Foundation Design of Skirted Foundations and Anchors in Clay
Authors: Andersen Knut H. , Jostad Hans Petter 
Published: October 2008

Abstract
The paper describes the principles of skirted foundations and anchors in clay and their applicability for various types of offshore platforms and different types of loading. Procedures for penetration analyses and calculation of capacity are presented. It is argued that skirted anchors can carry both wave loads and large permanent pull-out loads.

Introduction
Skirted foundations and anchors have proven to be competitive alternatives to more traditional foundation solutions like piles and drag anchors in various types of soils and for a wide range of fixed and floating offshore platforms. Skirted foundations have been used for gravity platform jackets, jack-ups, subsea systems and seabed protection structures, and skirted anchors have been used for floaters (process barges, production vessels, loading buoys, storage buoys) and tension leg platforms. Skirted structures also have the potential of being used for several other purposes. Examples of a skirted anchor and a skirted jacket are shown in Figs. 1 and 2.
Some main reasons for the success of skirted foundations and anchors are that they give potential for:significant cost savings compared to more traditional foundations and anchors. Skirted foundations and anchorsmay be cheaper to fabricate, need less expensive installation equipment, can be installed by controlled and simple marine operations, and require shorter offshore installation time.shorter anchor lines and accurate positioning of anchor. Skirted anchors have significant uplift load capacity, high positioning accuracy, and require no drag-in operation or proof loading. This reduces interference with mooring systems of other structures and with other platforms. It also makes them well suited for fibre rope applications.removal and reuse. Relocatable structures can be used atmore than one site and may make marginal fields profitable. Removal of the structure also provides a clean site after exploitation and accommodates environmental concerns.

Principle of skirted foundation and anchor concept

Geometry.
Skirted foundations and anchors are normally cylindrical units made by steel or concrete. The cylinders are open at the bottom and closed by a cap at the top (Fig. 3). The term "skirted foundation" is used herein when the concept is applied for fixed platforms, and "skirted anchor" when it is pplied for floaters. Each foundation or anchor may consist of a single cylinder or several attached cylinders (Fig. 4).

Installation.
Installation of skirted foundations and anchors are based on the principle that they penetrate partly into the soil under weight. Further penetration is achieved by pumping water out from the top of the cylinder, creating an underpressure inside the cylinder. The difference between the hydrostatic water pressure outside the cylinder and the reduced inside water pressure gives a differential pressure that acts as a penetration force in addition to the weight (Fig. 3).

Short term capacity.
After penetration, the water outlet is normally closed, and skirted foundations and anchors may achieve substantial capacity, both for vertical downward loads, horizontal loads, vertical uplift loads, moments, and combinations of these loads. The capacity to carry wave loads is in clays governed by an undrained shear failure in the soil, and the capacity depends on depth of skirt penetration, cylinder diameter, soil strength and the combination of horizontal, vertical and moment loads.