There are 28 pipelines and umbilicals (including one power cable) to be decommissioned at Brent. They measure approximately 103km in length, with the shortest being just 0.03km and the longest 35.9km. They are grouped into four different categories:
Rigid pipelines are generally used to transport oil or gas to shore. They are usually made of steel, coated in a layer of concrete to provide stability. By the very nature of these materials, the pipelines are rigid.
Flexible pipelines are made of steel, plastic or various composite materials. They have small or medium diameters and are mostly used for shorter lengths of pipeline.
Umbilicals are typically used to inject chemicals into, or transmit control signals to, the subsea production system. They are usually of small diameter and pliable structure, similar to those of flexible pipelines.
Power cables provide electricity for the subsea system. They are similar to umbilicals but contain large electrical cores.
What do the regulations say about pipeline decommissioning?
The main legislation in the UK covering pipeline decommissioning is the Petroleum Act 1998 and the Pipeline Safety Regulations 1996. There are no international guidelines on pipeline decommissioning.
The UK Department for Business, Energy and Industrial Strategy (BEIS) has issued Guidance Notes on Decommissioning which include the decommissioning of pipelines. These require each pipeline to be assessed individually and you will be able to find full details of the recommendations for each of the Brent pipelines in the Pipelines Technical Document when the Decommissioning Programme for the field is submitted. If a pipeline cannot be re-used, all feasible decommissioning options should be considered and a Comparative Assessment (CA) made to arrive at the recommended option. Every pipeline in the Brent Field will therefore be subject to an individual CA.
Each pipeline will be flushed
As part of the decommissioning process, the pipelines will be flushed and pigged (the process of passing a mechanical device through a pipeline to scrape the inner surfaces) where appropriate, to remove any remaining oil, gas, wax and other contaminants. The waste will be collected at the receiving platform in tanks and transported to shore for treatment and disposal.
What are the decommissioning options for pipelines?
We have identified 14 pipelines where the decommissioning options are simple and straightforward, and the solution more obvious based on the BEIS Guidance Notes. The relatively fewer, simple decommissioning options for these lines were compared and assessed using a qualitative or narrative-based method. For simplicity, we refer to these as “qualitative” pipelines.
For the other 14 pipelines, the options are more numerous and complex, with no preferred decommissioning option being immediately apparent. We have called these “quantitative” pipelines since we subjected each of them to comparison and assessment using a quantitative method. This approach is consistent with the expectations of the regulator.
For qualitative pipelines there are two groups of decommissioning options:
- Leave the pipeline in place in its existing trench or under the existing rock dump, with any protruding end removed and/or covered with rock to reduce the risk of snagging by vessels; or
- Remove the line completely, either by reverse installation or cut and lift.
For quantitative pipelines we identified nine technically feasible options, although not every option was applicable to each pipeline. These include:
- Leave tied in at both ends;
- Leave tied in at platforms, trench and backfill remote end;
- Leave tied in at platform, rockdump remote end;
- Trench and backfill whole length;
- Rockdump whole length;
- Recover whole length by cut and lift;
- Recover whole length by reverse S-lay (where the pipeline is taken aboard a vessel and cut into smaller sections for transportation to shore);
- Partial trench, backfill and isolated rockdump; and
- Partial rockdump
Comparative assessments and many studies
We have carried out a separate CA of each of the 28 Brent pipelines. Each assessment considered the construction, condition and position of the pipeline and the feasible decommissioning options.
In accordance with BEIS guidelines we examined each decommissioning option in terms of five criteria: technical feasibility, effects on society, environmental impact, risk to safety, and cost.
The CAs were informed by a number of studies that were either carried out by ourselves or commissioned from independent experts, for example: Anatec, which carried out a study of safety risk to other users of the sea; Mackay Consultants, which looked at the socioeconomic effects on leaving equipment behind on fisheries; and Atkins, which worked on pipeline degradation. These included a pipeline degradation assessment, an assessment of the safety risk to fishermen, a soil and trenching analysis, a rock-dumping assessment, and an environmental impact assessment.
What do our comparative assessments recommend?
Most decommissioning options involve a balanced judgement between two or more BEIS criteria. In the case of the pipelines, most of the balanced judgements are between technical feasibility and/or cost on the one hand and the long-term safety risk/long-term environmental impact on the other.
The 14 simpler, qualitative pipelines include all the flexible lines, umbilicals and the electrical cable, as well as four rigid pipelines. Our recommended options are to remove nine of these pipelines and leave five in place - in one case under rocks.
The 14 more complex pipelines are all rigid. We recommend that 13 of these remain in place as is on the seabed, with the other one to be left on the seabed after being retrenched.
All the lines included in the Decommissioning Programme are intra-field pipelines, and critical infrastructure in the form of the main gas and oil export pipelines will remain. Bypass plans have been put in place to support production from other fields post decommissioning.
Are the pipelines a risk?
An important criterion for all the decommissioning options is the level of risk they present to other users of the sea, especially fishermen.
There is currently a 500 metre exclusion zone around each Brent platform, which will continue to apply after decommissioning is completed. The platform legs and the pipelines will be clearly marked on all relevant sea charts, and will be logged in the FishSAFE database, which warns fishermen about snagging hazards in UK waters.
Currently, the exposed sections of pipelines are crossed regularly and safely by bottom-trawling fishing gear. To assess the risk of whether pipelines could present a potential snagging hazard when they degrade, we commissioned two studies: one to quantify the extent of fishing along each Brent pipeline and the speed and angle of the vessels crossing it, and the other to assess the long-term degradation of each pipeline.
These studies have informed our CA and recommended options. We have held frequent meetings with fishing and marine authorities and fishermen’s associations to present our findings and receive their input.
Who is responsible for Brent after decommissioning?
Shell U.K. Limited and Esso Exploration and Production UK Limited are the joint equal owners of the Brent field. Under current legislation both companies will be responsible for inspecting and monitoring the pipelines after decommissioning.