Traditional pipeline isolation plugs are controlled using a hydraulic umbilical, which passes through a modified pig trap door. Pigging pressure is contained using a stripper packer, a rubber gland which is energized hydraulically onto the umbilical, creating a seal. The umbilical is pulled through the stripper packer by the plug's pigging action. However, this mechanism has limitations:
- The umbilical stripper interface has a 68-bar operational limit, which means the plug can only be deployed in a pipeline pressure of less than 65 bar. But the minimum operating pressure of some new pipelines is 110 bar.
- Tension and drag on the umbilical restricts the maximum deployable distance from the pig trap to 200-400 meters, dependent on the number and severity of beds to be pigged.
- Internal pipeline maintenance, such as valve removal, requires the umbilical to be temporarily disconnected.
- Pigging with an umbilical must be smooth. Gas contamination of the pigging fluid can cause the plug to surge forward during the pigging operation, resulting in damage to the umbilical.
Longer run requirements
Pipeline pressures are increasing and pigging run requirements are longer. PII Tecnomarine has developed a remote plug to overcome umbilical limitations without compromising the plug's essential design features. The initial design, for Statoil's Åsgard riser base, may begin service in 2000 or 2001. The key requirements were:
- 200 bar working pressure, 250 bar test subsea pig trap
- High location accuracy
- Communication range of up to 5 meters
- 60 mm pipe wall thickness setting spool
- 30-day deployment maximum.
The basis for the remotely operated plug evolved from the design principles of the tethered isolation plug, which can be either a single or dual module pig. For the latter version, the locks used to hold the plug in position are packaged in one module, while the elastomer seals used to block the pipeline are packaged in the other. Both modules are connected using a structural ball joint. The single module plug has the same locks and seals but packaged into the one module.
Tethered isolation plugs are pigged to location using water or glycol. They are connected to the control console via a five-core composite umbilical, which passes through a stripper package mounted on a modified pig trap door. Location can be achieved by umbilical length, or through use of an isotope or magnet attached to the front of the plug. Once at the desired setting location, it is held static by pigging control until the locks have gripped the pipe.
Design features of the umbilical plug which had to be retained for the remotely operated isolation version included:
- The plug must remain set and safe when differential pressure is across it.
- No single fault in the control system can stop the plug from being unset and retrieved from the pipe.
- The plug must be held at the set location by balanced pressure until the locks are set.
- Any penetrations in the pressure head must be protected by a velocity fuse, so that no single fault allows product to bypass the seals.
- Plug seals must be tested to full pipeline pressure differential.
- The annulus void, set pressure, and pipeline pressure are to be monitored regularly during the construction phase.
- Hydraulic set and unset actuation.
Limitations that had to be overcome were:
- Subsea launch with an umbilical.
- The glycol effect on the umbilical tends to increase the friction with the stripper packer, causing increased pigging limitations.
- Maximum pigging distance of an umbilical plug is limited by the drag on the umbilical.
- Some maintenance tasks necessitate breaking of the umbilical using a midline disconnect coupling.
Åsgard added the following demands:
- Location requirements to within 50 mm.
- No single failure could be allowed to interfere with a successful deployment (an aborted deployment was too costly).
- A 200-bar operating pressure.
In the remotely operated isolation zsystem design, a third module connected to the back of the lock module houses electronic systems (communication and control), the fluid control circuit, fluid storage and batteries. The need for the plug to operate under any single failure was solved by incorporating a two-channel system, each one having its own battery, fluid control circuit, through-pipe wall com munication system, location system, and embedded computer.
Only one of these channels is active at any time. The decision over which is active is controlled by the operator and transmitted through a third channel. Outside the pipe, a subsea communication unit is connected to a surface vessel with an umbilical. There are also two channels for through-wall communication and umbilical up-link in this unit as well as two power lines.
To retain the self-energization and passive unset features of the tethered plug, the control system default state had to leave both the set and unset sides of the actuation cylinder open to vent. This allows the plug's inherent safety features to operate unimpaired.
