in the Project SR-1354 (Massachusetts Institute of Technology Joint Industry Project) “Grounding Protection of Tankers,” and the Advanced Double Hull Project of the Carderock Division of the Naval Surface Warfare Center. These methods, however, do not 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 the width of the double side. The local behavior of the internal members (web frames and stringers) 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 recent design of double-hull tankers has used the results of some research into damage reduction, such as attention to structural details. However, the exercise of new design tools permits 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.

Recommendations Perform the following tasks:

• Review the results of extensive studies that have been carried 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 Maritime Technology computer program for computing damage, the Dynamic Load Approach and Safehull computer programs of the American Bureau of Shipping for fatigue analysis, and the related research under the shipbuilding program and the double-hull investigations at the Carderock Division of the Naval Surface Warfare Center.

• Using results from 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-stiffening-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 the 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-shell 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 (e.g., 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.

• If necessary, recommend a Phase II for additional model or full-scale tests where the first and third tasks have defined important gaps in the data.

Duration 2,000 labor hours over 1.5 years

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 risks.

SSC Strategy Research of double-hull vessel technology

Background The double-hull design of tankers and ships is used to provide an extra margin of safety 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. 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 project¹ under the U.S. Navy Manufacturing Technology program on double-hull failure modes will 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-hulled 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, so that the potential for failure is reduced.

Duration 2,000 labor hours over 1.5 years

Objective Evaluate the effect of fillet weld-design on structural rupture resistance to groundings in order to develop weld-design guidelines that would enhance tanker capability to resist pollution in groundings.

Benefit The tearing of fillet welds in the grounding of tankers can impact the rupture resistance to groundings of the double-bottom structure. This study will evaluate the risk resulting from fillet-weld failure and provide appropriate guidelines to the designers and builders.

SSC National Goals

• Support the North American maritime industry in shipbuilding, maintenance, and repair.

• Improve the safety and integrity of marine structures.

SSC Strategy

• Development of better design tools

• Development of principles of design for production

• Research of double-hull vessel technology

Background A review of a number of tanker casualties has shown that the strength of welds can have a major impact on the amount of energy absorbed. In some instances the fillet welds have held; in others, they have not. This project will analyze varying grounding scenarios and propose a rational approach to improve weld-strength effectiveness. A 3-year joint Massachusetts Institute of Technology-Industry Program on Tanker Safety is progressing at the university. The study covers the development of a computational model of the process of a steady tearing of hull plating. However, the computer code DAMAGE from the university's project will not explicitly consider fillet-weld performance. The project will be developing the basic technology that can be applied in this project.

Recommendations Perform the following tasks:

• Evaluate the risks of differing-strength fillet welds under various grounding/stranding scenarios. Validate the findings.

• Establish the cost-effectiveness for different-strength fillet welds as above.

• Incorporate weld-strength characteristics into the DAMAGE computer model.

• Prepare guidelines for designers and builders.

Duration 2,000 labor hours over 1.5 years

Objective Determine the flammability, smoke, and toxicity properties of composite materials, and identify promising candidate material systems for ship and offshore platform structures designed to meet international standards for fire-safety and environmental protection.

Benefit The study will provide needed understanding of the opportunities and limitations of the application of composites in compartments subject to fire hazards. It will provide the stimulus for encouraging the development of new materials and provide the opportunity to evaluate promising new fire-resistant materials.

SSC National Goals

• Reduce marine environmental risks.

• Improve the safety and integrity of marine structures.

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

Background Materials used to construct and outfit commercial ships must meet stringent requirements for flammability, while still meeting naval architectural requirements for light weight and low cost. The International Convention for the Safety of Life at Sea, 1974, as amended, requires the hull, superstructure, structural bulkheads, decks, and deckhouse to be constructed of steel or other equivalent materials. “Equivalent material” is further defined in the convention as any noncombustible material that by itself or because of insulation provided has structural and integrity properties equivalent to those of steel at the end of standard fire tests. Further requirements are also established for bulkhead, ceiling, and deck materials. The lack of generation of smoke and toxic combustion products in a fire is also important for successful fire safety. Paradigms about the performance of composite materials in a fire are often inconsistent with test results. Tests on fiberglass/epoxy pipe, for example, have demonstrated the safety merits of the use of fiberglass pipe in firewater systems on offshore platforms, and fiberglass/epoxy firemain pipes are now used in most parts of the world, including the United States, the North Sea, and the Middle East. The United States, Norway, and the United Kingdom are highly active in assessing the safety of materials in fire, and this project should take advantage of U.S. and foreign developments to ensure that the study does not duplicate prior efforts. The program should concentrate on commercially available materials. Phenolic resins have shown promise in past studies; however, a host of new materials have recently become available, including several manufactured in the United States.

