Taut Leg Moorings Using Polyester and VLA Anchors

Abstract

Abstract The use of catenary moorings systems in deep and ultra deep water requires long and heavy mooring lines. As a consequence the floater has to support a large weight and the mooring footprint becomes large and may interfere with neighbouring infrastructure. Installation of these mooring systems becomes very expensive due to the vessels that are required. For deep and ultra deep water mooring systems the more appropriate moorings system is the taut leg mooring, using shorter and lighter mooring lines. The mooring loads arrive at an angle between 30 and 45 degrees at the bottom, requiring the anchor points to resist not only horizontal, but also considerable vertical loads. This paper discusses the use of synthetic ropes and anchors in deep water mooring systems. The characteristics of the ropes and anchors are presented and how they can be used in both catenary and taut leg mooring systems. When oil and gas exploration and production took place in shallow to deep water, the most common mooring line configuration was the catenary mooring line consisting of chain and wire rope (Fig. 1). For exploration and production in deep to ultra deep water, the weight of the mooring line starts to become a limiting factor in the design of the floater. To overcome this problem new solutions were devised consisting of synthetic ropes in the mooring line (less weight) and/or a taut leg mooring system (Fig. 2). The major difference between a catenary mooring and a taut leg mooring is that where the catenary mooring arrives at the seabed horizontally, the taut leg mooring arrives at the seabed at an angle. This means that in a taut leg mooring the anchor point has to be capable of resisting both horizontal and vertical forces, while in a catenary mooring the anchor point is only subjected to horizontal forces. In a catenary mooring, the restoring forces are generated by the weight of the mooring line. In a taut leg mooring, the restoring forces are generated by the elasticity of the mooring line.