APPENDIX L Research on Double-Hull Vessel Technology since 1990
This appendix describes major research since 1990 aimed at enhancing understanding of the structural behavior of double-hull tank vessels, as well as improving ways to reduce potential outflow after an accident. Much of this research was planned or initiated before the promulgation of the Oil Pollution Act of 1990 (P.L. 101-380) (OPA 90), but the results have been obtained only during the last six years. The first section focuses on structural research on tankers. The ultimate objective of this research is the development of methods and data that will facilitate the design of hull structures with long life and good performance in accident situations. The second section addresses double-hull tanker design concepts that offer alternatives to conventional double-hull tanker construction.
Major research projects in double-hull technology since 1990 have been conducted principally in the United States, Japan, the Netherlands, Denmark, and Norway. In 1991, Japan initiated a major seven-year structural research program on the prevention of oil spills from crude oil tankers under the Association for Structural Improvement of Shipbuilding Industry. Most of the research in the United States has been performed by the Massachusetts Institute of Technology (MIT), the Interagency Ship Structure Committee,1 and the Society of Naval Ar-
chitects and Marine Engineers. Structural research efforts in other countries have also been reported at international meetings, such as the International Ship and Offshore Structures Congress and the International Symposia on Practical Design of Ships and Mobile Units.
Structural Design Research to Reduce the Effects of Fatigue on Ship Life
As noted in Chapter 6, some problems with fatigue cracking of high tensile steels in double-hull tankers were encountered in the late 1980s. A double hull tends to be stiffer than its single-hull counterpart; this can affect residual stresses induced during construction and local stresses due to operational loads, both of which can result in initiation of fatigue cracks.
Advances in finite element stress analysis techniques have made it possible to obtain accurate and detailed stress estimates. For the most part, analyses of this type are now carried out routinely as an integral part of the design process by shipyards producing double-hull tankers and are no longer regarded as research studies. Structural details at welded junctions have been analyzed and redesigned to improve fatigue life. Experimental research in this area is under way to document the development of fatigue cracks in various joint designs. The large-scale tests conducted at the Krylov Shipbuilding Research Institute in Kiev, sponsored by Lloyd's Register of Shipping, represent one such example (Violette, 1995). The application of fracture mechanics to improve the fatigue life of ship hulls has already paid dividends. Nevertheless, there appears to be considerable potential for further progress in this area.
Structural Responses to Collisions
Since V.U. Minorsky's efforts in the late 1950s to correlate the interpenetration of colliding ships using accident data (Minorsky, 1959), there has been continuing research aimed at more accurately accounting for the structural details and approach characteristics of colliding ships. Although the early approach to predicting penetration largely depended on relatively simple energy accounting, the most recent methods are based on detailed analysis of plastic buckling, collapse, and fracture. The importance of post-collision ship motion and wave generation in the energy balance is now recognized. The evolving methods are applicable to all types of ship structures, including double hulls. The goals of this research are to allow the designer to evaluate the performance of competing designs in a variety of critical accident scenarios and to refine designs to meet specific performance goals.
Collision analysis has been greatly aided by modern nonlinear finite element methods, which have been increasingly used for research in this area during the past five years. Nonlinear finite element methods are now starting to be used to
optimize double-hull designs with respect to plate thickness; steel strength; and positioning of inner and outer hull plates, side stringers, and transverse webs. Verification of analytical procedures using scale-model tests and actual collision data—where available—is a necessary part of the approach because of the inherent difficulty in modeling highly contorted collapse modes and the relatively crude criteria that are still employed to model plate- and weld-fracture during crushing.