The workshop sponsored by the SSC entitled “Use of Composite Materials in Load-Bearing Marine Structures” (SR-1331) strongly recommended that work be done on the fire characterization of composite materials, including their flammability, smoke, and toxicity properties.

Recommendations Perform the following tasks:

• Perform a comprehensive review of all prevailing International Maritime Organisation and U.S. Coast Guard regulations and standards.

• Survey the existing database on fire performance of composite materials, including data from non–U.S. programs. The contractor should also solicit industry for promising new fire-resistant materials with low smoke and toxicity characteristics. This survey should consider matrix resins as well as insulation materials and should include all possible solutions, including thermoset and thermoplastic resins and phenolic-based materials.

• Survey and identify appropriate test methods for evaluating the performance of composite materials for marine structural applications.

• Conduct limited fire, smoke, and toxicity tests on promising new materials for which data are not available.

• Identify applications where fire-resistant composite materials could provide enhanced safety, structural efficiency, and competitive economic advantage. For example, a fire-resistant resin might be used to construct a composite structural frame or bulkhead, composite deckhouse, or accommodation module.

• From a human-factors viewpoint and considering fire and smoke standards, recommend maximum acceptable smoke and toxicity threshold levels for the applications studied above.

Duration 2,000 labor hours over 1.5 years

Objective Determine the effects of long-term marine exposure on the mechanical properties of polymer-based composite materials, and assess corrosion at metal/polymer interfaces to ensure safety and integrity of vessels incorporating composite materials.

Benefit This work will prevent the potential catastrophic failure or need for costly repair of composite structures used in the marine environment due to environmental degradation.

SSC National Goals

• Reduce marine environmental risks.

• Improve the safety and integrity of marine structures.

SSC Strategy Structural reliability engineering

Background Increased use of polymer-based composite materials in marine structures raises questions concerning the effects of long-term marine exposure on the degradation of composites and also the effects of these composites on the corrosion of metals. When polymer-based composites were first used in aircraft, they absorbed water, which resulted

in a decrease in their structural capability. Had exposure experiments been conducted prior to their use, some of the later problems that resulted could have been prevented. The National Aeronautics and Space Administration exposed a number of composite test specimens in low earth orbit prior to extensive usage of the material in such applications. These specimens were later retrieved and tested. In this manner, the sensitivity of these materials to atomic oxygen was discovered and accounted for in later designs, thus preventing potential problems.

In the discussion at the National Conference on the Use of Composite Materials in Load-Bearing Marine Structures (SR-1311), it was noted that there are virtually no published data on the environmental effects of immersion in sea water. This led to the development of a recommendation to establish a database for the extension of environmental modeling to marine structures with respect to sea water, temperature, pressure, aging, salt spray, ultraviolet radiation, and marine organisms. A program sponsored by the National Science Foundation at Texas A& M University has begun to address this problem, but much more work still remains to be done in this area. The marine environment does have unique constituents to which composite structures have generally not been exposed for long periods—microbes, marine growth, numerous ionic species, organisms, and others. It is necessary to determine what effects these factors may have on mechanical properties and, thus, provide a database that will allow for the ability to account for these effects in design.

