Estimation of pull out capacity of suction anchors in soft clays using numerical modelling

Offshore mooring requires temporary foundations/anchors in water depths exceeding 1000 m. Suction anchors / piles are feasible and preferred solution in such large water depths and find widespread applications in the offshore oil industry. Suction anchors are large cylindrical (inverted bucket type) structure open at the bottom with a sharp cutting edge and closed at the top. Such foundations are placed on the seabed and allowed to penetrate under self-weight; the rest of the suction anchor is sunk into the seabed by pumping water from inside thereby creating a differential pressure on the inner side. National Institute of Ocean Technology (NIOT), Chennai, India proposes to install an offshore desalination plant at approximately 1000 m water depths. Also, mining of gas hydrates from greater water depths require mooring points. Suction anchors are considered as a suitable option for such mooring. Numerical modelling has been carried out using PLAXIS-3D for simulating the suction anchor behavior and estimating ultimate pull out capacity under various loading conditions and soil type. The validation of the model is done by comparing the results with field tests carried out at Royapuram fishing harbor. It is observed that the model conforms to the ultimate pullout load obtained from the field tests and hence can be used for estimation of pull out capacity of suction anchors at other location having different soil conditions.