Small diameter natural gas pipelines with lower internal pressures inherently are difficult to inspect due to the lack of head pressure working as a brake for the tool. The small surface area of the pipeline creates a need for higher pressure in order to centralize inspection tools and lower the amount of speed excursions. When including debris, tight radius bends and wall thickness changes the likelihood of speed excursions is increased.
It is common to use a tether solution for these assets; this will insure 100% data collection. However, in order to run a tether inspection the asset must be empty of all product and not containing any hydrocarbons. Often also needing to be exposed below ground and cut into multiple sections. The additional work for a tether inspection is costly and time consuming to the client. Inspecting assets in operation does not require any downtime and little to no gas flow upset.
A client has asked us to inspect a 3-inch gas pipeline. The line has a nominal wall thickness of 3.96mm and a maximum wall thickness of 7.62mm. In addition to the wall thickness the pipeline, there were 5D bends in the heavy wall sections. The line pressure was 4000kPa at a maximum with a gas flowrate of 1000m3/hr. Gas lines in 3 inch are very challenging due to the low surface area of the tool. Often inspections in gas fail due to areas of over speed.
ROSEN provided a custom solution with a low friction setup, to lower the drag forces. The combo design utilized was able to capture both MFL-A and Geometry features in the pipeline. The recommendation was to keep the tool at 4000-5000 kPa and 800-1000 m3/hr. This would give the tool enough velocity to get through the areas with thicker wall thickness and small radius bends.
There was one section of the pipeline where the velocity reach above 5.0m/s, which is the maximum for MFL-A technologies. The average velocity of the inspection was 2.36 m/s with one area reaching 5.8 m/s. 99.86% of the pipeline length was within the acceptable velocity range. This resulted in a successful inspection. This was a cost savings to the client not having to dig bell holes for a tethered inspection. This was the first 3 inch combo inspection in Gas for ROSEN.
A 23km 6 inch Natural Gas emulsion line with low operating pressure. The line has a nominal wall thickness of 4mm. The pipeline has multiple riser sections with 1.5D bends. The line MAOP is 4000kPA and the line operates at 1550 kPa and a flowrate of 4167m3/hr. The pressure could not surpass 3000 kPa for the inspection due to emergency shutdown pressures on the compressors. The flow rate could be manipulated to suit the inspection. This low pressure causes a difficult to inspect with conventional inspection tools due to the heightened possibility of over speed.
After reviewing the details of the pipeline and calculating the flow requirements necessary for a successful inspection. ROSEN utilized one of our low friction services, with recommendations for the client to build between 2000 kPa and 2500 kPa line pressure and have a gas flow of 3750 m3/hr. This would have the tool average at 2.5 m/s, which is ideal in low-pressure situations.
The client was able to have the inspection done on this line without getting a temporary increase of MAOP or adding additional pressure units to the pipeline. The inspection was successful with one area of over speed at the receiver station, reaching 5.38 m/s. The inspection recorded 99.7% of the pipeline while staying under the 5.0m/s threshold, which is critical for MFL-A technology. The average pressure during the inspection was 2358 kPa with an average velocity of 2.41 m/s.
A 24km 8 inch Sour Natural Gas emulsion line with low maximum allowable operating pressure of 3200 kPa. The line has a nominal wall thickness of 4mm with a maximum of 8.92mm in the riser sections. One particular riser section had multiple 3.0D bends in a short section of pipe. The line has to run at less than 2500 kPa due to pressure restrictions on the compressors with a flow rate of the pipeline is 1250m3/hr. The low pressure in an 8 inch pipeline is often a cause of over speed situation through bends and at wall thickness transitions.
After reviewing the details of the pipeline and calculating the flow requirements necessary for a successful inspection. ROSEN utilized one of our low friction service tools paying close attention to ideal velocity. ROSEN recommended that the client to provide 6250 m3/hr flow with 2000kPa line pressure resulting in 2.2 m/s gas velocity. A natural gas pumping unit in addition to the flow from the well delivered the additional gas volume needed for the inspection.
The client was able to have the inspection done on this line without getting a temporary increase of MAOP and without flaring any of the gas on the receiver side. It was also completed without building additional pressure on the line. There above ground riser sections with the 8.92 mm wall thickness showed increases in velocity throughout the run with the area of back-to-back 3.0D bends going over 5.0 m/s. 98.76% of the pipeline was under the 5.0m/s threshold required for optimal feature detection and sizing. The average pressure throughout the inspection was 1930 kPa with a maximum of 2118kPa in the area with back-to-back bends.