Of specific interest with regard to metal/composite interfaces is galvanic corrosion that can arise from the use of graphite-reinforced composites. Since graphite is a conductive material, its use in systems containing dissimilar metals could lead to unexpected coupling of adjacent metals and severe galvanic corrosion if any of the graphite fibers are exposed and in contact with these metals. Processing, wear, or degradation of the composite with time can result in exposure of graphite fibers, which can significantly accelerate corrosion of metal in contact with the graphite. In addition, it is recognized that cathodic reaction products liberated on graphite can lead to a degradation of the reinforcement/matrix interphase or cause blistering of the composite. Therefore, it is necessary to characterize also the nature of this degradation process and its effect on mechanical properties of the composite. It is important to identify what metals and alloys are galvanically compatible with graphite. In addition, electrical isolation methods and design and fabrication processes need to be reviewed in order to make graphite-reinforced composites a viable design choice in situations where the composite will come in contact with metals.

Recommendations Perform the following tasks:

• Survey composite materials, and make recommendations on those to be tested. Summarize available long-term exposure data.

• Conduct experiments on the recommended materials to expose standard composite specimens (i.e., tension, compression, shear, and interlaminar specimens) to marine environments (both simulated and natural sea water). Test the materials after varying exposure times of up to 30 months. Some of the exposed specimens should be tested under load. In addition, an acceleration procedure, such as high pressure, should be identified and

employed for some specimens, and results should be evaluated and compared with the real-time data.

• Perform microanalytical, fractographic, and other analyses judged to explain the mechanism of any environmentally influenced property degradation.

• Based upon the information obtained, identify models that need to be developed to account for the observed behavior.

• Review polymer-based composite-processing methods in order to determine the probability of graphite fibers being exposed at composite-metal mating surfaces. If possible, recommend modifications to current processing methods to ensure that fibers are not exposed at the surface of the composite.

• Characterize the extent of damage experienced by graphite–reinforced composites due to cathodic processes as might occur when these materials are galvanically coupled to metals that are more electrochemically active.

• Identify engineering metals and alloys that are galvanically compatible with graphite, and investigate methods for electrically isolating graphite-reinforced composite/metal interfaces.

Duration 3,500 labor hours over 3 years

Objective Develop fracture mechanics test methods and analytical models for the crack-arrest assessment of marine weldments.

Benefit The results of this project will enhance the durability, reliability, and safety of marine structures through a more realistic and economical fracture-mechanics approach to toughness assessment of weldments.

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 At the present time, fracture-mechanics estimates of toughness and analytical models are used to assess the crack-initiation toughness of structural steel weldments. These models address the initiation of brittle fracture in the deposited weld metal or in the heat-affected zone. Further, it is assumed that (1) such an approach is a conservative, lower bound to the actual damage-tolerance of weldments, because a crack initiated in the weld metal or heat-affected zone will grow immediately into the base metal where it is arrested, and (2) the crack-initiation toughness and crack-arrest toughness are equal for a given material, crack length, and near-crack-tip stress-strain field. However, whether or not the growing crack is arrested in the base metal depends

on the velocity and length of the growing crack, the stress-field magnitude and environment ahead of the crack, and the crack-arrest toughness of the base-metal. Also, the crack-growth may be accelerated by unexpected thermal shock, local brittle zones, and pop-in behavior. Finally, the crack-initiation toughness and crack-arrest toughness are not equal for a given material, crack length, and stress-strain field.

A fracture methodology based on crack-initiation toughness alone is often insufficient to assess the reliability and safety of structures. Ultimately, the only reliable assessment of damage-tolerance of a structural weldment to avoid catastrophic fracture will combine the fracture mechanics assessments of both crack initiation and crack arrest.

Recommendations Perform the following tasks:

Phase I

• Develop a small-scale fracture-mechanics test method to estimate the crack-arrest toughness of steel weldments.

• Develop a fracture-mechanics model for the assessment of crack arrest in marine structures.

Phase II

• Corroborate the crack-arrest model with full-scale tests of representative marine weldments.

Duration Phase I - 1,000 hours over 1 year

Phase II - 2,000 hours over 2 years

Objective Provide guidance to ship structural designers regarding bow design to improve producibility and to reduce cost, weight, and required maintenance.

Benefit The results of this project can produce reduced ship cost, weight, and required maintenance.

SSC National Goal Support the U.S. and Canadian Maritime Industry in shipbuilding, maintenance, and repair.

