Tanker Spills: Prevention by Design (1991)

As noted repeatedly in Chapters 5 and 6, the committee was unable to identify one preferred tank vessel design or even to define specific standards for how a vessel should perform in an accident. The difficulty was not due to an overly cautious approach to the problem, or a lack of willingness on the part of the committee, but rather to an insufficient knowledge and technology base to resolve the problem. During its deliberations, the committee posed numerous questions for which answers seemed inadequate, or lacking entirely. Based on these and related discussions, literature reviews, committee studies, and assessment of the needs of the maritime community and society as a whole, the committee identified several areas where research is needed. These will be detailed in the following pages.

First, it should be noted that a growing number of research projects on improving tank vessel design and operations already are under way or planned. Any new research should take these projects into account.

The committee is aware of studies and research efforts, mostly in other countries, that clearly are the result of the EXXON VALDEZ accident and the U.S. government's interest in legislation aimed at improving tanker design, which culminated in the Oil Pollution Act of 1990, enacted on August 18, 1990. These include:

• A study on oil outflow from collisions and groundings was completed in May 1990 under the joint auspices of the Norwegian Maritime Director-

ate, the Norwegian Shipowners Association, and Det norske Veritas. The study was sponsored by the Royal Norwegian Council for Scientific and Industrial Research (NTNF). A report of this work prompted the committee to commission additional work by DnV extending the basic work done by NTNF. The Norwegian studies assessed oil outflow due to collisions and groundings in ships of various designs; they did not evaluate other accidents or consequences such as fire, explosion, structural failure, or personnel safety.

• A study focusing on three designs is being conducted by Committee RR761, formed by the Japanese government. Membership includes shipowners, shipbuilders, academicians, and government representatives. The study has concentrated on the evaluation of double hulls (particularly grounding outflow), the vacuum system for minimizing grounding outflow, and the intermediate oil-tight deck as a means of improving grounding and collision protection. Results of the studies were presented to the committee and were helpful in drawing conclusions and recommendations. Like the Norwegian study, the work of Committee RR761 addressed only oil outflow from groundings and, secondarily, collisions. It did not estimate effects from fire, explosions, structural failure, safety considerations, etc.

• Studies by shipowners. Various shipowners' associations have conducted international studies of all aspects of oil pollution from tankers (Lloyd's Register of Shipping for the American Petroleum Institute, 1990; International Chamber of Shipping and the Oil Companies International Marine Forum, 1990; the International Association of Independent Tanker Owners, 1990a). The results have been presented to the committee.

A common theme in the reports just described has been a recommendation for prompt, in-depth research into improved tanker design. The committee is aware of several ongoing or planned research projects seeking to meet that need, as follows.

• The Royal Norwegian Council for Scientific and Industrial Research has announced an in-depth study to build on the work of its 1990 study. The new study, to continue through 1994 with a budget of around $28 million, will seek to design a ship that reduces air pollution, sea pollution from operations, and sea pollution from accidents. The study will seek: (1) a reduction in probability of accidents through maneuvering and navigation measures; (2) a reduction in the consequences of accidents through hull design and safety systems; and (3) improved handling of oil spill cleanup.

• A seven-year program is planned by the Japanese government. This project has a budget of about $20 million to investigate new tanker designs that will minimize oil spills from collisions and groundings, and a budget of about $15 million to study the purification of exhaust gases from ships.

In addition to these comprehensive, government-supported efforts, numerous other, smaller programs have been initiated or are under development.

As noted in Chapter 2, naval architects traditionally have not designed tank vessels, at the detail level, to withstand groundings and collisions. While the technology base is adequate and there are internationally recognized standards for the design of vessels to assure their integrity during normal operating conditions, current design practice does not address in-process accident behavior, which aircraft and automotive industries refer to as "crashworthiness." The state of the art in understanding fundamental forces and structural reactions during tank vessel accidents is limited, particularly for groundings. Similarly, although computer models exist for estimating structural damage in particular scenarios, these models have not been validated with actual accident data or full-scale testing.

To better understand actual vessel performance, and to move in the direction of establishing performance standards, more basic knowledge is required. "Performance standards" have meaning only if there is a mechanism to test various proposed designs against these standards; there must be a way to quantitatively predict damage to any design, and the vessel's post-accident performance.

Present analytical methods rely on simplified assumptions and limited numbers of primitive control parameters, local to the site of impact. Consequently, damage projections are but gross approximations, lacking both the detail accuracy required to quantify results beyond directional ranking, and the sensitivity to reflect any but the most major changes in assumptions and/or control factors.

The committee has taken a strong position that performance rather than design criteria should be developed for a spill-free tanker. Design criteria tend to "fix" technology at a point in time, thus inhibiting innovation and removing the incentive to advance ship technology and design. Performance standards tend to promote new development in terms of structural and operational innovations that will result in meeting or surpassing the standards.