Location to within 50 mm is achieved through a beacon signal, transmitted continuously through the plug's through-wall communication system. This signal is received by two antennae on the subsea communication unit (SCU) which is placed up to 5 meters from the pipe with its midpoint in line with the plug antenna at the set location. The SCU's antennae are 1.6 meters apart at either end. The received signal at each antenna is converted into a DC voltage, which is proportional to the distance between the transmitting and receiving antennae. The stop location is identified by the two antennae voltages matching.
To position, command, and monitor the tetherless remote isolation plug, it was necessary to source a through-pipewall communication system. This would have to operate effectively in fluid and gas pipelines at pressures up to 250 bar for the in-plug component and the same pressure for the external component. The device's task is to locate accurately the plug at the work site and provide a non-invasive bi-directional communication highway through steel pipe walls up to 75 mm thick, with an effective range in seawater of up to 5 meters.
The use of acoustics and radio isotope technology were examined for this task, but were both discounted.
Conventional radio waves in the LF/HF/ MF/VHF/UF and microwave bands perform extremely poorly in both steel and seawater media due to the high electrical conductivity term (sigma) relating to both media.
However, at frequencies below 350 Hz in the ELF region, propagation characteristics arise which are useful for communication in seawater. In general, the lower the frequency of an electromagnetic wave, the more effectively it propagates in the conducting media of steel and seawater.
Extremely low frequency electromagnetic (ELFEM) signaling was therefore identified as the most likely means of meeting the remote plug communication operational requirement.
In 1989 Trident Underwater Engineering (TUE) Systems undertook development of a system of bi-directional through-pipewall communications for demonstration in a fully subsea operating scenario. Development was funded by TUE, while Statoil and Shell co-funded testing. The final offshore test showed reliable bi-directional communication with a test pig in a 30-in., 28mm wall buried steel operational gas pipeline in the North Sea in 80 meters of seawater. The range achieved was 10 meters. Further, the system proved to be immune to virtually all sources of man-made and naturally occurring electromagnetic noise.
In 1997, TUE was asked to supply a location and communication system to meet the specification for a PII Tecnomarine 40-in. remote isolation plug, which was duly accomplished and updated for the current version developed for Åsgard.
When the plug is launched, a high-powered ELFEM transmitter is initiated. This transmits a periodically interrupted signal through the pipewall. On the SCU, an array of receivers detects this signal from the approaching plug and computes its position in the target area, allowing the plug's location to be determined to within 50 mm when halted. When stopped in the target position, a command from the operator switches the beacon transmitter off such that isolation can proceed, using command mode. Should all three command channels fail (due to umbilical breakage), the beacon transmitter will start up again, allowing the plug to be located by other TUE equipment mounted on an ROV.
Setting the plug
The lock and seal modules operate the same way for the remotely operated plug as for the tethered version. Actuation cylinders are pressured in the remote plug. The locks are driven up the lock bowl first, thereby fixing the plug location. Then the seals are compressed out to the pipe wall. Pressure buildup in the annulus due to seal setting is vented automatically to tail pressure.
While the tethered plug can test both seals to full differential pressure by venting the annulus void to atmospheric pressure, the remote plug does not have access to atmospheric pressure until the pig trap is vented. Therefore, both seals must be tested separately. The secondary seal is tested first by venting the pig trap fully while retaining pipeline pressure in the annulus. This generates a full working differential across the secondary seal. Any leak shows up as a drop in the small annulus void. The annulus void is then vented to the tail and locked in. Now the primary seal sees full working differential pressure and any leak is quickly identified by a rise of pressure in the small annulus void.
In conclusion, the remotely operated isolation plug has overcome the tethered plug's limitations. It can be used to isolate a 200-bar pipeline pressure, and the 400 meter operating range limit no longer applies. The new system also offers a failsafe leak-free form of isolation for pipeline maintenance tasks without having to de-pressure the pipeline to 65 bar.