TANKER SPILLS

PREVENTION BY DESIGN

Committee on Tank Vessel Design

Marine Board

Commission on Engineering and Technical Systems

National Research Council

NATIONAL ACADEMY PRESS
Washington, D.C.
1991



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Tanker Spills: Prevention by Design TANKER SPILLS PREVENTION BY DESIGN Committee on Tank Vessel Design Marine Board Commission on Engineering and Technical Systems National Research Council NATIONAL ACADEMY PRESS Washington, D.C. 1991

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Tanker Spills: Prevention by Design National Academy Press 2101 Constitution Avenue, N.W. Washington, D.C. 20418 NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the panel responsible for the report were chosen for their special competencies and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The program described in this report is supported by Cooperative Agreement No. 14-35-0001-30475 between the Minerals Management Service of the U.S. Department of the Interior and the National Academy of Sciences. Library of Congress Cataloging-in-Publication Data National Research Council (U.S.). Committee on Tank Vessel Design. Tanker spills : prevention by design / Committee on Tank Vessel Design, Marine Board, Commission on Engineering and Technical Systems, National Research Council. p. cm. Includes bibliographical references and index. ISBN 0-309-04377-8 1. Tankers—Accidents. 2. Oil spills. I. Title. VM455.N35 1991 623.8'245—dc20 91-12489 CIP © 1991 by the National Academy of Sciences. All rights reserved. No part of this book may be reproduced by any mechanical, photographic, or electronic process, or in the form of a phonographic recording, nor may it be stored in a retrieval system, transmitted, or otherwise copied for public or private use without written permission from the publisher, except for the purposes of official use by the U.S. government. Printed in the United States of America Cover: The Arco Marine, 122,000 deadweight ton, single hulled, tanker Arco Juneau serves the U.S. trade, including Alaska and West Coast ports. The Arco Juneau is a U.S.-flag ship, classified by the American Bureau of Shipping. With 17 years of service (as of 1991), it is about average in age for the U.S.-flag tanker fleet. Photo copyright © Vince Streano, Streano-Havens, Anacortes, Washington.

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Tanker Spills: Prevention by Design COMMITTEE ON TANK VESSEL DESIGN HENRY S. MARCUS, Chairman, Massachusetts Institute of Technology, Cambridge WILLIAM O. GRAY, Vice Chairman, Skaarup Oil Corporation, Greenwich, Connecticut DAVID M. BOVET, Temple, Barker & Sloane, Inc., Lexington, Massachusetts J. HUNTLY BOYD, JR., Booz-Allen & Hamilton, Inc., Arlington, Virginia JOHN W. BOYLSTON, Argent Marine Operations, Inc., Solomons, Maryland JOHN W. BURKE, Mobil Shipping & Transportation Company, Fairfax, Virginia THOMAS D. HOPKINS, Rochester Institute of Technology, Rochester, New York JAMES HORNSBY, Transport Canada (retired), Ottawa, Ontario, Canada VOJIN JOKSIMOVICH, Accident Prevention Group, San Diego, California SALLY ANN LENTZ, The Oceanic Society and Friends of the Earth, Washington, D.C. ROBERT G. LOEWY, NAE, Rensselaer Polytechnic Institute, Troy, New York TOMASZ WIERZBICKI, Massachusetts Institute of Technology, Cambridge Liaisons DONALD LIU, American Bureau of Shipping, Parmamus, New Jersey JAMES M. MACDONALD, United States Coast Guard, Washington, D.C. BERT MARSH, United States Navy, Washington, D.C. FREDERICK SEIBOLD, Maritime Administration, Washington, D.C. DANIEL F. SHEEHAN, United States Coast Guard, Washington, D.C. Staff DONALD W. PERKINS, Associate Director LAURA OST, Editor GLORIA GREEN, Project Assistant

