A Case Study of Using Very Large Skirt Can for Self-Installing Platform Foundations
Authors: Alessandrini V. , Lebois G. 
Published: October 2008

The 35,000 tonne BP Shah Deniz Alpha offshore platform in the Caspian Sea (Azerbaijan) is a Technip TPG500 jack-up style permanently installed facility utilising very large (30m diameter by 12.3m-high) and heavy (1350 tonne each) "skirt cans" as foundations which was installed in April 2006 (see Fig. 1).

Fig. 1: BP Shah Deniz Platform using very large skirt cans (available in full paper)

The specific Project context brought with it a new set of technical challenges for the foundation design. The selection of the TPG500 concept was influenced by the fact that the Shah Deniz Project was running in parallel with the multiplatform ACG development, and construction & marine resources in the Caspian would be stretched. Two of the beneficial features of the TPG500 concept are that it lends to being fabricated in component parts, which can then be brought together for final assembly, which is what was done (components parts in Singapore and final assembly in Baku) and that it does not require heavy lifts or floatover operations offshore.

At the offshore platform location very particular soil conditions were found, i.e. dense sand with a weak clay layer of poor geotechnical characteristics between -4m and -6m, requiring foundations skirts to penetrate into the soil to a depth of 8m instead of classical spud cans (see Fig. 2). In order to provide sufficient strength and safety factors for all conditions of the fixed platform life, a diameter of 30m was found necessary.

Fig. 2: Particular soil conditions due to very soft clay (available in full paper)

Introduction

This paper describes the unique technical challenges of foundation design faced during this Project throughout all the Project phases (Engineering, Construction & Installation):InSitu design: geotechnical & structural design, long term settlement constraint, soil/structure interaction, impact on overall platform behaviour / structural integrity (fixity, stiffness, fatigue), the governing installation load case due to touch-down impact forces, earthquake aspect & conductor interactionFabrication & Mating: The cradle transportation phase, Leg/Foundation connection principles, the concept of a controlled submersion & mating by pendulum concept developed together with associated systems.The control of skirt penetration during the final platform installation phase with challenges such as: soil resistance curves, controlled "punch-through" of the weak clay layer, geotechnical issues, associated procedures and systems overview. Due to pre-installed template and pre-drilled wells, platform installation tolerance was +/-0.5m and +/-1°.

The 30m diameter by 12.3m-high skirt can foundations comprise (see Fig. 3):a lower skirted part of 8.75m depth fabricated from 30mm thick corrugated plate which penetrates into the soilan upper part -called the spud can- which is 3.55m-high and extends above the soil seabed soil after platform installation. This part consists of a stiffened tank plate structure and is connected to the platform leg. It is also fitted with internal watertight bulkheads in order to provide 7 tanks for ballasting/deballasting purposes.

Fig. 3: Skirt can view

The paper will also highlight the contingencies and redundancies that were considered necessary in order to provide the sufficient level of confidence to all concerned parties.