Catenary mooring with its dynamic effects

Abstract

The discoveries, explorations and productions of the oil and gas are going deeper. Most of discoveries for the last five years in Malaysia are in deepwater. Malaysia’s definition of deepwater is range from 200 meters to 1,200 meters, below 200 meters and above 1,200 meters is considering as shallow waters and ultra deepwater respectively. Due to the limitation of fixed platform operated and installed in deepwater, the floating structures systems will growth significantly. The use of mooring system to keep the floating structures in position relatively will also be grown. Mooring line dynamics effects have been ignored in most of the mooring analysis. The existence of the effects is always replaced by safety factor to the static analysis. This will result in unnecessary over sizing and cost as safety factor is always conservatives. The main objective of this research is to compare the maximum line tension arising from mooring line in static and dynamic. The static analysis is performed by solving the equilibrium equation on the line element. While, the dynamic analysis is considering dynamic effects mainly contributed from the inertia, drag force, buoyancy and weight calculated based on lump mass method. The mooring line is modeled by a series of springs. The external forces are applied on each nodal located at spring connection. As the interest is on the mooring line, floating structure is assumed to be operated in still water; therefore no effect on the floating structure is considered. MATLAB programming is developed in order to solve the above problems. Houbolt scheme is used to solve the multiple degree of freedom equation. From the result, it is concluded that in deepwater, the consideration of the dynamic effects on the mooring line is important for safety, technically and economically as the dynamic effect on the mooring line is increasing.