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Letter Report on a Technical Peer Review of the Buzzards Bay Risk Assessment (2013)

Chapter: Enclosure G: Description of Escort Tugs and How They Are Used

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Page 58
Suggested Citation:"Enclosure G: Description of Escort Tugs and How They Are Used." National Academies of Sciences, Engineering, and Medicine. 2013. Letter Report on a Technical Peer Review of the Buzzards Bay Risk Assessment. Washington, DC: The National Academies Press. doi: 10.17226/22480.
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Page 58
Page 59
Suggested Citation:"Enclosure G: Description of Escort Tugs and How They Are Used." National Academies of Sciences, Engineering, and Medicine. 2013. Letter Report on a Technical Peer Review of the Buzzards Bay Risk Assessment. Washington, DC: The National Academies Press. doi: 10.17226/22480.
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Page 59

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58 Enclosure G Description of Escort Tugs and How They Are Used Mission-specific enhanced performance escort tugs were first introduced as a vessel type after the Exxon Valdez oil spill. Before the spill, escort in Prince William Sound, Alaska, was done by conventional twin-screw tugs; tanker escort in Puget Sound involved a combination of conventional and moderate-horsepower tractor tugs. It was recognized after extensive study that the ability to apply an external force to a tanker that has experienced a rudder or engine failure in confined waterways would greatly reduce the risk of a tanker grounding or allision and hence greatly reduce the risk of an oil spill from breaching a cargo tank. A new type of vessel was designed for this purpose that could run at high speed to match tanker speeds, be highly maneuverable, and exert large forces when moving at speeds up to 12 knots through a towline. These escort tugs would trail the tanker in close proximity either tethered or untethered to the stern. In some cases, depending on waterway requirements and ship characteristics, a second escort tug would be provided. The techniques of the tanker escort system have been refined over time in terms of tug design, tug equipment, and operational methods, and they are generally accepted as a significant aid in reducing the risk of tanker groundings and allisions. Tanker escort systems are in place now on the West Coast of the United States and Alaska and internationally. The application of escort tugs in Buzzards Bay tug and barge operations is slightly different: speeds are lower, a barge is being escorted with a primary tug towing or pushing it instead of a single large tankship, the option to push on the barge is available in modest weather conditions, the additional risk of a broken towline is addressed, and a continuous tether to the

59 barge is not used. The service is required by the state of Massachusetts and by USCG regulations as found in 33 CFR § 165.100, which indicate that, when the primary tug has a nonredundant propulsion system, the barge “must be accompanied by an escort tug of sufficient capability to promptly push or tow the tank barge away from danger of grounding or collision. . . .” The USCG requirements apply to single-hull barges, and the state of Massachusetts requirements apply to double-hull barges as well.

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At the request of the Massachusetts Department of Environmental Protection (MassDEP), the TRB/Marine Board Committee to Review the Buzzards Bay Maritime Risk Assessment has evaluated the methods and conclusions of the Buzzards Bay Risk Assessment (BBRA).

The BBRA was originally commissioned by the United States Coast Guard and MassDEP. The committee’s task was to provide an independent technical review of the BBRA focusing on its scope, methods, and supporting data. The committee finds that the choices made in the formulation and execution of the study bring into question the conclusions of the risk assessment on technical grounds and that the ranking of risk mitigation options is not justified and could be reversed with slightly different and more defensible methods or assumptions.

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