SPECIAL REPORT: Advanced isolation technology allows subsea valve replacement without purge

May 2, 2011
Advanced pipeline-pressure isolation technology allowed Shell Philippines Exploration & Production to replace an export riser shutdown valve at an offshore natural gas production platform without the time and expense of depressurizing and purging the entire export pipeline.

Gerald Misajon
Shell Philippines Exploration & Production
Makati City, Philippines

Rolf Gunnar Lie
T.D. Williamson Asia Pacific Pty. Ltd.
Singapore

Advanced pipeline-pressure isolation technology allowed Shell Philippines Exploration & Production to replace an export riser shutdown valve at an offshore natural gas production platform without the time and expense of depressurizing and purging the entire export pipeline.

Shell faced three alternatives apart from the technology it ended up using.

The company could live with a defective emergency shutdown valve (ESDV) at the platform. This choice would effectively rule out proper boundary isolation for shutdown purposes or isolation of the export pipeline in emergencies and was clearly undesirable.

Shell could also have replaced the ESDV with a single flapper-type valve for isolation from the gas-export pipeline. Shell's permit-to-work safety system, however, would not allow this single-isolation option.

Finally, Shell could depressurize and purge the entire gas-export pipeline. Accomplishing this would have required blowdown of the entire 504-km gas export pipeline followed by full nitrogen purge of the line, involving pigging from the offshore gas platform to the shallow-water platform. This blowdown and purging would likely involve large-scale flaring or venting at both platforms, with negative environmental effects.

Valve replacement would follow completion of this process.

Once valve replacement was complete, the gas-export pipeline would need to be purged of air via nitrogen pigging from the offshore gas platform before hydrocarbons were introduced.

Background

Shell discovered the primary ESDV on its Malampaya shallow-water platform was defective.

The Malampaya deepwater gas-to-power project reduces the Philippines' oil imports and provides a stable supply of energy and a clean source of power, meeting up to 40% of Luzon's energy requirements. This made it critical the shutdown be kept to a minimum and isolation of the export pipeline be carried out without complication or delay.

The project is developing the Malampaya and Camago reservoirs via deepwater subsea wells connected by wet-gas flow lines to a shallow-water processing platform. Shell owns and operates the Malampaya SWP. The gas from the SWP travels 504 km through a 24-in. OD gas-export pipeline to the Malampaya onshore gas plant (OGP) at Batangas for final processing, and fiscal metering before dispatch to customers downstream.

The Malampaya development consists of the following pipelines (Fig. 1):

• Two 16-in. OD two-phase flowlines, transporting wet gas more than 30 km from subsea manifold to platform.

• A 24-in. OD export pipeline moving gas from the platform 504 km to the OGP at Batangas.

• A 24-in. carbon steel condensate export pipeline from the platform to the catenary anchored-leg mooring (CALM) buoy.

A major shutdown, planned for more than 1 year, took place February-March 2010, to conduct a range of rejuvenation work, including replacement of the main ESDV and a neighboring manual block valve on the export pipeline.

This article details that shutdown.

Shell deployed pressure isolation of the line downstream of the valves. The isolation method used T.D. Williamson Inc.'s SmartPlug tool. As it was the first pressure isolation of its kind to be undertaken at the Malampaya platform by Shell, concerns existed regarding its viability.

The ESDV, a 24-in. Class 1500 actuated ball valve on the export riser, and a nearby manual block valve both required removal and replacement (Fig. 2). The ESDV and the manual block valve are the primary means of isolating the platform from the export pipeline, requiring another means of isolation to allow their safe replacement.

A TDW engineering study raised concerns about whether the line was piggable, an intelligent inspection tool having been damaged the previous year. This was the first pressure isolation of its kind undertaken at the Malampaya platform, and it needed to be completed within the 30-day shutdown window, during which additional major maintenance procedures would take place.

TDW completed the engineering study and site visit in early 2009, about 1 year before the planned 2010 shutdown. The fixed and strict 30-day window raised the premium on careful planning of tool transportation, custom clearance, provision of pumping services, and compliance with safety regulations.

Isolation system

The standard SmartPlug tool seals against 200 bar (about 2,900 psi) operating pressure. Two plug modules perform seal and lock. Each module provides these functions independently of the other (Fig. 3).

The highly piggable, bidirectional plug train can travel in either direction inside the pipeline. The tool can isolate—and travel within—any pipeline medium, including natural gas, crude oil, distillates, treated or untreated water, glycol, and diesel. The two independent plug modules featured in the standard SmartPlug system typically travel on a spring-loaded wheel system and can isolate full pipeline pressure, allowing for a double-block isolation anywhere in the pipeline system. Use of a third plugging module sometimes takes place for postrepair hydrotesting.

