When a riser is damaged, the implications are far-reaching, often resulting in a complete halt in production. Riser repair takes on many forms, depending on the extent of damage and circumstances.

Operators seeking to repair a riser associated with a live gas pipeline are faced with the daunting challenge and expense of removing all product from the pipeline to ensure that repairs are carried out safely. The process is complicated, involving degassing the pipeline prior to repair work and depressurizing, flooding, and de-watering the line before it can be brought back online.

Before production could resume, the operator needed to repair the riser. The objective was to cut and remove the damaged riser section and replace it with a new one. (Images courtesy of TDW Offshore Services)

To offer operators an alternative, TDW Offshore Services has developed a method that makes it possible to leave product in the line at a safe operating pressure while repairs are being made. This approach means degassing the line is unnecessary, which results in dramatic savings in time and costs.

Turning adversity to opportunity

In 2007, a vessel passing through the North Sea collided with the southeast face of a satellite platform jacket, damaging the 12-in. export riser. To avoid a damaging discharge, production from the platform was immediately shut in via the emergency shutdown valves, which left the pressure in the pipeline at approximately 4 bar g.

Because the field had been shut in since the accident occurred, production remained at a standstill until repairs could be made. Each day the field was not producing translated into lost production and associated revenue.

Although TDW’s solution required a great deal of logistical planning to ensure that crews and equipment would be ready to commence repair work as planned, by retaining TDW to provide a repair solution, the operator stood to benefit in several ways. First, the operator could avoid the time-consuming and costly practice of flooding the entire pipeline in preparation for repair work. Second, the product could be safely left intact at 4 bar g in the remaining pipeline. Third, the equipment required to carry out the repair work could be handled easily by the platform crane in spite of its limited capacity. In sum, TDW was selected to carry out the repair work because it offered the most efficient, safe, and economic solution.

To repair the riser, TDW used multiple high-friction pigs to seal off the damaged riser section.

Before production could resume, the operator needed to repair the riser. The objective was to cut and remove the damaged riser section and replace it with a new one. To facilitate the repair process, TDW formulated a low-pressure solution to isolate the damaged section of the pipeline riser from the export pipeline gas inventory. Isolating the damaged section of the riser allowed the damaged riser section and associated topside pipework production system to be replaced.

Monitoring pressure during low-pressure isolation

TDW developed a solution using its range of specialized pipeline pigging, pig tracking, and remote communications technology. The approach involved using multiple high-friction pigs to seal off and replace the damaged riser section and pipework.

The solution consisted of the following components:

• A custom-designed pig trap and pigging spread;

• A high-friction pig train furnished with the SmartTrack remote tracking and pressure-monitoring system;

• A subsea remote tracking and pressure-monitoring system;

• A topside tracking and monitoring system with radio link to the dive support vessel; and

• A pipeline isolation ball valve.

In August 2009, TDW used its remote-controlled SmartTrack technology to isolate the damaged riser section from the gas inventory in the export pipeline without venting or flooding the pipeline or displacing the pipeline inventory. The company used a three-module high-friction pig train to create isolation against the gas pressure in the pipeline. The first step was to verify and record the pipeline inventory gas pressure and close and isolate the emergency shutdown valve (ESDV) 050. The team then removed the redundant topside pipework located upstream of the ESDV and installed a temporary spool and 12-in. valve upstream of the ESDV. Leakage over the ESDV was monitored closely with a view to minimizing pressure buildup in the spool.

Using the pig trap and pigging pump, the team launched the high-friction isolation pig train (HFIPT), pigging with water to the predetermined isolation position within a straight spool section of the vertical riser. Using TDW’s remote tracking technology, technicians onboard the dive support vessel (DSV) tracked the position of each pig to verify that the HFIPT was below the damaged section of riser designated for replacement.

Communication skids were positioned over the three pigs and connected to the pig monitoring system to allow the team to monitor the downstream pressure of each isolation pig continuously throughout the operation. Using its innovative “through-pipe wall” communications technology makes it possible to send isolation integrity data by radio link to a DSV.

When the damaged topside pipework was removed, technicians replaced it with new pipework. Once installation was complete, divers were deployed from the DSV. The section was cut and removed using a crane onboard the DSV. A mechanical connector was then locked onto the existing riser, and the new riser was attached to a crane on the platform and lowered down to rope access workers, who installed it on the topside pipework closing spool and to the existing riser located above the HFIPT.

Once the new riser section was installed, TDW verified that the ESDV and new 12-in. valve were operating properly and fully open. After purging the riser and topside pipework, the team used a pigging pump to slowly increase the water pressure to begin pigging the HFIPT downstream away from the platform to the launcher. They then closed the ESDV and 12-in. valve and increased the pipeline gas inventory pressure to prevent the HFIPT from moving forward. After recovering all of the pigs in the temporary pig trap, the team closed the ESDV and new valve. Using the platform crane and remotely operated vehicle, the TDW crew removed the pigging equipment and demobilized.

All of the offshore operations were carried out in about 10 weeks, from May 20 to Aug. 2, 2009.

Making a milestone

By using its remote tracking and pressure monitoring technology, TDW made it possible for repairs to the damaged riser to be made while maintaining a continuous flow throughout the duration of the operation. This project was the first in which TDW had monitored pressure during a low-pressure pipeline riser isolation operation. Not only was the team able to isolate the affected riser section effectively and efficiently, but the successful application of this solution meant that the operator experienced considerably less downtime. Most importantly, the entire repair process was carried out without environmental incident.

For TDW, this low-pressure isolation operation represents a milestone in terms of delivery and innovation. For the industry, the successful execution of this project introduces a new option for riser repair.