Physical measurement of a slow drag of a drag embedment anchor during sea trials

Anchor drag during operation of offshore structures could significantly alter the initial load design characteristics of a mooring system. Hence an estimation of anchor positions during operation is essential to identify whether slow or abrupt anchor motion occurs and might require the redeployment of an anchor. During storm conditions, monitoring of mooring tensions and structure motions at the South West Mooring Test Facility (SWMTF) revealed the slow drift motion of one anchor. This facility is a surface buoy with a three-legged, compliant mooring system designed to investigate mooring system behaviour for Marine Renewable Energy (MRE) devices. This paper presents i) some methods to identify the deployment anchor positions: numerical model, acoustics diver survey, and towed sonar ii) the analyses procedure, and estimations of slow drift anchor motion. The findings indicate that one drag embedment anchor moved slowly during a moderate but prolonged and isolated storm, before embedding again. The work demonstrates that anchor position can be accurately monitored and that anchor motion is not necessarily due to excessive peak loads.