Novel Dynamically Installed Fish Anchor - Diving Upon Loading in Calcareous Silt

Abstract

Abstract This paper proposes a novel dynamically installed ‘fish’ anchor, adopting a geometry taken from nature, for 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 (LDFE) 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 dived 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. Regardless of the padeye offset ratio and mooring mudline inclinations, the diving efficiency of the fish anchor, which dictates the potential gaining capacity, was significantly higher than that of the OMNI-Max anchor. The normalised net capacity of the fish anchor was significantly higher than obtained with the torpedo anchors regardless of mooring mudline inclinations, and comparable to that obtained with an OMNI-Max anchor for mooring mudline inclination 0°, after allowing for loading-unloading cycles experienced by the OMNI-Max DIA prior to a nominally monotonic loading test. Dynamically installed anchors have yet to be used in calcareous silty sediments (e.g. offshore Australia). This is primarily because the anchor tip embedment depth in calcareous silt has been found to be only half of that in clay due to the naturally higher undrained shear strength gradient and high dilation-induced bearing and shaft resistance. During subsequent loading, the anchor then pulls out of the seabed, without diving. To achieve adequate capacity under operational loading, deeper penetration that allows anchor diving and better diving potential are therefore critical in calcareous silt. The fish anchor was found to dive in calcareous silt for mooring mudline inclinations < 38°, while by contrast the OMNI-Max anchor generally did not dive. As such, the fish anchor has the potential for efficient anchoring to allow economic development of oil and gas reserves in deep water with calcareous seabed sediments.