PIG-TRAIN ISOLATION DESIGN DEVELOPED FOR NORTH SEA ESV WORK

Aug. 26, 1991
Terence A. Cooper Total Oil Marine plc Aberdeen Total Oil Marine plc (TOM) has developed and successfully employed a high differential pressure, high sealant pig concept as an isolation technique for both topsides and subsea emergency shutdown valve (ESV) installations.
Terence A. Cooper
Total Oil Marine plc
Aberdeen

Total Oil Marine plc (TOM) has developed and successfully employed a high differential pressure, high sealant pig concept as an isolation technique for both topsides and subsea emergency shutdown valve (ESV) installations.

In use, the concept required less production downtime than conventional flooding methods (Fig. 1). And the technique, which employs oversized discs, prevented migration of gas to the worksites and provided a contingency differential pressure capability of 11 bar gauge (barg; approximately 160 psig) for the topsides ESV installation and 15 barg for the subsea ESV installation.

In the past 2 years, TOM, which operates the North Sea Frigg field transportation system as well as the Alwyn North field, has replaced two 32-in. platform barrier ESVs and installed one 24-in. subsea barrier ESV.

FRIGG SYSTEM

The Frigg gas-transportation system (Fig. 2) consists of two parallel 223-mile, 32-in. subsea pipelines, the manifold compression platform (MCP01), and the St. Fergus gas terminal.

Third-party users of the transportation system include fields in the Tartan-Ivanhoe-Rob Roy area, whose gas enters at MCP01; Odin and the Frigg satellites; East Frigg and North East Frigg; and Alwyn North.

Gas from these latter fields enters the system via the Frigg platforms-TCP2 in the case of the Norwegian fields and TP1 for Alwyn North.

The ownership of the transportation system is as follows:

  • Frigg field installation is operated by Elf Aquitaine Norge AS on behalf of both U.K. and Norwegian associations (Fig. 2).

  • Frigg pipelines: U.K. association-Elf (U.K.) plc (66.67%) and TOM (33.33%); Norwegian association-Elf Aquitaine Norge AS (26.42%), Norsk Hydro Produksjon AS (32.87%), Total Marine Norsk AS (16.71%), and den Norske stats oljeselskap AS (24.00%).

  • Alwyn pipelines: Elf (U.K.) plc (66.67%) and TOM (33.33%).

  • Frigg field transportation is operated by TOM on behalf of both the U.K. and Norwegian associations (Fig. 2).

TOM's objective in developing the isolation system was to secure a safe environment to carry out "hot and cold" works such as welding, grinding, and cutting within several operational constraints.

These constraints included double-block isolation for hot work activities and continuous monitoring of pipeline conditions.

These also included minimal recommissioning problems associated with the introduction of water into the gas pipeline and isolation systems being piggable where possible for both installation and retrieval.

Topsides and subsea valve installations presented two different operating environments necessitating variations in the isolation technique used.

PIPELINE ISOLATION-TOPSIDES

In April 1989, a local consulting firm was commissioned to evaluate isolation systems which met TOM's isolation philosophy and were available for the first week of August 1989.

From the results of the study and knowledge of the industry experiences during the early half of 1989, TOM was in a position where no single vendor could guarantee a completely piggable self-contained isolation system capable of passing through 3D (3x diameter) bends.

Following McKenna & Sullivan's (M&S) experience with a bidirectional pig train, acquired during the installation of a subsea ESV, TOM developed with M&S an isolation system which consisted of two parts: primary pipeline isolation to allow cold cutting and secondary riser isolation to allow welding.

After depressurization but not degassing, pipeline isolation was accomplished in each riser with a series of bidirectional pigs separated by slugs of nitrogen, gelled diesel, and diesel forming a pig train (Fig. 3), referred to as primary dynamic seal (PDS).

The PDS was designed to achieve isolation not only as a result of the pig train's capability to form a complete seal and provide a differential pressure holding capability but also by dynamic protection under possible emergency scenarios such as inadvertent pipeline repressurization or pipeline rupture.

In such circumstances the seal would initially hold significant differential pressure before ultimately becoming mobile. When mobile, it would retain its sealing capabilities and, by virtue of an increase in hydrostatic head, exhibit an increasing pressure-holding capability if the movement occurred up the riser.

