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BB/9SD-Q A Study of the Effects of Transport Route Profiles Upon the Fatigue Failures of Ships Objective Determine the major effects of transport route profile on ship structural failure, and develop data recording and analytical techniques to facilitate consistent ant! continuing analysis of the effect. Benefit Reduce long-term ship repair and maintenance costs. SSC National Goal · Improve the safety and integrity of marine structures. · Reduce marine environmental risks. SSC Stra~y Prevention research including damage-tolerant structures, structural monitonng, and human factors Background There is evidence that specific transport routes have unique effects upon a ship's structural failure patterns. One study, conducted in 199l,~ presented conclusive evidence that crude of] tankers on the Valdez, Alaska, to California route have unique structural failure patterns. The starboard side of a ship on this route apparently experiences greater stress; the result is a significant increase in structural failures~oth the frequency of failure and the sevens of the failure are increased starboard. The Amencan Bureau of Shipping has also noted similar increases of failures on the same route; they stated that a ship in the California-Alaska trade is 40 percent more vulnerable to structural failures than a ship in North Atlantic service. A ship designer or owner with a precise understanding of how a transportation route can impact fatigue life in a ship can effect methods to minimize the impact. In the California-Alaska service, for example, steps could be taken to strengthen the starboard side of the ship dunng design or prior to construction. This early recognition of the problem would reduce long-term repair and maintenance costs. The project would establish failure-event data base collection systems, incIllding formats. The project would recommend methods of analysis so that the ship owner could monitor the occurrence of fractures dunng operation and recognize trends of the events. Finally, there we be an attempt to develop stress prediction models of the specific trade routes, which wall use the collected data as a basis for comparison. The final result of the project would be the ability to better design ships for specific trades. ~ Graduate student research project performed by Mr. Sanjay Venna, University of Michigan, during summer, 1991, while sensing as summer intern with West Coast oil company. 2 Memo, dated May 11, 1993, to the Design Work Group from Subrata Chal;rabarti, Attachment B describing presentations of Agespy Research Presentations at SSSC Spring Business Meeting. as
Recommendations The project would occur in three phases. Phase ~ Survey reported fracture data that is obtained from companies that have agreed to cooperate and furnish the information. Data for ships operating on several routes should be examined. Potential candidates for routes to be examined are: · Califo~a-Alaska; · Japan-Alaska; · United States-Northertl Europe; · Persian Gulf~outh Afoca-United States; and · Duluth-Chicago or Duluth-Lake Erie. Phase 2 · Analyze the data relative to severity of failure and location on the ship. · Evaluate this information against ship's particulars and the operating scenario. · Use the collected data as a basis for comparison to develop models of predicted stress levels that wall occur in the specific trade. Phase 3 Prepare the final report that wall present al] data analyses, recommended methods of data collection and analysis, and the results of the stress- prediction mode] studies. Duration Phase ~ 2,000 labor hours over ~ year Phase 2 2,000 labor hours over ~ year Phase 3 2,000 labor hours over ~ year SY/9~D-S Consistent Stochastic Analysis Procedure for Design of Floating Marine Structures Objective Develop and implement a consistent stochastic analysis methodology integrating structural and hydrodynamic analysis of floating manne structures subject to waves. The structural analysis of the methodology should be finite-element based and suitable for production use In design. The goals of the project are to unify the stochastic analysis of both hydra- and structural- dynamics and to make a stochastic computational methodology available for adoption by industry. This would encourage the design community to use consistent stochastic structural and hydrodynamic analysis methodologies where appropnate. Benefit The pnma~ benefits of this program are to update current structural and hydrodynamic modeling, analysis, and design procedures and to contribute to more- consistent optimum design procedures through the use of consistent stochastic analysis for both hydra- and structural dynamics. SSC Goal Support the U.S. maritime industry in shipbuilding, maintenance, and repair. 86
SSC Strategy Development of better design tools, such as computer-aided design systems and artificial intelligence Background A major objective In the analysis of a marine structure is to estimate the extreme response behavior and fatigue life for the design process. The traditional approach for analyzing floating structures is to determine the wave-induced random motions and the associated inertia and hydrodynamic loadings based on rigid body motion of the vessel. These loads are then applied, usually quasi-statically, to a deterministic structural model. Hydroelastic models deal with phenomena induced by the interaction of inertia], hydrodynamic, and elastic forces caused by flexibility of the structure (such as sagging, hogging, and torsional motions). These models can provide a more-accurate analysis procedure and are more compatible with refined reliability design procedures. The