Prediction of Seabed Trench Formation Induced by Anchor Chain Motions

Recently there has been significant interest in the development of seabed trenches associated with anchor chains in front of suction anchors. The issue was first highlighted by Bhattacharjee et al. (2014) in relation to the Serpentina FPSO located offshore Equatorial Guinea. The subsequent implications of the Serpentina experience have been significant, such that the geotechnical design of suction anchors now often includes allowance for the potential formation of an anchor chain trench extending down to the anchor padeye.

Previous publications relating to anchor chain (mooring line) trenching have focussed primarily on observational case histories (e.g. Serpentina FPSO) and analysis of the impact of seabed trench formation on suction anchor in-place holding capacity. Furthermore, recent work by others outlined the results of 1g and centrifuge model anchor chain tests and the use of these results in the preliminary development of a chain model encompassing the progression of erosion and trench wall collapse with time.

This paper describes a new design tool which permits identification of the primary mechanism leading to the formation of seabed trenches running along the embedded section of mooring lines in front of anchors. The model provides a direct approach to estimating the extent of seabed trench formation that may arise from mooring line motions under both normal (operational) and extreme (storm/cyclonic) conditions. As inputs the design tool requires the full mooring line layout/configuration from the fairlead (located at the floating facility) down to the anchor padeye, the range of design loads at any point along the mooring line and the seabed soil properties.

Through an example study the paper demonstrates the ability of the design tool to back-analyse seabed mooring line trench formation at existing installations and predict the extent to which mooring line trenches may develop for new sites. The ability to perform site-specific assessments of trenching risk should in turn allow the industry to move away from including blanket allowances for trenches which, sometimes unnecessarily, mean that much larger anchors are planned than actually required.