Fish Anchor Testing in the Swan River

Abstract

Abstract OTC-28901-MS proposed the novel dynamically installed "fish" anchor in 2018, adopting a geometry taken from nature, for potential economic and safer tethering of floating facilities in deep water. Every cross section of the fish anchor shaft is elliptical, leading to very low drag resistance during free fall through the water column, and also low resistance in penetrating the seabed sediments. The padeye is fitted on the widest part of the shaft to mobilise the maximum resistance area under operational loading. The fish anchor embedment depth during dynamic installation, and capacity under both monotonic and cyclic operational loading in calcareous silt were assessed through centrifuge model tests and large deformation finite element analyses. 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 dove deeper, reaching penetrations 20 to 60% greater than achieved during installation. By contrast the torpedo anchors (for all mooring mudline inclinations) and the OMNI-Max anchors (apart from a single test with mooring mudline inclination of 0°) pulled out directly without diving, reflecting insufficient free-fall penetration in calcareous soil. This paper provides a follow up reporting the performance of the fish anchor through field tests in the Swan River, Perth. A 1/15th scale model fish anchor was fabricated with dry weight being 0.304 kN. The anchor was tested at five different locations. At two shallow water locations (water depths 1.1 and 1.9 m, respectively), the tests were performed from the Burswood and Maylands jetty. At relatively deeper water depths of 2.91∼4.73 m, the tests were performed from a barge. The riverbed soils consisted of clay, silty clay, silt and sandy silt. The impact velocities were 5.9∼11.7 m/s. The normalised tip embedment depths were even greater compared to those achieved from centrifuge tests in calcareous silt. Under operational monotonic loadings, the fish anchor dove, as opposed to pull out of the riverbed, for mooring angles ≤ 37∼47°. Interestingly, in contrast to non-diving torpedo and suction caisson anchors, the diving fish anchor resulted non-elliptical failure envelopes, which have been expressed mathematically. The ultimate capacity was 3.5∼15 times the weight of the anchor submerged in water for taught and catenary moorings.