INTRODUCTION

Most forms of subsea installations have, as a common feature, a requirement of physical contact directly or indirectly with the sea bed The nature of that sea bed greatly influences the design and installation technique adopted, and consequently the economics of the project Furthermore, incorrect assumptions about the sea bed can necessitate expensive remedial measures, since little flexibility is available to adjust the design at the tune of installation

Despite these factors, site-specific knowledge of the sea bed is often unavailable for subsea installations. Designs based on sod conditions at platform sites many kilometres away are not uncommon This chapter describes some of the influences that the geotechnical conditions can have on the design and operation of subsea installations, and demonstrates the desirability of the acquisition of site-specific soil data

FOUNDATION LOADS

For the purposes of classifying their basic foundation loads, subsea units can be divided into three categories

  • units which are set completely below sea-bed level,

  • units which are set at or slightly below sea-bed level,

  • units which float above the sea bed and are anchored to the sea bed

In each case the purpose of the foundation member is to support the loads Imposed by the unit and any external forces applied to the unit safely, effectively and at an acceptable deformation. The first stage in deciding on the type of foundation unit to be adopted (and hence on the soil data requirements) is to establish the magnitude of the loads for the various options under consideration

Fully recessed sea-bed units are currently only adopted where local conditions render surface units vulnerable to damage Typically, silos may be adopted where iceberg scouring of the sea bed can occur Such units, by their nature, do not attract high accidental lateral loads, and silos are generally able to support their own weight and that of the unit by external-skin friction very adequately They therefore provide their own foundation, and the only geotechnical areas for particular study are those governing their installation.

Surface units represent the vast majority of existing and planned subsea installations The first generation consisted of a wellhead with a rigid riser and a production tree affording well control. The next stage was the development of manifold units to permit collection from a number of satellite wells and to allow transportation to adjacent topside production facilities. Current and future developments include sea-bed separation units, thus making possible long-range sea-bed transportation and the use of modular 1 atm units serviced by submersible Unless requiring the special protection afforded by category (a) above, these units are all normally placed on the sea bed and require support by the sea bed.

Vertical loads generated by these units and their protection cages are generally much lower than the range of loads normally associated with platforms. Vertical components of foundation loading arise from

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