Jacket removal and transport with Heavy Lift Vessel (summary)

Bas Hamer Thesis Report TUDelft - HMC

Jacket removal and transport with Heavy Lift Vessel xiv

Summary

In the coming 10-20 years many oil & gas platforms located on the North Sea are expected to be abandoned annually as the oil & gas fields have been depleted. With this perspective Heerema Marine Contractors is developing new methods for the removal of offshore structures. The objective of this study is to investigate the feasibility of transporting a steel jacket construction hanging in the cranes of a Semi Submersible Crane Vessel (SSCV). For this study the North West Hutton jacket is taken as reference jacket. The leading motions and loads of the jacket only suspended by the cranes are determined and a concept for extra fixation between jacket and SSCV is worked out in more detail.

The complete new jacket removal sequence will contain the following steps:

1. Removal of the complete top-side of the platform;

2. Preparations on the jacket for attaching the lift rigging (if required); 3. Jacket leg cutting;

4. Lifting of the jacket to the desired transport height; 5. Fixation to the SSCV;

6. Transportation of the jacket hanging in one piece in the cranes of the SSCV to a nearby decommissioning facility;

To assess motions of the jacket during transport two separate analyses are done: 1. The determination of the static displacement due to tow resistance of the partly

submersed jacket. In accordance with HMC a transport speed of 2 knots is used for this research;

2. The determination of the most probable maximum displacements for all directions of the jacket due to the interaction with ocean waves;

These two aspects are combined to get the total of displacements of the jacket during the transport operation. Extra fixation between jacket and SSCV is needed because the

displacements of the jacket exceed the limits in cross direction and to get more control over the movements between jacket and SSCV. It is also desirable to maintain a save distance between jacket and SSCV during transport.

The “Wishbone concept” is evaluated as the best concept to provide extra fixation between the jacket and the SSCV. This concept consists of two steel pipes that are connected to the launch legs of the jacket. The pipes are connected by a universal joint with two axes to the stern of the SSCV. In this way extreme bending moments on the stern of the SSCV are avoided. An important part of the Wishbone concept is the slings. The slings will suppress the rotations and displacements between the jacket and the SSCV.