Following confirmation that stability and effectiveness of the primary isolation had been established, cold cutting work to remove the ESVs was undertaken, thus allowing the installation of the secondary isolation.

The secondary isolation consisted of an inflatable hyperbaric sphere with provisions for monitoring pipeline pressure between the two isolations and the sphere inflation pressure.

The sphere and umbilicals were designed to allow "pigging out" on completion of welding.

PIPELINE ISOLATION-SUBSEA

In January 1990, TOM embarked on a fast-track project to install a subsea platform barrier ESV for the Alwyn North 24-in. gas export line. In the subsea application, the isolation consisted of three parts: primary pipeline isolation to allow cold cutting, pipeline device to allow pressure monitoring, and secondary pipeline isolation to allow welding.

  • Primary pipeline isolation. Following its use for topsides valve installation, the high-differential pig train was evaluated for the primary subsea pipeline isolation. The fundamental objectives remained the same, that is, isolating the line for ESV installation, while leaving the line full of gas at ambient seabed conditions, and providing a safe working environment for construction. The operational conditions were somewhat different.

    The main design change from the MCP operation was the change to water as the propelling medium. This was required to provide a safe working environment for the divers, and it would also give much better control over the positioning of the pig train.

    Because water was to be used for propelling the train, it would also be used as the main pig-separation medium. This would, however, require the inclusion of recommissioning fluids within the pig train.

    The design premise for the pig train was also altered by the construction work being subsea. It was intended that the pipeline would be vented down to static-head pressure subsea, approximately 13 bar (189 psi). With the pig train in position and the pipeline cut, the pig train would be in dynamic balance with 13-bar gas pressure on one side and 13-bar static head on the other.

    The pig-train design differential pressure holding capability was based on the emergency situation of pipeline rupture or topsides leak at Frigg causing pipeline depressurization.

    The full static head would then be acting across the pig train and the divers could potentially be sucked into the pipeline if the pig train moved. It was therefore decided that the pig train should be designed to hold the full static-head pressure (13 barg) plus a factor of safety.

    Because of the cumulative nature of the differential pressure across the pigs, the safety factor required can be relatively low because, in losing one pig for example due to damage, we only lose a small percentage of the entire systems' capability. The design requirement for the pig train was therefore set at 15 barg.

  • Pressure monitoring device. In the subsea valve installation case, the secondary isolation with umbilicals could no longer be used as a pipeline-pressure monitoring device because of its remote location from the platform.

    TOM developed with M&S a piggable device with acoustic instrumentation which provided information of pipeline conditions between the two isolation systems.

  • Secondary pipeline isolation. Following confirmation with the pipeline-monitoring device that stability and effectiveness of the primary isolation had been established, cold cutting of the pipeline commenced.

    The secondary isolations, consisting of hyperbaric weld spheres which can be pigged out, were installed by the divers.

TRIALS

In early 1989, manufacturers and users of pipeline pigs could not state accurately the required pressure differentials to drive pigs. In addition, information on the accumulative pressure differential effect of pigs in series did not exist.

For these reasons, onshore pigging trials for both projects were necessary in order to configure pig discs to achieve desired sealing capabilities and desired pressure differential holding capabilities, determine the number of pigs required for the pig train, evaluate wear characteristics of high differential pressure pigs, evaluate material compatibility with pig train fluids, and evaluate pigtracking techniques.

The test loops which were designed and built for both projects utilized pipes and fittings identical to the offshore installations: thick and thin pipe-wall sections were used to simulate the risers and subsea pipelines, respectively.

One factor that could not be simulated in the test rigs was that of pipeline roughness factor and hence friction factor because of internal pipe smoothness as a result of pigging and effects of condensate on pipe smoothness.

Safety factors were built into the pig-train design to accommodate these problems.

MCP01 PIPELINE ISOLATION

The objective of MCP01 pipeline isolation was to replace existing ESVs on the 32-in. pipelines both incoming from Frigg and outgoing to St. Fergus because of an unacceptable leak rate across them.

DESIGN, DEVELOPMENT

The design of the isolation system concentrated on two distinct areas:

  • The design differential and high-sealant pigs were developed with standard bidirectional pig components because of availability and proven reliability.

