High capital expenditure, and lifelong financial and other risks associated with pipelines can make it important to contain or reduce operating expenditure while keeping revenue-earning oil and gas flowing. Zero downtime is important therefore, and ‘live repairs’ enable this. They can improve cost efficiency while maintaining acceptable standards, reliability and safety levels.
“Live repair can eliminate or reduce disruption or financial liabilities that could otherwise result from removing and replacing pipes,” observed Asle Venås, global segment director for pipelines, DNV GL - Oil & Gas.
“Delivery disruption can have large consequences if a pipeline operator has a contractual obligation to keep gas flowing. Shutting down supply to a liquefied natural gas facility could cost such a plant around a hundred million dollars, for example.”
Live repair techniques
DNV GL is a technology leader for live repair. It has lately been involved in many, using various techniques such as welding, epoxy grouted sleeves, Grouted Tees, welded or mechanical sleeves, and pipeline clamps.
Epoxy grouted sleeves involve installing a steel sleeve around a non-leaking pipeline, then introducing a quick-drying end seal before injecting epoxy grout between sleeve and pipe. Grouted Tee is a similar technology but has a branch connection and seal enabling pipeline contents and high pressure to be contained. A single Grouted Tee can isolate a minor leak or encapsulate a damaged or deteriorated branch connection. Multiple Grouted Tees can provide a flow-stop and bypass arrangement to enable replacement of larger sections of pipeline.
Grouting is simple and does not require on-site welding, so full flow can be maintained during repair or intervention. Usage of Grouted Tees has expanded to include repairs on thin-walled and cast iron pipelines, and on subsea pipelines in up to 200 metres (m) of water depth.
“It is worth noting that the total cost of Grouted Tee connections is usually less than traditional, welded Tees at larger diameters, and they also benefit from a 40-year minimum design life compared with the 20 to 30 years that is common for a pipeline,” Venås said.
Design standards usually specify heavy wall pipe for sections subject to higher external forces, at road and rail crossings for example. Replacing standard with heavy wall pipe is very expensive if a pipeline has to be decommissioned during repairs, and could even be hazardous when flow is stopped while a bypass is welded into place to isolate a leaking section.
An epoxy grouted sleeve involves zero downtime, can address most onshore pipeline defects, and is one of few repairs suitable for defects in girth welds that join pipes end-to-end around their circumference.