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Tanker Spills: Prevention by Design MARINE BOARD BRIAN J. WATT, Chairman, Joy Technologies Inc., Pittsburgh, Pennsylvania ROBERT N. STEINER, Vice-Chairman, Delaware River Port Authority, Camden, New Jersey ROBERT G. BEA, NAE, University of California, Berkeley, California JAMES M. BROADUS, III, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts LARRY L. GENTRY, Lockheed Advanced Marine Systems, Sunnyvale, California ROBERT TURNER HUDSPETH, Oregon State University, Corvallis, Oregon MARCUS J. JOHNSON, Sea-Land Service, Inc., Iselin, New Jersey JUDITH T. KILDOW, Tufts University, Medford, Massachusetts BERNARD LE MEHAUTE, University of Miami, Miami, Florida WILLIAM R. MURDEN, NAE, Murden Marine, Ltd., Alexandria, Virginia JOSEPH D. PORRICELLI, ECO, Inc., Annapolis, Maryland PAUL A. SANDIFER, South Carolina Wildlife Marine Resources Department, Charleston, South Carolina JERRY R. SCHUBEL, State University of New York, Stony Brook, New York PETER R. TATRO, Johns Hopkins Applied Physics Laboratory, Reston, Virginia GEORGE P. VANCE, Mobile Research and Development Corporation, Dallas, Texas DON WALSH, International Maritime Inc., San Pedro, California EDWARD WENK, JR., NAE, University of Washington, Seattle, Washington Staff CHARLES A. BOOKMAN, Director DONALD W. PERKINS, Associate Director DORIS C. HOLMES, Staff Associate

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Tanker Spills: Prevention by Design The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Frank Press is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Robert M. White is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Samuel O. Thier is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Frank Press and Dr. Robert M. White are chairman and vice-chairman, respectively, of the National Research Council.

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Tanker Spills: Prevention by Design Preface Each year thousands of tons of crude oil and petroleum products are spilled in U.S. waters as a result of many minor, and a few major, tank vessel collisions, groundings, and other accidents. Although the average annual spillage represents 1/500th of 1 percent of the total amount of oil moved through U.S. waters, the effects can be costly, as well as devastating to the environment. Reducing the risk of oil spills is critical to continued public acceptance of tank vessel operations, particularly in light of the nation's growing consumption of oil and projections for increased seaborne imports. As part of the overall effort to promote maritime safety and environmental protection, the U.S. Coast Guard regulates some aspects of U.S.-flag tankship design, crew qualifications and training, and vessel traffic and routing. The Coast Guard also enforces national and international shipping regulations in U.S. waters. The March 1989 grounding of the EXXON VALDEZ, and the subsequent release of nearly 35,600 tons (11 million gallons) of oil in Prince William Sound, Alaska, renewed longstanding debate over the need for more pollution-resistant tank vessel designs. In the aftermath of the Alaska spill, the largest ever in U.S. waters, the Coast Guard asked the National Research Council to assess whether alternative tank vessel designs would improve maritime safety and environmental protection. A committee was convened under the Marine Board to conduct a comprehensive review of the safety, economic, and environmental implications of alternative designs, and to determine how these designs might affect the overall consequences of accidents. The intent of the study was to assemble technical information and recommendations that could be used by

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Tanker Spills: Prevention by Design the Coast Guard in determining design requirements for tank vessels operating in U.S. waters. The committee was charged with: collecting and analyzing data to update what is known about tank vessel accidents, their environmental consequences, and the effectiveness of alternative designs in preventing oil pollution; identifying and assessing the technical concerns about alternative tank vessel designs, including concerns about the effects of alternative designs on salvage, safety, and ship operations; considering whether new materials or design approaches or any other developments merit research; and elucidating the safety, environmental, and economic costs and benefits of alternative new or retrofitted tank vessel designs. Passage of the Oil Pollution Act of 1990 (P.L. 101-380), on August 18, further defined the committee's task. The Act states that "… the Secretary [of the Department of Transportation] shall determine, based on recommendations of the National Academy of Sciences or other qualified organizations, whether other structural and operational tank vessel requirements will provide protection to the marine environment equal to or greater than that provided by double hulls, and shall report to Congress that determination and recommendations for legislative action" (Title IV, Sec. 4115). The committee was composed of members with expertise in tank vessel design and construction, tank vessel operation, maritime salvage, maritime safety, vehicle dynamics, structural engineering, economic analysis, risk assessment and management, environmental and maritime law, and maritime environmental protection. The principle guiding the constitution of the committee and its work, consistent with the policy of the National Research Council, was not to exclude members with potential biases that might accompany expertise vital to the study, but to seek balance and fair treatment. The committee was assisted by representatives of the Coast Guard, the Maritime Administration, the American Bureau of Shipping, and the U.S. Navy, who were designated as liaison representatives. Reducing the risk of oil spills entails the ability to achieve the following fundamental goals: (1) an adequate understanding of spillage scenarios (e.g., groundings, collisions, etc.); (2) reduction of the likelihood of such scenarios; and (3) reduction of the extent or magnitude of spillage from such scenarios. Risk reduction can be accomplished through various means, of which improved tank vessel design is only one. The relative importance of design, in relation to other factors influencing risk of accidents, is open to debate. In any case, vessel design is a significant influence on pollution risk, along with such other factors as vessel maintenance and operations practices, sea conditions, traffic density, vessel speed, and crew competence.

