Gravity Based Structures – often referred to as GBS, are support structures which are held in place by gravity. A common application for a GBS is an offshore oil platform, and some of the earliest offshore structures use this technology.
The act of decommissioning an oilfield facility such as an offshore platform involves the safe plugging of the oil well underwater, and deconstruction and disposal of the platform structure and other equipment used in offshore oil production.
Decommissioning offshore oil and gas platforms presents inherent environmental and socio-economic impacts for which there is no single solution. Older structures such as GBS cannot be decommissioned effectively without appropriate consideration of the platform’s dynamic behavior during removal.
The decommissioning of GBS structures is a complex undertaking, due to their interconnected components. This component includes storage cells, concrete legs etc.
The chart below highlights the sections of a GBS structure that are considered in a complete removal project.
To protect the environment, the decommissioning process for GBS structures is best done via a systematic procedure to avoid contamination and accidents.This sequence is as follows;
STAGE ONE: INSPECTION
The integrity of the overall structure must be assessed. The major areas to look out for include drill cuttings accumulated on the storage cells and the description of the cell content. This operation can be carried out by an ROV (remotely-operated vehicle) or AUV (autonomous underwater vehicle)
STAGE TWO: REMOVAL OF CUTTINGS
Based on collected inspection results, an engagement plan for cutting removal is put in place.
It is important to know that this operation must be performed without compromising the integrity of the cells or initiating cracks on the wall.
A possible solution to remove drill cuttings will be to introduce a suction dredger with the sole purpose of safely removing the cuttings and storing them in the barge. The dredger performs this operation by water-jetting at a low pressure to add seawater to the drill cuttings, creating slurry that will increase the efficiency of cutting removal.
This is done with a careful approach to avoid turbulence that may affect the underwater visibility. A secondary inspection is then performed to confirm the removal
STAGE THREE: STORAGE CELL CONTENTS REMOVAL
The concrete storage cells used for oil storage may contain some oil sediments that could be harmful to the marine environment if dispersed. The GBS oil storage cells are expected to be comprised of the following;
- Attic oil: a small volume of crude oil trapped in the top of each cell above the oil export line.
- Interphase material: an emulsion of oil in water with some sediment particles, laying beneath the attic oil
- Water: a mixture of seawater and produced water
- Sediment: a viscous mixture comprising of water, oil and sand particles
In order to remove the attic oil located inside the storage cells, a hole should be drilled. This operation requires a systematic approach as follows:
- Step 1: Installation of the location plate: This device is installed on top of storage cells to provide a firm grip for the drilling tool during the drilling operation. This is to avoid undesired cracks around the hole being drilled.
- Step 2: Drilling the core and installation of the anchor hub: The actual drilling operation will be performed using the drilling tool which will then be replaced by the anchor hub which mechanically locks into the walls of the core hole and provides the fittings for subsequent tools.
- Step 3: Installation of a double block and valve assembly: This component effectively seals the cells.
- Step 4: Actual drilling of cell dome to attic oil: A second drill is then being introduced to perform the actual drilling of the cell dome down to the attic oil space.
- Step 5: Installation of Transfer hose: once the drilling operation is completed, a hose designed to transfer the oil from the cells to a floating tanker. The operation should be performed efficiently with the aid of a pump tool.
STAGE FOUR: CELL CONTENT INTEGRITY MANAGEMENT
After the attic oil has been successfully transferred, an ROV engages in the completion phase of the cell operation. The isolation valve is then closed, and the transfer hose is flushed with seawater into the storage cells.
The transfer hose will be disconnected and removed along with the pump tool. A retrievable mechanical plug is then set through the valve block into the hub. This is tested again, and a debris cap lock is set onto the anchor hub to keep it safe from contamination.
STAGE FIVE: EXCAVATION OF THE GBS FOUNDATION
To excavate the GBS foundation, a strategic approach also needs be adopted. A work-class ROV will first excavate the GBS base after the cell contents have been removed. The structure will be refloated and towed back to shore for dismantling. A systematic approach for this operation is highlighted below:
- Step 1: Removing the conductors: The GBS structure is expected to be heavier than the original float-out, resulting from additions of conductors and sediments. Therefore, the removal of these conductors is necessary to reduce the weight. This will be done by a diver/ROV
- Step 2: Sealing all penetration: A diver or ROV will seal all penetration areas to prevent the water flooding into the structure during the refloating operation. Thus, the GBS should be free of cracks and external penetrations.
- Step 3: Installation of new control system: A new system would be required to monitor pressure in the cells, monitor and control the break out of GBS and monitor and control its draught and orientation during the tow to the shore. These systems are fitted onto various parts of the GBS using an ROV.
- Step 4: Ballast control system: All the water should be removed from the cells and legs using the hose fitted to an ROV to achieve the required buoyancy. The hose would displace the water with are at a high-pressure.
- Step 5: Equipment to break the under-base suction: The space between the base of the GBS and the underlying seabed should be pressurized to move the GBS and its skirts from the seabed.
A zip-trench tool is attached to an ROV where the water is injected under high pressure through the tool under all the base compartments. The trencher excavates the sand, gravel and muds that hold the GBS firm.
Once the base of the GBS is cleaned and ready to float, rock-dumping should be reinforced around each of the GBS to prevent high pressure water escaping.
- Step 6: Refloat and tow the GBS: As the water in the cells and leg is displaced by high pressure air and sealed, a combination of buoyancy forces and under-base water pressure would lift the GBS from the seabed and it will rise to the seabed. Once the structure is stable on the surface, it is towed near the shore site for the full dismantling process.