The floating installation method of offshore platform topsides has become an effective installation method since the 1980s. After years of practice and exploration in the industry, not only the weight of the installation structure is increasing, but also the installation method is constantly challenging the worse construction sea conditions. The floating method solves the contradiction that the module construction cannot be expanded due to the limitation of lifting capacity, improves the efficiency of platform construction, is more stable than lifting, improves the construction quality, reduces the offshore operation time, and reduces the offshore operation risk.
The typical installation process of float over method includes the loading of topside, maritime transportation, construction standby, installation ship entering the jacket notch, ship position adjustment before docking, topside and jacket docking, ship position adjustment after docking, and installation ship exit. Taking the installation of jacket topside by floating method as an example, the implementation and precautions of the above links are briefly introduced.
The topside shipment is the starting point of the installation process of the float over method, and the shipment is generally carried out with the help of the skid (Figure 1.1). The decisive factors affecting the shipment design include the weight, size, center of gravity position, tidal range, wharf height and water depth of the topside.
After the completion of loading, the topside must be bound to the deck of the installation vessel before the offshore transportation can begin. Stability is a key issue for towing. The wider and deeper the hull, the better the initial stability and stability range. However, the increase of initial stability will reduce the rolling period, and the acceleration of the center of gravity of the topside will increase accordingly in wind and waves, which will generally increase the difficulty of binding design. In addition, the width of the installation vessel will directly affect the width requirements of the jacket notch, and a larger vessel width is an adverse factor for jacket design. Therefore, the hull width should be as small as possible on the premise of meeting the towing stability.
After the completion of offshore towing and before the commencement of ship entry operation, the installation vessel needs to make a series of preparations, such as cutting and clearing of lashing structure, starting of mooring and positioning monitoring system, starting of load transfer system, ballast adjustment of installation vessel, etc. In the standby state, the installation vessel must be positioned by the mooring system (except for the dynamic positioning installation vessel). The design of standby mooring system depends on the oilfield layout, sea conditions and other factors.
After the preparation is completed, if the scheduled installation window period is right, the ship entry operation can start. The installation process of large offshore platform topside is shown in Figure 1.5. The installation vessel will enter the jacket installation groove from the standby position.
