In this paper, a new chain-soil interaction model is presented considering the open cavity depth and soil back-flow characteristic. The bearing capacity coefficient increases from 0 at the mudline to 10.5 over full backflow depth instead of 5.14 to 7.6 without considering the cavity depth and soil back-flow effect. The friction force expression on the chain is presented considering the contact area varying with the backflow of soil. The results show that the cavity depth has influence on the reverse catenary profile, its effects decreases with the chain tension at the mud line increasing.
In recent years, the development of marine oil and gas resources has been advancing to the deep waters, hence new requirements have been put forward for the anchor foundation of floating offshore platforms. The designing trend of floating offshore platform is to install taut-leg and semi-taut mooring systems. The pullout force produced by the deep water floating structure is transmitted through the chain and acts on the padeye of the plate anchor, which is the main reason for the bearing capacity of the anchor. The chain inclination at the padeye determines the relative magnitude of normal force, tangential force when load is acting on anchor, and these components determine the failure mode of anchor. During the pullout process of the anchor chain, the anchor chain starts to slide and cut through the clay with the applied pullout force. Anchor chain is affected by tangential force and normal force and forms a reverse catenary profile in cohesive soils, which is contrary to anchor chain in water. Vivatrat (1982) proposed a method to predict the profile of the chain, but it does not provide the details of the calculation process. It was pointed out (O'Loughlin etal 2015; Xiong, 2016) that recent field observations have showed that the trenching phenomenon will significantly reduce the anchor system capacity. The chain-soil interactions affect the reverse catenary shape of embedded anchor chain in clay. The study of the reverse catenary profile can help to improve the embedded performance of the anchor, accurately predict the track of the anchor, and solve the problem of accurate positioning of the anchor in the installation.
It was noted (Hawlader, 2015; Gaudin, 2014) that the bearing mechanism of the anchor chain in the soil is similar to that of the pipeline and the T-bar penetration mechanism. According to pipeline and T-bar related mechanism, during the pipeline penetrated into a certain depth, due to the undrained shear strength of clay, an open cavity above the pipeline is formed before soil fully backflow embedding the pipe. Cavity will further influence the tension acting on the anchor and the fairlead, which is related to the safety of the mooring system.