Double Hull Design Improves Safety But Isn’t a Panacea for Oil Spills from Tankers
Since the Exxon Valdez oil spill, many nations, including the U.S., have enacted policies that require all new oil tankers to be constructed with a double hull. In the U.S., the Oil Pollution Act of 1990 sets out a schedule for the tanker and tank barge fleet to gradually phase out all single-hulled vessels and replace them with double-hulled vessels. Phase-in schedules vary by vessel type and location, but according to industry estimates, the Alaska tanker trade could be 100% double-hulled by as early as 2010, and no later than 2012.
A single-hulled tanker carries oil directly within the hull structure, while a double-hulled tanker has separate tanks within the hull structure (See photo below). 
In a double-hulled tanker, the cargo space that carries oil is surrounded by a ballast water space, which provides a buffer between the oil storage container and the outer hull. In order for a double-hulled vessel to spill oil from its containers, both the outer and inner hulls must be punctured or damaged.
A double-hull design may provide an additional safety benefit over a single-hull design during certain types of accidents, because with double hulls there are two layers of enclosures that must be punctured before oil can be released into the environment. But double hulls alone cannot prevent gigantic oil spills.
In August 1989, the Coast Guard testified to Congress that double hulls on the Exxon Valdez would have reduced the size of the spill by 60 percent at most, and perhaps by only 25 percent. Even in the most optimistic case—a 60 percent reduction—double hulls would have allowed 4.4 million gallons of oil to escape into Prince William Sound, still a catastrophic spill. A 25 percent reduction would have meant an even bigger release of 8.3 million gallons of oil.
There are both pros and cons associated with double hull tanker designs:
- Double-hulled tankers may spill less oil than single-hulled tankers during incidents where the vessel runs aground at low speeds.
- Double-hulled tankers are typically less effective at preventing or minimizing oil spills that occur during high-speed groundings or during collisions.
- Double-hulled tankers have no added prevention value for oil spills that occur during transfer or loading operations.
- The configuration of the tanks within the double hull is an important factor in determining how much oil would spill during a hull breach.
Double Hull Structural and Design Problems
While double hulls may reduce the amount of oil spilled from a tanker, there are aspects of double-hull design, construction, operations, and maintenance that may actually increase their risk of involvement in an accident or oil spill. The complex design and structure of double-hull tankers can make them more susceptible to maintenance and operations problems. Double-hull tankers can be prone to catastrophic structural failures, particularly if they are not maintained and operated to regulated standards.
A 1998 National Research Council report highlighted some of the stability, structural, and safety concerns for double-hull tankers. Because of their design and construction, double-hull tankers may operate with global stress levels 30% higher than single-hull vessels. These higher stresses increase the risk of buckling failure, and this risk increases over the life of the vessel because of corresponding reductions in plate thickness caused by corrosion. The higher stresses can also increase the likelihood of developing small fatigue cracks. Fatigue cracks, which may occur in all types of vessels, can propagate over time and if no action is taken (repairs, etc.), then a major structural failure may occur.
The Center for Tankship Excellence, which compiles data on oil spills from tankers and analyzes this data for trends in causality, reports that hull structural failure is responsible for the majority of oil spilled during tanker casualties. For the Alaska Class double-hull tankers, fatigue cracks and fractures have been observed in deck plate covers, anchors, and mooring bits.
Increased Corrosion of Double-Hulled Vessels
A double-hull tanker may be subject to accelerated corrosion in the space between the hulls. This space is used to carry ballast water, typically seawater, that stabilizes the tanker during trips when the cargo tanks are empty. When these ballast spaces are empty, condensation may also form. In any type of vessel, sea water or condensation against bare metal is corrosive. To manage this type of corrosion, the space between the hulls is covered with a protective coating. This coating minimizes corrosion as long as it stays intact, but as it ages, it may chip or fail and allow for corrosion to occur.
New International Maritime Organization (IMO) performance standards for ballast tank coating went into effect in 2008; however, the improper tank coating may continue to cause corrosion in all vessels. Consequently, the ballast space in a double-hull tanker requires timely inspection for coating failures and timely repair as indicated by the inspection.
Photo: Steel plating taken from a cargo tank in double-hulled tanker showing corrosion caused by microbes.
Oil Spill and Accident Trends for Single and Double-Hulled Tankers
Double hulls are not an absolute prevention measure because they cannot protect against spillage under all circumstances…nothing can. While double hulls can reduce the severity of an oil spill, they cannot interrupt the chain of events that may cause the accident to occur in the first place.
Several factors may actually increase the likelihood that a double-hulled tanker will experience problems that could lead to an accident or oil spill:
- Double-hulled tankers are more susceptible to hull fatigue, corrosion, stability issues, and a range of other design flaws.
- Because of their complex design and structure, double hull tankers are potentially more susceptible to problems associated with poor maintenance and operation.
