Motion Compensated Float-Over Installation with the Use of Ampelmann Systems

Abstract

Abstract Float-over installations offer opportunities to install heavy topsides in remote areas beyond the lifting capacity of available crane vessels, minimizing offshore hook-up and improving the overall project schedule. The topside is placed on a barge or heavy transport vessel, maneuvered in between the slot of the pre-installed substructure. By ballasting the vessel in combination with a hydraulic lowering system the topside is lowered onto the substructure. During the installation procedure large horizontal loads are transferred onto the substructure. These are often the determining load conditions for the design of substructure and foundation. Furthermore, environmental conditions at locations such as West-Africa do not permit year around floatover installation, and even in the favorable season daily fluctuations may result in delays in the installation process due to weather down time. The Ampelmann is a platform on six hydraulic cylinders, which is capable of compensating the motions of a vessel in six degrees of freedom. The platform is invented for marine personnel transfer and a prototype is currently being constructed. By placing the topside on multiple Ampelmann systems, the motions of the vessel can be compensated, resulting in an increase of the available weather windows for installation. Because the vessel can move freely under the topside, no horizontal loads will be transferred onto the substructure. This significantly decreases the design criteria for the substructure and temporary provisions required during the mating procedure. The topside will be placed on four Ampelmann systems with a central passive cylinder and four passive supports. The passive cylinders can carry the static load of the topside resulting in a reduction of the required dynamic power. The fact that topside is not moving with the vessel has an influence on the motion characteristics of the vessel, which is simulated and taken into account. The required dynamic power of the system is determined resulting in the technical feasibility of this new installation method. By comparing the cost and benefits of the Ampelmann float-over system with conventional float-over systems, an assessment is made of the financial feasibility. Introduction Installation of integrated topsides with the use of a crane vessel is restricted due to the maximum lifting capacity and availability of these vessels at certain geographical locations. Installation of the topside in separate modules relaxes the requirements for these heavy lift vessels, but hook up and commissioning time increase resulting in higher cost, particularly in remote areas such as West-Africa. An alternative to using a heavy lift vessel is the float-over method. When using the float-over method, the topside is placed on a barge or heavy transport vessel and maneuvered in between the slot of a pre-installed substructure. The vessel is then ballasted until the load of the topside is transferred onto the substructure. In locations exposed to wind seas or ocean swells, hydraulic jacks are used. These jacks can be retracted in a short period of time, reducing the exposure to impact loads before, and creating an air gap after the load transfer to safely remove the vessel from the jacket slot and eliminating the chance that the vessel impacts the topsides. In West-Africa, where many float-over installations have been and will be performed, the environmental conditions do not permit year around float-over installation. Even in the favorable season the SSW swell waves, which are generated by storms in the South Atlantic region, may occasionally result in large vessel motions, seriously affecting the number of available weather windows to perform the op