The Single Steel Drilling Caisson: A Novel Approach to Bottom-Founded Structures in Arctic Waters
- John Fitzpatrick (Dome Petroleum Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 5-8 October, San Francisco, California
- Publication Date
- Document Type
- Conference Paper
- 1983. Society of Petroleum Engineers
- 4.3.4 Scale, 4.2.3 Materials and Corrosion, 5.1.1 Exploration, Development, Structural Geology, 4.1.2 Separation and Treating, 1.10 Drilling Equipment, 4.5 Offshore Facilities and Subsea Systems, 1.2.3 Rock properties, 5.1.2 Faults and Fracture Characterisation, 2.4.3 Sand/Solids Control, 4.1.9 Tanks and storage systems, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 4.2 Pipelines, Flowlines and Risers
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In 1982, a mobile, bottom founded steel caisson drilling system, was conceived, constructed and installed on an underwater sand berm in 31 m of water, in the active shear zone of the Canadian Beaufort Sea. Discussed below are the principal design criteria, the ice/structure/foundation interaction, the construction and installation, and the post set down stress analysis of the steel caisson.
In the summer of 1981, Canmar, a subsidiary of Dome Petroleum, successfully completed a concrete caisson retained Petroleum, successfully completed a concrete caisson retained island in 21 meters of water in the Canadian Beaufort. The use of concrete caissons reduced significantly the sand volume which would otherwise have been required. Also, they minimized the effects of wave and current erosion during the open water season and provided a stable platform for the rig system at an elevation of plus 8 m. In addition the feasibility of constructing underwater side slopes of 1:5 was definitely established.
As a result of the experience gained through the design, installation and operation of the multi-unit concrete caisson system, Canmar developed a new concept for year round drilling from a subsea berm. The system, called the Single Steel Drilling Caisson (SSDC), offers several operational improvements to conventional islands. The principal advantages are:
1. The system allows permanent installation and hook up of the drilling rig package, and mobilization is much simpler, quicker and safer.
2. The high freeboard of 17m gives greater protection from sea spray and ice ride up.
3. The elevation of the top of the submerged berm at -9 metres results in an improved situation with respect to berm stability and erosion protection (fig 1).
The SSDC was fabricated by modifying the forward half of a 231,150 D.W.T. Very Large Crude Carrier (VLCC). The principal dimensions of the finished system are:
Length: 202 m Breadth: 53 m Depth: 25 m
The lightweight displacement of the SSDC is about 42,000 tonnes which consists of the following quantities:
Existing Steel 18000 tonnes New Ice Strengthening Steel 7000 tonnes New Topside Steel and Fuel Tanks 4000 tonnes New Ice Strengthening Concrete 13000 tonnes TOTAL LIGHTWEIGHT 42000 tonnes
The SSDC, when installed on a subsea berm and fully ballasted, has an effective ground contact force of 175,000 tonnes. It is this contact force together with friction that provides a reaction to the horizontal ice loads that are applied to the side of the structure. The system, in simple terms, has to contend with two basic forces.
i) A gravity load of 175,000 tonnes.
ii) A horizontal load of 175,000 x tan 30 = 100,000 tonnes, where 30 is the angle of friction of the foundation soil.
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