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Tanker Spills: Prevention by Design This study describes how alternative tank vessel designs might influence the safety of personnel, property, and the environment, and at what cost. Tank vessel design and operation, and risks worldwide, are addressed. To provide a context for its technical analysis, the committee addressed the feasibility and ramifications of implementing the various design options. In selecting designs to be considered, the committee included certain operational options that could minimize the amount of oil spilled in an accident, particularly if the procedure had structural implications. An example of such an operation is reducing the level of cargo carried in tanks. The committee also considered emergency mitigation options and associated equipment when related to ship design; cargo handling systems, for example, can help minimize oil spillage when placed at certain locations on the vessel. Among the options not addressed were: means of averting accidents, such as improved navigational aids; methods of altering the form of the cargo, such as mixing it with dispersants or jelling agents; and responses to oil spills, such as cleanup operations mounted from the vessel once cargo has been spilled. Risk never can be eliminated entirely, but it can be reduced at some cost to a value acceptable to society. Society enjoys the benefits of oil transportation but now demands a substantial risk reduction, for which it presumably is prepared to pay reasonable costs. This assessment addresses ocean-going oil tankers and barges over 10,000 DWT. Combination carriers "trading wet" (carrying oil) also are included. Chemical carriers and liquefied gas carriers were not addressed, because their cargoes, operations, and rules are vastly different from those of oil tankers. The committee met seven times beginning in late 1989. The effort to gather and analyze information reached around the world, reflective of the breadth and complexity of the oil transportation system. The committee received more than a hundred contributions consisting of background material and suggested tank vessel designs, including some from Europe and Japan. Presentations were solicited from the tanker and barge industries, oil companies, the Coast Guard, ship classification societies, shipbuilders, university researchers, and environmental groups. In addition to reviewing existing studies and data on accidents and tank vessel design, the committee requested or conducted new investigations of the theoretical effects of collisions and groundings, the pollution-reduction potential and cost-effectiveness of alternative designs, and risk reduction potential. The committee also toured the EXXON VALDEZ during its repair in drydock and the double-hulled CHEVRON OREGON, in port for maintenance. The report is organized into four parts. Chapters 1 and 2 provide a basic overview of the tank vessel industry, covering traffic patterns, casualty history, design and operational principles, and the regulatory framework. Chapters 3, 4, and 5 comprise the technical core of the report. These

