At 1,222km and with an annual capacity of 55 billion cubic metres, the Nord Stream pipeline in the Baltic Sea is the longest offshore natural gas pipeline in the world. This project involved the design, fabrication, test and operation of a subsea dry diver-assisted hyperbaric welding system at depths of 80 metres and 110 metres using a butt-weld TIG process to join sections of the 44-inch pipe in critical tie-in operations at two locations offshore.
To ensure all the equipment is correctly positioned for welding, four teams of divers work in a gas-filled habitat sealed around the pipe ends in rotating eight-hour shifts – once the tool is ready for use, the actual welding process is automated and can take up to 34 hours. These welds are called ‘golden welds’ and the procedure involves the removal of the lay-down heads that seal the pipeline ends, before cutting and bevelling the ends, welding the pipes and carrying out non-destructive testing of the weld.
The Nord Stream pipe has an outer diameter of 44 inches, which is a significant increase in the size capacity of previous pipeline repair systems. CSS Isotek Ltd had to design, fabricate and test tools that were capable of welding this size of pipe at the target depths. We supplied new 44” orbital tracks, new drives and inclinometers for the dry hyperbaric TIG welding heads, together with upgraded dry hyperbaric TIG welding heads, wire-feeders and arc viewing and weld inspection cameras. Our team also designed and built a new power and control module for use with the diver-assisted TIG welding habitat. Our software specialists upgraded the weld and H-frame control hardware and software and supported the offshore welding operation.
Working to critical timescales we completed this project successfully; the ability to perform dry hyperbaric welds on the 44-inch pipeline was crucial for the opening and operation of the Nord Stream pipeline. The equipment now forms part of the suite of tools in Statoil’s Pipeline Repair System (PRS) and has since been used for a number of tie-in operations in the North Sea. It remains available for contingency use on a 28-day mobilisation basis.