MPU Heavy Lifter for Offshore Platforms Built Using DYWIDAG Post-Tensioning Systems

MPU-Heavy Lifter, MPU Enterprise AS

A large part of the world’s crude oil deposits is not located on the mainland, but offshore, under the sea bed, primar - ily in the coastal and flat areas of the continental shelf. The extraction of the oil in these deposits is carried out using offshore platforms. Since this form of oil and gas extraction has been carried out for decades, a large number of these platforms have become outdated and have been decommissioned or sunk into the sea.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.

One of the first major projects concerning the disposal of such decommissioned platforms is the dismantling of the satellite platforms around the Ekofisk oil and gas field in the North Sea, near the coast of Southern Norway. 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 Task

Is there a way of recovering 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 Concept

 The MPU Heavy Lifter consists of a U-shaped hull structure with four 25m high towers – see Figure 1. 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. The towers of the “MPU-HL” have a diameter of approximately 20m. Special flushing tanks located in the towers can develop a lifting capacity of 8,000t within a very short time (within one wave length). This is achieved by the tanks’ “dumping” water, which simultaneously allows operations even in difficult weather conditions. Aside from the single lifting capability, this feature is one of the major advantages in comparison to ordinary steel lifting vessels. Comparatively low construction and maintenance costs as well as the stability, robustness and durability of  he structure are additional advantages of the proven concrete technology. Completely posttensioned lightweight concrete was chosen for the structure of the “MPU-HL” in  order to keep its weight as light as possible and to allow fast semi-submersible ships to transport it from one region of the world to another.

The Tests

Due to the extremely large amount of reinforcement and little room in the concrete sections, the decision was made to use threaded couplers and “T-bars”. In order to make sure that these methods of installation for complex reinforcement and casting methods (climbing formwork for walls of the hull and gliding formwork for the towers) corresponded to the special requirements of this structure, comprehensive full scale tests were carried out in advance. Figure 2 shows a full scale test 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. DSI was involved in these tests from their beginning. Static load transfer tests were carried out at the Delft University of Technology.

They proved that “oversized” plate anchors had to be used in order to achieve the required load without crushing the relatively “soft” Liapor aggregates in the concrete.

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. Figure 4 shows the state of construction in March 2008. 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.

Summary

 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 offshoreprojects 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.