Heerema Marine Contractors (HMC) have removed and will be removing various platforms from the North Sea. For these projects Heerema has developed an unconventional method to remove and transport jackets: Jackets are lifted as one single piece and transported to the recycling yard whilst being suspended from both cranes of the Heavy Lift Vessel (HLV) Thialf. Two purpose built structures at the stern restrain the jackets from horizontal motions during transport. In summer 2010 three eight-legged jackets were removed and transported using this method (see Figure 1). The jackets weighed approximately 5,000 metric tonnes each and stood in 70–80m of water. The jacket removal resulted in load cases that were never considered during jacket design. Jacket strength appeared very marginal when cutting the jacket into several sections, but by lifting the jacket as one single section, all members remained connected and ensured a stable structure. Other benefits were reduction of the offshore project duration, the subsea cutting scope and the required vessel spread. Risk on weather downtime was reduced and safety improved by preventing back loading operations in an offshore environment. The transport distance with the jacket suspended from both Thialf cranes ranged from 200–300 nautical miles (1 1/2–2 1/2 days sailing). The final cuts and the jacket lifting required relatively low sea states. The wave climate for transport was determined with the assumption of a preceding weather sensitive operation, which is different to a transport that assumes a start at a random time. Model tests for these design sea states have been performed to accurately assess the Thialf dynamic behavior at its shallow transit draught. Additional analyses were performed to confirm the vessel-jacket dynamic interaction. During transport the so called ‘restraints’ gripped around the jacket corner legs, restraining the structure horizontally and preventing side loads on the cranes. During the three transports, motions have been measured and dynamic behavior corresponded well with the analyses. Removing and transporting these jackets as a single piece was a unique operation. The method worked well and resulted in a predictable, safe and time efficient jacket removal. This paper will address the removal method, including structural aspects, model tests and full scale verification.

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