Technology Assessment of Deepwater Anchors

Abstract

Abstract The move of offshore oil and gas development into deepwater has required that the mooring systems for floating drilling and production platforms/vessels shift from catenary mooring systems to taut-leg mooring systems, requiring the need for a low cost deepwater anchor that can withstand major uplift mooring forces and be designed and easily installed to the design penetration with a high degree of reliability. Many deepwater anchor concepts have been proposed to meet the above requirements in recent years. The four most widely known concepts are discussed in this paper to represent the entire range of concepts in terms of operating principles. The selected anchor concepts discussed herein include two relatively proven anchor concepts, the suction caisson and the vertical loaded (drag embedment plate) anchor (VLA), and two developmental anchor concepts, the SEPLA (Suction Embedded Plate Anchor) and the Torpedo/Deep Penetrating Anchor (DPA). Each anchor type considered in this paper has a different level of technology maturity. The suction caisson is presently the preferred anchor for taut-leg mooring systems for permanent facilities and is probably the most mature in terms of installation experience and prediction of holding capacity, but there are economic issues associated with the fabrication and installation of suction caisson anchors due to their large size. The VLA is probably second in the overall level of maturity from the standpoint of prediction of holding capacity and installation confidence, but there are installation issues and limitations associated with the size, number, and hence cost of marine vessels required to drag the anchors to design penetration, to key the anchors, and to proof load the anchors. he SEPLA and Torpedo/DPA anchors trail in level of maturity and require the most technological development to attain a mature state of practice in the future. This paper will review these four anchors that show the greatest opportunity to serve the needs for deepwater mooring and describe the state of practice in terms of design and installation reliability. The objective of this paper is to discuss the uncertainties (areas requiring further development) of each anchor, the advantages and disadvantages, and to offer the author's opinions of future technology development direction and focus. The general conclusions regarding future development of the practice of deepwater anchors are as follows:Torpedo/DPA anchors appear to be the most promising option for improvement in cost reduction and simplifying installationFuture research activities are recommended for Torpedo/DPA anchors in the following order of priority:analytical penetration studies,analytical studies to determine the optimum number, size, and configuration of fins,small and full scale field testing to measure penetration and holding capacity in various soil profiles and optimize installation procedures, anddesign method verification and documentation. Introduction The move of the oil and gas industry into deeper waters has focused much attention on mooring and anchoring systems. Thus, the type of anchor used with a mooring system has become an increasingly important part of development scenarios involving floating installations.