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Tanker Spills: Prevention by Design chapters discuss the physical phenomena and engineering issues related to tank vessel design and present the committee's technical evaluation of 17 alternative tank vessel designs. Chapter 6 completes the committee's overall analysis with an economic assessment of the most promising designs. Chapter 7 examines the need for research. The committee's conclusions and recommendations, contained in the Summary, are based on presentations made to the committee, the professional experience of committee members, reviews of past studies and new investigations developed by the committee, and deliberations by the committee. The report was reviewed according to the criteria established by the NRC's Report Review Committee. The committee gratefully acknowledges the many individuals who contributed to the study. The broad scope of this report, and the difficulty of obtaining comprehensive information related to numerous topics, demanded an unusual effort on all fronts. In particular, the project could not have been accomplished without the help of the following people: Sean Connaughton of the American Petroleum Institute, who provided extensive background information; Thomas Allegretti of the American Waterways Operators, who provided information about the barge industry; Donald Liu of the American Bureau of Shipping, who helped explain the complex tank vessel design process; liaison representatives from the U.S. Coast Guard who offered continuing guidance, including Captain James MacDonald and his associates Stephen Shapiro, Daniel Sheehan, and Captain James C. Card; and other liaison representatives, including Frederick Seibold of the Maritime Administration and Lieutenant Commander Bert Marsh of the U.S. Navy. In addition, the committee appreciates the assistance of those who provided data and information for analysis by the committee: David St. Amand of Booz-Allen & Hamilton, Inc., who prepared tanker activity graphics from data compiled by Lloyd's Maritime Information Services Ltd.; Ian White of The International Tanker Owners Pollution Federation Ltd., who provided ship and events data for major oil spills; Richard Golob of World Information Systems Inc. of Cambridge, Massachusetts, and the personnel of Cutter Information Corporation, of Arlington, Massachusetts, who provided quick response information about specific accidents; David Palk and his associates at Clarkson Research Studies Ltd. (London), who supplied world fleet statistics; Constantine Foltis, naval architect, Arlington, Virginia, who researched accident damage data from selected grounding and collision reports; and Arthur McKenzie, Director, Tanker Advisory, who provided much data about U.S. and world double-hull tank fleets. Cost information was obtained from those who made presentations to the committee as well as from several persons who provided written reports. The committee also appreciates the extensive cost analyses for VLCCs provided by John Parker, Chairman, Harland and Wolff Shipbuilding and Heavy Industries Ltd., Belfast,

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Tanker Spills: Prevention by Design Northern Ireland, and by Mitsubishi Heavy Industries Ltd. The committee also appreciates the comments received from many ship salvors who responded to the committee's survey concerning tanker salvage. The committee acknowledges all who submitted proposals for alternative tank vessel designs and the many members of maritime industries and environmental community who contributed their insight on various issues. Special thanks is extended to those who made presentations at committee meetings, listed in Appendix J. Finally, the Chairman wishes to thank all members of the committee for their time and expertise, especially those who made an extra effort to obtain or analyze information on specific topics, to attend extra meetings or work sessions, or to prepare drafts of particular sections of the report. These special contributions were essential to the interpretation of a difficult subject and the production of a well-informed report.

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Tanker Spills: Prevention by Design In view of the above, the committee's judgment was by necessity a key element in every aspect of this report. However, the committee did reach a strong consensus on its findings, as follows. DOUBLE HULLS SHOULD REDUCE POLLUTION FROM GROUNDINGS AND COLLISIONS When OPA 90 requirements are fully implemented over the next 25 years, double hulls should save (in absence of other risk-reduction measures) an estimated 3,000 to 5,000 tons of oil spillage per year in U.S. waters from collisions and groundings, based on available historical data. The savings represent roughly half of the current average annual spillage from vessel accidents in U.S. waters. The added transport cost would be more than $700 million a year, or on the order of one cent per gallon transported. On the basis of cost-effectiveness, the double hull is among the best values of the designs evaluated by the committee. Double hulls are particularly effective in low-energy (typically low velocity) groundings and collisions. Of the alternatives considered by the committee, this option is the only one that places two structural barriers between the entire cargo and the ocean; thus, the double hull can prevent outflow as long as the inner barrier is not breached. In addition to providing sufficient between-hull spacing for grounding and collision protection as well as sufficient accessibility to all spaces for maintenance and inspection, double-hull designs must incorporate, to achieve full potential, adequate outer plate thickness, and cargo and ballast tank sizes and arrangements that assure adequate stability when the ship is damaged. Minimum thickness criteria for outer plates should be established promptly; in the interim, the minimum should be no less than the outer plate thickness required of new single-hull vessels. Accessibility of void spaces for inspection and maintenance is a significant concern, which grows with the number of compartments (as in double hulls). A spacing of about 2 meters minimum between hulls in tank vessels over 20,000 deadweight tons (DWT) should allow room for inspection. More research is needed to determine the spacing between hulls that best satisfies all concerns. To provide pollution protection, inter-hull spacing should be approximately the ship's beam divided by 15 (B/15) or 2 meters, whichever is greater. However, shipowners should not be required to make double-bottom heights (whether as part of a double hull or as an independent design) much greater than 3 meters; higher double bottoms should contain permanent fixed access to ensure adequate inspection of the underside of the inner bottom. Further study of void space dimensions in small tank vessels (including those under 20,000 DWT) should be conducted to determine the impact of the "B/15 or 2 meter" criterion.