SSC Strategies

• Development of better design tools and information systems

• Development of principles of design for production, improved engineering analysis, and evaluation

Background The structural design of a ship's bow is a complex matter. In this context, the term “ship's bow” refers to the forward 10 percent to 15 percent of the ship's length. The bow shape is largely driven by hydrodynamic considerations, (e.g., calm water resistance, slamming, deck wetness, etc.), but other requirements are also imposed, such as internal space, anchor handling, and collision resistance. The bow structure represents

a significant portion of the total hull structure weight and cost. Contributing factors include the additional strengthening required to resist slamming loads and the poor access for fabrication resulting from the inherent shape. Sufficient access for inspection and maintenance is also a problem on hulls with fine bows.

Development of the structural configuration of a ship's bow is still very much an art due to the complexity of the imposed requirements, many of which do not lend themselves to analysis. Ship structural designers with extensive experience in ship bow design are few and far between in the United States and Canada, and most of those are at or past retirement age. There is a need to capture the expertise of these individuals and upgrade it with lessons learned from state-of-the-art foreign designs to provide design guidance to the coming generation of domestic ship-structure designers.

Recommendations Perform the following tasks:

• Prepare an annotated bibliography of recent references pertinent to bow structural design.

• Collect data on recent foreign-ship bow-structure designs and note lessons learned.

• Interview U.S. and Canadian experts on the detail design of ship-bow structure.

• Identify the requirements that bow structure must satisfy, available analysis techniques where applicable, desired structural configurations from the standpoints of producibility and inspection/maintenance, and other points that would be of value to the bow structural designer.

• Document all the above findings in a ship-bow structural-design guidance document.

Duration 1,000 labor hours over 1 year

Objective Facilitate the introduction of weldable primers in the North American shipbuilding industry by evaluating how weld integrity is affected by these coatings, establishing a fitness-for-purpose approach for acceptance and developing a standardized evaluation/acceptance test.

Benefit The ability to weld over primers, eliminating base material preparation prior to welding, could be improved by establishing standardized acceptance tests developed in this project. Shipyards would be able to compare the weldability of candidate shop primers from literature provided by the paint companies and proceed with expedient utilization of more weld-tolerant paints. Standardized tests would also provide incentives to the paint companies to provide more weld-tolerant primers. This project will also provide assurance that such acceptance tests are based on weld properties and sound engineering principles. The economic benefits in advancing this technology should contribute to restoring the U.S. and Canadian shipbuilding industry.

SSC National Goal Support the North American maritime industry in shipbuilding, maintenance, and repair.

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

Background The ability to weld over primers, eliminating cleaning prior to welding, can enhance the productivity of the shipbuilding industry. However, welding through preconstruction primers has not generally been applied successfully in North America, and, where it is used, weld integrity is still an issue. Current codes use porosity levels as the basis for acceptance or rejection of a qualification test. There is a proliferation of new weldable coatings available from paint manufacturers and of new flux-cored electrodes capable of producing less porosity when welded through such coatings. Most of these originate from overseas sources. With the variety of new coatings that have unknown alloying with these new electrodes, the validity of porosity as the only measure of acceptability has been questioned. For example, the interaction between the flux-cored weld deposit and new paint chemistries has not generally been assessed as far as weld properties are concerned.

Currently, there is no standard way for paint manufacturers to test new coatings. A proposed qualification test has been recommended by the Materials Technology Centre in a report to the Canadian Coast Guard.¹ All weldability testing is currently performed directly by each shipyard, and there is no index by which a particular product can be qualified or different products can be compared.

Recommendations Perform the following tasks:

• Prepare a series of steels having primers of controlled thicknesses, then produce fillet welds using newly developed flux-cored electrodes designed for welding through primers to produce a range from zero porosity up to currently rejectable levels of porosity. Evaluate the welds produced with a series of selected tests to establish the properties of fillet welds as a function of porosity produced.

• Recommend a standard weldability test for evaluating new weldable coatings and newly developed flux-cored electrodes.

• Recommend quality control and qualification rules for assessing the acceptability of welding procedures used for weldable primers.

Duration 2,000 labor hours over 1 year

Objective Develop a method for determining the sea-operational profile of surface ships for the purpose of assessing their structural reliability, and suggest typical profiles.

