Alternative Configurations and Materials for Deep water Mooring. Results From a Three Year Joint Research Effort

Abstract

Abstract. Mooring of floating platforms in deep waters is a challenging effort which calls for new solutions. A deepwater catenary mooring system has a rather low horizontal stiffness resulting large vessel excursions and correspondingly long risers to accommodate the excursions. A taut leg mooring system with polyester fiber ropes reduces the vessel excursions by a factor of 2–5. This may have significant impact on the riser design and costs. A three year research programme addressing the challenges related to taut mooring systems (TMS) has recently been completed. The research programme has been funded jointly by the Norwegian Research Council, oil companies and contractors. The programme has been focused on taut mooring systems comprising synthetic fibre lines and anchors which allow for vertical loading. The research effort has been divided into four main tasks: Characteristics of synthetic fibre ropes; System design and costs; Anchor design and testing of anchors; Installation and retrieval of mooring systems. The paper gives a general overview over the activities and conclusions from the research programme. In particular the tests included in the research programme has added a lot of valuable information for designers of taut mooring systems. More details on the findings are found in accompanying papers. Introduction. As oil and gas activities are moving into increasingly deeper water, mooring of platforms becomes a more severe challenge. Traditional steel catenary mooring systems made from steel wire and chain are expensive and adds large weights to the platform. Further the restoring stiffness provided by such systems are reduced with water depth, allowing for large horizontal offsets of the platform during extreme weather conditions. This makes the riser solutions more expensive. To obtain reduced weights, costs and horizontal offsets, taut mooring systems (TMS) using synthetic is an interesting alternative. The TMS installed on a semi submersible is illustrated in Figure 1. The potential of TMS with synthetic ropes are well documented in several recent publications. To summarise the main advantages are:Less weight from the mooring system which is important for deep water applications.A stiffer mooring system gives less horizontal offsets (typically 1/4 of a Steel Catenary Mooring (SCM). This has implication on the riser design.Less footprint of mooring system, and, in contrast to steel systems; no potential hazard to subsea equipment if the line breaks and falls down to the sea bed.Easy handling due to low weight.Cheaper than conventional systems in deep water. Synthetic fibre ropes have been used for offshore applications for several years. A review of offshore mooring applications are given by Banfield and Casey (1998). They also summarise experimental work performed during recent years to characterise and quality synthetic fibre ropes for offshore mooring. Data on fibre ropes are also given by Banfield and Hearle (1998) and Isnard and Filch (1998). A key work was performed by Del Vecchio (1992). Several joint industry projects have been launched to improve the understanding of fibre rope moorings.