The condition status and integrity impact of main oil and gas transmission lines is today adequately addressed by a number of orpanisations who have developed technological know-how and applicational experience in the use of sophisticated ‘free swimming’ inspection PIGS capable of performing comprehensive ultrasonic Bi electro-magnetic assessment of pipewall condition over tens of kilometres while propelled by the differential pressure effect of fluid flow However, the most safety sensitive and production critical sections of pipework are ofien those adjacent to process plant or those sited in the vicinity of urban areas and, as such, these usually extend over relatively short distances and offer only a single point of access. Their critical status demands a clear and accurate understanding of their condition, of the degradation mechanisms at play and the propagation trends affecting ongoing reliability. These conditions certainly apply to offshore installations where risers and J-tubes provide the interface between the production platform and its main arteries. The hazard potential of these items of plant has always been recognised by both operators and regulators - but attention to their integrity has been justifiably more focused since the Cullen Enquiry, which highlighted that their proximity to work areas, escape routes and interactive plant elements, was cause for concern.
Quite apart from these safety considerations, plant operators are only too aware of the commercial implications of lost production caused by planned inspection and maintenance and, in the worst case, by unscheduled downtime to effect emergency repair.
Unfortunately, the PIG technology which proves so effective for long range pipeline inspection does not lend itself to the assessment of condition of these more critical piperuns for a number of technical and commercial reasons For instance, although it may be viable to amortise the costs of deployment of these very sophisticated and logistically complex tools over (say) a 100km ‘spread’, the same economies do not apply when only perhaps 0, Skm of pipe are involved.
There are also problems from a technical point of view Most fundamental of these is the fact that the method of propulsion used relies upon differential pressure and this means that although PIGS can be ‘uni-directionally’ conveyed from point to point (or trap to trap), they cannot be effectively deployed and retrieved from a single access point without restricting the flow Also, because they are reliant on fluid flow as their means of propulsion, they are not sufficiently controllable, nor do they run at sufficiently constant velocity, to achieve the very high orders of coverage and measurement resolution necessary to determine the condition of such safety critical components Furthermore, where electro-magnetic techniques are used, the effects of acceleration in the ‘start-up’ condition and speed of motion in normal operation, introduces variables which adversely affect wall thickness measurement reliability