    The increase in both differential pressure and sealing ability was achieved with various combinations of oversized discs from 100% of ID to 110%.

  • The hyperbaric sphere was designed with a built in bypass facility which allowed both pressure monitoring and a purging facility. It was also necessary to replace the in-built valve with an external one to allow remote pressurization-depressurization of the sphere.

The requirement for constant monitoring of both the sphere pressure and the atmosphere below it meant that a relatively complex umbilical system had to be designed to ensure continuous operation during valve installation, with one set of umbilicals running through the valve and another straight out of the pipeline, coupled by a quick-release mechanism.

A test loop consisting of light and heavy-wall 32-in. pipe and a 90 bends was built to measure differential pressures for various pig configurations. Static leak checks were performed to establish liquid-retention properties.

Initially, pigs were assembled with oversized discs in a standard bidirectional pig configuration.

These gave marginally higher differential pressures than the standard pig but little improvement in sealing capability.

Further trials indicated that increases in differential pressure could be obtained by supporting adjacent discs and blocking them together. This reduced the amount that individual discs could flex and therefore increased the differential pressure dramatically.

Problems were encountered with discs tearing under high stress, and the configurations had to be modified.

Several other factors arose in the trials which proved relevant and were developed further.

The tensioning of the bolts was found to be critical in preventing a leak path for liquid through bolt holes which were incorrectly sealed. Further, while it had been intended to develop a high differential pressure pig and a high sealant pig, the two features were found to coexist in the same pig.

The force exerted by the high differential pressure pig on the pipewall was found to enhance the pigs' sealing properties.

Trials were then carried out on trains of several pigs to check that the differential pressure across the train was in fact a summation of the individual differential pressures and also to check the pressure decay and stabilization times for the train to simulate the offshore scenario.

OFFSHORE PIG TRAIN

Offshore on MCP01 two pipeline isolations were required, one for the import gas line and one for the export gas line. Each isolation consisted on one pig train and one hyperbaric sphere. The pig train was designed with the following parameters (Fig. 4):

  • The front part of the train would aim to provide a barrier to prevent migration of gas towards the worksite.

  • The second part of the train would provide two functions:

    1. Differential holding capacity which would provide a large factor of safety in the event of inadvertent pressurization or pipeline rupture.

      This was achieved first by use of high differential pressure pigs and by use of slugs of liquid between the pigs to create a static head.

    2. Condensate swabbing of the pipeline walls by the pig train liquids.

A foam pig was used at the front of the train to contain a slug of diesel gel which would increase the sealing efficiency of the first pig. A 2-km slug of nitrogen would provide an inert buffer to minimize the risk of any gas diffusing through to the second half of the train.

The second portion of the train was made up of three high differential-pressure pigs separated by slugs of liquid.

The first of these was diesel gel to increase sealing efficiency, and the second was diesel. The length of these slugs was calculated to give 90 linear m of liquid, or approximately 7 bar of head.

The design specified that the pig train should be positioned just beyond the bottom riser bend with a slug of glycol 10 m up the riser. The level of glycol could be closely monitored in order to detect any movement of the pig train.

INSTALLATION, RECOVERY

A total of 300 tons of equipment was required for the isolation, requiring a deck area of 20 m x 20 m offshore. The equipment location was approximately 80 m from the pig traps, which included a 30-m change in elevation.

Upon the first pig train being launched into the gas-import line, two main problems surfaced:

  1. Loading of the pigs required a push of approximately 7 tons to achieve a seal in the parent pipe.

  2. Positional control of the pigs when being launched into an atmospheric gas line proved to be impossible.

When being launched with nitrogen, the pig would hold until pressure built up and then release, traveling an indeterminate distance before stopping. When propelling the pigs with fluid, the static head created in the riser caused the pigs to drop to the bottom of the riser.

The pig train actually ended up beyond the riser bend some 200 m further forward than anticipated. However, this proved to have little effect on either the efficiency of the pig train or the outcome of the operation.

The installation of the second pig train into the gas-export line was carried out with the same problems of pig control.

Problems were encountered in the launch of the last pig which seemed to stick in the riser while bypassing nitrogen. Subsequent inspection upon recovery revealed some damage (Fig. 5) to the pig rubbers, possibly caused by a ball valve not fully opening.

