A numerical macro model to simulate the whole life response of anchors for floating offshore renewable energy systems

The offshore renewable energy (ORE) industry is developing new solutions to enable floating facilities that can operate further from shore where more ocean space is available and stronger wind resources can be harnessed. Future ORE facilities will involve arrays of devices that connect and will transmit loads to the seafloor via mooring and anchoring systems. Therefore, it is essential to have a reliable estimation of the capacity that the anchoring system can provide for the variety of loads that are transmitted via the mooring lines. This paper provides a model for soft soils (i.e. soft clays, loose-medium silts and sand), where the capacity can evolve with time due to the sustained loads and variable components of the cyclic loads, which vary due to environmental conditions and the characteristics of the floating system. The model is referred to as a 'macro-model', meaning that the response of all soil elements around the anchor are defined by a representative value of strength and other properties at a single node. The model captures 'hidden' anchor capacity enhancements from (i) 'whole-life' changing soil strength, (ii) viscous effects on soil strength and (iii) added soil mass effects, which are usually absent in geotechnical foundation design. It is shown that these effects can be efficiently modelled and integrated into existing numerical analysis packages to provide a new basis for assessing through-life changes in geotechnical anchor capacity. This enables a better understanding of the fully coupled soil-anchoring-mooring behaviour of floating infrastructure over its operational lifetime.