Stress corrosion cracking causes pipe failure
- The Castero 10 is trenching a pipeline north of the Lennox field in Liverpool Bay.
- A welding station on the firing line aboard the Castero 6 in Liverpool Bay.
- The lay vessel prepares 12-in. line for installation.
The Liverpool Bay Development represents a UKP 1.1 billion investment by operator BHP Petroleum and three partners - Lasmo, Monument, and PowerGen. From the outset, it was clear that development would present unique technical challenges - operating in shallow waters 24 km north of the Welsh coast.
In June last year, six months after oil first flowed from the Douglas field, a problem with the gas re-injection pipeline caused oil production to be halted from the neighboring Lennox field. "Managing the pipeline fault was a key issue for us," commented Liverpool Bay Asset Leader
Neil Ferguson, who joined the project at the start of the year said, "It's been a case of taking things one step at a time. We took time to assess what had happened, why it happened, and how to prevent it from happening again."
Field layout
The Liverpool Bay development comprises four fields - Douglas (oil), Lennox (oil and gas), Hamilton (gas), and Hamilton North (gas) - all yielding sour oil and gas with high levels of mercaptan. Lennox, Hamilton, and Hamilton North are served by unmanned satellite platforms linked by pipelines and communications systems to the central production facilities at Douglas.Hydrocarbon production from the three unmanned fields are sent to Douglas. The oil is processed and sent through a 17-km pipeline to a floating offshore storage installation The gas is dried and transported through a 34 km pipeline to the main gas processing terminal at Point of Ayr on the North Wales coast, and thereafter exported by onshore pipeline to PowerGen's new Connah's Quay Power Station at Deeside.
The Lennox gas re-injection pipeline runs from Douglas to Lennox. The line returns gas to the field for injection underground after it has been removed from the oil during processing. Immediately after a problem in the re-injection flow was recognized, the line was taken out of service.
Then, a remote-controlled underwater camera was used to survey the entire length of the Lennox pipeline. It showed pipeline faults confined to a 1.2-km section situated at the lowest point on its 32-km route between Lennox and Douglas. A dive team was mobilized to remove the sections of faulty line, which were taken to shore for detailed metallurgical analysis. Unused pipe was also examined and test welds undertaken.
Problem analysis
The results of the analysis showed that the combination of extreme pressures, high levels of hydrogen sulfide, water-saturated gas, and the properties of a section of the pipeline itself had resulted in sulfide stress corrosion cracking.Lengthy debate and further assessment then ensued as the team probed how best to remedy the problem. A decision was made to replace the entire pipeline. A pipeline team was formed to oversee the process, drawing together experts in decommissioning, radiographic analysis, dredging, pipelaying, welding, metallurgy, process engineering, and instrumentation.
Construction work began last October when European Marine Contractors was awarded the contract to replace the 12-in. diameter line. While the new pipeline was being installed by the contractor, the tie-ins were contracted to Coflexip Stena Offshore, which moved in after the sections of pipeline were re-laid to complete the task. Tie-ins at Lennox required the use of the LM Balder, an anchored barge with up to 24 air divers on board operating in carefully controlled diving teams.
The weather conditions in the Irish Sea during the pipeline re-laying were a constant source of concern. Excellent early progress was offset by bad weather during the winter months, Despite this, however, the tie-ins were completed in early April. The work included replacement of the 16-in. associated gas pipeline subsea barrier valve at Lennox.
With a water depth of only 7.5 meters at Lennox, at its lowest tide, access dredging was required to clear a channel for the lay barge to approach the area. The total volume dredged during pipeline re-laying was in excess of 500,000 cu meters. A rigorous welding development program was undertaken, which exceeded the contractors' normal standards in order to meet BHP's standards for strength and durability.
Pipe-laying finished on schedule in March of this year. Two months of commissioning work then followed before Lennox finally came back on stream on schedule at the end of May. It was less than 12 months since the problem had first been detected.
Initial output from the field was restricted to around 10,000 b/d of oil to allow further commissioning to take place on the Douglas production platform. As that work progressed, production was slowly increased. Production is currently running at around 20,000-25,000 b/d of oil and is soon expected to reach higher levels.
"Given the nature of the problem, bringing Lennox oil back onstream so quickly was a significant achievement - and marked a critical milestone for the project," said Ferguson. "Liverpool Bay has recently made some big strides in terms of reliability and production, and we're confident we can achieve - and possibly even better - the targets we have set for ourselves," he concluded.
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