Samenvatting

Damen Shipyards is always on the lookout for new emerging niche markets. It appeared one of the most promising emerging niche markets is the offshore oil and gas infrastructure decommissioning industry. Therefore Damen produced a preliminary design of an offshore oil and gas infrastructure decommissioning vessel.

This decommissioning vessel is designed for the decommissioning of topsides from fixed steel platforms installed offshore. The vessel is equipped with a heavy lifting construction in order to be able to lift the topside. The method for heavy lifting is not defined yet, and therefore the functionality of the decommissioning vessel is not proven. By this thesis, the method of the heavy lifting construction will be defined and the functionality is proven according to the requirements of Damen Shipyards.

In cooperation with Damen, three methods for decommissioning have been established;
 decommissioning by a heavy lifting crane;
 decommissioning by a forklift construction;
 and decommissioning by a lifting frame.
By an analysis to the three methods and the assessment to the dimensions of offshore platforms, a choice is made among the three methods according to the set of requirements by a selection matrix which rates the methods with respect to each other. By this analysis occurred the lifting frame meets the requirements the best and is distinguished by the less sensitivity to weather conditions, the less negative impact on the stability and fastest operational speed. This method is further developed.

The functions of the lifting frame have been accessed and by this assessment several studies are carried out according to;
 the diversity in airgap;
 the tidal range;
 the motion compensation systems;
 the weight distribution of the topside;
 and the occurring forces for operational and survival conditions.

By this results a lifting frame is developed which consists of active compensation by cylinders, a height adjustable frame in order to adapt to the airgap and the ability of lowering the topside in order to discharge the drive components and sea fasten the topside before transport. The function is proven and the design complies with the set of requirements.
The final design is developed at conceptual level and the functionality is proven in general. It is of importance a next design iteration is done for this concept. However, it is of importance the specific movements of the decommissioning vessel and dimensions of platforms is determined, which is not complied by this project. It is also recommended to study the applicability of stamps instead of lifting beams in order to reduce preparations to the topside before decommissioning.
Figure 1 Final design lifting