Effects of Double-Hull Requirements on Oil Spill Prevention: Interim Report (1996)

One of the major concerns in congressional deliberations about OPA 90 was the ability of the maritime oil transportation industry and the shipbuilding industry to build and safely operate an economically viable double-hull fleet. Prior to OPA 90, double-hull tankers tended to be small vessels, typically product or chemical carriers rather than crude oil carriers. The imposition of the double-hull design on the entire tanker fleet by OPA 90 (at least on most of the fleet trading with the United States) has focused attention in the maritime oil transportation and shipbuilding industries on how to build safe, economical double-hull vessels.

The committee is seeking information on issues raised during discussions leading to OPA 90 on the design, construction, maintenance, and operation of double-hull tankers. This information will be reviewed, particularly in light of the operational experience of much larger double-hull vessels placed into service in the past five years. In addition, there has been considerable worldwide research in double-hull technology, primarily analytical and experimental studies on groundings and collisions. In the final report, the committee will address the implications of current research on groundings, collisions, and fatigue on current and future double-hull tank vessel designs. To address these questions effectively, the following steps will be taken:

• The present practices, concerns, and trends in the design, construction, maintenance, and operation of double-hull vessels, particularly in comparison to single-hull vessels, will be discussed and evaluated.

• Research studies and findings will be summarized, and results from the studies that influence double-hull design will be noted and discussed.

• Representative as-built, single-hull and double-hull vessels of various

The problems and concerns raised in 1990 regarding double hulls primarily dealt with effectiveness of the design in preventing pollution, the increased risk of fire and explosions, possible instability of damaged vessels, perceived salvage difficulties, increased hazards to personnel in double-hull spaces, concerns about ship structural integrity and the extensive use of high-tensile steels, and concerns about corrosion in double-hull spaces.

Data pertinent to issues related to design, construction, inspection, and maintenance have been obtained from survey questionnaires sent to owners and operators of double-hull vessels, shipyards, classification societies, and naval architectural organizations. However, the data and service experience from operating double-hull vessels in the last five years may not be sufficient to determine if anticipated problems will actually materialize.

Major research projects in the area of double-hull technology since 1990 have principally been carried out in the United States (by MIT, the Ship Structure Committee,¹ and the Society of Naval Architects and Marine Engineers [SNAME]), and in Japan, Denmark, and Norway. Structural research efforts in other countries have also been reported in the technical literature, such as the International Ship and Offshore Structures Congress (ISSC) proceedings. The results of these and other research will be assessed for significant effects on the design of double-hull tankers.

As an indication of the scope of research on double-hull designs, 30 papers related to the topic were presented at the International Conference on Technologies for Marine Environment Preservation (MARIENV '95) in Tokyo, Japan (September 24 to 29, 1995), sponsored by the Society of Naval Architects of Japan. Almost two-thirds of the papers were presented by authors from Japan. The remaining papers were by authors from Denmark, Germany, South Korea, the Netherlands, Norway, and the United States. Although it is not unusual for

most of the papers at a conference to originate in the host country, the number of reports presented at this conference reflects how much research relevant to tanker design is being conducted in Japan. In 1991, Japan initiated a seven-year structural research program, under the Association for Structural Improvement of the Shipbuilding Industry (ASIS), on the prevention of oil spills from crude oil tankers.

The three major areas in structural research on tankers are collisions, groundings, and fatigue cracking.

Since the late 1950s, when V.U. Minorsky attempted to correlate the interpenetration of colliding ships using accident data, researchers have tried to account for the structural details and approach particulars of colliding ships (Minorsky, 1959). Early predictions of penetration were based largely on relatively simple energy accounting, but the most recent methods are based on detailed analyses of plastic buckling and collapse. The evolving methods are useful for analyzing all types of ship structure, including double hulls.

Collision analysis has been greatly aided by modern nonlinear finite element methods. In the past five years, research in this area has made increasing use of these methods, and they are now being used to optimize double-hull designs with respect to the positioning of the inner and outer hull plates, the side stringers, and the transverse webs. Verification of the procedures using large-scale model tests and actual collision data, is a necessary element of the approach because of the inherent difficulty in modeling highly contorted collapse modes and the relatively crude criteria still used to model plate-and weld-fractures during crushing.

