Corrosion within a pipe can arise from many sources, in both sweet and sour conditions, with the main causative factors being the interplay of entrained water, acids and bacterial activity. For example, in a sweet environment, the presence of carbon dioxide and water can lead to the formation of carbonic acid whilst, in a sour environment, it is hydrogen sulphide reacting with water which produces a corrosive acid. The rate of corrosion can vary greatly and is influenced by the presence or absence of protective coatings, turbulent flow, and the generation of passivating corrosion products, the use of corrosion inhibitors and the maintenance regime of the pipe.
Modern Pipeline Inspection Gauge (PIG) systems are capable of detecting the onset of corrosion and monitoring its spread, providing valuable data on the location and potential consequences of the problem. However, once the presence of a defect is known, there is a requirement on the operator to assess and mitigate against any effects it may have. Existing means of dealing with a corrosion or similar small defects are complex, or expensive, or difficult to deploy, or all of the foregoing, and usually require significant time to implement. TWI Yorkshire’s Friction and Forge Processes section were thus tasked with developing a friction stir welding technique that can be deployed on a specialised PIG to perform a patch repair inside a 36” diameter, sub-sea, steel pipe, with the oil still flowing.