Benefit The sea-operational profiles will be used in reliability analyses of ship structures. Therefore, the reliability and cost-effectiveness of the resulting structures can be improved.

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

SSC Strategy Development of reliability-based design criteria for ship structures

Background The definition of loads for a ship requires the knowledge of its sea-operational profile. This profile can be defined based on its mission. The interactions among the mission definition, profile definition, ship operators, and loads can greatly affect the reliability of the ship's structure. The sea-operational profile can be expressed in the form of a percent histogram for the combinations of heading, sea state, and ship speed. The resulting profile is random in nature. This randomness and other associated uncertainties need to be determined.

A method is needed for determining operational profiles of ships that are suitable for structural reliability techniques that were developed in previous SSC projects or are currently being developed in projects SR-1344 and SR-1345. The expected outcomes of this project are a method for determining operational profiles of ships and an assessment of a typical profile with associated uncertainties. The typical profile can be developed using either analytical techniques, field surveys, or both.

Recommendations Perform the following tasks:

• Select classes of vessels and missions to determine their profiles.

• Develop a method for assessing the profiles and their randomness.

• Illustrate the developed method for selected classes.

Duration 1,500 labor hours over 1 year

Objective Develop guidelines to assess accurately the condition of the existing coating system and to determine an optimal coating maintenance program.

Benefit Results of this project would reduce maintenance costs, enhance safe operation of tankers with respect to structural integrity and pollution prevention, and improve the consistency of coating evaluation and maintenance planning.

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 Strategy Improved structural inspection techniques

Background In view of the large increase in ballast-tank surface area in new double-hull tankers, the requirements by the International Association of Classification Societies to coat the ballast tanks of new tankers, and more-stringent survey and maintenance requirements by international regulatory agencies, such as the International Maritime Organisation Enhanced Survey Programme for Oil Tankers, there is an increased need to assess accurately during inspection the coating effectiveness and to determine optimal coating maintenance and replacement program. This problem is of great importance to ship owners for several reasons. There is a large cost associated with coating maintenance and replacement in ballast tanks of double-hull tankers. There are serious consequences to vessel structural integrity and environmental pollution if coatings are not maintained. There are also large costs associated with repair of steel wastage due to corrosion. Several institutions have defined rating systems for coating effectiveness and recommended coating maintenance and replacement strategies based on these ratings. For instance, the American Bureau of Shipping and the Tanker Structure Co-operative Forum “Condition Evaluation and Maintenance of Tanker Structures,” classify coating conditions as “good,” “fair,” and “poor” based on percentage and location of coating breakdown and hard scale. Det Norske Veritas has published “Guidelines for Corrosion Protection of Ships,” in which coating maintenance requirements and coating useful life are defined based on percentage of surface area with breakthrough rust. Due to difficulties in quantifying the degree of coating failure by visual inspection, an optimum decision for repairing or replacing the coating is not always made. The assessment of coating failure will, in many cases, depend on the inspector' s experience.

In order to obtain accurate evaluation of coating effectiveness and improved coating maintenance, it is necessary that more specific and clear guidelines be developed for coating evaluation. In addition, strategies for coating maintenance should be developed. This project is complementary to the ongoing SSC projects in the area of corrosion control, SR-1377, “Commercial Ship Design and Fabrication for Corrosion Control,” and SR-1366, “Corrosion Control of Inner-Hull Spaces.” These two projects address the selection, as opposed to the maintenance, of effective methods of corrosion control.

Recommendations Perform the following tasks:

• Carry out a literature survey on evaluation of coating performance and criteria for repair.

• Review current guidelines for condition assessment and maintenance of coating, including information generated by painting manufacturers, classification societies, ship owners, and shipyards.

• Develop guidelines for assessing coating condition in a clear and specific way. Provide necessary information (e.g., description, examples, pictures) that would allow inspectors of different levels of experience to provide consistent evaluations.

• Develop a methodology for determining optimum maintenance strategies for coating that would result in minimum life-cycle costs. This methodology should take into account various parameters affecting the problem, such as the remaining life of the vessel, frequency of repairs, another corrosion protection system (e.g., as cathodic protection), and heating condition. Alternatives for surface preparation (e.g., hydroblasting or grit blasting) and their associated costs should be taken into account.

