The evolution of the Caisson Retained Island (CRI) from early articial islands made from native materials is described, and (he need for a retaining structure is developed. The design history including design requirements and criteria are discussed. The relationship between CRI and other Beaufort structures, both proposed and built is examined.
The design selected for construction is described, as well as the essential features of arrangement, structure, machinery, instrumentation and monitoring. Background philosophy and modes of operation are mentioned.
Concluding remarks focus on the significance of the structure in the achievement of the petroleum potential of the Beaufort area.
The Caisson Retained Island is a platform for an exploratory drilling rig. The platform is a structural system composed of three elements founded on the seabed. These elements are a sand berm or base, a steel retaining structure, and a sand core of retained fill. These interdependent elements are of equal Importance, but as the new element is the retaining structure this will be the subject of discussion.
In 1972 the first sand fill island in the Beaufort Sea was completed by Imperial Oil. Although very successful as a drilling platform, it was regarded as being extremely limited in its range of applicability, particularly in its range of possible water depts. Even though these limitations have since proved to be Imagmary, design activity was undertaken and a range ofconcepts for drilling platforms was investigated.
The advantages of structures that reduced the amount of fill required for an artificial island were recognized and ideas were focused on retaining structures. In 1976, in recognition of their innovative approach to the design problem, a contract was awarded to Albery, Pullerits, Dickson and Associates, to develop the design of a steel retaining structure. The result wasa design package ready for shipyard bidding that had been model-tested in both wave- and ice-test basins. This design was called Caisson Retained Island.
As the perceived limitations on sand island construction had not materialized, and islands were being successfully constructed, the design was shelved. In 1979 the Issungnak island sited in 20 metres of water, was completed only after extreme battle with the environment. Severe late season storms delayed completion of the top of the island and ice prevented normal rig mobilization. While the construction activity, at limes heroic, triumphed, permitting a discovery well to be drilled, it demonstrated several lessons:
dependence on late season weather permitting critical operations is extremely risky;
speed of emergence through the sea level interface is a decided advantage; and
wave action is as serious a design consideration in the Arctic as is ice.
As the CRI is specifically designed to counter these conditions, it was obvious that its time had come. The design was reevaluated, compared with suggested alternatives and found to have superior economic factors. In 1981 a construction contract was awarded for delivery in 1982, to meet a 1983 deployment.