The Floatel Superior Loose-Anchor Incident and its Significance for Design and Operation of Semi-Submersibles

The combination of dynamic positioning (DP) and conventional anchoring of semi-submersible platforms in the oil industry has recently lead to a serious incident with large loss potential: the Floatel Superior platform suffered severe damage to its hull from a loose anchor in heavy seas during the night of November 6th–7th, 2012. This paper discusses the root causes leading to the incident. The investigation report from the Petroleum Safety Authority Norway (PSA) concludes that damage leading to the loose anchor had developed over several months, and various warning signs during that period were not heeded. The main reason for the damage was rooted in the combined operation mode where position-keeping by use of DP resulted in large environmental loads on the anchors while they were stowed in their bolsters along the vessel side in rough weather when the installation was at survival draught and operating on DP. The anchors on Floatel Superior had also been exposed to environmental loads in excess of the design capacity during extended transit two years prior to the incident. The incident itself was handled efficiently by the crew and emergency preparedness organizations, with 336 people evacuated by helicopter and the remaining 38 crew members transporting the damaged vessel to shore for repairs. In this paper, an immediate recommendation for removing anchors from the splash-zone while operating on DP is presented as relevant for other installations with similar class notation. In a longer perspective, the investigation found that both well-established design codes and their application in the actual design had shortcomings which played a role in the cause of events. The vessel design appears to have been performed with a principal focus on sub-units (anchor, winch, bolster, pendant-wire attachment) and to an insufficient extent on the combined system use under which it was to be operated. A long-established method of design for connections of auxiliary structures to the main hull structure also showed shortcomings in the event: the doubler plates or pads used to attach the bolsters to the hull did not function as weak links as intended.