STATS Group has completed a temporary isolation on a subsea gas export line in the South China Sea.
The company was contracted by COOEC Subsea Technology to complete the work on the Yacheng pipeline, a 780 km, 28 inch (711 mm) subsea gas export line that supplies gas to Hong Kong.
STATS was called in after an anchor damaged one of two subsea pipeline end manifolds (PLEMs) and left a severe dent leaving it unpiggable.
The damaged section was situated approximately 280 km from Hong Kong at a depth of 90 m.
COOEC initially implemented a temporary repair, installing a 400 m, 14 inch (356 mm) bypass between the PLEMs, which enabled the pipeline to continue operating at 70 bar while the permanent repair method took place.
The repair was completed by recovering the subsea pipeline from the seabed onto a pipelay vessel, while the pipeline remained pressurised at a shut-in pressure of 57 bar; it included the removal of the severely dented pipeline and both existing PLEMs and the 400 m of pipeline between them.
The pipeline’s crucial importance to gas supply to Hong Kong meant the shutdown period needed to be kept to a minimum and the repair completed without depressurising and flooding the entire line.
To ensure recovery of both ends of the pipeline onto the pipelay vessel without depressurisation or water ingress, STATS Tecno Plug™ and BISEP isolation tools were installed, enabling isolation of only the sections of pipeline being recovered from the seabed.
The isolation tools had to provide fully proved, fail-safe, double block and bleed isolation barriers from the pressurised pipelines, complying with industry guidance on isolation and intervention for diver access to subsea systems.
As a result of the defect, which made the pipeline unpiggable, it was not possible to pig isolation plugs through the pipeline from the permanent pig launcher, so STATS proposed the use of its dual seal BISEP isolation plug, which is installed at location through a hot tap clamp fitting.
It was deployed into the pressurised pipeline downstream of the damaged pipeline and provided a fully proved double block and bleed isolation barrier against the pipeline gas pressure, allowing the pipeline to be depressurised between the rear of the BISEP and the PLEM while the damaged section of pipeline was safely cut and removed.
With access to the pipeline gained, STATS’ dual seal flangeless plug launcher was attached to the bare pipe end allowing two remotely operated Tecno Plugs to be pigged into the pipeline.
The BISEP could then be unset and recovered, allowing the first Tecno Plug to be pigged 700 m into the pipeline and set, providing isolation of the pipeline gas pressure.
The second plug was then pigged into the pipeline and set a short distance downstream of the hot tap fitting; this allowed the pipeline to be cut behind the rear Tecno Plug and the removal of the BISEP, fittings and hot tap penetration.
At the opposite end of the pipeline, the PLEM provided double block isolation allowing the flangeless plug launcher to be attached to the open pipe end and another two Remote Tecno Plugs were pigged into the line 700 m apart.
Once both plugs were set and double block isolation was confirmed, the isolation certificate was issued and the second PLEM was removed and recovered to the vessel.
A pipeline retrieval tool was then used to recover the pipeline to the pipelay vessel allowing new pipeline sections to be welded to the existing pipeline before laying them back onto the seabed via a flanged laydown head.
The four Tecno Plugs in the pipeline were continuously remotely monitored from a diver support vessel while the pipeline was being isolated.
To prevent seawater flooding into the newly laid pipeline sections when connecting the pipeline to the new PLEMs, the rear plugs were repositioned by equalising the pressure, unsetting them and pigging them back to the newly attached laydown heads.
The plugs were then reset and their double block isolations were proved again so that the laydown heads could be safely removed by the divers.
Once the PLEMs were connected to the new pipeline, subsea receivers were fitted and used to recover both sets of Tecno Plugs out of each pipeline section.
After all of the plugs were confirmed to be in the subsea receivers, the subsea pipeline isolation valves in the PLEMs were closed and the receivers were recovered to the vessel.
The repair was completed by connecting the two PLEMs together with a final 25 m tie-in spool, which was then leak tested and dewatered.
The subsea pipeline isolation valves were then opened and the critical gas supply to Hong Kong was restored.
According to STATS, the pipeline was repaired as safely and quickly as possible, without depressurising or flooding the pipeline and ensuring that any seawater that may have entered the system was removed before the pipeline resumed operation.
For more information visit the STATS Group website.
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