This brief review shows our current understanding of how a waxy pipeline plugs with non-bypass pigging and how bypass pigging allows this problem to be overcome. Pipeline dewaxing with non-bypass pigs is responsible for blocking several pipelines every year. These are generally small bore lines, less than 14-inch for example.
The figure shows the non-bypass scheme where a plug is build up ahead of the pig. Inlet pressure then rises and the pipeline shuts down due to low of flow: –
The pig begins to push on the wax column. As the pig pushes on the wax, this causes the shear stresses between the wax and the pipe wall to increase, thus again increasing pressure. The analogy is with soil mechanics: –
The shear stress increases with normal stress or the pressure in the plug and acts to increase the pressure required to push the plug.
Due to the pressure gradient, oil is then pushed out of the wax, further compounding the problem as higher shear stresses are developed. Finally, the pressure required to push the plug is greater than the pressure available or the line MAOP and production halts.
Pipeline Research Limited is currently working on a mechanistic model based on this scenario to try and further understand the problem and ways to combat it in the field.
The problem is alleviated by using bypass pigging. The scheme is shown below: –
Our current understanding is that by maintaining a flow of oil or condensate through the wax, then the wax has room to move inwards as it is removed from the wall. Hence, the self-locking mechanism as described above with shear forces increasing all the time is not able to happen.
A further spin off is that much of the wax that enters the flow path of the bypass is flushed ahead of the pig and into the flow stream. The Continuity Principle – a method for determining pigging frequency – is used to ensure that the flow path is kept open and that the wax arriving cannot overwhelm the bypass flow.