The typical isolation-plug train carries a plug module at each end. These house the on board control and communication system and assist in driving the plug train through the pipeline. The control and communication system communicates wirelessly with the external extremely low-frequency (ELF) antenna placed on or above the pipeline.

The self-locking plug modules apply differential pressure against the pipe wall to maintain or intensify sealing and gripping once they've been expanded. Applying the system's hydraulic operating pressure to the actuator cylinder sets the plug. The actuator cylinder pulls on the actuator flange, forcing the slips up the tapered ramp on the slip bowl and wedging them between the outside diameter of the slip bowl and the pipeline's inside diameter.

After the slips come into contact with the pipe wall, the movement of the actuator flange against the pressure head compresses the packer radially, expanding it to seal against the inner diameter of the pipeline. The outer surfaces of the slips are machined with very sharp threads, allowing the slip teeth to uniformly penetrate the surface of the pipeline inner wall. Because this penetration is only a few thousandths of a millimetre deep, however, it is well within API tolerances for scratch marks. In fact, engaging only one third of the slips with the ID of the pipeline provides sufficient grip for isolation.

Each plug module is designed and tested to seal against the full pipeline differential pressure, making it possible to test sealing and gripping capability.

The ELF control and communication system on the pigging module bidirectionally conveys control signals and digital data through steel pipelines. Its antenna can lie either subsea or topside. The ability to amplify signal strength to allow a distance of 10 miles between antennas allows use on physically inaccessible or buried pipelines. The system's battery pack provides a minimum of four actuation cycles and status monitoring for a 30-day operation.

A computer system (Fig. 4) allows the operator aboard the platform or surface support vessel to send and receive commands and data. A subsea application often prompts acoustic communication between the SmartPlug antenna and the vessel instead of hard-wiring.

Isolation operation

The engineering study undertaken by TDW before the shutdown included a site visit, preparation of design premises, piggability study, pipe stress calculations, and formulation of operating procedure. The design premise completed following the site visit is a documentation of all pipeline data and issues associated with the operation. Reliable and accurate pipeline information is essential to the successful engineering and execution of pipeline isolation operations.

TDW reviewed the ID of all pipeline components, as well as the basic tool configuration and piggability study. The piggability study assessed the SmartPlug tool's ability to negotiate the pipeline in order to make sure it could be both safely pigged into the set location and retrieved to the launcher. The study also formulated contingency plans for pigging the tool more than 500 km to the OGP should unforeseen difficulties arise when pigging the tool back to the launcher.

A pipe stress analysis also checked the hoop and von-Mises stresses in the sealing area of the set location on each plug module against permissible use, as defined by DNV-RP-F113.1 Isolations on or close to the platform require Safety Class "High," with a maximum permissible von-Mises membrane stress of 66% specified minimum yield strength (SMYS) for the pipeline material.

Shell and TDW decided before the Malampaya shutdown to gauge pig the line from the platform to the OGP to ensure no obstructions existed. Shell had previously encountered a problem with the valve closest to the launcher failing to open properly and damaging an in-line inspection (ILI) tool. Though Shell had subsequently repaired the valve's actuator mechanism, the company wanted to be certain there were no obstructions. To prepare for this contingency, TDW ran the gauge pig from the platform to the receiver at the OGP to verify piggability along the entire line.

Mobilizing equipment

To allow sufficient time for onward transportation and preparations at the Malampaya platform, TDW transported equipment from Stavanger, Norway, to the Philippines about 6 weeks before the shutdown. In keeping with standard practice, TDW builds a SmartPlug system for every isolation operation. The entire SmartPlug assembly undergoes a full factory acceptance test before mobilization, replicating operational conditions, such as the pipeline ID and WT, and witnessed by a third party, typically Det Norske Veritas (DNV).

TDW structure tests each individual isolation plug to 1.43 times operating pressure at maximum ID before the acceptance test.

TDW deployed a team of five specialists to handle isolation at the Malampaya platform about 1 week before beginning the shutdown.

Launching, pigging

Fig. 5 illustrates the prelaunch procedure for the SmartPlug tool, following required planning and hazardous operations meetings. External pumping services provided by BJ Services in Singapore depressurized the launcher and prelaunched the tool into the launcher barrel, before closing the door and pressurizing the launcher.

A crew on the Malampaya shallow-water platform prepares to launch a pipeline pressure isolation tool that will allow replacement of two downstream valves (Fig. 5).

TDW considered water, nitrogen, and export gas already in the GEP as the pigging medium, having used these options in the past, before choosing nitrogen. The choice allowed shutdown of the gas-export line and its being gas free while the plug was being set. An external nitrogen pump can also control pigging better than a gas-export compressor. And finally, although the volume of inhibited water and duration of exposure would be minimal, its presence presented potential corrosion problems, particularly since the launcher was ill-equipped to handle any mercury-contaminated water that may have been recovered.

