Physical and theoretical modelling of embedded mooring line–seabed interaction in sands

Abstract

The most common mooring configuration for floating facilities is a catenary system. The final section of the mooring line is embedded in the seabed and forms an inverse catenary between the sea floor and anchor padeye. The inverse catenary absorbs part of the mooring load through friction and influences the magnitude and inclination of the load transferred to the anchor. This study sets out an improved model for embedded mooring line–seabed interaction in sand, based on model scale experiments conducted in a geotechnical centrifuge. The experiments reveal the influence of embedded line dimensions and sand density on the inverse catenary shape and resistance. This information is used to calibrate and refine an improved theoretical model that uses cone tip resistance as the input, to estimate the embedded line shape and tensioning response. The value of the new model is illustrated by a case study that highlights the influence of mooring line–seabed interaction on anchor capacity, due to the strong influence of the embedded line dimensions on the inverse catenary shape. Careful selection of the anchor and embedded line combination allows the load inclination at the anchor padeye to be optimised, and the embedded mooring line–anchor system capacity increased for a given anchor size, allowing improved reliability.