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Tanker Spills: Prevention by Design The latest single-skin tank vessel designs exceed international criteria meant to ensure adequate stability following hull damage. The introduction of more ballast compartments subject to flooding (as with double hulls and related concepts), and larger cargo tanks, could result in designs that are less stable, while still meeting current criteria. The committee is persuaded that current criteria do not ensure adequate stability following high-energy (high-speed) accidents. The criteria can and should be revised to ensure that double-hull and other possible new designs are not less stable following damage than single-hull tank vessels. MERITS OF OTHER DESIGN ALTERNATIVES FOR NEW VESSELS A total of 17 design concepts, as well as 3 combination concepts, were evaluated by the committee. Alternatives and combinations that received all evaluations—technical, outflow performance, and cost—were: double bottom double sides double hull hydrostatically balanced loading (hydrostatic control) smaller cargo tanks intermediate oil-tight deck with double sides (IOTD w/DS) double sides with hydrostatic control double hull with hydrostatic control The committee did not identify any design as superior to the double hull for all accident scenarios. Double bottoms and smaller tanks do not offer comparable pollution-reduction potential, although each is cost-effective. Double sides are not cost-effective. The committee sought to establish whether there were design alternatives that would meet or exceed the pollution control and cost-effectiveness of state-of-the-art tankers built to current standards (codified in the International Convention for the Prevention of Pollution from Ships, or MARPOL). OPA 90 establishes a different standard, specifying that design alternatives "provide protection … equal to or greater than that provided by double hulls." However, there are no generally accepted criteria for evaluating the equivalency of two designs. The difficulty is that the results of any analysis of the performance of tank vessels depends on the assumptions and the particular accident scenario analyzed. Different designs may perform better in some situations and worse in others. The committee considered adding an operational feature, hydrostatic control, to various structural alternatives for new tank vessels. This feature makes use of the laws of physics involving hydrostatic pressure to reduce or prevent

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Tanker Spills: Prevention by Design oil outflow resulting from bottom damage. The combinations considered were double hulls with hydrostatic control, double sides with hydrostatic control, and a conventional single hull with hydrostatic control. These concepts are not considered as desirable as the double hull; the primary reasons are: the effectiveness of hydrostatic control depends on operators' strict adherence to rules, rather than on a permanent feature of vessel design and construction; and hydrostatic control does not provide complete protection against oil outflow, due to tidal variations or wave action following a grounding. Therefore, the committee does not favor hydrostatically balanced design options for new tank vessels. In theory, another design concept, which also employs hydrostatic principles, could perform better than the double hull in certain circumstances and scenarios. The IOTD w/DS could spill less oil in high-energy groundings (under circumstances that would have penetrated the inner hull of the double hull) and in some collisions (because it employs wider side tanks). This design is unique in that the pressure of seawater forced into the vessel (rather than oil flowing out) in the event of tank rupture would be significantly greater than the pressures available in all other design alternatives. Another favorable attribute of this design is that it employs less structure in void spaces than double hulls; this should result in less risk of corrosion and easier inspection. However, significant factors may undermine the pollution-prevention benefits of this design, including the advantages of its hydrostatic pressure features. Because the cargo tanks have a single bottom, these tanks would be penetrated in groundings more often than would cargo tanks within a double hull; furthermore, when the bottom of an IOTD w/DS vessel is penetrated, at least some oil would be spilled (i.e., poorer performance than the double hull in low-energy groundings). Moreover, the spillage could be aggravated by tidal or wave action or ship motion subsequent to an accident. The cargo tanks of the IOTD w/DS would be a greater challenge to inspect than a single-hull vessel built under existing standards. An undetected structural failure of the middle deck of the IOTD w/DS would effectively transform the vessel into a conventional single-bottom tanker with equivalent outflow potential. Failure of mechanical features (i.e., valve actuators) that isolate upper and lower tanks, and operational mishaps (i.e., human error) that compromise the separation of upper and lower tanks, would eliminate the hydrostatic balance integrity of the ship. Finally, this design increases operational complexity with respect to loading, unloading, and preparation for docking. Weighing the pros and cons, some committee members consider the IOTD w/DS a practical and innovative application of available technology, which, if treated as an equivalent to the double hull and allowed to trade in com-

