Other infrastructure such as pipelines may also need to be removed as part of the decommissioning process.
Decommissioning options will be influenced by each individual development and external circumstances at the time of decommissioning and may include:
- finding an alternative use for part or all of the structure;
- recycling part or all of the structure;
- final disposal onshore of part or all of the structure;
- leaving the structure in place;
- toppling the structure on location; and
- disposing of the structure elsewhere at sea, such as an artificial reef or deep-sea disposal.
Decommissioning a petroleum production facility of any sort will always include plugging and abandoning the wells so that no fluids can escape after the site has been decommissioned.
The main objective of plugging and decommissioning a well is to avoid reservoir fluids (oil and gas) leaking to the environment.
This process requires containing the formation fluids (such as oil and gas) within the well and ensuring the correct structure or ‘plug’ is in place to restrict any movement of fluids. A series of barriers is placed inside the well at various depths determined by engineering criteria. These barriers ensure that the well integrity is maintained over time.
Well barriers have the capability to withstand the different loads they will be exposed to during the life of the well.
In plugging and abandoning a well, the objective is to have the determined barriers set in perpetuity.
Several techniques are used to install or set the barriers and different materials qualify for use in constructing these barriers.
The techniques include, but are not limited to, cement placement through pipes, plugs placement through wireline or pipe or cement squeezed into the formation. In recent times, resins have also been used more widely as a plugging material, and sometimes resin is combined with cement to create a flexible mixture.
A range of technical decommissioning and disposal options exists for offshore facilities, each of which must be evaluated on a case-by-case basis. The scope of consideration should include a broad range of aspects and not only focus on the vicinity of the structure.
Decommissioning options include:
- complete removal, disposal onshore or offshore
- partial removal, disposal onshore or offshore
- abandonment in-situ
- alternate use, either within the oil and gas industry or other industries / purposes.
The method of removing and disposing of a structure depends on factors such as the type of construction, size, distance to shore, weather conditions, the complexity of the removal operation and the environmental impact. It must also consider the safety of workers.