Assessing the integrity of nitrogen-filled pipelines with ILI tool runs

“Agility” is a big thing in business these days – being able to respond quickly to changing conditions. In pipeline operation, that includes being able to safely return idled lines to service in a timely manner so you can take advantage of opportunities for them to carry product again. That product might be hydrocarbons, another fuel such as hydrogen, or a greenhouse gas like carbon dioxide. But assessing the integrity of idled lines can present unexpected challenges – particularly when it comes to running inline inspection (ILI) tools through the nitrogen that is used to monitor idle lines for leaks and as a blanket to protect them from corrosion.

A recent project that we supported points to some of the challenges involved in running ILI tools with nitrogen.

Several years ago, a major global energy company turned to us to help engineer, plan, and execute the product displacement and nitrogen packing of a pipeline – its days of transporting a single high-value refined product were over. Read more about that here. More recently, they returned to us with a new mission: help run gauge pigs and combination ILI tools through the multiple segments of the idled 450-mile line in preparation for its return to service.

HT Engineering worked closely with the company’s project manager and project engineer to synthesize requirements from management, technical, and operations stakeholders, and develop an execution plan that provided the company’s management with confidence that the project could be completed safely. The team initially proposed a plan to run multiple pigs simultaneously that would minimize nitrogen costs and project duration. However, personnel constraints and risk concerns drove the final plan to run one pig at a time.

During the planning process, HT Engineering built a numerical hydraulic model to calculate the nitrogen flow rates and pressures required to successfully run the pigs. Multiple iterations of planning, modeling, and validating were needed to satisfy the operator and ILI vendor stakeholders. The model allowed the team to optimize nitrogen injection and release locations, plan injection and release operations, and verify that pressures and pig speeds could be kept within acceptable ranges for the inspection of each segment.

In addition to the execution plan, HT Engineering worked with the local operations personnel and the ILI vendor to develop detailed work procedures and facility schematics that guided the team step by step through each of the gauge pig and ILI tool runs. After each operation, we helped to capture lessons learned and updated the subsequent procedures accordingly, allowing the team to improve its performance as the project progressed.

While the team conducted the ILI operations, HT Engineering worked on site, supporting the company’s project manager by tracking procedure completion and monitoring operating conditions. By monitoring and analyzing the nitrogen injection and release rates, line pressures, and pig location and speed, we were able to provide the PM with the information and recommendations he needed to run the operations safely and recognize emerging problems quickly.

One such problem occurred when an ILI tool’s drive cups wore out part way through the first smart pig run. The worn cups allowed the nitrogen to bypass the pig, which stalled in the line. After the team successfully blew down a section of line and cut out and replaced the pipe that contained the ILI tool, HT Engineering helped the company and vendor subject matter experts diagnose the cause and design a solution.

The cause turned out to be the abrasive fine grit in the pipeline combined with the lack of a lubricating liquid, causing a much higher wear rate than the ILI vendor had expected. The solution was to use drive cups with metal inserts that protected the polymer from abrasion and allowed it to maintain a seal.

A second problem resulted in speed excursions that made another of the early runs unacceptable. Most lines designed for liquid contain a range of pipe wall thicknesses along their length. It takes more differential pressure to move an ILI tool through pipe with a smaller inside diameter, and so the tool would slow or stop in sections of heavy wall pipe until the differential pressure built up enough to push it through.

While the pig stopping or slowing isn’t desirable, it’s usually not cause to reject a run. The real problem happened when the tool reached the end of the heavy wall pipe sections. With the high differential pressure that had built up and the sudden reduction in resistance, the tool would enter the thinner wall section like a champagne cork popping from a bottle. In one instance, the tool recorded highway level speeds.

The solution in this case was simple. The client had been reducing nitrogen injection to maintain a constant pressure on the upstream side of the pigs and increasing the release of nitrogen on the downstream side to lower the pressure to achieve the difference needed to get the pig through the heavy wall pipe. This created a “weaker spring” in front of the pig when it left the heavy wall pipe that was unable to prevent it from accelerating to unacceptable speeds.

For future runs, we instead held the pressure in front of the pigs constant and increased the pressure behind them when necessary, maintaining a “stronger spring” in front of the pigs that prevented them from exceeding their acceptable speed range.

Ultimately, despite the two required reruns, the client’s project was successful. With HT Engineering on the team, our customer safely and successfully ran five-gauge pigs and five combination ILI tools in nitrogen in the idled liquid pipeline. These inspections provided information about the idled pipeline’s condition that was used to prepare it for eventual reactivation.

What’s different about running ILI tools in nitrogen?

