In a Nutshell:
Safely operating a pipeline requires keeping track of many different factors, including maintenance and repairs. Regulations offer guidance on what elements must be considered; one of the key factors for gas pipelines is humidity. The standard in Mexico, NOM-001-SECRE-2010, requires humidity in gas pipelines to be below
110 mg/m3. This case study details the drying of a 42” natural gas pipeline in Mexico, specifically the last 230-km section of the line, which had exceeded acceptable humidity levels.
In the case of this Mexican asset, the mountainous terrain surrounding this specific pipeline causes water to collect in areas of low elevation, which increases humidity and poses a threat to asset integrity. In-line drying activities are also affected. Increased water and humidity levels can render simple pig runs ineffective for two primary reasons: 1) Displacing liquid upwards is complicated. 2) Due to the elevation changes along the pipe route, cleaning tools would seal less effectively and much of the liquid would bypass the pig.
WHAT IS THE SOLUTION?
This is a classic example of why a full cleaning program is more effective than just one run. In this case, the plan devised by ROSEN experts included a dewatering configuration to push as much liquid as possible out of the pipe, followed by a series of bare foam pigs to absorb remaining liquid like a sponge. So, in short: dewatering first, then drying.
The program began with a dewatering train consisting of two ultimate cleaning pigs with multiple sealing discs and brushes, five low-density foam pigs, and three high-density foam pigs. This initial dewatering train, propelled by the gas flowing through the pipeline, would push not only liquid out of the line but also dust and debris. The sludge subsequently created could not be sent downstream into the next pipe sections. Therefore, a separation and filtration system was installed at the receiver end. This system first separates the sludge from the gas and then filters smaller particles from the medium. These products can then continue to move downstream without causing damage to the rest of the pipeline system.
Figure 1 – Tool arrangement of dewatering train
After a successful dewatering run, subsequent drying tool trains followed, each time reducing the levels of moisture in the pipeline. In total, three additional tool trains travelled through the line before it reached acceptable humidity levels. In order to ensure the moisture in the pipeline was continuously decreasing after each tool train, a handheld dew point meter was used at each valve along the pipeline. During the operation, excess gas was vented or flared at eight vents located along the pipe route, at which point humidity measurements could be taken.
Drying of Gas Station
In addition to drying the pipeline itself, the operator required a drying service for the gas station. Not being able to navigate the pipe from the inside, to dry this section of pipe, ROSEN technicians injected nitrogen directly into the system at a high flow rate. The properties of nitrogen allow for the drying of the system without interfering with the product.
Figure 2 – Humidity levels during the full cleaning program
With the various drying activities completed, the pipeline section was back to operating within the required specifications (below 110 mg/m3), making this a successful drying campaign. To ensure continued safe operation, our ROSEN experts recommended periodic cleaning and in-line inspection to enable effective integrity management decisions throughout the lifecycle of the pipeline.