After installation of the primary isolation, the residual nitrogen was vented to atmospheric pressure. Without the means to measure the level of glycol in the riser, pressure monitoring of the pipeline became the only means to determine the pig-train stability.

At the end of 24 hr, no movement of the train or migration of gas had been detected, and it was decided to cold cut the line.

Following removal of the redundant ESVS, the modified hyperbaric spheres were installed. Both the sphere inflation and pipeline pressure monitoring continued throughout the construction period.

On completion of the valve installation, it was planned to remove the hyperbaric spheres. Part of the umbilical incorporated a steel wire to enable the spheres to be winched out of the line. As part of the sphere installation procedure, it had been felt necessary to install a centralizing device above the sphere to keep the umbilicals away from the pipewall during welding.

A "slice" of a foam pig was used for this operation. During recovery, the support for this foam pig failed and the sphere became lodged on the foam pig in the bend configuration at the top of the riser.

Being above the new ESV and the first PI point, it was possible to blow this sphere out with nitrogen after removal of the umbilical and pull wire. The other sphere was removed without major problems.

Recovery of the pig trains started with gas pressurization from the remote end of the line. Initial movement of the pig train took place at about 10.8 bar. As the pig train came into the riser, the static head caused by the slugs of liquid brought the pig train to a halt, and the pig train slowly moved up the riser in small movements.

Careful control of the back pressure allowed the first two pigs to be brought into the trap in a controlled manner. Subsequent draining of the diesel gel was complicated by a blocked drain line, and the limited number of pig trap connections meant that the pig trap had to be drained via the door.

Sampling of the intermediate nitrogen slug indicated that no gas had migrated past the first interface, and this was vented to atmosphere. Receipt of the final gel slug and the foam pig also caused some problems because the foam pig had broken up.

This pig was eventually recovered by venting gas via the flare system to create sufficient velocity to bring the pig in.

NORTH ALWYN LINE ISOLATION

Following the 1988 Piper Alpha accident (OGJ, July 11, 1988, p. 20; May 7, 1990, p. 65), TOM decided to install a subsea platform barrier ESV on the 24-in. gas export line from the Alwyn North (B) platform.

FURTHER DEVELOPMENT

The design objective was to refine and further develop the high differential pressure concept obtained from the MCP01 works.

A test loop consisting of light and heavy wall 24-in. pipe was built to measure differential pressures for various pig configurations. The program also included wear tests, required because the whole pig train would have to travel approximately 700 m with the leading pigs traveling some 3.5 km.

Envisioned were more stringent leak testing (because of the inclusion of both water and recommissioning fluids) and material compatibility tests, given that the urethanes would be subjected to 30 days' contact time with a variety of fluids.

Initial pigging runs, however, had a dramatic impact on this proposed program. The first pig runs with the pigs developed for the MCP01 operation came out severely damaged. Subsequent minor modifications to the pigs showed no major improvement with pigs still incurring damage.

An evaluation of the pipeline parameters for both Alwyn and MCP01 revealed a marked difference in the step changes in wall thickness for the two systems. While the 32-in. line had wall thickness changes of only a few millimeters between thick and thin wall, the difference on the 24-in. line was 10 mm.

The effect of this change was exacerbated by the smaller pipeline diameter giving an even larger reduction in diameter in percentage terms.

The pressures experienced during the initial Alwyn test runs had been largely as expected in the thin wall pipe but significantly higher in the heavy wall.

Consequently, the pig design had to be considerably modified to achieve a piggable design. This led to similar differential pressures being achieved in the heavy wall pipe for Alwyn as were achieved in the light wall on MCP, these being slightly less than the threshold at which the pigs started to incur major damage.

As a result, differential pressures obtainable in the light wall pipe were considerably lower.

With a pig that was now piggable through the full loop, it was then necessary to move on to durability testing to establish the differential pressure capability of the pig after running various distances from 700 m up to 3,500 m. This was a lengthy operation given the test loop's length of 100 m.

Earlier results had indicated that all the wear must be obtained by running in one direction to simulate the offshore situation correctly because wear occurred only on one side of the discs.

The pig therefore had to be removed from the bottom end of the loop and reloaded on each occasion.

Running the full distance of 3.5 km led to an approximate 50% reduction in differential pressure experienced in the light-wall pipe at the start of the run decreasing from 45 psi to 22 psi.