Full-scale collision tests have been conducted using two inland waterway tankers, each of approximately 1,000-metric ton displacement, in a collaborative effort with support from a number of Dutch and Japanese groups (Vredeveldt and Wevers, 1992, 1995; Wevers et al., 1994). These tests were accompanied by detailed numerical simulations (Lenselink and Thung, 1992). A series of four impacts were conducted wherein one tanker fitted with a nominally rigid bow struck the other tanker's side at 90 degrees. Two of the impacts were against side sections of the ship having a single hull, whereas in the other two collisions the tanker was struck in side sections having a double hull. Data were recorded on penetration depth, collision force, strains in critical locations, and all six rigid-body motions of each of the ships. In addition, observations on cracking, which largely occurred along weld lines, were reported. The accompanying numerical simulations were successful in replicating major features of the collision, with the exception of crack patterns. The experimental data will be available for calibration of analysis methods in the future. Among the conclusions drawn from the joint Dutch-Japanese research were these: fracture initiation is dominated by the welds and is poorly characterized; the hydrodynamics of both ships during collision must be modeled correctly if penetration and collision forces are to be predicted accurately; and a sizable fraction of the energy dissipated in a collision goes into wave generation.
Structural Responses to Groundings
Most aspects of structural failure in tanker grounding incidents can be analyzed by the same methods used to analyze ship collisions. However, hull-girder failure (i.e., "breaking the back" of the tanker) and hull tearing are features specific to grounding that require specialized approaches. Hull-girder failures due to grounding have been examined with the aid of increasingly powerful numerical models within the last five years. Issues studied include whether dynamic effects contribute significantly to hull-girder collapse and the influence of friction between hull and seabed. The computational models are in reasonable accord with model and full-scale grounding tests, such as those undertaken in Denmark (Paik and Pedersen, 1995).
The U.S. Navy has conducted 1/4-scale model tests for strandings (loadings normal to the bottom of the hull) and groundings (combined normal and in-plane loadings) (Sikora and Bruchman, 1992; Melton et al., 1994; Rodd and Sikora, 1995; Sikora et al., 1995). These tests were part of a comprehensive program that
also considered preliminary designs of double-hull vessels for both naval and commercial use, with particular attention to efficient producibility and long fatigue life. The grounding tests were accompanied by analytical work based on nonlinear finite element structural models similar to those used to analyze the Dutch-Japanese collision tests.
Initial efforts to assess the resistance of underside hull plates to tearing by a protrusion, such as a rock jutting up from the seabed, have been undertaken at MIT (Wierzbicki, 1995). This effort is couched within the framework of fracture mechanics, where the energy required per unit length in the tearing of a plate plays a central role in the analysis. The tearing energy for steel plate must be measured independently in a simulated tearing test. Then, the length of the underside rupture is estimated by accounting for the combined energy dissipated in the grounding from tearing and from plastic deformation of the hull during interaction with the protrusion, with due allowance for other mechanisms of energy loss. The mechanics of problems combining large amounts of plastic deformation and fracture are unusually challenging. Part of the difficulty in applications to ship hulls lies in the fact that the tearing energy constitutes a relatively small proportion of the total energy dissipated in the grounding, yet this energy is critical in determining of the extent of the tear.
The integration of a sound fracture analysis approach into collision and grounding analyses would constitute a major advance in the analytical tools available to assess and design double-hull tankers. The U.S. Navy has identified this as an important goal necessary to improve prediction capabilities for the design and residual strength analysis of ship structures against accidents and aggressive attacks. Observations suggest that most cracks will initiate and propagate in weldments. Criteria currently employed for the initiation of cracking during a collision or grounding are usually based on the attainment of some critical plastic strain locally in the weld or plate material. The validity of such criteria remain poorly established. Once a crack has initiated in a region of intense deformation, its subsequent spread requires a fracture mechanics analysis using the relevant fracture properties of the weldment or plate. The joint Dutch-Japanese study cited above found that cracks that formed and propagated outside the immediate penetration region had to be accounted for if accurate predictions for collisions or groundings were to be achieved. Observations concerning the tendency for cracks to initiate in welds highlight the importance of weld quality.
