Introduction
High Friction pigging can aid in pipeline installation and maintenance in many ways. A high friction pig can be used to simultaneous install and flood a pipeline. The pig allows the line to be flooded which overcomes the buoyancy but stops the floodwater from reaching the lay vessel: –
Another possible application is the use of High Friction Pigs during repairs or replacements such as valves where only a small pressure difference is required to be held by the pig. Instead of using specialised plugs, High Friction Pigs are considered an alternative. A safety factor is generally applied between the actual pig differential and the head difference required.
This review details some of the design issues associated with these pig types: –
- How to maximise the differential pressure across the pig;
- How to minimise the wear on the discs which acts to reduce the differential pressure;
- A possible design route for such a pig development.
How to Maximise Pig Differential Pressure
A model has been developed to determine the force and friction exerted by the seal and the pig in the pipe wall. The model can then be used to calculate pig differential pressure, with the following seal variables: –
- Seal diameter;
- Seal thickness;
- Seal hardness (Estimated as a compressive and tensile Young’s Modulus);
- Pig clamping flange diameter.
The differential pressure (DP) across the pig can be maximised by altering the physical design of the seals. Whereas it is clear that increasing the thickness and the diameter of the seals will increase the differential pressure, such action can also result in damage to the seals and/or the pig. Therefore, the parameters are varied to increase the pig differential pressure but without causing undue stress.
The most effective parameter to vary is the flange diameter. An optimal flange diameter can be achieved so that the differential pressure can be maximised on this basis: –
How to Minimise Wear Effects
Once the pig has been design to support a required differential pressure, then it is necessary to ensure that this can be maintained over the length of the pipeline. The model has been extended to allow abrasive wear to be taken into account with representative wear rates.
The figure below shows a typical differential pressure against distance graph. It may be necessary to return to the seal design again and increase the differential pressure that the seal can support across the pig, in order to take the effects of wear into account. An iterative approach to the High Friction Pig is proposed.
Estimation of differential pressure against distance has been verified for a number of different line sizes (10″, 36″ and 42″), with and without pipeline internal coating.
The Design Process for High Friction Pigs
Other aspects in the design process include ensuring that the seals do not buckle and that they do not pull out of the flanges because of high drag with the pipe wall. Compression set is also a problem and this is generally considered during testing. These aspects are taken into account in the overall High Friction Pig Design.
