A New Technology for Increasing the Load-Carrying Capacities of Offshore Foundations in Soft Clays
Authors: Micic S. , Lo K.Y. , Shang J.Q. 
Published: October 2008

An experimental study on electrokinetic improvement of the load-carrying capacity of offshore foundations embedded in soft clays is conducted. The paper presents a summary of method and results of two series of electrokinetic tests conducted on natural and simulated marine clays in small-scale and large-scale laboratory testing facilities. The results demonstrated that the load-carrying capacity of the foundation models increased up to three times after electrokinetic treatment.

Introduction

Recent developments in the offshore oil and gas industry have resulted in an increasing number of offshore construction projects that include different types of offshore platforms. If the seabed in the construction areas consists of soft marine clay deposits, these structures may encounter serious geotechnical problems. These soil deposits are widespread in the oceans throughout the world and are often characterized by the low shear strength and high compressibility that usually result in extensive settlement under structural loading. The construction and installation costs of the structures are enormous, and the consequences of failure can be catastrophic. To overcome the foundation problems caused by difficult soil conditions, there are two alternatives, i.e., increasing the size of the foundation or improving the soil. Electrokinetics (EK) may be used to strengthen the soil when the latter alternative is considered in design and construction. This study investigates the feasibility of electrokinetic strengthening of soft marine clays, and consequently increasing the load-carrying capacities of foundations embedded in such soils.

Over the last decade, skirted foundations have been increasingly used to support large offshore structures such as gravity platform jackets, jackup rigs, semi-submersible platforms, floaters, tension leg platforms, sub-sea systems and other structures. Skirted foundations are used to resist both compressive forces from fixed jacket structures (e.g. Sleipner T platform) (Sparrevik, 1998) and tensile forces from floating or tension-lag platforms (Snorre TLP platform) (Fines et al., 1991). They have been used in the past in water depths that range from 70 m (e.g. Draupner E heavy jacket in the North Sea) (Tjelta, 1995) to about 1000 m (e.g. semi-submersible platform at Marlim Field offshore Brazil) (Mello et al., 1998).

Skirted foundations are large, hollow, cylindrical foundation elements that are usually made of steel. Their capacity to carry loads depends on factors such as depth of skirt penetration, cylinder diameter, soil strength and the combination of horizontal, vertical and moment loads. When soft soils are encountered in a site, however, the load-carrying capacity is governed by an undrained shear failure in the soil. Therefore, the undrained strength of the soil becomes one of the major concerns in the design of skirted foundations.

Skirted foundations are installed by penetrating the skirts into the seabed, first partially under self-weight, and then by creating an underpressure inside the cylinder (Andersen and Jostad, 1999). A thin zone of soil along the skirts will be remoulded during installation.