Development and Use of High-Differential Pig Trains for Pipeline Isolation
Authors: Cooper T.A. , Graves L.J. 
Published: October 2008

ABSTRACT
This paper describes a safe isolation technique for gas pipelines to enable construction activities, without complete decommissioning and recommissioning, The technique can be applied to valve installation or replacement, bypass of structures or platforms, and modifications such as to tie-in of pipelines to existing gas systems.
This paper will examine the development and use of a technique employed on three major North Sea Projects since 1989, namely the High Differential Pig Train.

INTRODUCTION
Since 1989, activity in the area of localised pipeline isolation in the North Sea has increased dramatically, especially for the installation of topsides am. subsea emergency shutdown valves.
In April 1989 Total oil Marine PIC (TCM) commissioned a study to evaluate isolation systems which net the Company' s isolation philosophy and would be available for an August shutdown to replace pipeline riser emergency shutdown valves.
From the results of the study and from other Operator's experiences TCM were not satisfied that a completely safe isolation system was readily available. Options available were:Completely decommission pipelinecancel proposed shutdownDevelop a safe isolation technique in 6 weeks
Option C was preferred since option A would involve unacceptable loss of production and option B would leave unacceptable isolation valves in service.
As a result of McKenna and Sullivan's (M & S) experience in pipeline isolation and commissioning TCM developed jointly with M& S a technique which net the Company philosophy within 6 weeks. This technique has since been successfully deployed on numerous North sea Projects.
The parameters for design were based on creating a safe work environment for hot and cold work to:–provide a safe isolation for construction activities–provide the ability to monitor pipeline conditions–minimise recommissioning problems associated with the introduction of water into gas pipelines–comply with Governmental Safety and Guidance notes
The technique developed was based on using modified bi-directional pigs to from an isolating system capable of withstanding suitable differential pressures without moving and providing the necessary sealing to act as an interface barrier between inventory gas and the worksite. The isolation technique resulted in reduced production outage and lower construction costs compared to full pipeline decommissioning / recommissioning.
This paper outlines the development and use of the technique on three TCM projects on the Alwyn and Frigg Gas Transportation system (see Figure 1).

Project 1 : 1989

CHANGEOUT OF TOPSIDES ESDV?S (Emergency Shutdown Valves)
TCM developed with M&S an isolation system which consisted of two parts:Part 1 Primary Pipeline Isolation to allow cold cuttingPart 2 secondary Riser Isolation to allow Welding

Part 1 Primary pipeline Isolation
Pipeline isolation, after depressurisation but not degassing, was accomplished in each Riser using a series of bi-directional pigs separated by slugs of nitrogen, gelled diesel and diesel forming a pig train (figure 2) referred to as Primary Dynamic Seal (PDS).