basic methodology is weld developed, at least for the linear case. One study revealed that one of the pnma~y questions to be resolved from a research perspective involves the integration of these tools for coupled stochastic analysis. Work has been done in this area, and a few computer codes for coupled analysis have been recently developed (in the United Kingdom and China). However, the coupling In these models is crude, and further development is needed, as the codes are not suitable for production use. Another deficiency of today's models is that although the hydrodynamics are modeled stochastically, the entire structure is modeled deterministically. With the advances in the field of stochastic finite elements, it would be advantageous to incorporate randomness in the structural mode] via stochastic finite-element methodologies to make the overall analysis more consistent. Recommendations Perform the following tasks: A multiphased program to develop and implement a methodology for integrated structural analysis and hydrodynamic analysis based on stochastic finite-element methodologies should be initiated. The project would occur in three phases: Phase ~ . Surrey leading design fines to ascertain their existing computational tools, including commercial and proprietary software. For the new tools to be most useful, they should fit as well as possible into the existing program structures. · Survey the literature on the state-of-the-art methodologies for stochastic finite-element analysis that are potentially applicable to existing and anticipated future structural analysis programs for marine structures. . Develop modeling and analysis procedures for floating marine structures using stochastic fin~te-element methods. Phase 2 · Develop and implement a theory for the coupling of the stochastic structural and hydrodynamic models. One possible approach here is the development of 87
stochastic "interface" elements, opposite sides of which would interface with the structure and the fluid discret~zations. · Develop a series of benchmark examples to verify the accuracy of the theory and the correctness of the implementation. The benchmarks win also serve to help others implement this methodology or competing ones. Phase 3 · Conduct a parametric study on the influence of randomness of structural parameters on the overall stochastic design of marine structures. · Solve a complex, realistic ship-structure/manna-structure problem and compare deterministic and stochastic finite-element method results. Duration Phase ~ 2,000 labor hours over ~ year Phase 2 2,000 labor hours over ~ year Phase 3 2,000 labor hours over ~ year SKC/9SD-T Nonlinear Rolling of a Lightship Tanker and Other Shallow Draft Structures Objective Evaluate the rolling motion of a shaBow-draft vessel, which cannot be determined by a conventional analytical tool. Develop a method to predict this highly nonlinear motion and assess stability. Benefit Capsizing of floating vessels in roll is a common phenomenon, especially in high seas. The present analysis does not work weld for a shallow-draft vessel. Development of an improved analytical too] wall help determine the vuinerabili~ of a shallow-draft tanker and other floating vessels in high seas and evaluate their stability cutena. SSC National Goal Improve the safety and integrity of marine structures. SSC Strategy · Development of better design tools · Improved eng~neenng analysis and evaluation Background Most ships and offshore floating structures experience natural frequencies in roll that fall within the range of appreciable wave energy. The hydrodynamic damping in roll is often small and nonlinear. Under these conditions, vessels are susceptible to instability and even capsizing. For shallow-draft vessels, the conventional method of analysis for roll motion is generally inadequate. The roping motion of ships in a seaway has received extensive attention In research. However, a unified approach does not exist to study this motion of a floating vessel. A practical method to evaluate the stability characteristics of the vessel is therefore needed. It should study the instability characteristics of the vessel, as well as ~8
the speec! and heading of the vessel which may make the vessel more unstable under certain conditions. It should consider both the eject of waves and vessel speed at different headings for establishing stability cntena. The effect of random seaway on the roll stability, the probability distn~ution of roll amplitudes, and the sensitivity of the roll motion to variations in system parameters should also be studied. Recommendations Perform the following tasks: · Review past work on rolling motion of floating vessels with special emphasis on shallow-draft vessels and corresponding stability condition. · Determine the areas of uncertainties in terms of draft of vessel versus seventy of the sea-states. · Evaluate methods of predicting roll motions of these vessels in regular high waves. · Establish a practical method to determine stability cr~tena of roll of these vessels. · Extend the analysis to random waves, and determine the probability distribution of roll angles. · Perform sensitivity analysis of dynamic response to and stabilizer with variations in system parameters. Duration 2,000 hours over I.5 years PW/9SD-V Probability-Based Design (Phase 5~: Novel Hull Forms and Environments (93-~) Objective Develop prototype probability-based design criteria for novel marine structures. Benefit Results of this project would provide the basis for reliability assessment and probabiliW-based design for novel marine structures. Background A 5-year research program to apply reliability technology to marine strictures and to develop probability-based design criteria for ship structures has been initiated by the SSC. The program consists of a sequence of five projects leading to probability-based design for both ships and novel marine structures and to related reliability issues as well. This project is the fifth phase of this program. The first phase of the program is a demonstration project (SR-1330~; the second defines loads and load combinations (SR-1337~; the third, implementation of design guiclelines for ships, is an SSC FY 1992 project (SR-1345~; the fourth (9~2) summarizes the present state of the art In rebabili~ design. This fifth-phase project, proposed for FY 1995, wild address reliabilitr-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 ~9
