Installation and Capacity Analyses of Offshore Torpedo Anchors

Abstract Torpedo anchors are dynamically installed anchors for the mooring systems of floating platforms and received the name of "torpedo" due to the cone head and the fin tail. Commonly used torpedo anchors have a diameter of around 1.0m (typically between 0.7 to 1.3m) and a length-to-diameter ratio of less than 20. During an offshore installation, the anchor will be lowered down to a height from the seabed about 50-100m. Once released from the releasing mechanism, the anchor will fall through the water column and penetrate into soil reaching the target depth within 10s. The target embedment is usually about 2-3 times the anchor length and will determine the holding capacity of the anchor. Torpedo anchors have been deployed multiple times in offshore Brazil as mooring anchors for floating production platforms, as flowline/riser initiation anchors, and as subsea well conductors with significant economic values. Despite the successful applications in offshore Brazil, the adoption of torpedo anchors in other offshore regions is rather limited if any. One possible reason might be the limited experience in the integrated study for torpedo anchors from installation to the final geotechnical capacity evaluation. Hence, the objective of this paper is to present the installation and geotechnical holding capacity methodologies for offshore torpedo anchors. Existing models, including the anchor hydrodynamic model in water, the anchor penetration model in soil, and the anchor geotechnical holding capacity model, are presented and assessed with the associated uncertainties quantified. Then, the design of a 120-tonne torpedo anchor is presented covering the installation and the final geotechnical holding capacity. Recommendations for future studies are provided in the final section to further mature and promote the wider adoption of the torpedo anchor technology.