Structural design approaches used today ensure that tankers have sufficient strength to withstand the loads encountered in regular operation, but there are no provisions for the loads encountered in accidents. Similarly, the outflow performance of tankers is based on tank subdivision only, and no consideration is given to the performance of the structure in collisions and groundings. The development of tools that could be used to design tanker structures for good performance in accident situations will provide an important advance in the design of tankers. The research described above has this objective, although much work
is still required before the results of research efforts can be translated into practical design tools.
Other than work being conducted by the U.S. Navy, the research of Wierzbicki and his coworkers at MIT represents the main activity in the United States dedicated to the development of advanced analysis methods for ship structures along the lines indicated above. Nearly all of Wierzbicki's effort is supported by industry sources, mainly from abroad. The committee is concerned that important research opportunities may be missed due to the absence of any significant U.S. agency funding for work on the development of analysis tools for structural integrity of ship structures. Most of the powerful computer codes used to analyze the nonlinear deformation of structures have been developed in this country, and the expertise needed to extend them to include effects such as collapse and fracture also resides in this country.
Alternative Double-Hull Tanker Designs
Several design concepts have been developed since 1990 that offer alternatives to "conventional" double-hull tanker construction. The concepts can be divided into three major categories: (1) designs to improve producibility, (2) designs to improve outflow performance, and (3) designs to reduce maintenance costs.
A number of designers have proposed unidirectionally stiffened double-hull tanker designs in which the amount of transverse structure has been minimized to maximize productivity in construction. This concept was proposed before double-hull tankers became mandatory (Okamoto et al., 1985). The unidirectionally stiffened structure improves construction productivity by reducing the number of structural joints and by allowing maximum use of automatic welding. A disadvantage of this design is that a unidirectional structure requires smaller tanks than a conventional tanker structure, thus increasing the subdivisions and the weight of steel in the vessel. The unidirectionally stiffened double-hull concept has been applied to small product tankers built in the Far East but has not been applied successfully to larger tankers to date.
The U.S. Navy has undertaken a study of a unidirectionally stiffened double-hull design (advanced double-hull concept) with emphasis on fatigue life and producibility as well as resilience to collision, grounding, and attack (Melton et al., 1994; Sikora et al., 1995). The MarC Guardian tanker project—supported by the Carderock Division, Naval Surface Warfare Center, and Advanced Research Projects Agency (ARPA) Maritech—is proposing a unidirectionally-stiffened design concept that uses slightly curved plating for the outer and inner hulls of a vessel, thereby eliminating the need for local plate stiffeners. The spacing between hulls,
the spacing of longitudinal girders, and the spacing of transverse structures can be standardized for various vessel sizes by using curved plates (Goldbach, 1994).
Improved Outflow Performance
Five European shipbuilders—Astilleros Españoles, Bremer Vulkan, Chantiers de l'Atlantique, Fincantieri, and Howaldtswerke Deutsche Werft—have cooperated in designing an ecological tanker concept called the "E3 tanker" (Paetow, 1992). The name refers to several tanker designs that provide varying levels of protection against oil spills. The arrangement and construction of a "standard E3" concept do not differ from a typical very large crude carrier (VLCC) double-hull tanker design. However, the "superecological E3" tanker design has small cargo tanks and double-hull dimensions that were optimized to reduce the probability of oil spills using statistical data on damage extent and damage locations on the hull. So far, one standard E3 tanker has been built in Spain (Gutierrez-Fraile et al., 1994).
NKK Corporation and World-Wide Shipping Agency have proposed an alternative double-hull design concept in which the double-hull spaces are dry void spaces and ballast tanks are arranged in the inner hull in a manner similar to that in a single-hull tanker. The concept aims to eliminate concerns associated with the operation and maintenance of double-hull tankers. The increased initial cost is offset by lower maintenance costs (Akita et al., 1995). Such a design would have to be larger overall to provide for adequate ballast in addition to void tanks.
All of the double-hull tanker designs described above are still at the concept stage, with the exception of small unidirectionally stiffened tankers built in the Far East and the standard E3 tanker built in Spain. They have not yet been proven to be competitive alternatives to conventional double-hull tanker designs.
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