Anchors for Deepwater to Ultradeepwater Moorings
Authors: Colliat J-L. 
Published: October 2008

From a review of current applications, suction piles and vertically loaded plate anchors (VLAs) appear as the two preferred solutions for deepwater mooring systems, in up to about 1500m of water depth, and for both temporary mooring of exploration drilling rigs and permanent mooring of production units. From this experience, some general trends for application in up to 3000m of water depth are suggested.

The availability of recognised design methods will facilitate further applications of suction piles and VLAs in deepwater to ultradeepwater moorings. In the paper, emphasis is put on the required field load testing work for improving the present design practice for suction piles and VLAs used in permanent moorings in soft deepwater clays. Finally, within the on-going R&D studies on new anchor types, the freefalling torpedo anchor is identified as the most promising anchor concept, in particular for use in temporary moorings for MODUs beyond the 3000m water depth limit.

Introduction

The rapidly growing offshore activity in ultradeep waters (i.e. between 1500 and 3000m of water depth), requires the need to increase the mooring capability of the existing fleet of exploration drilling or floating production units (FPUs), with the objective to maintain the procurement and installation costs of the mooring system at a reasonable level, without decreasing the reliability of the mooring components. The selection of the mooring system for a given floating unit, which has become a key parameter in deep waters, should be based on an integrated study, including not only the type of mooring line and anchor, but also the installation procedure at the site and the type of installation vessel(s) which will directly influence the final cost of the complete as-laid mooring system.

For deepwater developments, apart from the TLP currently applied in up to about 1200m of water in the Gulf of Mexico, and excluding the piled compliant tower used in moderate water depths, dry completion and production units under consideration include semi-submersible FPUs, FPSOs/FSOs, and deep draft floaters such as the SPAR buoy (Fig. 1).

To date, the driven pile is still the main type of foundation or anchor for the TLPs and SPARs. The low soil resistance to driving encountered in soft clays at deepwater sites makes the use of relatively small hydraulic hammers efficient, with the hammer weight not being an installation issue. However, the need for alternative solutions is related to the costly pile driving fieldworks with large crane barges, as well as to the present limit of hydraulic hammers at about 1500m of water. According to hammer manufacturers, this limit should probably be extended to 2000m in the near future, with fabrication of new umbilicals and winches, but more costly modifications would be required for up-grading the hammers for depths beyond 2000m.

With the exception of the lateral mooring system of the Auger TLP in the Gulf of Mexico, the only two catenary moorings for deepwater FPUs were those for the FPSO-II and FPSO-VI, installed off Brazil in 1400 and 800m of water in 1997 and 2000, respectively.