BOP and Wellhead Protection/Subsea Arctic Caisson

ABSTRACT This paper describes the equipment and procedures for economic placement of a 20-foot diameter caisson in the Arctic sea floor for the purpose of placing the well head and Blow Out Preventer (BOP) stack below the sea floor to avoid ice damage. Reverse circulation induced by air lift and a hydraulic-motor-driven, large-diameter bit are used to drill a 20-foot hole. The caisson sections are made of heavy corrugated steel. The drill string serves as the running tool for installation of prefabricated caisson sections which follow the bit down hole and upon reaching desired depth are released and the bit retrieved. A Variably Buoyant Guide base (VBG) is installed inside the caisson. This VBG will:provide a large load-bearing surface,minimize foundation loading by deballasting and supporting casing and well head loads with its buoyancy,protect permafrost from thermal inversion from sea water by means of an external jacket through which refrigerated brine is circulated. The riser is used for running the VBG and remains in place to drill the 36-inch hole, thus allowing cuttings recovery and drilling fluid control. The VBG is deballasted upon landing the 3D-inch casing. Deballasting supplies a support for the 30-inch casing in the event of cement bond failure from permafrost deterioration. INTRODUCTION The advance into offshore Arctic drilling has presented problems previously not encountered in conventional offshore drilling in warmer latitudes. The problems encountered include scouring of the ocean floor due to ice pressure ridges and deterioration of permafrost when penetrated. The scouring problem has forced the well head and well control equipment to be sunk to a safe level below the mud line with the aid of a large-diameter hole cased with a segmented 20-foot-diameter caisson. The permafrost must be disturbed as little as possible to allow good hole definition and cement bonding, which has led to the development of programs for refrigerated or chilled drilling fluid and special arctic cement. Well heads in the Canadian Beaufort Sea have been protected by drilling large "Glory Holes" in the sea floor so that the Blow Out Preventer stack can be place below the point of grounding of an ice ridge. This method requires the drilling of several large diameter holes in a cluster. The sea water melts down the permafrost barrier over a period of several months time. This process allows enough time for a large hole to be cleaned out by means of an airlift dredge. This procedure is time consuming and requires several months delay between preparing the "Glory Hole" and the spudding of the well. In developing this design, the major goal was to devise a system for installing the Blow Out Preventer stack beneath the sea floor in the variety of bottom soil conditions expected while using a minimum amount of rig time and additional equipment.