DRILLING THE WELL
The drilling location is generally indicated by a survey vessel, using a marker buoy, prior to the arrival of the drilling vessel. The rig is towed onto the location and anchor-handling tugs are used to drop the anchors in a pre-scribed pattern. The anchors are tensioned to ensure that they are securely set into the seabed,
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then slacked off and adjusted to obtain the final position and heading of the rig. This whole operation may take a few hours or a few days, depending on weather conditions. The drilling rig may be held in position over the well by using anchors or by using dynamic positioning techniques. If anchors are used, great care must be taken to ensure that the anchors do not damage seabed pipelines.
The condition of the seabed directly beneath the rig will generally have been checked by a seabed survey before the rig arrived on location, but a final check is generally made with an ROV prior to running the equipment.
1.2 Running the Temporary Guide Base (TGB)
The first stage in the drilling operation is to establish an anchor point, on the seabed, for the 4 guidelines (3/4" or 7/8" diameter wire) which are used to guide drilling tools and casing from the rig to the seabed. The guidelines are attached to a Temporary Guide Base — TGB which is the first piece of equipment to be lowered to the seabed. The guidelines are attached to the base at a 6ft radius from the centre and are kept in tension.
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The TGB is positioned in the moonpool of the rig and a special running tool, run on drillpipe, (Figure 3) is latched into the base. The running tool has 4 pins which engage J-slots on the internal profile of the 46” slot. Sacks of barite or cement are loaded onto the base, to increase its weight to 25000-30000 lbs, and it is lowered to the seabed on drillpipe. When the TGB has landed on the seabed the running tool is unlatched by rotating the drillpipe by 1/8 of a turn to the right. The running tool and drillpipe can then be retrieved. A level indicator (bull’s eye) on the TGB indicates whether or not the structure is lying in a horizontal position on the seabed. If the TGB is level the tension on each guideline is then adjusted to about 2000 lbs.
A 36" hole is drilled to a depth of 100-200ft. below the seabed. The bit is guided down through the TGB by means of a Utility Guide Frame (UGF) fixed around the drillpipe just above the bit and attached to the guide wires (Figure 4). Once the bit has been guided through the TGB and the first 30ft. of hole has been drilled the UGF is pulled back to surface.
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The 36" hole may be drilled using an 18V2" bit and 36" hole opener, or a pilot hole may be drilled and opened out to 36" diameter on a second run. The hole is drilled with sea water, with the drilled cuttings settling onto the seabed (no riser or BOP is installed at this stage). Having drilled to the required depth the hole is displaced to mud to prevent debris from settling onto the bottom of the hole when running the 30" casing.
Figure 4 Running the Drillbit to Drill the 36" Hole