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OCR for page 85
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
OCR for page 86
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
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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
OCR for page 88
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
OCR for page 89
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
OCR for page 90
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
OCR for page 91
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
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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
Representative terms from entire chapter:
labor hours
