EHS Management

The Oil Threat from Sunken Vessels

But given the difficulty of confirming information about submerged shipwrecks, there are many uncertainties about the estimates.  For example, the NOAA says that 47 of the 87 priority vessels have unknown or unconfirmed locations.  Unconfirmed locations include vessels where divers have reported finding a shipwreck, but definitive identification of the shipwreck has not been made.  Also, there are numerous instances of vessels being misidentified, particularly in areas where several vessels of similar size and age were lost.  The NOAA also concedes that there may be many nonpriority shipwrecks that, in fact, contain substantial amounts of oil. 

Even with the uncertainties, the large number of sunken vessels and the difficulty and expense of physically assessing the risk and attempting to recover the oil necessitate a prioritization scheme.  That’s the objective of an NOAA program called the Remediation of Underwater Legacy Environmental Threats (RULET).  The RULET database, which was updated in 2012, contains post-1891 wrecks with steel, iron, or concrete hulls that are either tank vessels (tankers or tank barges) or are at least 200 feet (ft) long or 1,000 gross tons in size.  To better understand the possible risks, the NOAA developed risk scores for the priority vessels.  Scoring is based on a broad multidisciplinary, weight-of-evidence approach that combines the historical evidence, archaeological interpretation, salvage engineering with pollutant fate modeling, and ecological and socioeconomic risk assessments.  Building upon RULET and using the scoring system, the NOAA has identified 6 vessels as a high priority for a most probable discharge (10 percent of the oil volume) and 36 vessels as a high priority for a worst-case discharge (100 percent of the oil volume). 

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Determining the risk presented by a shipwreck involves assessment of many factors.  Here’s a partial list:

•              Volume of oil.  The NOAA classifies a major oil spill as one involving 2,400 barrels (100,000 gallons) or more.  The NOAA states that a small number of shipwrecks may have contained hundreds of thousands of barrels of oil.  Since there are no airtight data about how much oil the vessels contained, RULET assumes they had full bunkers and cargo when they left port.  Although the report focuses on oil, the NOAA notes that thousands of sunken vessels contain other hazardous substances (mercury and unexploded ordnance are two additional substances of high concern), which must be considered in assessing risk and developing response plans.

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•              Oil type.  Crude oils range from light to heavy (low density to high) and have a broad range of viscosities.  More viscous oils are more persistent in the environment, float longer, and are more likely to impact shorelines than light fuels.  Therefore, the more viscous the oil, the higher the risk associated with the wreck.
•              Did the vessel burn?  The vast majority of potentially polluting shipwrecks lost in U.S. waters can be tracked to a 4-year period between 1941 and 1945 when Japanese and German submarines sought to destroy tankers and freighters along the relatively undefended U.S. coasts.  Multiple World War II reports reveal how survivors could not escape a ship because of burning oil surrounding the ship or how the survivors had to swim hundreds of feet to get away from oil escaping a doomed merchant vessel.  The more burning reported, the more likely there were breaks in the hull or tanks that would have increased the potential for oil to escape from the shipwreck.  The NOAA increases the risk for vessels for which no burning was reported. 
•              Structural breakup.  Vessels that are believed to be broken into three or more pieces are regarded as low risk.  However, the NOAA points out that even vessels broken into three large sections can have significant pollutants on board if the sections still have some structural integrity.  One example cited frequently in the report is the SS Jacob Luckenbach.  On July 14, 1953, this 469-ft freighter collided with the SS Hawaiian Pilot and sank in 178 feet of water approximately 17 miles west-southwest of San Francisco, California.  In 2002, the decaying wreck was identified as the source of intermittent oil spills that had occurred along the coast for nearly 30 years.  Cumulatively, these spills killed thousands of seabirds and other marine life.  In 2002, the U.S. Coast Guard (USCG) contracted a salvager to assess, locate, and remove the remaining oil from the hull. The assessment included development of a 3-D model of the vessel.  Some oil remained in the tanks, but much of it had migrated extensively within the wreck via corroded vents and piping, and oil was found in over 30 compartments on the vessel. The USCG led the response operation that removed approximately 2,380 barrels of oil.  However, some of the compartments were inaccessible and some oil was left on board.  The SS Jacob Luckenbach is an often-cited example of a potentially polluting wreck and a landmark in the development of underwater assessment and removal technologies.

See tomorrow’s Advisor for more factors.

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