This paper describes the design, development, testing and installation for an eight leg Taut Leg Mooring (TLM) for the MODU Ocean Confidence. The mooring system is being used to provide station keeping for the rig while conducting drilling operations for BP in the Gulf of Mexico. The TLM uses parallel strand torque-neutral polyester rope provided by Marlow Ropes, CSO Aker Marine Contractors' (CAMC) patented Suction Embedment Plate Anchors (SEPLA) and Ballgrab subsea connectors provided by BSW Ltd.
The system is designed to be recovered and reinstalled many times. CAMC performed the initial installation in three phases, the first two using a single anchor handling vessel. The first phase pre-installed the anchors and a receptacle for a subsea connector. The second phase attached ground chain and polyester with a subsea connector to the anchors and suspended the polyester rope in the water column using surface buoys. The third phase inserted additional polyester into the preset moorings and attached the moorings to the MODU. The installation was completed on September 26, 2001, making it the first full polyester mooring system to be used in the Gulf of Mexico.
The system is provided to BP on a lease arrangement, with BP and CAMC sharing the risk for the success of the project. Sections of the polyester will be removed and inspected on a periodic basis. It is expected that the success of this important project will lead the way in obtaining more general acceptance of the technology and will pave the way for allowing its use for additional MODU moorings, and for permanent moorings in the Gulf of Mexico and other areas of the world. Other firsts for this project include the use of Ballgrab subsea connectors in a mooring system and SEPLA anchors for a full eight-point mooring system.
As the offshore oil and gas industry moves into deeper water, the need for cost effective, efficient moorings becomes a large issue, especially for long term development drilling and floating production systems. While Dynamically Positioned (DP) Drilling Vessels can be well suited for exploration drilling and some development drilling, mooring systems can provide cost and technical benefits.
Numerous papers, studies, articles and presentations have covered both the technical and economic benefits of polyester taut leg moorings (see References 1, 2 and 3 for three examples). The main benefits revolve around weight reduction and polyester mooring system compliance compared to steel systems. These two factors lead to substantial reductions in operating tensions, maximum line tensions, vessel offsets, anchor radius, and installation loads and costs.
Once it was identified that polyester taut leg moorings held great promise, the move was on to prove the technology. Many research projects and Joint Industry Projects (JIPs) studied polyester to determine its properties and characteristics. Most of these studies were conducted in the laboratory to determine properties needed to design a mooring, such as strength, elasticity, and fatigue.