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Tanker Spills: Prevention by Design merce to the United States, would reduce pollution in several classes of accidents, including high-energy groundings (which have caused some of the largest oil spills). Others of the committee hold the judgment that the gap between theoretical principles and practical application is wide, that the design and its implementation are unproven, and that, pending more extensive evaluation, the nation deserves designs employing physical barriers, such as double hulls, over those that continue to employ a single hull, such as the IOTD w/DS. Some committee members recommend that the U.S. Coast Guard extend the committee's evaluation of the IOTD w/DS alternative to determine whether this design is equivalent or superior to the double hull standard of OPA 90. The results of this analysis should be presented to the Congress and the IMO for action, as appropriate. Committee members who are skeptical of this design recommend that a more extensive, critical evaluation be conducted, and the results evaluated by the IMO, before the Coast Guard takes action relative to OPA 90. Any of the design options considered by the committee, if fully implemented in U.S. waters, would add roughly one or two cents per gallon to the cost of oil transported, which might be reflected in a similar increase in the cost of gasoline to consumers. This amounts to total annual costs of at least $340 million, and, at most, roughly $2 billion, depending on the design chosen. Other design alternatives may be proposed in the course of future research. New proposals should be considered. Regardless of designs chosen, certain features should be standard on all new tank vessels. Towing fittings of a design recommended by IMO would help in towing a vessel, if that should be necessary after an accident or machinery breakdown (conventional mooring fittings are not as sturdy). All new tankers and barges should be built with towing fittings mounted on the bow and stern. In addition, all new tankers should have a reliable onboard system for transferring cargo from a damaged tank to an intact tank or another vessel. Practical concepts have been developed utilizing available equipment for this purpose. These features are likely to be less applicable to barges, as they are unmanned. In addition, IMO and the Coast Guard should prohibit the placement of cargo piping in ballast tanks, to reduce the danger of fire and explosion due to hydrocarbon vapor leakage. Finally, the passive vacuum system (for use on fully loaded cargo tanks) deserves further research and development. FEWER OPTIONS ARE AVAILABLE FOR BARGES In general, fewer pollution-resistant design options are appropriate for barges. Barges are unmanned and their attendant tugs have minimal crew

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Tanker Spills: Prevention by Design complements; thus, the dedicated presence of skilled personnel for operational control is limited. Also, barges carry a wide range of petroleum products of varying densities and properties. The diversity in cargo-loading levels (depth of oil or product in tanks) required to implement hydrostatic control implies crew vigilance, knowledge, and presence to a degree not consistent with barge operations. At the same time, towed barges are less maneuverable than self-propelled ships and hence, more damage prone. Physical structural barriers for secondary containment, such as double hulls, double sides, or double bottoms, are more reliable for barge operations. The intermediate oil-tight deck might be adaptable for barges, but would require carefully controlled loading and discharge practices. DOUBLE HULLS NEED NOT INCREASE INCIDENCE OF FIRES OR EXPLOSIONS, IMPAIR POST-ACCIDENT STABILITY, OR COMPLICATE SALVAGE Various issues related to double bottoms, double sides, and double hulls (or any design incorporating large void spaces) have caused considerable controversy. Increased risk of fire and explosion, possible vessel instability after an accident, perceived salvage difficulties, and increased personnel hazards are cited by critics of these designs. The committee received various arguments along those lines but little conclusive evidence. Void spaces involve risk of fires and explosions due to the potential accumulation of hydrocarbon vapors, which can enter through cracks or pits in bulkheads adjoining cargo tanks, or through a defect in the cargo piping system. Although double hulls have significantly increased void spaces compared to single-hull tankers (with up to three times the shell and bulkhead area exposed to corrosion), there is no reliable evidence of increased incidence of fires or explosions in existing double-bottom or double-hull ships. However, the risk cannot be ignored; planned maintenance and thorough inspection are critical. Coast Guard inspections already are considered barely adequate, and owners must be relied upon to conduct complete inspections. Additional requirements for particular structural configurations, especially those increasing the need for proper maintenance and inspection, will add to the existing workload. Furthermore, most existing double-hull vessels carry petroleum products, which usually are less corrosive than crude oil. In double-hull crude carriers, diligence will be needed to monitor for corrosion from the cargo tank side as well as the ballast tank side, especially on horizontal surfaces. Means of detecting and combating the problem should be assessed as well. Double-hull designs have large void spaces exposed to potential damage, flooding, and additional weight, which implies possible instability following an accident. However, tankers designed to international (MARPOL)

