Novel Fish Anchor and Suction Caisson Anchor in Calcareous Silt

Muhammad Shazzad Hossain

Very unique characteristics of calcareous seabed materials have challenged economic and safer extraction of oil and gas hiding in deep water seabed adopting advanced technologies. This presentation will cover performance of the novel dynamically installed fish anchor and suction caisson anchors in calcareous silt.

The benefit of using dynamically installed anchors has already been proven in clayey seabed sediments in offshore Brazil and in the Gulf of Mexico. However, application of these anchors in calcareous seabed has been restricted by the lower embedment depth achieved during dynamic installation. During subsequent loading, the anchor then pulls out of the seabed, without diving. Hossain developed and patented a dynamically installed fish anchor, adopting a geometry taken from nature. During dynamic installation, the normalised tip embedment depth of the fish anchor was typically three times that for the torpedo anchors and 50% greater than that for the OMNI-Max anchors. Under operational loading, the fish anchor was found to dive deeper. The diving efficiency of the fish anchor has also been confirmed through field testing in the Swan River.

The behaviour of a stiffened suction caisson anchor and a mooring chain under cyclic loadings in calcareous silt has been investigated. Half-caisson tests and subsequent particle image velocimetry analyses allowed for revealing the failure mechanisms during installation and inclined loading. From full-caisson tests, post-cyclic monotonic capacity was found to be up to 35% higher compared to the pure monotonic capacity. The contribution of the chain-soil interaction on the cyclic performance and on the capacity was quantified. Potential trenching around the chain was monitored.

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