Subsea Drilling Machine

Overview

Geoprober has developed a subsea drilling machine in response to a challenge presented by Shell to devise a method of gaining access to the large concrete storage tanks beneath Shell’s Brent Delta platform. The Brent platform is one of three gravity based structures installed on the Brent field which is located 100km North East of the Shetland islands.

Challenge

In the Brent field there are 64 gravity based cells, 16 of which are beneath the Brent Delta platform. The cells are at 4 bar lower pressure than the surrounding sea water and are 60 meters high with a 20 meter diameter. Over their lifetime the cells have been used for a number of purposes including the storage of export crude oil. As part of the abandonment program, it is expected that the access holes drilled by Geoprober will be used to conduct sampling and, if required, remediation of the cell contents.

Solution

The Subsea Drilling Machine was developed as a bespoke solution and consists of two main components, the base plate and the drilling stack. The base plate will be pre-installed with diver assistance on top of the target cells. Within the base plates are sealing arrangements that ensure once access is gained to the cell, a double barrier can be maintained between the external environment and the cell contents at all times.

Once the base plates are installed on the target cells, the drilling stack is run from a vessel onto guide posts of the selected base plate. The electro-hydraulic control system on the drilling stack allows dogs on the drilling stack connector to be actuated and a dual seal with the base plate to be engaged.

Within the drilling stack is a base plate connector, annular preventer, annular seal, reciprocating gripper and a drilling assembly. The stack is connected to surface via a 3000 V umbilical with 8 fibre optic lines. The electro-hydraulic control system consists of a control pod, three smart valve packs, a junction box and an electric motor driving two hydraulic motors.

Once the drilling stack is landed out, the drilling motor is then started and the reciprocating gripper pushes the torque tube with the attached core barrel and drill bit into the concrete cell top to start cutting. Once the gripper has traveled the full cylinder length, the annular preventer grips the torque tube while the grippers relax and the gripper cylinders travel back up before repeating the process. This is continued until the core bit has broken through into the cell.