Eight of the papers presented at MARIENV '95 dealt with structural integrity in collisions. A recent paper listing some of the representative work in this area is “Collapse of a Ship's Hull Due to Grounding” by J.K. Paik and P.T. Pedersen (Paik and Pedersen, 1995). In addition, Dutch-Japanese full-scale collision tests were carried out in the Netherlands with two 1,000-ton inland waterway tankers. Four collision cases were studied for ship structural resistance and ship movement.

Most aspects of structural failure in tanker grounding incidents can be analyzed by the same methods used to analyze ship collisions, but hull-girder failure (i.e., breaking the back of the tanker) and hull tearing are features common to groundings that require specialized approaches. Hull-girder failures due to grounding have been examined with the aid of more 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 the hull and the seabed. The computational models are in reasonable accord with

model and full-scale controlled grounding tests. Initial efforts to assess the resistance of underside hull plates to tearing by a protrusion, such as rocks jutting up from the seabed, have been undertaken by T. Wierzbicki and colleagues 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 independently measured in simulated tearing tests. Then the length of the underside ruptures 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.

The mechanics of combining large amounts of plastic deformation and fracture are unusually challenging. Part of the difficulty in applying of models and tests to ship hull performance is that, although tearing energy represents a relatively small proportion of the total energy dissipated in the grounding, it is critical in determining the extent of the tear. Although difficult to develop, the integration of a sound fracture approach into collision and grounding analysis would constitute a major improvement.

Experimental test programs to evaluate ship groundings include the following:

• The U.S. Navy conducted analytical studies and large-scale model tests for strandings (loadings normal to the bottom shell) and groundings (combined normal and in-plane loadings). This program consisted of preliminary designs of double-hull vessels, grounding model tests, fatigue testing, and an analysis of the producibility of double-hull structures.

• Denmark has undertaken full-scale tests to evaluate soft groundings (Paik and Pedersen, 1995).

• The Dutch and Japanese have jointly conducted one-third-scale model tests to simulate bottom-raking damage of single-hull and double-hull tankers (Vredeveldt and Wevers, 1992; Lenselink and Thung, 1992; Wevers et al., 1994; Vredeveldt and Wevers, 1995).²

Large ships of any kind must be carefully designed to reduce the incidence of fatigue cracks, which usually start at points of high stress concentration in the hull, typically at junctions where one plate is welded to another. Some issues are specific to double hulls. In particular, double-hull vessels tend to be stiffer than single-hull vessels, and this can affect both the residual stresses induced during construction and the local stresses from loads in operation. Another contributor to potential fatigue problems, which is not specific to double hulls, is the increased

use of thinner plates of higher-strength steels.³ If appropriate design modifications are not made, the thinner plates will be subject to higher stresses and will, therefore, be more likely to develop fatigue cracks.

Fortunately, advances in finite element stress analysis techniques have made it possible to obtain more accurate and detailed stress estimates. 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. Experimental research in this area is under way to document the development of fatigue cracks in joints of various designs, for example, large-scale tests conducted at the Krylov Shipbuilding Research Institute in Kiev, sponsored by Lloyd's Register of Shipping (Violette, 1995). As was the case in collisions and groundings, the capability of dealing with crack initiation and growth has not progressed nearly as far as the capability of dealing with stress, deformation, and buckling. Applying fracture mechanics to ship hulls appears to be a fruitful area for research.

The present design will be summarized based on ship data received from shipowners and operators, shipyards, classification societies, and naval architects on double-hull tankers built since 1990. Design concerns about double-hull tankers were primarily based on the trend towards the greater use of higher-tensile steels and the importance of designing structural details properly to minimize the risk of fractures and the possible leakage of oil into the ballast spaces or into the sea. Concerns have also been raised about the need for classification societies to examine strength standards for design, with a view to making tanker structures more robust.

Since 1990 some classification societies have been actively examining standards for tanker structures. The American Bureau of Shipping, for example, has developed new structural criteria and a structural software system called SafeHull for tankers (and bulk carriers). SafeHull is based on a first-principles dynamic load approach in which realistic dynamic loads are considered in the criteria. These loads include wave loads, inertia loads of the ship and cargo, sloshing loads of cargo in partially filled tanks, and cyclic fatigue loads. Finite element stress analysis is also used to evaluate the reaction of the overall structure to these combined loads. SafeHull takes into account corrosion effects expected to occur

in the hull during service life, as well as the possible modes of failure resulting from yielding, buckling, and fatigue. Lloyd's Register of Shipping has also introduced its ShipRight analysis to improve the safety of new tankers. Many of the classification society rules now have fatigue criteria and fatigue assessment methods applicable to structural details to ensure fatigue strength. The International Association of Classification Societies has also been active in establishing unified requirements to improve the design and construction of double-hull tankers. The effects of these standards on double-hull design will be assessed through an analysis of replies to questionnaires from classification societies, from direct interrogation, and from information provided by classification societies and from published documents.

