A new technology now allows pressure isolation anywhere in a pipeline, enabling repair or maintenance of offshore oil and gas pipelines without shutting down the line, losing inventory, or losing production from downstream platforms. Plugging Specialists International AS (PSI) of Stavanger, Norway, under contract to Rockwater (Halliburton), has successfully installed two high-pressure, remotely operated (tetherless) pipeline pressure isolation tools, called PSI SmartPlugs.

Performing work on a subsea pipeline previously meant shutting down the line. There were several severe consequences to this action. First, gas inventory had to be flared off, and oil inventory had to be displaced to a safe downstream location. Second, shutting down and re-commission-
ing could take many weeks, meaning substantial loss of production.
A tool existed which could be inserted into the pipeline to isolate pressure at a given point, but it could only be operated and monitored via umbilicals, or "tethers", running from the rear of the tool, through the launcher door, to an external control panel. These tethers interfered with the pipeline work, and restricted piggability of the tool to only a few dozen metres and large radius bends, making them useless for all but a few projects.

Tetherless
PSI developed a new technology that could not only send data and instructions through the pipewall without tethers, but a system that could use these instructions to perform tasks inside the pipeline, and re-send data back out. As a result, PSI's new technology allows communication and data transmission through the pipewall, using extremely low frequency (ELF) electromagnetic signals.
PSI then placed the hydraulic system and pressure monitoring equipment inside the SmartPlug. The motors, valves and sensing equipment were coupled to an internal onboard computer and communication system. The result is the PSI SmartPlug, a mechanical high-pressure pipeline isolation system without tethers, requiring no holes in the launcher, and with virtually unlimited pigging distance and ability to traverse various pipeline impediments and bends.
The PSI SmartPlug is launched into the pipeline like an ordinary pig, and has similar pigging characteristics to an intelligent pig. Once the PSI SmartPlug reaches the intended location within the pipeline, an antenna is lowered to the seafloor over or near the pipeline at the plug location. The antenna on the seafloor is connected to the topside vessel either by cable or via an acoustic modem, and connected to a laptop computer which sends and receives instructions and data to and from the SmartPlug. Once the tool is set and holding pressure, the communication system continues to monitor all plug and pipeline pressures and parameters. So how does the system work in action?
When ONGC wanted to replace a 26" gas riser and 30" oil riser and install SSIVs in each line, they knew that shutting down the pipelines to which they were attached was undesirable, since downstream platforms exported product into these same lines. Blowing down the lines would have shut down all downstream production, resulting in loss of production from several platforms.

Pigging distance
Traditional tethered plugs were not an option, since they were not capable of negotiating the number or severity of the bends involved, nor were they able to traverse the pigging distance required, due to the weight and friction of the tether.
Rockwater, the contractor responsible for replacing the risers, became aware of the PSI technology, and, after careful evaluation of the PSI system, awarded PSI a contract to deliver a 26" SmartPlug and a 30" SmartPlug.
These tools were manufactured and tested in six weeks and eight weeks respectively. Each SmartPlug had to be able to negotiate several 3-D bends, be pigged over 600 metres to the desired point of isolation, and be able to isolate against 80 bars pressure.
On 7 October 1999, the 26" tetherless SmartPlug was inserted and pigged into position. The internal hydraulic system actuated the SmartPlug, after being given instructions from the topside computer sent via electromagnetic signals through the pipe-wall
The isolation took approximately 1.5 hours from launching of the SmartPlug through completed setting.
With the tool being set, downstream pressure was maintained at its 80 bars operating pressure, which allowed all downstream platforms to continue operating while upstream pressure was reduced to atmosphere.
During change-out of the riser and valves, all pressures were monitored (upstream, downstream and in the annulus between the two plug modules), with all information being sent electromagnetically through the pipewall, and read on the topside computer. The risers and valves were thus changed-out without interfering with production other than the flow through the risers being replaced.
The change-out was completed on 17 October, and the SmartPlug was unset and pigged back out. Total loss of production was 10 days, and only for the platform where the work was performed.
The same procedure was performed on the 30" oil line, with the SmartPlug being inserted on 19 October. Again, the tool was set via remote tetherless instructions and all downstream pressures were maintained, allowing the downstream platforms to continue production while work was performed on the 30" riser.
The 30" riser replacement was completed, and the SmartPlug unset and removed on 28 October 1999, losing only production through the riser being replaced for 9 days.
Without this new technology, the pipelines would have been bled to atmosphere, taking several days just to pig inert substances to rid the lines of content. In addition, shutting down of production on all downstream platforms would have been required, resulting in the loss of the entire pipeline contents. All platforms would then have had to shut down during replacement of the risers, and would have remained shut down for several more days while the lines were dried.
Since each of these pipelines is approximately 250km long, and each line has several other platforms producing into it downstream, it is estimated that this new technology saved wasting 500km of contents (both oil and gas), involved no pollution to atmosphere, and saved over 30 days of lost production.
PSI faced an even more difficult problem which arose in Abu Dhabi for ADMA-OPCO.
ADMA operates a 30" line, which runs 93km from an onshore processing plant to a subsea tee-piece connecting one platform, then another 11.3km to another platform. This last 11.3km needed to be replaced, a job expected to take between 12 and 18 months.

Entire pipeline
In the meantime, the 11.3km section was not able to maintain operating pressure, so either the 11.3km section had to be isolated at the tee-piece, or the entire pipeline would have had to shut down, which would then have shut down the first platform connected at the tee-piece.
Since the 11.3km piece was not usable, the second platform would have to be shut down. However, shutting down the entire line would have meant loss of the first platform as well, whereas use of a SmartPlug would allow the first platform to save 12 to 18 months of production.
Again, a traditional tethered plug could not be pigged more than a few meters, so shutting down two platforms seemed imminent. ADMA learned of the SmartPlug technology and gave PSI an order to provide a SmartPlug that could negotiate not only the 11.3km to the set position, but also a 1.8D bend and an S bend. It would then have to isolate pressure for 12 to 18 month, unset itself, then pig through the tee-piece and the 93km to the onshore receiver.
On 7 November 1999 the SmartPlug was inserted into the pipeline on the platform at the end of the 30" line, pigged through the bends and stopped at the 11.3km point, just before the tee-piece. The subsea antenna was placed over the pipe, and instructions were given via ELF to the SmartPlug, causing it to set. It continues to date to isolate the 11.3km section from the downstream tee-piece and platform. Once the 11.3km section had been isolated, the second platform was and is able to continue in operation while the new line is being laid.

© 2000 Underwater World Publications Ltd.