Heave compensated floatover operation

Abstract

Feasibility study of an alternative method for a heave compensated floatover installation. An alternative floatover method is desired to perform the installation and decommissioning projects that the Pioneering Spirit (PS), a newly built multi-purpose vessel from Allseas, is not able to operate. The alternative method focusses on platforms that the PS cannot operate due to the vessels shape. This shape causes restrictions for the width, length, water depth and air gap of the platform. The latter indicates the height of the topsides above mean seawater level. Topsides installed in West-Africa often have a small air gap, too small to be handled by the PS. An alternative method is therefore required if these topsides are to be installed or decommissioned by Allseas. The aim of this study is to investigate the possible gain in workability by implementing a heave compensation system in a conventional floatover operation. In this preliminary study the mating phase of the floatover operation is examined. The study is based on a well-documented floatover operation: the Liwan 3-1 topsides (26,300 tons) installation. The Nigerian swell sea is considered to simulate the West-African conditions. The heave compensation concept consists of three main components: hydraulic cylinders to lift the topsides, pneumatic pressure vessels to provide passive actuation and accumulators to connect the cylinders with the pressure vessels. The topsides is supported by four of these systems that are placed in a rectangular configuration on the barge. A model is developed to find the influence of the heave compensator on the hydrodynamic behaviour of the barge and topsides. The model uses a frequency domain analysis that is performed with a linear hydrodynamic model. The heave compensation systems are modelled as linear spring-damper systems. To compare the heave compensated installation and the Liwan 3-1 installation a benchmark value for the installation limit is used. This value is based on the most critical loading condition, being the horizontal impact forces on the substructure. The maximum installation velocity of the topsides is found by simplifying the substructure and leg mating units configuration, this leads to a maximum velocity of 0.4 m/s. The results of the heave compensation system are promising. The heave compensated scenario almost doubles the workability in head waves, from 46% without heave compensation to 88% with heave compensation, based on the year-round sea statistics of Nigeria. In beam sea conditions the gain is even higher, from 6% in the uncompensated scenario to 44% with the heave compensation. These workability gains are obtained using a heave compensation system with a pressure of 200 bars, 2,600 m3 of pneumatic storage and 20% of the critical damping. The dimensions of the three main elements are approximated. This leads to a system that contains 24 hydraulic cylinders, 8 accumulators and 96 pressure vessels. Combined with the deck support frame these elements weigh 9,000 tons in total and cover 46.5% of the deck area. The Liwan 3-1 operation contains a deck support frame of 4,000 tons that covers 14.8% of the deck area. The workability can thus be improved significantly in the mating stage of the operation by the use of the heave compensation system, but this would require the implementation of a larger and more complex system. However, the gain in workability might be worth this investment.