A comparative evaluation of the producibility ⁴ of double-hull and single-hull tankers will be carried out based on the results of the survey questionnaire sent to shipyards. Expert testimony presented before the committee by representatives of shipyards in Japan, South Korea, and the United States will provide additional information. A significant part of producibility, the importance of coatings and their application during the construction process, will be addressed in the same manner.

When OPA 90 was passed, several concerns were raised about the difficulty of maintaining and inspecting double-hull tankers. The 1991 NRC report by the Committee on Tank Vessel Design discussed corrosion in ballast spaces, structural cracking, safe access to ballast tank spaces, and quality of inspections (NRC, 1991).

Although double-hull vessels, both tankers and other vessel types, were in service at the time, there were no large crude oil carriers in the VLCC (very large crude carrier, approximately 200,000 to 300,000 DWT) with double-hull construction. In fact, double-hull tankers have usually been chemical tankers and product carriers. Therefore, little data from past experience were available to evaluate maintenance and inspection of large, double-hull crude carriers. Today there are a number of large crude oil tankers with double hulls in the world fleet. Because these tankers have been in service for a few years, some information on their maintenance and inspection can now be gathered. In addition, the committee will seek out and review examples of experience in liquefied natural gas (LNG), liquid petroleum gas (LPG), and liquid ammonia (NH₃), as well as chemical carriers service that have application to crude oil double-hull tanker maintenance and service.

The committee will collect and evaluate data to identify current maintenance and inspection practices for double-hull tankers; current concerns regarding maintenance and inspection; and future trends in maintenance and inspection practices and possible effects on future designs. The differences between double-hull and single-hull maintenance and inspection will be assessed. The data supporting this assessment will be based on a survey of double-hull tanker owners and operators, as well as of shipyards building double-hull tankers; expert testimony by representatives of classification societies, operators, and shipyards; current research; and relevant literature.

Questionnaires have been sent to owners and operators, shipyards, and naval architectural organizations (see Surveys to be Conducted and Evaluated later in this chapter). The questionnaires include questions on tank inspections, coatings (types, current practices, and experiences), corrosion in ballast spaces, differences in maintenance and inspection practices for single- and double-hull tankers, accessibility to spaces, and the ability to maintain gas-free spaces. Copies of the questionnaires are provided in appendix B. Representatives of Lloyd's Register of Shipping, the American Bureau of Shipping, Hitachi Zosen (a Japanese shipyard), Hyundai Heavy Industries, Ltd. (a South Korean shipyard), and Newport News Shipbuilding Company (a U.S. shipyard) have presented expert testimony to the committee.

After collecting and evaluating the data, the committee will draw conclusions about the current state of double-hull maintenance and inspection.

In addition to the concerns regarding the maintenance and inspection of double-hull tankers, other operational concerns were identified in the 1991 NRC report, namely the stability of double-hull tankers, the effect of bottom and side voids in salvage, and the risk of explosions and fires.

Relying on the current experience with double-hull tankers, the committee will collect and evaluate data on current operational practices for double-hull tankers, current operational concerns, and future trends in operational practices, and possible effects of operational experience on future designs. Operational differences between double-hull and single-hull tankers will be assessed using data from a survey of owners and operators of double-hull tankers and on current research and literature.

The questionnaire mentioned above has been sent to owners and operators of double-hull tankers. In addition to maintenance and inspection, the questionnaire solicits information on operational safety (e.g., stability during loading and discharging), access to ballast spaces, the ventilation of ballast spaces, operational procedures established for double-hull tankers, experience on current double-hull designs (the use of high-tensile steel and structural performance), design recommendations, and the advantages and disadvantages of double-hull tankers.

