A major technology development project is currently in progress aimed at applying friction stitch welding to subsea applications in deepwater. Friction stitch welding is a solid phase process which involves drilling a hole, typically 12 mm in diamter, into the parent material. A rotating metal stud is inserted into the hole and an axial force is applied to it, filling the hole with friction weld metal.
The hole filling process takes between 4 seconds and 20 seconds, depending on material thickness. Welding proceeds along the joint by drilling and filling a series of overlapping holes. For pipeline repairs and tie-ins, the drilling and welding head is mounted on a rigid orbital clamp and rotated around the pipe circumference.
Friction stitch welding has a number of important advantages for deep diverless welding of subsea pipelines:
- The process can be used directly in water without the need for a dry welding habitat.
- It is not sensitive to increased water depth, unlike the electric arc welding processes, which are currently used for dry hyperbaric.
- The use of friction stitch welding would remove the need for the costly hyperbaric welding qualification test required for arc welding, as well as the time and cost to install the welding habitat.
- The process operates in a fully mechanized mode and consists of a series of hole drilling and filling operations.
- There are fewer variables and parameters to be controlled than for hyperbaric arc welding processes and the system is well suited for development as a remotely operated completely diverless pipeline repair and tie-in method.
Cost savings
A deepwater diverless pipeline repair system based on this process will provide major cost saving compared with the use of mechanical connectors for repair contingency. This allows pipeline operators to have more flexible repair procedures and avoids the costs associated with keeping a set of mechanical connectors for repair situations.
The system would pay for itself by being used for diverless tie-ins and hot tapping operations on a routine basis for a wide range of pipe sizes, while being available for repairs should the need arise. Cost savings will also be achieved over existing diver-operated hyperbaric welding tie-in systems.
A major technology development project involving the system is currently in progress with Stolt Comex Seaway and GKSS, the German research center, and supported by the European Commission. The develop-ment is aimed at building and testing the stitch welding module for subsea pipeline repair.
The module is designed to operate with the remotely operated pipe alignment equipment called Matis (modular advanced tie-in system) already developed by Stolt Comex Seaway for flanged connection of pipelines in deepwater without divers. It is intended that, for hyperbaric welded connections or repairs, the flange bolting module will be replaced by a stitch welding module.
Pipeline owners and offshore operators have been invited to participate in the project by having friction stitch welding parameters developed by Stolt Comex Seaway and GKSS to suit the wall thickness, grade and specifications for their pipelines. Assisting with organizational information about the project is Dr. Ian Frazer, the corporate R&D manager at Stolt Comex Seaway in Aberdeen.