What circumstances might call for a need to run ILI instruments through nitrogen? Often, a liquids line is idled and packed with nitrogen to help detect leaks and protect the line from corrosion. Then, as in our story above, there comes a need to return the line to service, and a integrity assessment is needed to understand the line’s condition and plan any needed repair work.

What often happens is that pipeline operators accustomed to running pigs in liquids are unaware of what’s different about running tools through a gas such as nitrogen. Three of the biggest issues are:

1. Liquids are relatively incompressible, gas is very compressible. Liquids provide a “strong spring” upstream of the tool that pushes it along. Liquids also provide a “strong spring” ahead of the tool that prevents it from surging ahead too fast.

But, as we saw in the story above, gases like nitrogen are a weaker “spring” and may not provide enough pressure to move the tool ahead if it hits a constriction. If a tool is stopped, gas will build up behind it, increasing pressure. It may take time to build enough pressure in the gas to overcome the resistance. Then, when the tool does move past the constriction, it may surge forward against the weak “spring” in front of it too quickly to gather quality data, possibly ruining the run.

2. Consequences of line failure are different. Another “gas pipeline vs. liquid pipeline” difference shows up in the consequences of a pipeline failure. In the case of a pipeline failure in liquid service, the product will spray quickly from a hole or crack in the pipe, and will be quick to come to a neutral pressure. However, a pressurized gas release will last much longer, and may cause a much longer tear in the pipe, which results in a greater direct safety hazard to workers and the public than a liquid release.

3. Tool abrasion may be greater in gas. In a liquid line the cargo usually provides some lubrication value, so the tool slides past restrictions. Liquid also provides lubrication against grit in the line that can otherwise abrade the tool’s drive cups. Nitrogen doesn’t provide that lubrication, and can “dry out” and embrittle some polymers.

Working with nitrogen

Some tips for success that HT Engineering has discovered while working with liquid and gas lines – including running tools in liquid lines filled with nitrogen, are as follows.

• Select ILI technologies that can operate in a gas environment, or plan to run the tools in a slug of water or liquid product. Ultrasonic tools need a liquid couplant to work, while magnetic flux leakage and electro magnetic acoustic transducer tools work well in gas.
• Ensure the tool vendor designs the tool’s drive and other cups to accommodate the conditions in the pipeline. This may include use of steel inserts in the polymer, and polymers that are compatible extended exposure to nitrogen.
• If available, use tools that have speed control; they can automatically allow some gas to bypass the tool and slow it down if needed. This can help avoid unacceptable excursions and reruns.
• Nitrogen is held at about -330 degrees F. It must be warmed before it is released into the pipeline, because that kind of cold (just a bit colder than Michigan’s Upper Peninsula in winter!) can embrittle pipeline steel. For many pipeline personnel, working near liquid nitrogen is an unfamiliar situation for which training, good procedures and safe practices are needed.
• Releasing nitrogen comes with a host of considerations. Temporary manual (continuously manned) control valves are typically used rather than automated valves and permits and filtration may be required – or drive the selection of one release location over another.
• Safety considerations on site include having a windsock at the release location to help understand dispersal of the N2. It usually isn’t a problem – the gas dissipates in even a light breeze – but it can settle in low areas if there’s little or no wind. Gas monitors at both ends will provide alerts if there are hazardous gases or the oxygen level is low.
• To provide a smooth, problem-free tool run, it’s important to consider the tension between two priorities: (1) Higher pressure in the line will keep the tool moving through at a steadier pace due to the “stiffer spring” ahead and behind; it’ll push through constrictions more easily with lower-amplitude hang-ups and surges. (2) On the other hand, there are risks from having the pressure too high – including leaks and ruptures in the line. Work with your inline inspection vendor and internal stakeholders to select the allowable pressure ranges for the runs.
• There may be liquids in low spots in a purged line; be sure that the tool you choose can deal with liquids and still provide good data. Have a way to deal with the liquids at the tool receipt point, such as a vac truck, and at the nitrogen release location, such as a frac tank. Be prepared for slugs of liquid that may come through at the receiving end, as well as product vapors that come out with the N2.

Getting the help you need

HT Engineering has had significant success helping pipeline operators plan and execute projects that involve running ILI tools in nitrogen, and we’re familiar with both liquid and gas pipelines. We also have experience working with nitrogen vendors, tool vendors and other industry specialists, and we know how to get best results from them.

We also have experience with all aspects of the nitrogen ILI process: planning, numerical modeling, preparing detailed procedures for all aspects of the work, designing temporary facilities for injection, release, launch, and receive sites, tracking pigs, data gathering, tracking, and analysis, troubleshooting and root cause analysis, problem solving, and closeout documentation.

Contact us if you need help determining an idled line’s integrity so you can get that line ready to move product and generate revenue again.