The significance of performance standards can best be explained using the example of the automotive industry. Safety standards were adopted in the United States in the early 1970s. All passenger cars currently are required not to exceed 20g deceleration (g is 9.81 m/sec² at the Earth's surface) in a 30 mile-per-hour head-on collision with a rigid barrier. There are also other

requirements concerning, for example, intrusion of the steering wheel into the passenger compartment. Each auto manufacturer took a slightly different approach to meeting these standards, and the highly competitive market resulted in many innovative and ingenious approaches to crash-worthiness. While the performance standards for cars are checked and enforced through mandatory compliance tests, no such full-scale tests are possible for tank vessels. Therefore, an alternative, purely analytical method should be developed to accept or reject a given tank vessel design.

Because full-scale testing of tankers is prohibitively expensive, it is more practical to establish relative performance standards with respect to a chosen design, rather than absolute criteria. Such a reference vessel could be, for example, a conventional MARPOL tanker (the reference for the present study), or a suitable double-hull ship (the new standard for new tankships traveling in U.S. waters).

In the automobile industry, establishment of worldwide performance criteria was preceded by a decade of research and development by regulatory agencies and the industry. Interested parties within the maritime community could launch a similar research program. One focus of such a program would be to establish equivalence criteria for choosing alternative designs, based, for example, on the amount of cargo lost. The effectiveness of alternative designs could be examined in a wide range of collision and grounding scenarios. Additional small-scale and possibly full-scale static and dynamic tests would be necessary to calibrate various theoretical models and to obtain the scaling laws.

Tank vessels vary significantly in structure, even within similar design types. Differences in basic structural arrangements, scantlings, and detail, both individually and cumulatively, will result in different failure sequences and post-accident vessel conditions. Experience proves this, but present technology cannot project it beyond gross generalities. Basic theoretical research is needed into relevant material behavior leading up to microelement failure, and progressing through major structural energy dissipation into ship structures. Models of detail sequence behavior must be enlarged (1) to encompass the full variety of ship structures, (2) to reflect the global changes during the accident process, and (3) to account for the cargo-structure-water interface phenomena as the vessel's integrity is assaulted.

At present, dynamic progressions are defined through rudimentary static study of single-step movement from the intact state to local detail failure. This is an inadequate and misleading approach, if there is to be any attempt

to meet dynamic performance standards of integrated structures. The path of structural failure follows the movement of destructive force as it stresses contiguous local structure into plastic deformation, first rupture and, ultimately, failure. As the dissipating force moves into peripheral structure, the process is repeated, moving like cracks in splitting sheet ice, seeking out the weakest path of resistance before dissipating in non-destructive stressing of steel. The projection of path and extent of destruction remain quantitative unknowns.

To achieve an understanding of structural behavior that is adequate to development of vessel performance standards, specific needs must be met, as follows:

• Improved analytical techniques are required to understand the hull rupture initiation process, a key controlling factor in whether cargo will be spilled. Present analyses use simplified assumptions and limited numbers of parameters. In properly designed hulls, rupture may not be inevitable; to that end, further research also should be directed toward developing innovative hull materials and structural configurations.

• Improved capabilities are needed for predicting the vertical, lateral, and horizontal extent of damage sustained to the ship bottom (of both single and double hulls). Current methods use simplified assumptions. Improved methods also would help determine outflow from breached tanks, determine stability of damaged ships, and plan salvage of stranded ships.

To make design tools available, a development program would need to integrate theory, modeling, structural testing, and verification with historical accident data. Other than the last aspect, all of these are within the ability of academia and industry. Unfortunately, verification is not possible with existing databases and is not part of industry practice.

The preceding discussion points out another significant need. The committee found existing casualty databases incomplete and/or misleading: Of the dozens of accidents referenced in the study, not one is publicly documented to serve as an adequate resource for the most general accident description.

Records lack the detail documentation (vessel speed or description of grounding obstacle, for example) that would help relate a particular scenario to the exact damage done. Such information either is considered proprietary (frequently for legal reasons), or is recorded in an oversimplified form, or not at all. In addition, there is seldom any database link between general accident statistics (such as the number or volume of spills) and the supporting details (vessel description, initial cause of accident, extent of damage in structural terms). An accident analysis typically must

rely on manual search through incomplete records from numerous sources (such as the Coast Guard, shipbuilders, operators, classification societies, and marine casualty organizations). Therefore, better procedures are needed for inspecting newly damaged ships, including detailed determination of collapse/tearing of structure and full photographic coverage. This type of inspection, if mandatory, would provide an engineering and statistical data bank that could enhance understanding of the phenomena involved and lead to improved designs.

