Caisson Retained Island for Canadian Beaufort Sea-Geotechnical Design and Construction Considerations
Authors: Mancini C.V. , Dowse S.E.W. , Chevallier Jean Marie 
Published: October 2008

This paper reviews the development of the caisson retained island (CRI) concept for exploratory drilling in the Canadian Beaufort Sea and outlines the geotechnical design considerations for CRI construction.

Since 1973 Esso Resources canada has constructed sandbag retained and sacrificial beach type islands as drilling platforms. But the use of these islands is limited to relatively shallow water or areas with abundant borrow material. As exploration activity moved into areas where borrow material is scarce, economics favoured development of the CRI.

During 1982 the first Esso caisson was fabricated in Japan and transported to Tuktoyaktuk in the Beaufort Sea where it currently awaits deployment in 1983. As the caisson arrived in the Beaufort, construction of the CRI berm was nearing completion at the Kadluk location.

In geotechnical terms, the CRI design must ensure adequacy of the berm and the total structure. The major considerations for design include: the evaluation of stability under gravity and ice loads estimates of deformation due to seabed and berm settlement and the effect of filling rate for the CRI core on the differential water head relative to sea level.

INTRODUCTION

Oil exploration in the shallow waters of the Canadian Beaufort Sea has relied on the use of temporary artificial islands as drilling platforms since 1973. The islands are designed to resist ice forces and to minimize the erosional effects of summer storm waves. The ability to use islands throughout the winter is a major advantage over the limited drilling season of conventional drill ships and jack-up platforms. Thus, completion of the drilling program and full evaluation and testing of the well can be achieved in one season. Furthermore, delineation wells through directional drilling on the same island are possible at greatly reduced costs.

The principle types of islands constructed to date include the sandbag retained and the sacrificial beach types.

The sand bag retained island uses a berm of sandbags to minimize the volume of fill required in areas where sand borrow is scarce. It has been used effectively in relatively shallow water, up to 7 m depth. The sacrificial beach island is characterized by long gradual beaches and underwater slopes of about 1: 15. The beaches force storm waves to break and, thereby, dissipate their energy before reaching the island proper. In winter, the sacrificial beach promotes ice sheet failure which develops into a protective rubble field around the island. Secondary slope protection is provided by sandbags and filter cloth on the beaches and around the drilling surface.

Issungnak, located as shown in Figure 1, is the largest sacrificial beach island constructed to date and required two seasons to complete. Located in 19 m water depth, the island required 4 million cubic metres of hydraulic fill material. Following a successful drilling program in the winter of 1979, further construction in 1980 to permit the drilling of a directional well increased the total island volume to 4.9 million cubic metres.