Optimisation of mooring and riser system for deep water floating production systems including fatigue life requirements

摘要

Simultaneous optimisation of riser- and mooring systems for floating production units has been made possible by combining efficient design analysis tools for mooring and riser systems with a state-of-the-art nonlinear optimization code. This can contribute to improving the conceptual design work of anchor and riser systems. The fatigue load may become more critical than the extreme load in some cases, depending upon environment conditions, and floater- and mooring system properties. Thus, the minimum required service life should be included as a design constraint in the optimisation problem formulation. The feasibility of doing this has been demonstrated by implementation in the computer program MOOROPT. An efficient frequency domain analysis of vessel motions and of mooring line dynamics allows analysis of a large number of cases for each gradient evaluation in the optimum search scheme. The sensitivity to the number of seastates analysed is investigated, and it is shown that condensing the 'scatter diagram' from 1400 to about 40 - 60 cases, covering 8 direction sectors of environmental conditions may be acceptable for representing the fatigue life.