By Mark Sim, T.D. Williamson, Inc.
Of the many eventualities that operators of subsea pipelines must prepare themselves for, damage caused by anchor drag remains a top priority. When a passing ship’s anchor collides with a pipeline, it may pull the line out of alignment or damage the exterior. It isn’t often that the line is completely severed, but when it is, the results are dramatic. When an anchor recently sliced through a 26-inch high pressure subsea gas pipeline in the Mediterranean Sea, it resulted in failure of the line and a simultaneous l4% reduction in a parallel 20-inch subsea high pressure pipeline. (See Fig. 1) Because these lines were part of a common system consisting of parallel lines, delivery of gas was maintained, making it possible for the operator to implement a thoughtful, measured response.
Following the incident, the operator’s pipeline repair contractor looked to TDW Offshore Services (TDW) to isolate the damaged sections so that they could be removed and replaced. TDW planned to use a combination of its pipeline recovery tools (PRTs), SmartTrack™ remote tracking and pressure-monitoring system, and SmartPlug® pipeline pressure isolation tools to isolate the designated sections of the line.
The operation featured two common emergency scenarios that TDW was required to address: a piggable dent and a wet buckle. A “wet buckle” is a condition where the pipeline is breached and damaged, allowing water to enter, making the pipeline “wet.” A “piggable dent” describes a situation where there is limited damage to the pipeline so that a pig can still pass through the “dent” or obstruction.
When an anchor makes contact with a pipeline, it normally deflects the anchor, as it did with the 20-inch pipeline. Although TDW had repaired pipelines damaged by dragging anchors, this was the first time that it had been called in to facilitate repair of a pipeline that had been completely severed by one – as was the case with the 26-inch pipeline.
Assessing the damage
The effect of anchor drag damage could have been far worse. To illustrate, water depth drops to below 600m on one side of the pipeline. Had the line been completely flooded in this particular location, the decommissioning, repair and recommissioning of the line would have been a more complicated, drawn-out process.
Fortunately, the damaged pipeline was located in only 70m of water, with a favorable topography, so the full line did not flood. As a result, a wider range of repair options could be considered. Regardless of the solution, use of remotely-operated diver-less technology would be required.
Formulating a solution for decommissioning
During planning sessions with the pipeline repair contractor, TDW considered options to decommission and repair the two lines. Previously, TDW had designed and manufactured a custom SmartLift™ pipeline recovery tool (PRT) for a project carried out subsea in much deeper water. (See Fig. 2) By using the SmartLift PRT, it would be possible to recover and dewater the recovered pipe. The PRT’s internal gripping function is activated hydraulically by a remotely-operated vehicle (ROV) or diver, and self-locks mechanically after being set. Packers are activated simultaneously, to provide a seal. Dewatering is achieved by running a dewatering pig either from a launcher or to a receiver attached to the PRT. As part of the planning process, it was agreed that PRTs would be used, with delivery and design the primary issues.
Pigs tracked with through-wall communications technology
Because the 26-inch pipeline was blown down, a standard bi-directional pig was selected for use in conjunction with TDW’s SmartTrack system. By using this method, the pipeline repair contractor would be able to accurately track the pigs from each end of the line through to the damaged section. As an added benefit, in addition to tracking the pig as it travelled through the 120km and 30km lines, the SmartTrack system served as back-up to the pig detector on the PRT, so that positioning could be confirmed within +/- 5cm. With the SmartTrack system’s ability to communicate “through-wall,” technicians could read the pressure inside the pipe prior to removing the PRT.
Because the 20-inch pipeline was still flowing at operational pressures, a different approach was required. The best solution was to isolate it using specially designed pigs to prevent major seawater ingress during the cutting operations. It would also make it possible to insert the PRT and carry out the dewatering process.
TDW had to make certain that the line was safely isolated so that the damaged section could be removed, and the pipeline could be recovered and repaired. Although the recovery and repair procedure was identical to that used for the 26-inch pipeline, in this case, proper isolation and depressurization were critical.
A custom pig was to play a central role in the operation, so TDW developed a design with the requisite parameters. The plan was to send two multi-diameter, bi-directional pigs to provide a sufficient barrier so as to allow the damaged section to be cut and removed without the risk of uncontrolled seawater ingress. The two pigs would be positioned on either side of the damaged section.
Test rig custom-built to simulate subsea conditions
At its facility in Stavanger, Norway, TDW designed and built a test rig with a built-in dent in excess of the 14% actual deformation. (See Figure 3) Essentially, a 16-inch to 20-inch dual diameter pig was necessary for the operation. A number of configurations were tested to simulate the subsea conditions. Flip pressures, launch pressures, holding pressures, and pigging pressure across the dent were measured and optimized during the test to ensure proper performance offshore.
The objective of the trials was to verify tracking signals and pressure readings through the pipe at the site of the dent. Multiple delta-Pressure (?P) measurements were taken while running in straight pipe, through the dent, restarting in the dent, and flipping of the dent-passing pig in straight pipe. In addition, ?P measurements were taken while running in straight pipe for the second pig and combined pig train.
Executing the plan
After careful assessment, surveys to detect water and analyze oil characteristics were executed, along with an inspection of the subsea 26-inch line to determine where seawater flooding had occurred, and to collect soil data along parallel routes. To prepare the 26-inch line for isolation, the main contractor cut it, installed PRTs at both ends, and removed the damaged sections. (The pipeline repair contractor proceeded to repair the 26-inch line. The pipe ends were recovered, and an above water tie-in performed prior to laydown. The line was re-commissioned by drying, filling with nitrogen, and packing with gas before being re-started, allowing production to resume. This was followed by repair of the 20-inch line by the pipeline repair contractor, who employed the same process. The line was re-commissioned by drying, filling with nitrogen, and packing it with gas before it was re-started, allowing production to resume.
Operation completed as planned
The operation progressed as planned, and was completed on schedule. According to TDW, the SmartTrack system played a critical role, with its ability to function as a tracking and pressure monitoring system. “That we were able to accurately track positioning made it possible for the pipeline repair contractor to feed back information to the northern pump site,” said Rutger Schouten, Director of Operations and Technology for TDW. “This method provided better control as the pig train approached. In addition, each pig is equipped with a unique identifier system so there was no confusion when placing one pig on either side of the damaged section. Finally, pressure was read through the pipe wall, ensuring pressure equilibrium and a safe environment in which to make the cuts,” he added. After the cuts were made, TDW inserted the PRTs, and pumped air/nitrogen from either end to push the pigs to the PRT pig catcher, while simultaneously removing the water via the PRT dewatering ports.
By adopting a considered approach to planning, engineering custom pigs, and carrying out trials on the test rig, TDW completed a meticulous pipeline pressure isolation operation. By doing so, the operation paved the way for safe, efficient repair of this subsea pipeline, allowing full production to resume and gas to flow.
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