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Internal Weld Repair of Live Oil Pipelines

With many pipelines being kept in service beyond their original design life, and others being used to transport corrosive crude, the requirement to monitor and, if possible, repair internal corrosion defects is becoming more significant. Internal defects are difficult and costly to repair, especially for sub-sea pipelines, yet even these costs are small in comparison with the loss of revenue that ensues if it is necessary to close a pipeline whilst a repair procedure is performed[1].

 

 

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.

Making a friction stir weld requires, as the name suggests, plenty of friction – a commodity in short supply when the area of the steel pipe being welded, and the friction stir welding tool doing the repair weld, are both immersed in oil! Excluding the oil from the region of the pipe being welded not only proved impractical, but would also have introduced a further problem, namely a potential for fire or explosion due to the presence of residual hydrocarbon vapours. It was therefore necessary for TWI to overcome the issues arising from making a friction stir weld in a very lubricating environment, and to develop the necessary welding control parameters that would allow the process to be performed autonomously, in complete darkness, up to 200 miles offshore…

TWI’s Materials Performance and Ferrous Alloys section assisted by assessing samples of the welds made to confirm that the new FSW process parameters had not only produced a sound weld but, equally importantly, had not caused an uptake of hydrocarbons into the weld zone. This was shown to be the case. Further work will be performed by TWI’s NDT Validation Centre, to assist in developing the on-board weld validation system that the PIG will also carry.

In operation, the FSWBOT will be loaded into a pipe at the producing platform and travel through the pipe to the defect site. Once in position, it will deploy a milling cutter to remove the corroded area of the internal pipe surface and position a pre-cut steel patch into the prepared surface. The milling cutter will then be switched for a FSW tool and the patch welded into place and then milled flush with the pipe surface, restoring the pipe wall thickness and allowing full flow of product through the pipe.

 

[1] On 11 Dec 2017 it was reported by Ineos that repairs to a pipe forming part of the Forties Pipeline System would require the closure of the pipeline for three weeks and that more than 80 platforms would have to suspend production. https://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-42308437

Figure 1. Section through an 8mm deep friction stir weld made into a 28mm wall thickness, API-X65 steel pipe. The weld was performed under oil and is defect free
Figure 1. Section through an 8mm deep friction stir weld made into a 28mm wall thickness, API-X65 steel pipe. The weld was performed under oil and is defect free
Avatar Stephen Cater Principal Project Leader – Friction and Forge Processes

Stephen joined TWI in 2007 and is based at the TWI Technology Centre in Yorkshire. A metallurgist by training, Stephen is a principal project leader and oversees TWI’s research into both the friction stir welding of steel and thick-section aluminum. A winner of the Richard Weck Award for his work on the friction stir welding of steel, Stephen also has an interest in underwater friction stir welding and the development of new friction stir welding tools and tool materials.

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