Installation of Two Prototype Deep Penetrating Anchors at the Gjoa Field in the North Sea
Authors: Lieng Jon Tore , Tjelta Tor Inge , Skaugset Kjetil 
Published: July 2010

The installation of two full-sized 80ton Deep Penetrating Anchors (DPA™) in August 2009 marked the end of a technological qualification process for this free-fall anchor concept for mooring of offshore floating structures. These anchors are 13m in length with four meter wide fins and were installated at the Gjøa Field in the North Sea off the West coast of Norway. Maximum pullout capacity is approximately 700tons. The anchors will be used by the mobile drilling rig (MODU) TransOcean Searcher for drilling and completion of the wells at the Gjøa Field. They are designed to take larger loads than what may be realized by the present MODU, but next generation rigs will be operating at the field in the future where 84mm chain will be used with an MBL in excess of 700tons.

The anchors were installed with less than two degrees tilt and anchor tip penetrations 24m and 31m in a water depth of 360m using the AHTS vessel M/S Island Vanguard. Maximum velocity when they reached the seabed was 24.5m/sec and 27m/sec dropped from heights above the sea floor 50m and 75m, respectively. The successful installation and qualification of these anchors at the Gjøa Field has subsequently resulted in an anchor solution that can now be offered to the oil and gas industry as a viable alternative to present day solutions for anchoring of FPSOs and similar structures in soft seabed sediments.

Introduction

The R&D of the DPA concept has periodically been ongoing since around the mid 1990s in a gradual but thorough process through to qualification. Promising results from initial theoretical investigations involving hydrodynamic behavior and geotechnical design gave incentive to continue. Model testing in scale 1:25 [1] followed by 1:3 scale fjord [2] and offshore testing [3] has been part of the technical qualification process. However, this past year greater effort was focused on bringing the concept through a final technology qualification funded by Statoil, the Norwegian Research Council and own funding. The Deep Penetrating Anchor (DPA™) is an alternative anchor concept to present day solutions that offers cost saving potential in offshore moorings where soft clay seabed sediments are present as earlier reported by Lieng et.al [1,4]. Cost savings are primarily on reduced installation time and simpler and less costly installation vessels but also on fabrication. The anchor is simply dropped from a predetermined height over the seabed from say 50 to 75m, such that the anchor acquires a velocity of approximately 25-30 m/sec when it reaches the sea floor depending on anchor dimensions and mass. At such velocities, deep anchor penetrations are obtainable and controlled by the strength characteristics of the soil as the anchor penetrates into stiffer, stronger sediments. Gravity "shoots" the anchor into the seabed i.e. no external energy source is required for installation other than lifting capacity, which makes it unique in relation to other anchor solutions. There is normally no need for arduous subsea intervention i.e. only observation by ROV and for hook release to initiate free fall is required. The permanent mooring line can be attached to the anchor during drop installation. In this respect anchor installation is practically independent of water depth. This means substantial cost reductions in marine operations relative to today's plate and suction anchors.