TDW opened the launcher valve and pigged the tool to its location in the riser, tracking it throughout the 30-min operation.

Setting, monitoring

Tool setting followed this sequence:

• Setting the first plugging module at full line pressure.

• Depressurizing to 50% of the pipeline pressure from the launcher side.

• Setting the second plugging module.

After setting, controllers depressurized the launcher side to ambient pressure and then monitored the annulus pressure between the two plugging modules to verify sealing. TDW then issued an isolation certificate to Shell declaring the line sufficiently safe for valve replacement.

One important safety feature of a double block isolation system is the ability to monitor annulus pressure constantly to ensure its integrity. The entire launching operation—including prelaunching, pigging, setting, and monitoring—took less than 24 hr.

Testing, retrieval

The newly installed valves used KaMOS Kammprofile gaskets consisting of a solid metal core, serrated on both sides and covered with a soft, conforming, sealing material bonded to each face. KaMOS has developed a method of leak testing of flanges with gaskets that can measure whether tightness has been achieved on all sealing surfaces of the flange before it and the pipe are subjected to internal pressure. The method consists of pressurizing the annular space above and below the seal ring using test gas, allowing the joint to be tested as soon as it is made, so that problems can be addressed before the remainder of the piping is installed. This method makes it necessary to pressurize large volumes of piping.

Running the setting sequence in reverse unset and retrieved the SmartPlug tool: pressurizing from the launcher to 50% of isolation pressure, unsetting the first module, then pressurizing to the full isolation pressure and unsetting the second module. After unsetting, TDW pigged the plug back to the launcher, closed the launcher valve, depressurized the launcher, and retrieved the tool.

It then demobilized personnel and equipment from the Malampaya platform.

Safety

The entire Malampaya shutdown team attended "Implement the Plan on Paper" sessions during which all work parties met to discuss and understand the scope of the operation. Shell uses a Permit to Work (PTW) procedure at all operated facilities as part of its HSE management. It is a formal prescriptive procedure designed to provide clear instructions to the user and safeguard personnel carrying out work activities. Application of the Task Risk Management Process supports it. All personnel working within the PTW must demonstrate full understanding and compliance.

The SmartPlug system is designed to self-lock in the set position so that it is fail-safe as long as there is pressure differential across the plugging tool. It will continue to isolate the pipeline pressure, even if communications fail.

Constantly monitoring delta pressure through pressure indicators provides the operator with all relevant pressures for both the hydraulic system and the pipeline. Activated alarm switches can also alert the user if annulus pressure exceeds or falls below a preset value. Creation of a log file begins when the program commences, with all information stored for later retrieval. Unsetting and recovering the SmartPlug tool can also take place via an on board mechanical fail-safe system without the communication system.

DNV has given the methodology for using the SmartPlug isolation plug system a Type Approval Certificate2 and confirmed compliance with DNV OS—F101.3 Included in this certification is a detailed Failure Mode Critical Analysis study based on Fault Tree Analysis. DNV certifies each plug module before assembly of the train and witnesses testing of the assembled train before mobilization. Specially trained personnel perform all SmartPlug isolation operations.

Acknowledgments

The authors acknowledge the following for information concerning the Malampaya shutdown project and execution of the isolation work: Nathan Roberts, discipline lead, mechanical engineering, SPEX; Nathan Stephenson, project manager, Sarawak Shell Sdn. Berhad-Sabah Shell Petroleum Co. Ltd., Kuala Lumpur; and Bard Kloster, offshore supervisor plugging operation, TDW Offshore Services AS, Stavanger. OGJ

References

1. DNV-RP-F113, "DNV Recommended Practice—Pipeline Subsea Repair," October 2007.

2. DNV Type Approval Certificate P11273-Rev 2, "SmartPlug & SmartPlug Compact."

3. DNV OS—F101: "Submarine Pipeline Systems," October 2007.

The authors

Gerald Misajon ([email protected]) is maintenance engineer at Shell Philippines Exploration & Production. He has also served as graduate mechanical-design engineer for Amec Services Ltd. He holds a BS in mechanical engineering from De La Salle University, Manila.
Rolf Gunnar Lie ([email protected]) is regional business development manager-South East Asia at T.D. Williamson Asia Pacific Pty. Ltd. in Singapore. He has also served as regional manager for TDW Offshore Services LLC in Houston and engineering manager at TDW Offshore Services AS in Stavanger. Prior to joining TDW he worked as engineering manager for Plugging Specialist International. Lie holds a BS (Hons) in mechanical engineering (1993) from the University of Glasgow and is a member of TEKNA, a Norwegian association for chartered engineers.

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