and costly) marine structures have been designed, built, and operated for offshore of] 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 they cannot be based on extrapolation or interpolation of current design practice of egg structures such as conventional vessels. This project wail define the current data base, existing body of structural reiiabili~cy literature, and necessary elements to conduct (to greater or lesser accuracy) a probabilistic assessment of performance and safely of marine structures having special features or subject to special environmental loads. The main thrust of the project wall 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, American Petroleum Institute-Ioad and resistance factor design, 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 nI] tones of marina structures, and develop appropriate limit-state expressions. . . . Or · tuent~ty generic loaning models appropriate tor 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 I.5 years 90
BS/9SM-D Development of a Sensor for Evaluating Corrosion in Areas Not Easily Accessed for Inspection Objective Develop a system for monitoring corrosion and/or paint delamination In locations that are `difficuIt to inspect. Benefits The results of the program are expected to significantly enhance safe operation of ships. SSC National Goal Improve the safetr and integrity of marine structures. SSC Strategy Prevention research including damage-tolerant structures, structural monitonng, and human factors Background Much progress has been made recently In the area of design for the minunization 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 not easy to inspect, such as interhull spaces in some ship designs. What is needed is a nondestructive method that both identifies areas of corrosion and also 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 DC electrochemical measurements is the abilitr to probe corrosion of a m eta] surface beneath an organic substrate. 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. Recommendations Perfo~ the following tasks: · Identity 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 capable of measuring the parameters identified above. · Conduct field teals of the prototype sensor. · Make recommendations for Anal design and implementation of the monitor. Duration 4,000 labor hours over 2 years 9
9SM-M Development of Ductile Fracture Assessment Techniques for Small Defects in Ship Structure Components (9~9) Objective Conduct an experimental and analytical study of the techniques for determining and applying ductile fracture toughness criteria to ship structural steels containing small defects. Benefit The correct evaluation of fracture behavior in structures with small defects can reduce costly, overconservative design or repair practices in ship-structure components that may arise from an incomplete technology. SSC National Goal Support the U.S. maritime industry in shipbuilding, maintenance, and repair. SSC Strategy Structural reliability engineering Background Ductile fracture toughness is traditionally evaluated using J-~ntegral or crack-tip opening displacement as the characterizing parameter. These parameters are easily determined for test specimen geometries in which the crack length is a reasonable fraction of the specimen width. The reason for that restriction is that yielding must be limited only to the uncracked ligament area for the fracture toughness parameters to be correctly evaluated. ~ the specimen contains very small defects, Belying would spread outside the uncracked tip root ligament area, resulting in a gross section or general yielding condition. When this happens, the crack-tip stress field does not relate to the applied fracture parameters as it does for net section yielding, and a fracture toughness measured for standard-sized cracks may be overly conservative when used to predict fracture of structures with small defects. The prediction of fracture behavior for structures with small defects requires that the general yielding condition be included in the evaluation of the fracture parameter and its relationship to the crack-tip field. This evaluation can be made numencally using techniques such as f~nite-element analysis. Results of this analysis would serve as a starting point for an expenmental evaluation of the ejects of small defects on ductile fracture toughness in ship-structure steels. Recommendations Perform the following tasks: · Review the numencal evaluation of the relationship of the fracture parameters J and crack-tip opening displacement to the crack-tip field for short cracks under general yield conditions. · Develop a smalI-defect structural test-specimen geometry to use In fracture toughness testing, with a range of tension and bending loading modes. · Determine fracture toughness using both structural test specimens with small defects and conventional fracture-toughness ones. 92
