Study on Offshore Skirt Penetration Using the Geocentrifuge
Authors: Bezuijen Adam , Hjortnaes-Pedersen A.G. , Luger Dirk 
Published: October 2008

Abstract
The behaviour of deep skirts in clay has been stud, ied by performing centrifuge tests. A circular skirt pile with length 36 m, inside diameter 10.5 m and wall thickness 1.27 m was scaled 1:150 and 1:300. The tests involved installation in flight with a constant rate ofpenetration. The total force, friction force, the development of heave both in- and outside the skirt and generated excess pore pressures in the clay were measured both during penetration and dissipation afterwards. In-flight cone penetration tests were performed to determine the strength of the clay. The experimental results of force and heave have been compared with the results of the PIAPA computer program especially developed to predict the force and the heave during skirt penetration.

Introduction
Skirt foundations are often used underneath offshore platforms, when they are to be placed on soft soil. The soil in- and outside the skirt walls will rise (heave) when placing the skirt foundation. The penetration force and heave depend on the skirt geometry and soil properties and need to be known for the design of the platform. To investigate experimentally the fundamental mechanism and to verify calculation techniques, centrifuge tests were executed in the large Geo-centrifuge at Delft Geotechnics, described in [1]1. To this end, one idealized prototype skirt compartment was modelled and tested. In the centrifuge tests the model skirts were penetrated in flight in clay. Force and heave were measured. Excess pore pressures in the clay, generated by penetration, were also measured. After dissipation further penetration was included to measure the ultimate axial bearing capacity.
This paper will deal with a description of the models and the preparation of them together with the followed test procedure. The test results on force and heave obtained during the first penetration are presented and compared with results of calculations done with the program PIAPA. The calculation model on which the program is based is also described.

2 Description of the PIAPA program
The PIAPA program was developed specially for skirt penetration as it occurs during installation of an offshore platform. Therefore the capabilities of the program incorporate the calculation of the influence of different skirts sizes, etc. In the tests described in this paper there was only one compartment.

2.1 Calculation of penetration force
The total penetration force is calculated by adding the tip resistance force to the friction forces that develop at the sides of the skirt. The calculation of the tip force is based on the bearing capacity formula for purely cohesive materials.qt = Net * (1) Cu +q
Where qt is the local tip resistance, Nd the tip bearing capacity factor, c.. the undrained shear strength (cohesion) and q the overburden stress. The total platform tip resistance force is obtained by integration over the skirt tip area. The difference between PIAPA and most other programs is that account is taken of the extra vertical stress due to heave and friction. PIAPA enables the input of arbitrary cohesion, adhesion and weight profiles of the soil.