SECOND ORDER WAVE EXCITATION OF TANDEM-HULL VESSELS WITH IMPLICATIONS ON MOORING SYSTEM DESIGN

Monohull vessels with the ability to weather vane about their turret mooring systems are widely regarded as suitable platforms for hydrocarbon production in many of the world's offshore regions. A limiting factor however is their seakeeping characteristics which are generally inferior to those of semisubmersibles. Tandem-hull vessel offer a unique design of floating production system that bridge the gap between semisubmersibles and monohulls, yet retain the desirable characteristics of both vessel types. A tandem-hull vessel consists of a fully submerged lower hull for oil storage positioned a small distance below a surface piercing hull with inter-hull columns connecting the two. The size of the inter-hull gap plays an important role in the vessel's seakeeping. This paper presents a study on the influence of the gap on the hydrodynamics of the vessel. In particular the second order wave drift forces and moments are established. This represents a dominating factor in the design of the mooring system. The development is based on first order potentials which are evaluated by a boundary integral technique. Of particular interest is the effect on the numerical solution of the close proximity of sources between the two hulls. Comparisons between the mean second order forces acting on a tandem hull vessel with varying inter-hull gap are established and the influence of viscous effects is outlined. Calculated results are used to provide mooring loads for a particular field application and implications on the mooring system design discussed.