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APPENDIX D STRUCTURAL DESIGNS FOR NEW DOUBLE-HULL TANKERS T he world tanker fleet has gradually changed with the construction of double-hull vessels to comply with U.S. regulations that fol- lowed passage of the Oil Pollution Act of 1990 (OPA 90). Since the design of a double-hull tanker has an effect on its outflow performance in the case of a grounding or collision, it is important to be familiar with current design practices when selecting a double-hull tanker for use as a reference in comparing alternative designs. An overview of issues related to the structural design and arrangement of new double-hull tankers is provided in this appendix. STRUCTURAL DESIGN OF DOUBLE-HULLTANKERS Since only a few double hull-tankers were built prior to OPA 90 and IMO Regulation 13F, designers had to develop new structural layouts, as well as use the experience with double-hull designs for other vessel types, such as containerships. There was very little service experience available for most double-hull construction types and none for the very large vessels. Hence, designs based on requirements expressed in OPA 90 and Regu- lation 13F benefited little from past experience. To meet this need, the Tanker Structure Cooperative Forum (TSCF) developed and published the Guide to Inspection and Maintenance of Double-Hull Tanker Structures in 1995. The information in this guide is based on experience with operations as reported by the forum mem- bership, which included oil companies and classification societies. This information was collected for a limited number of existing double-hull tankers, double-side tankers, and double-bottom tankers, as well as other types of vessels with similar details. The guidance provided serves as an 128

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STRUCTURAL DESIGNS FOR NEW DOUBLE-HULL TANKERS 129 excellent resource for initial design efforts by indicating details that re- quire special attention in design and construction. For its 1998 study Double Hull Tanker Legislation: An Assessment of the Oil Pollution Act of 1990, a National Research Council study com- mittee conducted a survey of designers, builders, and operators of double- hull tankers to gather information on actual experience. The conclusion of most respondents is that double-hull tankers can be operated as safely as other designs, provided more attention is given to inspection and maintenance. Classification societies have also looked at their rules to determine how to evaluate double-hull tankers. Most of the societies have made use of detailed analysis methodologies that have become available because of advances in computer design. Experience gained through a better under- standing of the failure mechanisms of yielding, buckling, fatigue, and cor- rosion of single-hull tanker structures has been the basis for many of the requirements applied by the classification societies, and the societies have developed guidance for the evaluation of these failure mechanisms. The early double-hull tankers designed and built since OPA 90 and Regulation 13F are just now completing their first 10 years of service. Both operators and classification societies are currently gathering data on the performance of the structure of these tankers. In general, the expe- rience has been good, but there have been some areas in which either guidance found in the work of TSCF has not been followed or unexpected structural problems, such as microbial corrosion in cargo tanks, have developed. ARRANGEMENTS OF DOUBLE-HULLTANKERS Most new tanker designs have complied with the double-hull requirements of Regulation 13F and also meet USCG's criteria for double-hull designs under OPA 90. Appropriate provisions of these regulations define the minimum size of the double-hull space. The size of the space is based on the deadweight of the vessel for both the side breadth and the bottom depth. The minimum space for the separation of the inner skin is 2 meters for all vessels above 40,000 deadweight tons (DWT). Tank vessels in common use that are covered by the regulations are generally categorized by size as follows: Product tanker--about 40,000 DWT. Aframax tanker--about 80,000 DWT.

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ENVIRONMENTAL PERFORMANCE OF TANKER DESIGNS 130 TABLE D-1 Compliance of Tanker Double-Hull Designs Regulation Regulation Actual Depth of Actual Depth of Depth of Side Depth of Bottom Side Range Bottom Range Type (meters) (meters) (meters) (meters) Product 2.0 2.0 2.02.42 2.02.18 Aframax 2.0 2.0 2.02.28 2.02.63 Suezmax 2.0 2.0 2.052.7 2.582.8 VLCC 2.0 2.0 2.43.6 3.03.2 Suezmax tanker--about 135,000 DWT. Very large crude carrier (VLCC) tanker--about 250,000 DWT. All of these vessels will have a minimum space of 2 meters separating the inner and outer hulls of the double hull. The other important arrangement is the number of tanks: they range from 8 to 16 in number and can be further subdivided by one or more longitudinal bulkheads. Most designs have similar arrangements because both OPA 90 and MARPOL define double-hull and tank size. One significant change that has occurred at IMO since the work of TSCF is stability by design. The early double-hull designs in the Aframax and Suezmax size ranges had only one tank between the inner hulls, either to save weight or max- imize operational efficiency. This led to some vessels having stability problems, termed lolling, during the discharge of cargo. In 1997, the need to meet stability requirements led to modifications of the Aframax and Suezmax tanker designs, which now include centerline bulkheads to provide port and starboard cargo tanks. A number of other requirements specify tank sizes based on specific design arrangements. A raking damage stability requirement1 was added to MARPOL regulations to ensure the damage stability of double-hull tankers. A significant number of double-hull ships have been built for reg- istry outside the United States since OPA 90 and Regulation 13F entered into force. Typical spaces for the double hull for the product, Aframax, Suezmax, and VLCC tankers are indicated in Table D-1. FUTURE IMPROVEMENTS IN DOUBLE-HULLTANKER DESIGNS Although classification societies have adequate tools to assess designs and ensure the safe performance of double-hull tankers, there is room for im- 1This is a requirement to consider a long extent of bottom damage (about 60 percent of the ship's length) as one possible condition for damage stability calculations.

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STRUCTURAL DESIGNS FOR NEW DOUBLE-HULL TANKERS 131 provement in many areas. Among the critical areas are structural details and fabrication tolerances. Early design evaluation can also lead to struc- tural improvements. Another area for improvement is assessment of struc- tures that have been in service to provide feedback for new designs. Efforts now under way within many organizations to improve design details are expected to bring greater safety and reliability to tankers of the future. If and when newer alternative designs are considered, it will also be nec- essary to ensure that any new structural details are adequately evaluated.