Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering Transport Technology
R. Huinink Concept study for lifting equipment foundation on a topsides removal vessel. Masters thesis, Report 98.3.TT.5064, Transport Engineering and Logistics.
Since the problems with the removal of Shell's Brent spar, closedown and demantling of offshore constructions is of major importance to the offshore industry. The problems aroused by the abandonment of offshore constructions are large, on economical, technical as well as on environmental areas. Presently, the end of lifetime for a large number of fields is approaching and therefore need to be decommissioned. According to a European law the operators and oil companies are obliged to remove the platforms within a certain period of time after the production of the field has stopped.
Allseas Engineering BV has developed a new concept for the one piece removal of large topsides and jackets. This concept consists of a twin hull vessel equipped with large lifting gear and is designed for the removal of large North Sea platforms. The maximum weight of North Sea topsides to be removed with the Topsides Removal Vessel (TRV) is up to 29.500 t. and the jacket weight up to 10.000 t.
This thesis deals with the design of the foundation structure of the lifting equipment to the hull of a Topsides Removal Vessel. The boundary conditions for this research project are set by the dimensions and weight of the topsides to be removed, and the physical dimensions of the TRV.
First a better understanding of the reaction forces on the hull is obtained. Although motion compensation may be needed, which leads to variation of the reaction forces on the hull per lift frame, this thesis focuses on a static lift and therefore static forces.
To get favourable force conditions on the various support points, several concepts for support systems are evaluated, It is chosen to support the lift frames with three support structures on the inner side shell and both longitudinal watertight bulkheads. An hydraulic system is used for load compensation on the inner side-plating and the inner bulkhead. This results in an equal compressive force of approximately 3750 tonnes on these two supports and a maximal tensile force of 2400 tonnes on the outer longitudinal bulkhead.
With the reaction forces known, the local hull resistance to these forces are determined. The resistance for extra vertical deck loads on a typical tanker hull is relatively low. The large loads, applied during lifting of the topsides, cause large deflections in the tanker hull and collapsing or buckling of the structure is most likely. A simplified model is made for theoretical elastic buckling of the side-plating and longitudinal bulkheads. An approximation of the criticai buckling load on these elements is found using this model.
Subsequently, installation of extra plates on the side shell (21.5 mm) and longitudinal watertight bulkhead (23.5 mm) is required. The estimated costs for the installation of these plates are 12.5 mln dutch guilders. Furthermore a total of 68 additional webframe plating with flanges should be installed. The
estimated costs are 12.2 mln guilders.
With the reaction forces and additional hull strengthening known, various concepts for the foundation structure are generated. The structure aims to transfer large forces from the lifting equipment to the tanker hull. The concepts are evaluated afterwards using imposed selection criteria.
For the foundation structure on the side-plating of the tanker, a box-shaped shaft is designed. Regarding the dimensions of the topsides to be removed, the total length of this box is set to 92.5 m. The cost of this foundation is estimated to 4.5 mln dutch guilders.
The foundation on the longitudinal bulkhead of the tanker consists of a box-shaped structure. The total length of this box is equal to the first foundation. The costs are 7.4 mln guilders.
In contrast to previous support foundations, a tensile load is applied on the outer longitudinal bulkheads. For the foundation on this structure, a connection system using pins and hinges is used. Furthermore, extra hull strengthening is added for a more optimal force transfer from the lift frames to the hull structure. Furthermore, more detailed calculations should determine the minimal dimensions of this structure.
Alternatively, research should be done for the influence on the critical buckling load of applying the principle of the double hull structure. Finite Element calculations will have to verify the assumptions made in the critical buckling model. Afterwards, extra predefined hull strengthening should be added. The reaction forces for the various supports and thereby the shape of the lift frame should be recalculated after installation of the predefined hull strengthening. Reports on Transport Engineering and Logistics (in Dutch)
