Pre-commissioning oil and gas pipelines for field readiness requires a series of tests to validate the system’s structural integrity before introducing the hydrocarbons to be transported. Hydrostatic testing (hydrotesting) for leaks generally involve a flooding and pressurization process— the most important of all pipeline integrity tests. The flooding phase is typically carried out with a diesel-driven water pump, while the pressurization phase requires compressor capacity. Hydrotesting has long been employed to verify pipeline integrity. Several types of information are obtained through this testing process, such as flaws in the pipeline material, stress corrosion cracking and specific mechanical properties of the pipe.

To properly perform the hydrotest, the temperature of test water must first be stabilized and cooled to the pipeline’s organic temperature before the actual pressurization phase of the test can be initiated.

Hydrotesting is also the most time-consuming test, requiring several stages to properly perform the procedure. However, mechanical cooling strategies can be engineered to minimize the time to cool the test water used in hydrostatic testing.

Cooldown

Industry experience indicates that up to 10 days may be needed for test water to stabilize to an acceptable temperature. Considering the current economic climate in the energy industry value chain as pipeline operators try to efficiently link hydrocarbon supplies to demand centers, this length of time spent waiting for test water to cool down is often unacceptable. The costs associated with keeping crews on site are high.

This was the case with a contractor that needed help reducing the time to commission a newly constructed 30-inch natural gas pipeline in the Permian Basin.

Hot ambient weather can typically result in test water temperatures as high as 110 degrees Fahrenheit (F) during the summer in the basin. The temperature in the frack tanks containing the floodwater must therefore be reduced to about 70 F before flooding and pressurizing the pipeline.

Once the hydrotest is completed, the test water needs to be reflowed back into the frack tanks, and the full segment of pipeline tested must be dried according to stringent moisture specifications to prevent formation of hydrates, microbial contamination and other quality concerns. Against this backdrop, cooling down test water in a matter of hours instead of days can significantly reduce time and cost.

To illustrate the importance placed on cooldown strategies, a 400-ton chiller and 1-megawatt diesel-driven generator were utilized to engineer and install a water cooling system for a pipeline builder, bringing the hydrotest water temperature down to 60 F in just five hours. The mechanically cooled water was then flooded into the 4,600-foot segment of pipeline to expedite the completion of pressurization testing. Mechanical cooling using a chiller cut days off the planned schedule and reduced associated project crew costs.

Meeting specifications

To properly manage water flow from the chiller to the eight frack tanks, engineers installed custom manifolds and valves at the tanks. A 1,500-cubic-foot-per-minute, diesel-driven air compressor located on the opposite end of the pipeline provided 100% oil-free air to push the test water back into the tanks.

The same air compressor then dried the pipeline by pushing eight drying pigs through the line. To perform the final step of removing moisture from the piping system, engineers deployed a low-amperage desiccant dryer to reduce residual moisture to required minimum levels. This type of dryer configuration operates through a 120-volt current that is obtained by installing an inverter on the compressor batteries with the capacity to achieve the desired dew point below the hydrate formation curves, taking into consideration the temperatures and pressures typical of gas pipeline operations in the Permian.

Moisture and dew point targets specified by the pipeline operator typically depend on the type of hydrocarbon to be transported. In the case of the Permian Basin pipeline, the generally accepted pre-commissioning moisture limit for gas is about 40 pounds per million cubic feet, or a dew point of -40 F, predicating the selection of the diesel-driven desiccant dryer compressor package. With other pipeline hydrotesting applications, targeted dew points as low as -100 F have been achieved by cleaning, drying and reducing residual moisture.

In general, new pipelines have relatively more flexibility with specifications compared to existing pipelines with a history of maintenance and repair that need to adhere to more stringent dew points. For example, dew point limits are a major consideration in preparing the pipeline for startup to help prevent the potential for hydrate formation and contamination by microbial growth once the pipeline is in operation.

If the dew point discharge point meets the dryness criteria of -40 F, then the drying phase is complete.

Test options

Selection of compressed, oil-free air or inert nitrogen for propelling pigs in the drying phase depends on the volatility and combustion risks that are expected. If the risk of combustion has been eliminated, the use of compressed, clean dry air is preferred due to its efficiency.

The industry paradigm that “you pay for what you use and you use what you pay for” influences the decision on selecting compressed air for efficiently hitting required dew points in those instances where volatility risks have been ruled out.

The higher costs of inert nitrogen can be justified when pre-commissioning tests need to be conducted in a potentially volatile environment.

When nitrogen is employed at a site, the additional costs of nitrogen lost to vaporization from the time the trailer is filled and prepared for transport to the test site must be factored into the total costs. Whether employing compressed dry air or nitrogen, drying a pipeline requires relatively low pressure (less than 100 pounds per square inch).

In the Permian Basin and other expanding shale plays, section-by-section testing of the pipe’s integrity may require several pigging phases for cleaning and drying that involve using inert nitrogen in the first phase, followed by using dry air in the next phase. For example, in a four-step phase, two of the phases involve dry air and two involve nitrogen, particularly with gas pipelines that transport NGL, etc.

In any case, there are inherent risks that need to be considered, whether for new lines with less stringent specifications in place or for existing lines that have already incurred substantial maintenance and rework.

The upside

With all these bespoke pre-commissioning considerations, the Permian pipeline builder saw how a rapidly produced turnkey solution drastically accelerated the time line for pipeline pre-commissioning, from setup to fluid cooldown, in preparation of actual pressure testing up to 1.5x the actual pipeline maximum allowable operating pressure. The turnkey solution also could hold the pressure for 4 hours to 8 hours, followed by dewatering, drying and removing moisture.

The pipeline construction company had not previously used a mechanical cooling system, instead choosing to let the water cool organically in the pipeline. In addition to the significant reduction in hydrotest water cooldown time, the onsite solution encompassed the entire hydrotesting process, including 100% oil-free compressed air for dewatering and pushing pigs and low-amperage desiccant drying for moisture removal.

With the industry more focused on hydrostatic testing and maintaining the integrity of the aging pipeline infrastructure, more pipeline operators are outsourcing the multiphase pre-commissioning process to a one-source provider capable of delivering a comprehensive solution. Moreover, new shale-sourced hydrocarbon production has substantially shifted gas flow dynamics, predicating a need to increase the frequency of the hydrotesting phase for upgraded pipeline networks.

As a result of the hydrotesting capabilities available from a single source, the Permian Basin operator was able to noticeably compress the pipeline pre-commissioning process, drastically reducing the time and costs traditionally associated with this effort. That enhanced the project’s upside potential.

Forrest Marsh is oil and gas manager-North America for Aggreko Plc.

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