Research leading to establishing a mandated damage-assessment protocol should be undertaken; complete and accurate cause-and-effect data would enable development of an accurate vessel performance model. This research would require the cooperation of engineers, ship repair facilities, shipowners and operators, and legal experts.

Ship structural behavior following an accident is influenced by cargo and ballast status. Tank contents affect not only total vessel mass, but also the performance of the containment structures as they process damaging forces. The effect may be either to inhibit or to exacerbate structural failure—and resulting oil outflow. The interaction of tank contents with the sequence of structural failure is neither understood nor taken seriously into account; the issue is no more than an aside in post-accident investigations. Research into the impact phenomena of tank contents (or lack thereof) will be required to quantify post-accident vessel status accurately.

The computational models to be developed for predicted potential oil outflow from a damaged ship should link kinetic energy, structural configuration, and the extent of damage. The next step is to determine the relationship between extent of damage, relative hydrostatic pressure, tank size, and oil outflow. A first step has been taken (the Det norske Veritas method used in Chapter 5), using a very simplified model for describing the ship in conjunction with a probabilistic approach. A more accurate and complete analysis, based on past accidents in combination with a simplified description of probable events, may be appropriate.

In addition, more research is needed to understand the dynamic mixing of sea and cargo in a damaged cargo tank following an accident. Forces contributing to this process, which can lead to greater cargo outflow than explained by hydrostatic pressure, are poorly understood.

Calculation of the reserve strength of partially damaged hulls is an essential aspect of managing groundings, as the ship must be unloaded, salvaged,

and brought to a repair dock. Procedures for evaluating progressive collapse of a hull girder, subjected to a combination of still water and wave loads, need further development and application. Such models have been developed for offshore oil platforms. Some non-linear finite element programs can be used, but present codes need further refinement if they are to be practically applied and more widely used.

A carefully planned and fully instrumented full-scale grounding test would contribute substantially to understanding of tank vessel structural response under traumatic stress. Each year a number of tankers reach the end of their useful life cycle. Possibly one or more of such tankers could be converted into experimental ships and subjected to a controlled grounding.

An analogous crash test of a fully instrumented Boeing 707 was sponsored by the Federal Aviation Agency with industrial involvement. The knowledge gained from the ''test to destruction" of an instrumented surplus tanker would advance all of the structural research areas that have been identified.

The technology base must be enhanced across the spectrum, from exploratory research to proof-of-concept development.

The result of the research projects described should be a reliable mechanism that can, irrespective of a vessel's details, accurately project structural and cargo behavior as a function of vessel design, in any selected accident scenario. Whether the accident is a grounding or collision, the model must perform within the envelope of the given operating environment.

The needs include: (1) an integrated micro-understanding of the dynamics of ship structural failure, and related factors; (2) long-range research in failure theory (interactive behavior of over-stressed integrated structures in association with hydrodynamic loads); (3) protocols leading to mandatory engineering documentation of casualties; and (4) computational models resulting in outflow predictions.

The U.S. government is sponsoring a variety of research efforts related to different aspects of oil spill prevention and cleanup. The present study is one example. Some research will be funded under the Oil Pollution Act of 1990. However, at the present time, no comprehensive multi-year plan has been announced that would have significant impact on ship design focused to reduce or prevent oil outflow during accidents.

The government should cooperate in a comprehensive, multi-year research and development program that would result in computational models for predicting vessel structural response during an accident and consequential cargo outflow. These are the engineering tools basic to development of vessel performance standards.

The program should (1) define the documentation, procedures, and protocols that would remedy the absence of quantified engineering casualty data, (2) ensure adequate theoretical knowledge and application technology to design tank vessels to meet performance standards, and (3) achieve optimal pollution control by integrating use of design alternatives with operational and cleanup options.

The scope of such a program would require the cooperation of government, the engineering and computer science communities, legal experts, shipbuilders, shipowners, and classification societies. The project should be coordinated with foreign efforts through the IMO.

Det norske Veritas. 1990. Potential Oil Spill from Tankers in Case of Collision and/or Grounding: A comparative study of different VLCC designs. Report conducted for the Royal Norwegian Council for Scientific and Industrial Research, Oslo. DnV 90-0074.

International Association of Independent Tanker Owners. 1990a. Measures to Prevent Accidental Pollution. Oslo, Norway: The Association.

International Chamber of Shipping, Oil Companies International Marine Forum, and International Association of Independent Tanker Owners. 1990. Oil Tanker Design and Pollution Prevention. Study prepared for the Committee on Tank Vessel Design, Washington, D.C.

Lloyd's Register of Shipping. 1990. Statistical Study of Outflow from Oil and Chemical Tanker Casualties. Report conducted for American Petroleum Institute, Washington, D.C. Technical Report STD R2-0590.