Cleaning pigs in action

It is generally accepted that pipelines offer the most cost effective and efficient method of transporting liquids and gases, however, this assumption depends upon achieving the required throughput, coupled with the lowest capital investment to achieve the lowest operating costs.

It is also accepted that in order to achieve continuous operation, together with the required payback, is based upon ensuring that the optimum flow is achieved with the minimum amount of maintenance. Cleaning to remove substances which may damage the pipeline, disrupt or reduce the flow should be conducted with purpose designed cleaning pigs. The calculation of these projected costs can be further complicated by the fact that flow rates, and consequent deposition, can vary dramatically over time which means that the design of the pig type and/or the frequency of cleaning can change over the projected lifetime, and consequent pay-back, by a number of factors. The anticipated lifetime of asset may be extended by many years due to the development of new extraction techniques has significant extended production but at the same time altered the pigging requirements of mature reserves.

Subsea deepwater pipelines further exacerbate pigging variables which impact upon cleaning frequency and consequent pig design. Cold seawater temperatures can lead to increased deposition on the pipe-wall, with paraffins and hydrates of particular con-cern. The use of flexible pipe for connection jumpers, risers and flowlines is prevalent in subsea pipelines which can mean pigs need to be designed to run in multiple diameters. Further potential complications for pig design include valves, bends, multiple offtakes and wyes all have implications upon the pig design. For example, tight bends will reduce the pig body length however wye pieces will dictate a longer pig body length and the use of both may dictate an articulated pig design. Often the most cost-effective pigging solution is to pig from a launcher topside through to a receiver on land. One of the most expensive pigging solutions is the use of remote subsea production systems which may need extra expensive flowlines to create a pigging loop from the surface or through an even more expensive subsea launcher at the production site which will require periodic intervention by a surface supported system.

An example for changes in the relevant cleaning tool is a 36 in. offshore to onshore gas transporting pipeline which originally used inflatable magnetic polyurethane spheres for the removal of condensate. The preferred pigging tool then subsequently changed to a bi-directional pig. However, the volume of condensate being pigged out increased beyond the capacity of the slug-catcher. Consequently, a cup pig was designed to control the amount of condensate removed by the cleaning tool which proved very successful. It was then decided that the client wanted to increase the frequency of pigging and to semi-automate the existing very old launcher to minimise wear and tear on the isolation valves and closure. This requirement necessitated a conceptual redesign around a re-movable cassette followed by full scale testing to ensure the successful launch using either water or air as a launching medium with client witnessed testing with the production pigs.

Pipeline pigging phases throughout pipeline life

Various designs of cleaning tools or pigs are used over the course of the pipeline life cycle, although the function and design can vary greatly. Life cycle phases include:

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Read the article online at: https://www.worldpipelines.com/special-reports/20092024/cleaning-pigs-in-action/