The use of floating facilities and taut-wire mooring systems for offshore exploration requires high-capacity anchoring systems capable of withstanding vertical loading components. Drag-in plate anchors, or vertically loaded anchors (VLAs), were developed recently as an alternative to conventional drag anchors. In their normal loading mode, they act like embedded plate anchors. The performance of a model VLA was modelled in the centrifuge, with particular attention to the performance ratios (i.e. the ratio of installation load to pull-out load). Whilst the performance ratios obtained were much lower than expected, the testing proved that VLAs provide a simple and inexpensive alternative to other anchoring systems.
With the exploration for oil and gas reaching deeper waters, the use of floating production, storage and offloading (FPSO) facilities has increased, identifying a need for high-capacity anchoring systems. Conventional catenary systems have become too costly, and this led to the introduction of taut-wire mooring systems, where anchors are subjected to significant vertical loads. While large drag embedment anchors can have very high holding capacities and offer an economical and simple means of anchoring for a catenary system, their suitability for taut wire systems remains in question. As a result, drag-in plate anchors, or vertically loaded anchors (VLAs), were introduced about five years ago by anchor manufacturers: the Denla from Bruce Anchors and the Stevmanta from Vryhof Anchors. VLAs are installed like conventional fluke anchors by pulling up to a target installation load. The loading direction is then changed such that it becomes normal to the fluke, leading to a significant increase in anchor resistance. In this normal loading mode, the anchor acts as an embedded plate. An important factor to consider when designing VLAs is the performance ratio, which is the ratio of pull-out resistance to the installation load.
