A dual diameter pig train is used to dewater a 10″ x 14″ pipeline. The pipeline consists of a 1000m 10″ riser, 228.7mm internal diameter, expanding to 14″, 320mm internal diameter. The pig train consists of 5 pigs with DEG between the first 4 pigs and then a gas volume between the final two pigs. This is to allow the final glycol in the line to be collected and removed from the system.
50m after the expansion there is a wye piece with another 14″ line joining the main export line to shore. It is required to simulate the motion of the last pig in the train and care must be taken to insure that this pig does not reverse back into the wye piece due to compressible effects.
Whenever a pig experiences a change in sliding friction with the pipewall, in a gas pipeline, it will undergo transient motion. This can occur at a weld in a pipeline for example, where excess pressure must build up because of the pig stalling at the weld. This energy is then converted to kinetic energy and high velocities result.
In dual diameter pigging this can occur when there is a large difference between the pig differential pressure in the small diameter line and that in the large diameter line. This can cause high accelerations, excessive velocities, possible reversal of the pig in the large diameter line, and damage to the pig and/or pipeline components.
The volumes of gas either side of the pig will determine when and where the pig will reverse. A number of undesirable events may occur: –
- Reversal into the wye, damage to the pig or possible stuck pig;
- Pig stops in the reducer (a dual module pig with rear module bypass may stall due to flip of the seals);
- Possibility of the pig trying to reverse back into the reducer. This is a less likely scenario.
The following input was used in this Case Study: –
- Pig DP in the small diameter line = 2.5bar
- Pig DP in large diameter line = 0.1bar
- Pig reverse dP in the large diameter = 0.25bar
- System pressure = 40bar
The first figure shows the pig velocity and distance travelled for the pig with a large gas slug downstream. This shows the risk of reversal back into the wye piece. The propagation of pressure waves up and down the pipeline governs the timing of the reversal.
To solve the problem the gas slug length is shortened. This is shown by the second output trace for this case. It may be possible to alter other aspects such as system pressure (The higher the system pressure the more stable the pig motion), but this was not possible in this instance.