Analysis and Design of Suction Anchors Allowing for Trench-Paper

Paper

Pdf Summary

The paper examines how seabed trenches formed by mooring-chain movement affect the holding capacity of suction anchors. Because trench development is difficult to predict, the authors recommend conservative design assumptions: a full-width trench extending to the mooring-chain padeye, loss of soil resistance in front of the anchor, a possible gap behind it, and potential reduction in reverse end-bearing.<br /><br />To assess these effects efficiently, the authors extended AGSPANC, an upper-bound analysis tool for laterally loaded caissons, to model trenches. The tool was validated against independent finite-element analyses using PLAXIS 3D and ABAQUS. Four cases compared anchors with and without trenches, shallow and deep padeye positions, different trench shapes, rear-face gapping, and reverse end-bearing conditions.<br /><br />The three methods generally produced similar soil failure mechanisms, but their predicted capacities differed. AGSPANC typically gave higher capacities than ABAQUS—by about 10% for an intact seabed and up to 21% for a trenched case. PLAXIS generally predicted higher capacities than AGSPANC, except for one intact-seabed case; the reason for this discrepancy remains unclear. A trench substantially reduced capacity: in one deep-trench case, AGSPANC predicted a 70% reduction relative to the corresponding intact condition. Trenching also changed anchor rotation and could make a padeye position that was optimal for an intact seabed no longer optimal.<br /><br />The modified, web-based AGSPANC tool provides a practical means of evaluating trench effects and optimizing padeye depth, but the results underline the importance of conservative assumptions and further investigation of differences between analysis methods.

Keywords

suction anchors

seabed trenches

mooring-chain movement

holding capacity

AGSPANC

finite-element analysis

PLAXIS 3D

ABAQUS

padeye position

reverse end-bearing