Duration 1,000 labor hours over 6 months

Objective Develop a system for monitoring corrosion or paint delamination in locations that are difficult to inspect.

Benefits The results of the program are expected to significantly enhance safe operation of ships, improve structural inspection, and reduce maintenance and costs.

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

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

Background Much progress has been made recently in the area of design for the minimization of corrosion, and this knowledge is currently being implemented. However, corrosion detection and corrosion rate determinations for existing and inaccessible structures remain a problem. This information is especially important in areas that are difficult to inspect, such as inter-hull spaces in some ship designs. What is needed is a nondestructive method that both identifies areas of corrosion and lends itself to corrosion-rate determinations. One such method for evaluating corrosion, which has become quite popular within the past 10 years, is electrochemical impedance-spectroscopy, also referred to as AC impedance-spectroscopy. A distinct advantage of impedance techniques over direct-current electrochemical measurements is the ability to probe corrosion of a metal surface beneath an organic coating. Corrosion sensors based on impedance spectroscopy have been developed for a few specific applications where corrosion rates or other parameters of interest, such as coating deterioration, have been tracked with time under varying exposure conditions.

Because of recent advances in the technology of electrochemical sensors, including impedance-based devices, the probability for successful demonstration and

implementation is considered to be high. Some important issues that need to be considered in the design and implementation of electrochemical sensors include the following:

• identification of electrochemical parameters to be monitored;

• the type and size of the sensor;

• the number and placement of sensors; and

• data collection, archiving, and analysis.

Recommendations Perform the following tasks:

• Identify impedance parameters that are capable of measuring a broad range of damage under varying exposure conditions.

• Develop a prototype in situ electrochemical impedance-spectroscopy sensor and ancillary equipment capable of data collection, archiving, and analysis.

• Conduct field trials of the prototype sensor.

• Make recommendations for final design and implementation of the monitor.

Duration 4,000 labor hours over 2 years

Objective Collect data on stiffened panel collapse under combined in-plane and lateral loads due to tripping and buckling, performing experiments where data are insufficient and estimating uncertainty in strength models in these areas.

Benefit The assessment of uncertainty in the structural strength models can only be done when sufficient experimental data are available. However, these data are essential in an appropriate load and resistance factor design. This study will provide such data, which will help in the development of reliability-based design for ship structures.

SSC National Goals

• Improve the safety and integrity of marine structures

• Support the North American maritime industry in shipbuilding, maintenance, and repair.

SSC Strategies

• Development of better design tools

• Development of structural reliability engineering

Background Reliability methods are becoming the preferred method of design over the deterministic method in many engineered structures, including marine structures. Reliability methods take into account more information than their deterministic

counterparts in the design of structural systems. However, for a successful reliability method, these data must be obtained reliably, and uncertainties must be defined quantitatively. These areas were discussed in a recent SSC report, “Uncertainty in Strength Models for Marine Structures” (SSC-375). The report points out that insufficient data are available to determine uncertainties predicting collapse of stiffened panels. Therefore, carefully planned and coordinated tests are required involving these failure modes. The current Project SR-1378, “Strength and Stability Testing of Stiffened Plate Components, ” will generate a portion of these experimental data. Additional test data will be necessary to determine uncertainties in these failure modes.

Recommendations Perform the following tasks:

Phase 1 - Definition of needs and design of experiments

• Review literature to collect pertinent data on stiffened panel collapse tests, including SR-1378.

• Ascertain the area in which collapse-type data are lacking.

• Develop a test plan to acquire data in these areas of stiffened panel buckling under combined in-plane and lateral loads.

Phase 2 - Testing and design recommendations

• Conduct the tests planned in Phase I. Any deviations from the test plan should be approved by the project technical committee.

• Based on the results of this and other tests, as well as any other available data on stiffened panels, estimate the statistics of modeling bias of selected strength analysis algorithms. Use the methodology for estimating modeling uncertainties outlined in SSC-375.

• Recommend changes to currently used prediction models for the strength of stiffened panels under combined in-plane and lateral loads.

Duration

Phase 1: 1,000 labor hours over 1 year

Phase 2: 2,000 labor hours over 1 year