ABS Releases Prestige Report

ABS has released a report on "Technical Analyses Related to the Prestige Casualty.” It is available for download from the ABS website (www.eagle.org). The report assesses vulnerabilities that may have theoretically existed in the hull structure at the time of the casualty. It also addresses potential external forces that could have turned that vulnerability into a failure of the side shell, leading to the initial flooding of the starboard Nos. 2 Aft and 3 wing tanks. "It is almost certain that the initiating cause of the damage to the Prestige will never be definitively identified," said ABS President Robert D. Somerville. "The physical evidence was either destroyed or lost during the subsequent six day ordeal that the vessel went through after it was denied access to a sheltered area. That leaves the inquiry in the realm of hypotheses and speculation. The intent of the extensive, rigorous technical analyses that have been undertaken by ABS is to explore these various theories in the hope that some weighting can be given to the validity of each." The ABS President stressed the importance the classification society has attached to the investigation. "The livelihoods of thousands of people, and the pristine coast of Galicia have been devastated by this accident," he said. "And in the 25 years prior to the sinking of the Prestige only one ABS classed tanker had been lost at sea due to structural failure with that incident occurring during a severe typhoon in the Pacific Ocean. It is for these reasons that we considered it imperative that ABS look at all possible contributing factors to the loss of the Prestige to determine if any further strengthening of our class requirements should be considered." The technical analyses included consideration of: · Static load determination for the hull structure · Wave and ship motion induced load component determination · Structural analysis of the hull structure using finite methods · Determination of hull girder strength · Fatigue analysis of structural details · Contact damage assessment due to lightering operations · Effects of welding imperfections and residual stress · Breaking wave load analysis on side structure. These analyses found, inter alia, that the as-built hull structure of the Prestige fully satisfied both the 1973 ABS Rule requirements and the 2003 ABS Rule and IACS Unified Rule requirements for hull girder section moduli and hull girder strength. The vessel was properly loaded, had more than adequate hull strength for the reported conditions at the time of the casualty and would not have failed by either yielding or buckling, assuming no weakened condition existed in the hull structure. The analysis also confirmed that, in the weather and sea conditions prevailing at the time of the casualty, the maximum impact pressure that could have been imposed on the vessel's side shell from breaking wave impact load was significantly less than the impact strength of the side shell. "While intuitively ABS believes the most likely failure mode leading to the initial flooding of the starboard tanks was a combination of a weakened section of the vessel's structure, coupled with severe wave impact force, our analyses of potential factors could neither prove nor disprove the contribution of any of these," the Report states. "In the absence of physical evidence or eyewitness testimony, it may never be possible to pinpoint the contributing factors which led to the initial breach of the shell. This is unfortunate since it will both prevent specific corrective action and continue the speculation regarding the specific cause of the initial flooding." In assessing the many possible contributing factors the one that has been identified by the ABS analysis as warranting further consideration is the possible deformation or fracture of a section of the side shell plating or supporting structure due to the lightering activities of the Prestige in St Petersburg between the time of the last annual survey and its departure on its final voyage. "While no evidence currently exists that the Prestige did suffer such damage," the Report notes, "ABS does have damage survey reports describing similar damage that was experienced after lightering operations in 1989 by a sister ship of the Prestige. While ABS is hesitant to define lightering damage as the probable cause of the initial casualty, it does remain a more likely contributor than any of the other factors." Although the principal focus of the ABS technical analyses has been directed towards identifying the initiating cause of the damage, consideration was also given to the ultimate hull failure and resulting sinking of the tanker. Rigorous analysis of the vessel in the damaged and counter flooded conditions clearly showed that the vessel retained sufficient hull girder strength and buoyancy to remain intact and afloat if not subjected to sustained additional dynamic wave loads. However, its ability to withstand further hull damage was severely limited. The analysis confirms that, if the vessel had been afforded the protection of a sheltered area, the hull structure had substantial reserve strength against the total vertical bending moment at the time and would not have broken in two. "The sustained dynamic wave loading for the period while the Prestige was under tow, subsequent to the initial casualty, was the direct cause of the ultimate disintegration of the hull structure and subsequent sinking of the vessel," states the Report. The Report concludes that, "Had the vessel been afforded a safe refuge, protected from the wave bending moments and dynamic forces experienced in the open ocean, it would have remained intact and afloat for a sustained period, certainly long enough to lighter the oil cargo off the vessel and prepare it for subsequent repair." It is for that reason that the single definitive recommendation issued by ABS as a result of its rigorous technical investigation into the casualty is that "the international community of nations establish guidelines for the safe refuge of damaged vessels in order to prevent another damaged vessel becoming the cause of a regrettable and avoidable pollution event."