Torpedo shaped anchors may be the next cost-effective and robust anchor solution for offshore floating structures. These anchors are designed such that after release from a specified height above the seabed they are installed well into the seabed by penetration after reaching velocities up to 100 km/h during the drop and "free-fall" through the water. The anchors may typically have a dry weight of 50 - 100 tonnes and a height of 10-15 m.
Figure 1: DPA anchor for installation at the Gjøafeltet (left).
DPA installation set up (right) Source: Photo: Gemini 2009-04 , Figure: after Lieng et. al, 1999
One such system is the Deep Penetrating Anchor (DPA) proposed by the entrepreneur and former SINTEF geotechnical researcher Jon Tore Lieng. Statoil has been a key partner in the development of the DPA concept and recently two test anchors where successfully installed at Gjøafeltet which is under development by Statoil . The DPA development has also been funded by The Research Council of Norway and is described in more detail in  and . The Brazilian energy company Petrobras has also worked with a similar torpedo anchor concept since 1996.
Figure 2: Location of NGI instrumentation for the DPA tested in the Trondheimsfjord and at the Troll field in the North Sea.
NGI involved in instrumentation and numerical modelling
NGI has provided instrumentation for the testing of the DPA concept in the Trondheimsfjord and at the Troll field in the North Sea. Instrumentation was used to log acceleration, pressure, pitch and yaw during the "free-fall" and penetration phases. The instrumentation set up is shown in Figure 2.
NGI has also carried out an internal research project where anchor penetration into the seabed clay soil and the stress distribution around the anchor has been studied by a large deformation finite element analysis FEA . Large deformation FEA is a fast developing method for simulation of large deformation and penetration problems in geotechnics. In this work the Abaqus Updated Lagrangian method has been used together with a contact formulation for the anchor-soil contact.
Figure 3: Results from Abaqus FEA by Sturm & Andresen . Distribution of increased radial stress caused by anchor penetration (left). Distribution of increased shear stress caused by anchor penetration (right).
VIDEO 1: Distribution of increased radial stress caused by anchor penetration
VIDEO 2: Distribution of increased shear stress caused by anchor penetration
Figure 4: Results from Abaqus FEA. Distribution of increased radial stress in a horizontal section.
The soil being displaced during penetration causes increased stresses and pore pressures adjacent to the anchor. The stress and pore pressure distribution is used to analyze the soil re-consolidation with time after installation and to assess the "set-up" of shear strengths used for calculation of the anchor pull-out capacity.
NGI provides offshore soil investigations, field and laboratory testing. We deliver instrumentation and monitoring systems and interpretation of measurements. Through our comprehensive expertise in offshore geotechnics we can perform finite element analysis for the penetration and "set-up" assessment and the calculation of pull-out capacity.
||Lieng J.T., Hove F., Tjelta T.I. (1999). Deep Penetrating Anchor: Subseabed deepwater anchor concept for floaters and other installations, Proc. 9th Int. Offshore and Polar Engr. Conf. , pp. 613-619|
Sturm H., Andresen L. (2010). Large Deformation Penetration Analysis of Torpedo Shaped Anchor, Proc. of 7th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE), Trondheim.
Sturm H., Lieng T.I. and Saygili G. (2011) Effect of Soil Variability on the Penetration Depth of Dynamically Installed Drop Anchors, Proc. of the OTC Brasil 2011, Rio de Janeiro, Brazil