THE CONCEPT OF DEEP PENETRATING ANCHOR (DPA)
The Deep Penetrating Anchor (DPA) concept is a solution based on the need from the offshore industry to find more cost-effective solutions as oil and gas exploration moves into deeper waters.
Principles of Concept
The DPA is released freely from a predetermined height over the seabed where gravity is used as the installation force. A 75ton DPA for example, dropped from a height of say 50 to 75 meters, will acquire a velocity of ca. 25-28 m/sec when it reaches the seafloor. At such velocities, deep penetrations are obtainable into stiffer lower lying seabed sediments depending on their strength and deformation characteristics.
CFD analyses have been performed with the objective to compute anchor performance with respect to drag, terminal velocity and hydrodynamic stability. A finite volume technique was applied for solving the 3D non-viscous Euler equations.
Transportation and Installation
Transportation, either by transport barge or the installation vessel itself, is foreseen with the anchor lying horizontally as opposed to the vertical configuration when lowering to seabed for installation.
The penetration analyses for DPAs adopts the method formulated by True (1975). True analyzed soil failure around an advancing penetrometer to determine seabed undrained shear strength. Penetrometers used for tests carried out by the British Research Establishment (BRE 1998) were essentially slender torpedo shaped vehicles weighing up to 3 200 kg and impacting the ocean floor at velocities over 65 m/sec. Penetration depths of over 60 m were realized.
DPA Pull Out Capacity
Due to the DPA design, the vertical pullout capacity may be determined in the same manner as for tension loaded piles. Short-term vertical capacity can be estimated using API Recommended Practice for cohessive soils i.e. total stress analysis, while long-term drained loading is described by Janbu’s (1973) effective stress approach.
Retrieval and Removal
Retrieval is defined as the recovery of DPA as a contingency means during the installation phase and subsequent re-installation; while removal is defined as the permanent recovery of the anchor after completion of the operational phase.