The committee has also identified design requirements that may have conflicting effects on vessel characteristics. Satisfying all the design requirements may require operational restrictions. For example, improved intact stability characteristics could lead to reduced damage stability performance, even though all applicable regulations have been satisfied. A comparative evaluation of intact stability, damage stability, and oil outflow characteristics of alternative single-and double-hull designs will be carried out by Herbert Engineering. The evaluation procedure is described in the next section of this chapter.

After the data collection and comparative analysis have been completed, the committee will draw conclusions on the impact of the double-hull requirement on tanker operations.

During the preliminary design process, tank arrangements and subdivisions are developed to suit owner requirements as well as classification society, flag-state, and other relevant regulations. Within the bounds of the regulations, the designer makes decisions on the location of the cargo block, the size and location of tanks, and the dimensions of the wing tanks and double bottoms. These decisions affect a wide range of factors, including cost and weight of ships, oil outflow and pollution prevention, survivability, intact stability, and longitudinal strength of the vessel. Various trade-offs are made during the design process. For instance, increasing the wing-tank width generally reduces projected oil outflow values. However, it may have a negative effect on survivability because damage to large wing tanks will result in increased heeling and sinkage. This might necessitate more extensive subdivision of the wing tanks, which increases vessel complexity and costs.

Responses to the questionnaire regarding double-hull tanker designs indicate that shipyards have taken different approaches to optimizing their tanker designs. To assess relative improvements in the various designs compared to traditional single-hull vessels, the committee will assess the oil outflow, survivability, intact stability, and strength characteristics of double-hull tankers built in the last five years. Approximately 16 double-hull designs ranging in size from 40,000 to 300,000 DWT will be evaluated, and the results will be compared to existing single-hull tanker designs of similar sizes. The results will also be used to assess the effectiveness of current regulations regarding stability, damage stability, and outflow in maintaining consistent standards for safety and environmental performance.

As discussed previously, questionnaires concerning experiences with double-hull vessels were sent to owners and operators of double-hull tankers, shipyards

building double-hull tankers, classification societies, and marine architecture firms. The questionnaires and the list of companies to which they were sent are provided in appendix B.

Twenty-five owners and operators of double-hull tankers were sent questionnaires on the operation of double-hull tankers. The purpose of the questionnaire was to gather information on current operational experiences. The main areas covered in the questionnaire are operation, inspection and maintenance, design, and fleet information.

The committee will evaluate the questionnaires and, assuming the experience base is sufficient to validate operational concerns, draw conclusions on the effects of the double-hull requirement on tanker operations, maintenance, and inspection. Responses will also be used for a comparative evaluation of alternative designs.

A questionnaire concerning tanker design, construction and maintenance was sent to shipyards that build double-hull tankers, classification societies, and, with slight modification, to marine architects throughout the world. The areas covered by this questionnaire include ship characteristics data for double-hull tankers built or classed, percentage of high-strength steel used, and comparison of producibility of single-hull and double-hull designs of 90,000, 150,000, and 280,000 DWT sizes. Indicators of producibility may be based on differences in labor hours and cost for steel fabrication, machinery and outfitting, coatings, total construction time (from keel laying to delivery), and any other comparative data related to construction or producibility.

Questionnaire responses will be evaluated and conclusions will be derived in the following six areas: design trends, changes in use of high-strength steel, problems and solutions to producibility of double-hull versus single-hull designs, construction practices, coating concerns, and maintenance-related problems and solutions.

A review of the design information from questionnaire responses and from technical documents indicates that adequate information will be available to assess most of the concerns about the progress of the double-hull design since 1990. Data reviewed to date have not revealed any new or novel design features incorporated into double-hull designs since 1990. Most new designs were developed

to meet the current double-hull requirements of OPA 90 and IMO Regulation 13F and follow standard practices. However, one finding not previously identified as a potential problem in design is the sensitivity of certain double-hull tankers to instability in the intact condition, particularly if the cargo tank is very wide. ⁵

Because of relatively recent deliveries of large double-hull tankers, not much operational experience is available on long-term structural and operational effects, such as fatigue performance of the structure and corrosion protection of the ballast spaces. The committee will rely on experience from double-hull chemical and product carriers, as well as from the few double-hull crude carriers built before 1990.

After an exhaustive analysis of information from the surveys and expert testimony and a thorough search of the literature in the first phase of the study, remaining gaps in information will be filled by personal contact between committee members and individuals from various organizations and companies. This will be necessary because of ongoing changes in the process of gearing up for construction to meet OPA 90 and IMO 13F requirements.