Darran Pledger, STATS Group, UK, explores how combining equipment can lead to safe and efficient isolations for subsea pipeline repair and maintenance.
T
he global oil and gas industry is reliant on the long term dependability of pipelines. Pipelines are vital to the safe and efficient transportation of hydrocarbons and failures of these pipelines can have huge energy supply, environmental, safety and reputational implications. As the global demand for energy increases, so do the consequences of any potential pipeline failure. In many instances, pipeline operators encounter difficulties when attempting to isolate and depressurise a section of their pipeline to facilitate repair or maintenance activities, especially when suitably located valves are not available or are not functioning correctly. Often it is the isolation valves themselves that need to be replaced, repaired, or retrofitted. There are many reasons for removing a section of subsea pipeline. Sometimes a degraded or corroded section needs to be removed to increase the operating life of a pipeline. Other reasons for compromised pipeline integrity could be dents or gouging from external mechanical damage or highly stressed deformations. Although, more often the situation is that even well managed
pipelines with good integrity need to be isolated and have sections removed, to allow infrastructural reconfiguration, as offshore facilities are being decommissioned and removed. When a pipeline section must be removed, it may initially be assumed that to conduct the intervention with as low risk as possible, the entire pipeline needs to be fully depressurised, thus removing any pressure threat, and that the pipeline needs to be flushed and flooded to displace all hydrocarbons, so removing any environmental impact. However, in many cases achieving the lowest possible risk is not reasonably practicable; for instance, when the pipeline has many assets producing into it, or when venting off the line pressure, flooding the pipeline and then dewatering and drying the line, would lead to a production outage of many months and large discharges to the atmosphere or environment. If it turns out that it is not reasonably practicable to depressurise and hydrocarbon-free the pipeline, then it may be necessary to break containment with the pipeline pressurised and full of its production inventory. In such a scenario a safe, pressure capable
35
