The construction of pipelines internally clad with corrosion resistant alloy (CRA) places a high criticality on the quality of the root bead of the girth weld since it maintains the integrity of the thin CRA layer to the corrosive products being transported. This is against the backdrop of CRA materials which have difficult weldablity, are prone to the pick-up of residual magnetism and further, are challenging to inspect with conventional inspection methods due to the different material interfaces and physical properties.
The welding method employed over the last decade in the field, is to weld externally using specialist power sources with modified waveforms to minimise the heat input in order to achieve the required control - however such methods are highly susceptible to variations in joint fit-up and residual magnetism which can often make achieving the required quality level unpredictable.
Saipem have developed a new approach which overcomes many of these difficulties using an Internal Plasma Welding System (IPW). Now the critical root pass is deposited from the inside of the pipe by the IPW module attached to the internal line-up clamp. This results in welds that are:
consistently high in quality,
highly tolerant to variations in pipe fit-up and residual magnetism,
easily inspected visually with a camera, with radiography and automatic ultasonics due to it's extremely smooth profile,
can offer higher levels of productivity.
IPW Systems have been in continuous service on clad pipeline projects performed by Saipem worldwide since 2013 and most recently used for all offshore welds (approx. 130km) of the 28" clad pipelines which form part of the Kashagan Field Development in the Caspian Sea. Construction of these lines were completed in July 2016.
Saipem believe this approach represents a significant advance in the welding of clad pipelines.
This paper describes equipment and procedure development and goes onto to discuss the projects on which the IPW system has been applied.
