Geotechnical Considerations for the Installation of Gravity Base Structures
Authors: O&apos;Riordan N.J. , Clare D.G. 
Published: October 2008

The installation of a Gravity Base structure involves interaction between the disciplines of naval architecture, structural and mechanical engineering and geotechnics. This paper draws attention to key factors that can affect the performance of the foundation system in service. Careful examination of the installation process by the geotechnical engineer and integration with other elements of the platform design and construction is necessary.

INTRODUCTION

The API and DnV code requirements for foundation design of Gravity Base structures concentrate on the measures needed to ensure stability and serviceability once the platform is in place. In contrast, the transition from a floating structure during tow-out to a structure keyed into the seabed is left largely in the hands of the designer.

If the structure and foundation system are not adequately considered through all the stages of installation the long term performance of this structure will be impaired. Indeed the code requirements demonstrated in the design may be compromised if the installation sequence is not carefully analyzed.

In this paper, the installation of a simple platform of straightforward geometry is considered. This paper is intended to augment existing work in this area (Eide et aI, 1979). The site and structure parameters are deliberately chosen to focus on key stages during the installation. For example, the platform has skirts, it is to be installed within a water depth equal to its base width, in a situation where seabed scour can occur. It is assumed that prior to installation, the seabed has been surveyed and any large objects or drilling arising removed, in preparation for installation.

The principal stages to be considered are as follows:parallel installationinclined installation and control of weight and penetrationpenetration rate: prevention ofliquefaction scour during descent and at installation.

PARALLEL INSTALLATION

The majority of gravity structures has been installed using parallel installation techniques, as illustrated in Figure 1. The structure has been towed to its location, held by tugs; closed cells are flooded with water symmetrically about the platform's vertical axis and the platform descends onto the seabed.

The penetration of the soil is near-vertical and Lune and st John (1979) give a relationship for skirt penetration resistance, It is important to recognize that these parameters are associated with skirt penetration rates of around 0.004 to 0.019 cm/s and qc measured at 2 cm/s. Although clean sands and gravels are not sensitive to rate of shearing, clays can be very sensitive and allowance must be made if there are significant departures from the above penetration rates. To illustrate this, Figure 2 is taken from work by Dayan and Allen (1975). The bearing capacity number N given in Figure 2 is equal to kt.qc(d)/su(d) where su(d) is the untrained vane strength of the soil. Rate effects decrease with decreasing liquidity index, and increasing OCR. In addition to rate effects, the designer must ensure, as with plugging of open-ended piles, that the failure mode associated with the highest penetration resistance has been selected. This is illustrated in Figure 3 and is particularly important for inclined installation.