This chapter presents an overview of analysis methods and design procedures used for catenary mooring systems, with particular emphasis on their application to floating production platforms Modern trends in automated analysis procedures for mooring system design are described, together with an overview of new design features and operating techniques that are increasingly being utilized This includes considerations of the merits and drawbacks of active winch control, and the use of thruster assistance for direct reduction of net environmental forces or for control and damping of vessel offset transients.
Catenary mooring systems pose a classical engineering design problem in that the most important performance yardsticks (maximum vessel offset, system weight allowance on vessel and cost) are in direct conflict with the maximum allowable structural loadings This conflict is compounded by uncertainties in the results of analysis methods arising from two sources First, there is inadequate physical understanding of low-frequency wave drift forces on the surface platform and the corresponding low frequency radiation and viscous damping, which contribute significantly to maximum vessel offsets and line tensions. Secondly, dynamic analysis methods cannot readily account for the non-linear behaviour of mooring catenaries without some approximations. This is particularly true for multi-element mooring lines installed with clump weights. Furthermore, the requirements of certifying authorities are becoming increasingly stringent for the performance of intact and damaged (usually one line broken) mooring systems, in both operating and survival conditions.
A recent trend in the performance demanded from catenary moorings for floating production systems is also relevant. The design of such systems has traditionally evolved from floating drilling rig practice, in which the relatively small offset limits imposed by vertical tensioned manne risers have led to a quite specific and limited range of mooring line geometries, pre-tensions and operating philosophy. However, the much larger offset limits allowed by the use of flexible risers for floating production means that the mooring system for such platforms must be analysed and designed from a fresh perspective to effectively optimize mooring system performance, safety and cost under these conditions.
A mooring catenary relies on an increase or decrease in line tension as it lifts off or settles on the sea bed to generate a restoring force on the surface platform A spread of catenary lines thus generates a non-linear restoring force that increases with vessel horizontal offset and resists steady environmental forces. The equivalent spring stiffness imposed on horizontal vessel motions is generally too small to significantly influence wave frequency motions, although excitation by low-frequency drift forces can induce dynamic magnification in platform horizontal motions and lead to high peak line tensions. The longitudinal and transverse motion dynamics of the mooring lines themselves can influence their behaviour, particularly for long lengths in deep water. The role of thrusters within a(Fig. 1 is available in full paper) thruster-assisted mooring system also needs to be considered during analysis and design.