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Tanker Spills: Prevention by Design standards tend to be exceptionally stable and generally exceed current damage stability requirements. Double-hull vessels can be designed not only to meet requirements for damage stability, but also to survive more severe accident damage than currently required. The committee believes, however, that criteria for evaluating damage stability of double-hull tank vessels should be tightened to assure damage stability approaching that of conventional single-skin MARPOL tankers. Salvage concerns are only partly supported. Piercing of void spaces of loaded ships (whether at bottom or sides) will cause sinkage and/or heeling depending on the position and size of the void. This may or may not be a problem, depending on whether the vessel remains firmly stranded following an initial grounding. Greater amounts of flooded void spaces would help keep a stranded ship firmly aground, thus reducing the possibility of pounding under wave action. However, this would increase the amount of cargo to be removed to refloat the ship, or the amount of water to be blown from flooded voids. Most tankers grounding (or stranded as a result of grounding) at service speeds (14 to 16 knots) will remain firmly stranded; the possibility of losing the entire cargo from a stranded double-bottom tanker (where a single-bottom tanker would have survived) is remote. There are no salvage-related concerns that should limit the use of properly designed double hulls. PERSONNEL HAZARDS DIFFER WITH VARIOUS DESIGNS, BUT ARE VERY DIFFICULT TO EVALUATE Some design features may increase the hazard to the crew during normal operations. These include greater amounts of void spaces, which will have to be periodically inspected; void spaces lacking direct access to the weather deck (such as a double bottom or double hull); and greater numbers of cargo tanks, or cargo tanks located below other tanks (such as in the intermediate oil-tight deck design). There is insufficient evidence to judge the degree of personnel risk associated with various designs. The committee believes these concerns do not represent unmanageable risks in any of the designs considered, but rather constitute important factors that will demand continuing vigilance on the part of ship operators. EXISTING VESSELS WILL COMPRISE THE MAJORITY OF THE FLEET SERVING THE UNITED STATES FOR MANY YEARS The phase-out of single-hull tank vessels trading to U.S. coastal ports will begin in 1995, and over the following 20 years they will be replaced with double-hull vessels. Therefore, means for improving pollution control on existing vessels should be considered.

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Tanker Spills: Prevention by Design Serious consideration should be given to requiring that all existing crude oil tankers promptly meet the latest IMO provisions for pollution prevention for new tankers (MARPOL). Most significant are the requirements for segregated ballast tanks (SBT carry only ballast, never cargo), crude oil washing (a system for rinsing out cargo tanks with oil), and possibly protective location of ballast tanks. The 1978 conference for the MARPOL convention recommended that IMO set a date by which new ship requirements would be applied to existing ships, thereby enhancing protection against operational outflow, but, in fact, this action never has been implemented. A multi-year phase-in period would be necessary, as "pre-MARPOL" vessels would require shipyard work. Another possibility is a requirement for hydrostatic control, which could be implemented immediately without structural overhaul, although more research is needed to determine the optimal method of achieving hydrostatic control. Moreover, the design and condition of each vessel would have to be checked, and all bulkheads inspected, to ensure sufficient strength to withstand the added liquid cargo sloshing. Finally, leakage from a hydrostatically loaded tank vessel following a grounding may be exacerbated by wave action, current, falling tide, or vessel heel or trim (inclination). Implementation of this concept for the entire tank vessel fleet serving the United States would increase the number of vessels operating in U.S. waters, thereby potentially increasing the risk of accidents. Moreover, the cost of this measure could be substantial and warrants further study. The passive vacuum system might be another alternative, depending on the results of further testing. Retrofitting double hulls on existing tank vessels is possible, but potential difficulties could result from combining new and old structural members. Furthermore, retrofitting would be, in general, much more expensive than hydrostatic control. Replacement of the entire ship forward of the machinery space would entail fewer technical difficulties and would require less time in a shipyard; however, it may be more expensive than retrofitting a double hull. The committee urges prompt evaluation of a multi-faceted program to improve pollution control for existing vessels. One element would be the complete implementation of existing MARPOL requirements. The second element would be the consideration of implementing hydrostatic control. Additional emergency features, such as towing fittings and emergency cargo transfer systems that can help reduce the consequences of casualties, also should be considered. While the potential costs and benefits of such a program are difficult to evaluate, it is reasonable to conclude that such requirements might expedite the construction of new vessels with double hulls. Implementing hydrostatic control on existing vessels would reduce cargo capacity on the order of 15 to 20 percent. If all existing vessels had been built at present costs,

