Taut Leg Spread Moorings: A Cost-Effective Stationkeeping Alternative For Deepwater Platforms

Abstract

Spread moored floating production platforms are employed worldwide in the exploitation of offshore hydrocarbons. To date they have all employed catenary spread mooring systems (CSMS) using chain or combination wire/chain components. In water depths to 1500 feet and beyond, such simple wire/chain systems become increasingly inefficient and costly. To improve cost efficiency, tighten watch circles and 10wer vertical load on the platform several innovations have been introduced such as the use of submerged spring buoys (Ref. I), ground wire, and ropes made from synthetic aramid fiber. An aramid rope has yet to be employed in a permanently spread moored production platform as industry awaits a better understanding of the long term behavior of this material (Ref. 2). The installed cost of a deep water (XM remains high despite these innovations and a need exists for even more efficient spread moorings as industry looks to water depths of 3000 feet and beyond. The most promising alternative to the (XM to emerge in recent years is the Taut Leg Spread Mooring (TLSM) system, with short scope legs (Fig. 1) and where vertical uplift on the anchors is permitted (Ref 3, 4). This paper explains the operating principles of a TLSM, its performance sensitivity to variations of key parameters and shows the cost benefit versus a CSM by specific case studies. FUNDAMENTAL BEHAVIOR EXPLAINED To develop a cost efficient design, it is essential to understand the basic mechanisms by which a TLSM resists platform mean loads and wave induced motions. The behavior is most easily grasped by studying the simple taut leg system shown in Figure 2. k consists of a light weight, elastic mooring leg in water depth D stretched between the anchor point A and the fairlead point F separated by a horizontal distance L. The anchor point is fixed and resists horizontal and vertical forces. As the fairlead point translates away from the anchor point in response to an applied mean load, tension is induced in the leg as it stretches elastically between the points. An equilibrium position is attained when the moment created by the horizontal force components (H *D) is balanced by the moment created by the vertical force couple (V*L) between the anchor and fairlead points. As the line slope angle relative to the horizontal decreases, distance L increases requiring a smaller vertical force and therefore smaller line tension to resist a given horizontal force. However, line scope and therefore cost also increases. If horizontal force per unit of line volume is used as a measure of cost efficiency, it can be demonstrated that the most cost efficient line slope of a simple TLSM to resist a horizontal mean load is 450 (scope/depth ratio of 1.414). In a catenary system, mean offset is controlled by line weight and pretension. Lines composed of materials with lighter weights and with larger pretensions produce.