Although high pressure pipelines are designed, constructed and operated within the requirements of exacting Engineering Standards and Codes of Practice, defects continue to occur which may result in pipeline failure and this suggests existing preventative procedures are not totally adequate. British Gas, as the owner and operator of a major gas transmission system, has recognized this deficiency and developed new equipment which can be run on-line to monitor pipeline integrity, non-destructively, to standards comparable to those obtained by hydraulic pressure testing to yield..
To date over 6,000 km of onshore oil and gas pipeline have been inspected with the equipment and the results obtained clearly demonstrate that the required inspection performance has been reliably achieved. This work has now been extended to include sections of offshore pipeline in the Corporation's Rough and Morecambe Bay installations, where engineering features have required changes to vehicle design. Primarily, equipment capable of negotiating a wide range of bore sizes and changing wall thicknesses, combined with controlled operation in vertical pipe sections, and the capability of surviving under harsher environmental conditions, had been specified. All these exacting requirements have now been met, and the vehicle systems developed have been fully proven in onshore pipelines under operational conditions prior to commencing routine operation offshore early in 1985.
From a study of the experience gained to date, the development by British Gas of pipeline on-line inspection equipment which can equal, and in most cases out-perform, all other conventional methods of monitoring pipeline condition, has clearly been technically successful. When the information obtained from its use has been combined with the practices embodied in soundly based repair procedures and protective measures, overall expenditure on pipeline maintenance has been significantly reduced whilst standards of pipeline integrity have been maintained or, in most cases, improved.
At the present time there are estimated to be over 1,000 offshore oil and gas installations worldwide. In some cases several production areas are interconnected by a series of interfield pipelines to a central point from which a main export pipeline brings the oil or gas ashore; in others, major fields are served by a single pipeline. In all situations, however, the main trunk pipeline provides the critical link between the production area and the shore terminal.
All pipelines located both on and offshore are designed, constructed and operated in accordance with Standards and Codes of Practice which aim to ensure safe and economical operation for long periods of time. Despite this, however, defects can and do occur and if catastrophic failure is to be avoided some means for periodically assessing pipeline condition must be employed. Conventionally, for accessible pressure vesselswhich can be taken out of service, revalidation is completed using an over-pressure test, applied hydrostatically and meant to burst-out defects approaching significant dimensions at the operating pressure.