Photo: Undetected corrosion caused one of the more spectacular structural failures of a double-hulled tanker when the Greek tanker Kirki lost its bow and caught fire off the coast of Western Australia in 1991, spilling about 5.4 million gallons of crude oil.
To help better understand the relationship between single and double hull configurations and tanker accidents, a review of the State of Washington’s tanker incident database was conducted. The review shows that the proportionate number of oil spills by year from single-hulled vs. double-hulled vessels has changed over time. Until 1997, significantly more single-hulled tankers spilled oil in Washington than double-hulled tankers. During the past ten years (since 1999), double-hulled vessels accounted for more spills each year than single-hulled vessels. Obviously, this shift reflects, to a significant degree, the phase-out of single-hulled tankers in U.S. waters and the shift toward a predominantly double-hulled fleet. The highest number of oil spills from double-hulled tankers in any spill year (15 spills in 2001) is equal to the highest number of spills from single-hulled tankers in a single year (15 spills in 1992).
Double Hulls Don’t Prevent Human-Caused Oil Spills
While structural, design, and corrosion can cause tanker accidents and oil spills, oil spills are not caused by equipment failures alone. Often, the people or organizations operating and managing the equipment are at fault. Human factors – either individual errors or organizational failures – cause an estimated 80% of oil spills and marine accidents.
A 1998 National Research Council study that considers the double-hull requirements in the Oil Pollution Act (OPA 90) notes that the transition to a double-hull oil tanker fleet does not erase the need for prevention programs that address human factors. When human error causes an accident or mishap, safety equipment such as double hulls may reduce the amount of oil that spills, but it will not interrupt the chain of events that cause the accident in the first place. The major protection afforded by double hulls occurs in a scenario where a grounding or collision has already occurred, and double hulls would be expected to prevent oil from spilling or reduce the size of the spill. Human factors interventions work to prevent accidents and oil spills much earlier in the accident timeline – by preventing the critical failure or series of events that lead to the grounding or collision in the first place.
The study of human factors considers how human characteristics and behaviors are directly linked with the functioning of the technology people design, build, maintain, and operate. The human-technology relationship works in both directions, though. Not only do humans impact the way technology functions, but technology can also influence human decisions and actions.
As the vessels that transport oil become increasingly reliant on engineered systems and automated technologies, humans that operate these systems are subjected to new challenges that may actually increase accident risks. While accident probabilities with a technological basis can often be reduced through engineering, accidents that involve human-technology interactions are much more difficult to address and can never be completely eliminated.
Given the complex nature of human-technology interactions, and the potential for new tanker designs and technologies to feed into the potential for human error, additional safety measures such as dual escort vessels provide an obvious benefit in that they may be able to provide assistance before the tanker experiences a major casualty. This is likely the closest one can get to achieving a “fail-safe” system.
Red Herring: Any Call for Another Risk Assessment in Prince William Sound
A credible risk assessment involves an objective evaluation of risk in which assumptions and uncertainties are clearly considered and presented. When properly conducted, using scientifically valid methods and credible data sets, risk assessments can shed light on the inherent risks within a given system and can provide insight into the types of risk interventions that would best reduce that risk.
Part of the difficulty in designing and conducting a risk assessment is linked to the fact that both of the factors by which risk is measured – potential loss and probability of occurrence – can be extremely difficult to calculate and often times these risks simply cannot be accurately quantified. The chance of error in the measurement of these two concepts is significant and well-recognized.
A quantitative risk assessment was conducted in Prince William Sound during the mid-1990s, but the resulting report was extremely polarizing and was harshly criticized for both its methods and its results.
Within the Prince William Sound tanker system, the oil spill risks are already well understood. Another catastrophic oil spill could occur. Thanks to the hard work of industry, regulators, and citizen oversight, a sophisticated safety improvement program has been operating with good success for twenty years. As the transition to a double-hull tanker fleet nears completion, a few from industry have sought another risk assessment to determine whether the dual escort vessel system should be maintained for a double-hull tanker fleet. It would be ill-advised to conduct an unnecessary study for the purpose of justifying the reduction or elimination of the proven dual escort vessel system in Prince William Sound.
The Prince William Sound dual escort vessel system has evolved through an open process of give and take based on careful monitoring and study. The present dual escort system is the result of what has been, in effect, a “rolling risk assessment,” based upon the ongoing work of the industry, government, and stakeholders who have contributed to the design, operation, and continued refinement of the dual escort vessel system.
The people who have had to live with the consequences of the Exxon Valdez oil spill understand the risks all too well – and they recognize the value in the prevention system that has been in place for the last twenty years. Common sense dictates that when a system has worked for two decades and continues to work now, it should not be changed or weakened; it should be kept intact. Alaskans are in near unanimity that the current dual escort system must not be weakened at all and they are not inclined to buy into any recitation of probabilities in lieu of relying on the system that they have seen work, and work well, for nearly twenty years.
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Need for Dual Escort Vessels for Single and Double Hulled Tankers
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