Subsequently, static differential pressure tests were carried out on the worn pigs to check differential pressures and leak rates. Increasing the pressure slowly behind the pig to the anticipated move off pressure of 22 psi revealed that the pig started to creep forward at 18 psi.

Leak rates were then examined at this pressure and found to be minimal. The creep pressure of 18 psi was therefore taken as the value for use in the design of the pig train.

Material testing was also carried out on samples of urethane soaked in methanol, isopropyl alcohol (IPA), and condensate. This testing revealed a fairly significant weakening of the urethane, particularly in terms of tear strength, after soaking in some of the fluids.

It was then felt necessary to conduct further tests on actual pigs soaked in fluid. But, because no additional damage seemed to be produced and no lowering of the differential pressure occurred, the configuration of the pig remained unchanged.

TRAIN DESIGN

The pig train designed for the subsea ESV installation incorporated the following parameters:

  • The front part of the train would provide the main interface to prevent migration of gas towards the worksite.

  • The second part of the train would provide three functions: differential holding capacity which would provide a large factor of safety in the event of inadvertent pressurization or pipeline rupture, condensate swabbing of the pipeline walls by the pig train liquids, and pipeline drying with methanol on pig train recovery.

IPA was used at the front of the pig train for two reasons.

IPA would act as a solvent wash to remove condensate on the way into the line and, being mutually soluble, would then be washed off by the water, leaving a safer environment for the welding operation.

And, during recovery of the train, the IPA would enhance the drying operation, forming an azeotrope with any remaining water in concentrations greater than 90%.

The use of nitrogen within the pig train also required careful consideration.

While slugs of nitrogen were desirable to minimize diffusion of gas along the train, their use would create other problems. When launching the pig train for topsides isolation into a pipeline at zero pressure, it had been possible to vent the residual nitrogen pressure after launch of the first two pigs.

In the subsea scenario, this launching against a pressure of 13 barg would not be possible. The nitrogen slugs would therefore act as springs with the potential to push the pig train back toward the worksite after the launch pressure was reduced to static head pressure.

The pig train was designed with three pigs at the front separated by slugs of nitrogen. Again the main purpose was to reduce diffusion of gas towards the worksite.

This was then followed by four slugs of recommissioning fluid trapped between high differential pigs. A further eight high-differential pigs separated by slugs of inhibited water would complete the train.

A standard bidirectional pig would be added at the rear of the train to remove the hyperbaric spheres on the way out.

The lengths of all the liquid slugs were sized to give the necessary spacing when the train was received to ensure that none of the train left in the line would be in the other ball valves.

NITROGEN USED

The logistics of the offshore operation were once again complicated with large amounts of equipment required for deployment of the isolation system.

The problem of loading the high-friction pigs experienced on MCP01 was overcome by the design and manufacture of an hydraulic pig-loading tool. Problems were experienced with the launching of the pigs through the topsides in-line ball valves which were considerably oversized relative to the pipeline ID.

It proved impossible to launch pigs because of the liquid bypass of the pigs. The problem was overcome by the pigs being pushed past the valves with nitrogen finally displaced with inhibited seawater.

When it was initially perceived that the differential pressure across the train was lower than design, an additional two pigs were added to the train. Once the train was pigged into its final position, the differential pressure achieved was 21.4 bar, slightly more than predicted.

After stabilization, the pipeline was cold cut, and hyperbaric spheres were installed ready for the start of construction work.

Recovery of the pig train commenced with gas injection from Frigg. The inhibited seawater was dumped prior to receipt of the hyperbaric spheres. Various pig tracking and locating systems were used to identify the position of the pigs.

The hyperbaric spheres and pigs were received into the pig traps with no significant problems. The methanol and IPA were collected into reception tanks for disposal onshore. Examination indicated a concentration of 93 wt % methanol in the last slug.

The two slugs of IPA liquid ensured the pipeline was thoroughly "dried" and ready for operation.

The oversized valves did cause problems in receipt of the last nitrogen pigs, and it was necessary to employ larger venting facilities via the platform's vent system to bring these pigs back into the pig trap.

ACKNOWLEDGMENT

Contributing to this article was A. J. Barden of McKenna & Sullivan.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.