A System for Offshore Drilling in the Arctic Islands

Introduction

Seven major gas fields have been discovered in the Canadian Arctic Islands since exploration began in 1961. All the fields extend offshore, and seismic surveys indicate many prospective structures entirely offshore. A method has been developed for drilling these offshore structures by artificially thickening the ocean ice to support conventional land exploration rigs. Five wells have been drilled in water depths ranging from 200 to 905 ft Currently, work is under way to drill a well in 1,200 ft of water 20 miles offshore.

Rig Description

Three delineation wells and a wildcat well have been drilled with a Cardwell Model A-150 A converted Sky-horse trailer rig. It is designed to be transported by helicopter in 4,000-lb sling loads and is rated to 6,000 ft with 3 1/2 in. drillpipe. Total weight of this rig is 500 tons with full mud tanks, including the riser and hook load. Design ice-platform thickness for this rig is 12 ft.

A second rig was used on an ice platform during the 1975-76 drilling season. The rig is a Helihoist 1500 rated to 15,000 ft with 4 1/2 in. drillpipe. Its total weight was decreased to 845 tons, including riser and hook load, by removing some mud- and water-storage tanks. Design ice-platform thickness for this rig is 14 ft.

Ice Drilling Platforms

The limiting design consideration is total vertical deflection of the ice platform during drilling since flooding of the working area would make drilling difficult or impossible. A platform with adequate thickness, lateral dimensions, and strength to support the rigs safely and to provide a dry working area is required. To obtain the provide a dry working area is required. To obtain the required ice thickness, the natural ocean ice is thickened by flooding with water pumped from beneath the ice and freezing in thin layers.

The ice platforms are treated as flexural members with transverse loading only. Formulas applying to elastic, homogeneous, and isotropic plates on elastic foundations are used to calculate the stresses and deflections. Since ice is weaker in tension than in compression, the extreme fiber flexural tensile stress in the ice is limited to 50 psi or less. For the calculations, the ice elastic-modulus value obtained from field measurements is 600,000 psi.

To prevent flooding of the work area, creep deflections have to be typically less than 2 ft. The long-term deflections are calculated using a reduced elastic modulus of 25,000 psi. Stresses and deflections are also calculated for the unlikely event that the ice becomes completely separated along a line parallel to or across the rigs. Even in this case, reducing the loadings somewhat will assure the safety of the rig, prevent major flooding, and allow drilling to continue.

Construction of the Platforms

Sites Selected. The ice present at the start of construction at all sites, except for Jackson Bay G-16, was multiyear ice averaging 5 to 8 ft thick and with a rough surface topography.

Flooding. Flooding of the platforms starts in late November using electric submersible pumps placed in electrically heated wells.

JPT

P. 300