Drilling From Deepwater, Concrete Gravity Platforms
Authors: Cobbett James S. 
Published: December 2007

This paper discusses the operation of three different types of gravity platforms in the North Sea. When the Brent, Cormorant, and Dunlin fields platforms in the North Sea. When the Brent, Cormorant, and Dunlin fields were discovered, drilling required the use of a completely untried type of platform. Various design problems and their solutions are described. platform. Various design problems and their solutions are described.

Introduction

When oil exploration rigs first drilled in the northern North Sea, many companies knew that if oil was found, no established production method existed in such hostile waters. The announcement in Aug. 1972 by Shell U.K. Exploration and Production Ltd. of the discovery of the Brent Field heralded one of the most expensive and innovative oilfield development programs ever seen.

In the following year, Shell announced the discovery of two more fields in the far north, Cormorant and Dunlin. For these three fields, six platforms were ordered (summarized in Table 1). Five of these were of a type completely untried for drilling operations: gravity platforms. Unlike a conventional steel jacket, which platforms. Unlike a conventional steel jacket, which relies on piles driven deep into the seabed to keep it secure, a gravity platform relies mainly on its own weight contacting a large area of the seabed to prevent it from moving.

This paper discusses Shell's experience in operating three different types of gravity platforms (Fig. 1). (Shell Expro operates in the North Sea on behalf of Shell and Esso in a 50/50 partnership covering all areas where joint acreage is held).

Because economic incentives to start exploiting the oil as soon as possible were great, there was no time to think out every detail of platform design before construction began. Simultaneous design and construction became the order of the day. When drilling engineers began to look at the problems of drilling from a gravity structure, it soon was apparent that disturbing the platform foundation (the seabed) by drilling through it would weaken the structure itself. Avoiding undue disturbance of the seabed and near-surface sediments was a major concern. These gravity platforms were larger than any used before. The high derrick-floor elevation along with the great water depth could lead to difficulties in getting mud returns while drilling tophole - the second major concern. A third problem was to design the conductor and subsequent pipe strings to take wind and wave loads, either applied directly or through the platform. Fourth, each individual platform had its own individual problems resulting from differences in general layout and design.

Conductor String Design

The main criteria for the conductor strings wereto be strong enough to absorb environmental loads,to be strong enough to take vertical loads imposed by platform settlement,to be strong enough to be driven to refusal using the largest diesel hammer that could be used in a derrick (Delmag D-55, 144,000 ft-lbf),if possible, to be driven so deep that mud returns could be obtained at derrick-floor level without fracturing the formation at the conductor shoe when drilling out of the conductor, andto be large enough for a 20-, 13 3/8-, 9 5/8-, and 7-in. casing scheme.

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