Technology Assessment of Deepwater Anchors

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. The 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 authors' 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 installation • Future research activities are recommended for Torpedo/DPA anchors in the following order of priority: 1. analytical penetration studies, 2. analytical studies to determine the optimum number, size, and configuration of fins, 3. small and full scale field testing to measure penetration and holding capacity in various soil profiles and optimize installation procedures, and 4. design method verification and documentation.

Efficient Gravity Installed Anchor for Deepwater Mooring

“…This confirms earlier promise seen by Ehlers [2] with other gravity installed anchors like Petrobras's Torpedo Anchor. Typically a set of eight anchors can be deployed from a typical AHV.…”

“…Assuming the surface undrained shear strength of the soil on this Mississippi Canyon location was around 50-psf and using Equation [2] in conjunction with the installation penetrations, Table 2 shows estimated undrained shear strength gradients for each anchor location. Using the calculated undrained shear strength gradient, the assumed surface shear strength of 2.4-kPa (50-psf), and the post Hurricane Gustav surveyed anchor penetrations, the axial capacity is estimated using Equation [3].…”

Experimental Investigation of Installation and Pullout of Dynamically Penetrating Anchors in Clay and Silt

“…Although mostly used in soft clay, torpedo anchors have been used to moor mono-buoys and ships in shallow and deeper water in calcareous soils (Ehlers et al 2004). As they are being proven as an attractive and cost-effective alternative to more traditional anchoring solutions, with the progress of maturity, torpedo anchors will be applicable for other floating facilities, e.g.…”

“…The rapidly growing hydrocarbon exploration and extraction activity in deep and ultradeep water (now approaching 3000 m) and in emerging provinces has necessitated the development of alternative anchoring concepts. Among others, DPAs are the newest (late 90's; Lieng et al 1999Lieng et al , 2000Medeiros 2001Medeiros , 2002 and are identified as the most promising concept due to (a) an economic, simple and quicker installation process performed by means of single anchor handling vessel (AHV) with no external energy source or mechanical operations required, (b) (and hence) a lower sensitivity to increasing water depth, (c) a high capacity under vertical and horizontal loading with no severe constraints about accurate positioning of the anchors on the sea bottom, (d) a flexibility in geometry that can accommodate a wide range of seabed strength (Medeiros 2002;Colliat 2002;Ehlers et al 2004), and (e) a relatively lighter weight and inexpensive fabrication. Medeiros (2001) reported a related cost savings of 30% compared with conventional anchoring systems.…”

Failure Mode and Pullout Capacity of Anchor Piles in Clay

“…For TLPs, the loading angle θ of the pile is nearly zero, the pullout capacity is not affected by the location of the attachment point. For catenary and taut-wire mooring systems, the loading angle θ is typically in 45-60° (Randolph et al, 2005;Ehlers et al, 2004), the attachment point should be carefully selected.…”