Suction anchors have frequently been used as anchoring for floating production systems at offshore oil and gas fields around the world since the early 1990s. The anchors are considered reliable with no reported failures due to loss of holding capacity or dragging to date. In a recent paper published by Bhattacharjee et al., 2014, severe trenching in front of suction anchors due to anchor line movements has been observed in deep water at a field offshore West Africa. The soil at that location consists of soft clay with very high water content and high plasticity. This study provides results of holding capacity calculations where a trench close to a suction anchor is modelled and where the effects of load angle and set-up are particularly evaluated. Calculations are performed using full 3D Finite Element analyses. Results indicate that based on a generic soil profile considered typical for West African soils and the existence of a diameter wide trench down to padeye level in front of the anchor with an aspect ratio of 4 (embedment depth/anchor diameter), the holding capacity of the anchor can be reduced by 20-45% dependent of load angle.
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