Dismantling of an Offshore Platform on Shore using Post-Tensioning Lifter
A large number of oil and gas extraction platforms have started to become outdated and have been decommissioned or sunk into the sea. These proceedings are now to be stopped by international agreements aiming to protect the sensitive ecosystem of the sea. In the future, decommissioned platforms are to be dismantled in an environmentally friendly process on shore instead of being sunk. Furthermore, platforms that have already been sunk are to be lifted and brought ashore.
The dismantling of old platforms with conventional methods is more than complicated and is made even more difficult by the fact that the work often has to take place on stormy seas. The ideal would be to recover offshore platforms with as little effort as possible without using expensive and elaborate special ships. The Norwegian engineering company Dr. techn. Olav Olsen a.s. developed a special lifting vessel made of post-tensioned lightweight concrete for this purpose.
This special heavy lifter, known as “MPU-HL”, is being constructed by order of the Norwegian company MPU Offshore Lift ASA in the Keppel-Verolme dry dock near Rotterdam. For the first time, the MPU Heavy Lifter will allow the removal of the complete topside and jacket of the offshore platform, both in a single lift. Thus, dismantling will be considerably quicker and dependability on long periods of calm weather will be significantly reduced.
The MPU Heavy Lifter consists of a U-shaped hull structure with four 25m high towers. The total length of the heavy lifter is 87m and its width is 110m. It is semisubmersible and can develop a lifting capacity of 15,000t for topsides or 28,000t for jackets.
DSI has been involved in the full scale-tests of the MPU HL from the beginning. We tested in which horizontal posttensioning tendons and vertical loops have been tensioned. Choosing the suitable anchor type for the particular lightweight concrete used in this case was an additional challenge. Static load transfer tests were carried out at the Delft University of Technology.
Post-Tensioning DSI received a contract for the delivery, installation, stressing and grouting of all posttensioning tendons. Horizontal 19 strand DYWIDAG tendons are predominantly used in the bottom slab, the walls and the top slab. Additional vertical 19 strand tendons are used in the towers and some vertical 12 strand loops are used in walls. In July 2007, the first part of the bottom slab was cast. DSI installed the sheathing and plate anchorages prior to casting. Approximately 1,000t of 15,7mm diameter grade 1860N/mm2 must be installed, stressed and grouted in a very short construction time. The entire concrete hull structure is scheduled to be ready in August 2008. This is a real challenge for the Joint Venture “Kombinatie Heavy Lifter” and for DSI.
DSI is very proud of having received the posttensioning contract for MPU’s innovative heavy lifter. Due to the successful co-operation with Dutch companies during previous offshore projects such as the Ekofisk Barrier and the construction of the NAM F3 Platform, DSI Netherlands was also chosen as a partner in this project. In the past, DSI has contributed to a large number of offshore projects as a reliable and competent partner. DSI is looking forward to contribute with its know-how to the success of similar projects in the future.