Effect of Soil Variability on the Penetration Depth of Dynamically Installed Drop Anchors
Authors: Sturm Hendrik , Lieng Jon Tore , Saygili Gokhan 
Published: October 2011

Due to the increasing interest in the last years in cost effective and easy to handle anchoring solutions for floating offshore structures, a series of model tests with so-called Deep Penetrating Anchors (DPA™, Lieng [6]) has been conducted at the Norwegian Troll field. Prior to the tests the penetration depth of the anchors had been predicted based on a representative soil profile as it is commonly used in the FEED-design of geotechnical structures at the Troll field. However, the actual penetration depth achieved in tests was noticeably lower than predicted. Subsequent detailed soil investigations at the test site revealed a somewhat different soil profile than assumed in the design. Calculations accounting for the updated soil profile could explain the discrepancy between the measurements and the prediction.

In 1975 True [17] proposed an approach for the prediction of the installation process of cylindrical-shaped objects penetrating dynamically in predominately homogenous soft soils. This approach has been extended and implemented in a FORTRAN routine which can account for arbitrary soil layering and complex geometries of the penetrating object. In addition rate effects are considered more accurately where the soil shear strength is a direct function of the actual penetration rate.

This contribution presents a numerical study using the implemented model. The calibration is done by back-calculations of the drop tests performed at the Troll field. The model is then used for a sensitivity study by varying the soil properties in order to identify a reasonable application range of the considered anchor type based on a qualitative evaluation of the achieved capacity.

Introduction

The concept of DPAs or Torpedo Anchors (TA™) has emerged in the last decade due to the requirements of the oil and gas industry for reliable and cost effective anchoring systems. The anchors considered are torpedo-shaped steel structures with wings that are installed dynamically penetrating the seabed with an initial impact velocity achieved during an underwater free-fall phase. The potential of dynamically installed anchoring systems for the mooring of floating offshore structures has already been recognized by True [17] in the early 70s. The commercial employment of TAs, however, started first in the late 90s by Petrobras, offshore Brazil. While these anchors are used today mainly for the mooring of floating oil and gas facilities, they are also a potential foundation solution for floating Offshore Wind Turbines (OWT) when these move into deeper waters.

In order to have sufficient capacity, the anchors - generally 10 to 15 m long and 35 to 115 tons1 - should penetrate approximately 2 to 3 times their length into the seabed. The padeye where the mooring line is attached is located in general at the top of the anchor. Although this padeye position is beneficial for operational aspects and also affects only very little the total (vertical) capacity in case of taut moorings, the padeye position can be unbeneficial for catenary moorings. From studies on suction anchors is known that the highest total capacity can be achieved for padeyes located at the lower half of the anchor2, e.g. Andersen et al. [1]. Similar results have been found for anchor plates, e.g. Rowe and Davies [14].