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Tanker Spills: Prevention by Design the limited data available to the committee indicates that reducing cargo capacity to accommodate hydrostatic loading (if implemented immediately) would cost about $1.1 billion per year, while saving 3,700 to 4,300 tons of oil spilled annually. Existing vessels were built over many years at a wide range of costs, so the total cost could be considerably less than the dollar amount stated. The actual impact on ship rates would be influenced by the relationship between vessel supply and demand. If existing vessels were insufficient to meet the cargo capacity shortfall, then costs would increase until new vessels were built. This simplified model does not consider the additional person-hours necessary to monitor and enforce hydrostatic control, nor the increased vessel traffic resulting from carrying less cargo in each vessel. Furthermore, the benefits (and costs) of this program would diminish as new vessels replaced existing ships. In sum, this type of program for existing vessels deserves consideration because of the potential environmental benefits. The program should guard against encouraging retention of older vessels.1 It is outside the scope of this committee to judge whether similar benefits could be achieved at more or less cost through non-design measures (such as crew training, electronic display charts, or vessel traffic systems). A COMPREHENSIVE RESEARCH PROGRAM SHOULD BE MOUNTED IN THE UNITED STATES The state of knowledge regarding the precise circumstances and structural effects of actual tank vessel accidents is so inadequate that any assessment of design alternatives will produce results that are dependent on the chosen assumptions and accident scenarios—artificial rather than actual criteria. This is true even of this study. 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 are preferable, in that they tend to promote new development in terms of structural and operational innovations that would result in meeting or surpassing the standards. However, to achieve that goal, needs include: (1) an integrated micro-understanding of the dynamics of ship structural failure, and related factors; (2) long-range research in failure theory; (3) protocols leading to mandatory engineering documentation of casualties; and (4) computational models resulting in outflow predictions. Major research programs directed to design of a "spill-free" tanker are being mounted in other countries. Japan plans a seven-year study on alternative tanker designs. Norway plans a five-year study on tanker design, operations, and oil spill cleanup. The U.S. federal government, industry, and academia should cooperate in a coordinated, substantive research effort directed to:

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Tanker Spills: Prevention by Design performing a comprehensive risk-assessment study that would lead to establishment of future risk-based design goals for tank vessels with attendant compliance guidance; accomplishing the basic research needs noted; testing and evaluation of design concepts (including theoretical analyses, model tests, and field trials); advancing the capability to assess and value natural resource damages (better understanding of the environmental effects of oil spills and the feasibility and cost of restoration, and the development of accepted methodology for valuing environmental benefits would permit more reliable cost-benefit analyses of vessel design alternatives and other means of pollution control); and achieving optimal pollution control by integrating use of design alternatives with operational considerations. OPA 90 authorizes such a research program. The program should be coordinated with foreign research centers, notably in Norway and Japan, and the efforts of the IMO. A comprehensive research effort is essential if important, far-reaching decisions concerning the future of oil transportation by tanker are to be made on the basis of fact, as contrasted with informed opinion. NOTE 1.   Failure to require pre-MARPOL vessels to retrofit SBT requirements before employing hydrostatic control would give these vessels an economic advantage compared to newer vessels meeting MARPOL standards.

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Tanker Spills: Prevention by Design TANKER SPILLS

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