June 2011 Vol. 238 No. 6

TechNotes

Piggable Pressure Isolating System For Untapped Live Onshore Pipelines

In a perfect world every pipeline repair or maintenance site would just happen to have a valve on either side to use to isolate the work site. It is not a perfect world but the good news is that help has arrived. With the advent of new communications systems and piggable pressure isolation tools, that isolation capability now exists.

In other words, it is now possible to have an internal temporary “piggable valve” placed at any location (or locations) on a pressurized onshore pipeline at any time.

Pressure isolation – isolating the work site from the pipeline content under pressure using remotely controlled (tetherless) piggable plugs – is quickly becoming the norm for allowing repairs, or other work such as modifications or tie-ins, to onshore pipelines.

These tools, such as the Lineboar Piggable Valve™, allow repair and maintenance to take place without blowing down, evacuating or depressurizing the onshore pipeline, and without welding on or cutting a hole into the line using hot-taps or stopples. In other words, both the upstream and the downstream of a pipeline repair can remain at operating pressure, with the work site only needing to be evacuated of content.

Use of such tools is the equivalent of placing a valve at any point in the pipeline, without intervention into the pipe, without depressurizing or evacuating large sections of pipeline, and without leaving any trace that the tool/valve was there once the operation is complete.

Operating Mechanism
The concept is relatively simple. A tool that looks and pigs much like any ILI tool, but can also be actuated so that it transforms into a “plug“ which forms a complete pressure seal between the work area and the content in the line.

This “plug module,” along with a communication system in a separate housing attached to the plug module, is inserted into the launcher and pigged to the work site (or two systems which are pigged in so that they straddle the work site). Once the module is stopped at the desired location (see example in Figure 2) an operator sends a signal to the tool instructing the plug module to “set.”

This instruction is sent via a tetherless, through-wall, real-time control and communication system that can send data through ground cover and the pipewall. It is also able to monitor pressures in real time on each side of the tool during the operation.

Upon receiving the instruction from the operator, the tool begins to set itself using an internally powered mechanism that causes the plug module to contract.

As the entire module contracts, a set of segmented gripping wedges ride up a ramp until they engage the pipewall. As the tool continues to contract, the pressure plate at the end of the tool moves toward the ramp, with a sealing element between them. This compresses the sealing element which causes it to extrude until it, too, engages the pipewall. Once set, greater differential pressure across the tool transfers longitudinal pressure circumferentially, maintaining the seal by using the difference in pressure to continue to contract the plug module. At this point, the tool is set and is therefore isolating the content under pressure on one side from the work site on the other side of the tool.

At that point, the work site can be evacuated and work can commence. These tools are designed so that as the differential pressure across the tool increases so does the intensity of the set. They are also designed with a zero leakage tolerance, so that no content contaminates the work site. In other words, with the work site at zero psi and the other side of the tool seeing normal pipeline pressure, there is a differential pressure that acts to cause the plug module to continue to contract, making the seal more robust.

This procedure allows the operator to 1) save time, 2) eliminate the cost of displacing pipeline product between distant valves, depressurizing and emptying the line, and then re-commissioning the line. The only evacuation of product is that in the region between the isolation tools.

History And Current Capability
Similar tools have been used in offshore pipelines for many years with tremendous success. However, due to technical limitations and the requirements of higher pressure offshore lines, piggable pipeline pressure isolation tools have not typically been useable in onshore pipelines.

Two issues related to higher offshore pressures had to be overcome for use onshore. First, high- pressure tools have traditionally had to have a hard diameter very close to the ID of the pipe. Second, due to the need to isolate high pressure, the tool would exert a significant pressure against the pipewall.

By reducing the pressure capacity of the plug module, newer pressure isolation tools can be pigged and set in pipelines with as much as a 10% reduction in ID, without risk of deforming the pipeline. The latest version of these tools can isolate lines at lower operating pressures, and have the ability to open a bypass valve to fill the annulus between two tools after completion of the work.

With these improvements, onshore pipelines can be pressure isolated using piggable pressure isolation tools/piggable valves, providing an alternative to evacuating and de-pressuring the line or hot tap/stoppling.

The main benefit is that pipelines can now be repaired and maintained without loss of pressure or product and without welding, tapping or leaving any trace of the fact that an isolation occurred.

Since the communication system can function through ground cover, it is not necessary to remove cover at the tool site, but only the area necessary to perform the repair.

Like their offshore brothers, the isolation tools/piggable valves are bi-directionally piggable and are operated and monitored entirely via remote control. This eliminates the need to weld a fitting on the pipeline (i.e., no need to hot tap/drill into the pipeline), so nothing is left behind. There are no holes and therefore no potential future leak path and no issues regarding cuttings or coupons.

Typical Uses
Historically, these tools have been used to isolate pipelines during various types of repair or maintenance operations, including pig trap and valve maintenance, repair or replacement, mid-line repair or inspection, and lateral line installations and isolations.

Simple operations such as replacing a pig trap valve only require that one tool be pigged past the valve, before the Tee, so that the valve can be repaired or replaced without interrupting normal pipeline operation.

A mid-line repair can be performed using two tools, one set on each side of the work site so that pipeline pressure and content can be maintained on either side. This limits the distance over which content must be removed and line depressurized since the operator needs only to de-pressure and evacuate the short distance of pipeline between the tools at the repair site.

Once the repair is complete the tool is unset and pigged back to the launcher. It also can be pigged farther downstream to a pig trap or to the next repair site to perform more isolations if there are multiple repair locations.

The author can be contacted at rselden@bjbinc.com.