Experimental study of pullout capacity of plate anchors shallowly embedded in hydrate bearing sediments

Abstract

Plate anchors are commonly used in mooring systems of offshore facilities for gas and oil production in deep and ultra-deep water. They may encounter hydrate-bearing sediments widespread in marine continental margins. The pullout capacity of square anchors embedded shallowly in a hydrate-bearing sand is investigated with small-scale model tests using tetrahydrofuran (THF) hydrate as the substitute of gas hydrate. The results show that the pullout capacity of the anchors increases with the increase of the anchor width and the burial depth. The pullout capacity of the anchors can be estimated by a dimensionless breakout factor. This factor increases with the increase of the burial depth ratio (defined as the ratio of the burial depth to the width of a plate anchor), exhibiting a trend similar to that obtained numerically and experimentally from plate anchors shallowly-embedded in undrained clays. Hydrate dissociation brings detrimental effects on the pullout capacity of the anchors. The pullout capacity of the anchors drops to 0.3% of the initial value after the hydrates dissociate. This pronounced reduction highlights the need for further investigation on the effect of oceanic hydrate dissociation on engineering performance of offshore facilities as a result of natural or anthropologic perturbation.