Geotechnical performance of dynamically installed anchors in soft clay

As the development of oil and gas field exploitation moves into deeper waters, the industry is tending towards cost effective anchoring systems as an alternative foundation solution. The apparent success of the PETROBRAS “torpedo piles” in mooring a number of oil and gas facilities in Brazilian waters has prompted considerable interest in dynamically installed anchors. A dynamically installed anchor is rocket or torpedo shaped, with one such variant having a diameter of 1.2 m, a height of 13.25 m and a dry weight of 740 kN. After release from a designated height above the seabed (typically 20 m to 50 m) the anchor reaches a maximum velocity of 25 – 35 m/s and penetrates by up to 3 times the anchor length in soft clay by the kinetic energy obtained through free-fall and anchor self-weight. The uplift forces due to environmental loading are mainly resisted by the friction developed along the soil-anchor interface (i.e. between the soil and the shaft and flukes of the anchor).

With the exception of a number of field trials, (which have not been published in detail) very little performance data exists for this form of anchorage. Increased understanding of the geotechnical behaviour of these anchors in varying soil and loading conditions would evidently lead to increased industry confidence in the potential for dynamically installed anchors to provide significant cost savings. Given the lack of performance data currently available and the potential economic benefit of these anchors to industry, there exists a clear need for an experimental study to address the basic issues of predicting anchor embedment depth and subsequent holding capacity for a given anchor geometry, anchor drop height and seabed strength profile.

The proposed project aims to investigate the geotechnical performance of dynamically installed anchors in normally consolidated clay as this is the dominant deep water seabed deposit. Considering the scarcity of experimental data from field and model tests, the first objective of the proposed project is to develop an experimental database by conducting an extensive suite of reduced scale centrifuge tests on model anchors. Such a database would have considerable merit in the development of design charts and verification of numerical/analytical techniques that predict the embedment and eventual capacity of dynamic anchors under working and ultimate loading conditions. The second major objective therefore, is to develop a design tool that can be readily employed by industry for the prediction of anchor embedment and subsequent capacity.

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