Pipeline Isolation Using Remotely Activated Spheres
Authors: Newman W.J. , Kontou T.Y. , Aldeen Abdullah 
Published: October 2008

INTRODUCTION

Following the Piper Alpha tragedy in 1988, Shell UK Exploration and Production, Operator in the UK Sector of the North Sea for Shell and Esso, decided to fit subsea isolation valves (SSIV'S) to its offshore gas pipelines which had the potential to jeopardise the integrity of manned living quarters in the event of failure and were not already provided with subsea isolation valves. These lines all have large hydrocarbon inventories. Expro's two largest pipelines already had isolation valves, close to their respective offshore platforms, from inception.

For environmental, safety and operational reasons, Shell considered it beneficial to avoid flooding the lines with water and instead to use a plugging system to isolate the section of line to be cut. Difficulties were presented by the 30" pipeline, since it had design features which precluded the use of the high friction pig isolation system developed for other lines. Modifications to enable its use were considered prohibitively expensive, therefore a new isolation method was clearly required.

Two options were available:Development of a pre-inflated high seal sphere.Development of a remotely activated on-site-inflation sphere.

Due to the very high wear of the pre-inflated spheres when pushed along the pipeline to their required locations, which could result in leakage of the seal, the first option was eliminated.

The second option required the development of a through-pipe Communication system operating in an environment of either gas or liquid, or a combination of both. Several companies had already attempted this but those that achieved any measure of success had required internal pipewall to transducer contact. In the case of a 30" sphere this would be impractical as the stand off would be at least four inches due to skin thickness. In addition the sphere would be rolling and turning, therefore orientation of receiver to transmitter had to be non critical.

Despite the forementioned difficulties, Shell Exproconsidered it worthwhile to develop a remotely activated sphere isolation system and commissioned Acurite Ltd to produce a prototype.

Acurite Ltd are a consultancy previously retained by Shell Expro to engineer innovative solutions to a variety of pipeline problems. They also had already produced an outline design of an isolation sphere and were confident of overcoming the communication difficulties by using an adaptation of an electromagnetic technique which they had used in their NDT research.

DEVELOPMENT

Phase 1

The initial specifications required that the sphere could be:launched and transmitted as a standard sphere inflated to pipeline internal diameter.activated by an external signal, as it passes a given location, to slowly inflate from a self contained source.deflated at the end of the operation, up to three weeks later, for retrieval.

The size for a prototype was set at 20" to accommodate the longest readily available test loop, (at McAlpine premises in Ellesmere Port), which would be required towards the end of the project for dynamic testing.