The engineers will then determine specifically where it is necessary to prepare the seabed to avoid long freespans before pipe laying. It will also be decided where the pipeline must be buried and how obstacles or particularly sensitive areas can be avoided.

The pipelay vessels are capable of laying large-diameter underwater pipelines. The planned laying speed is up to three kilometres per day. They have a large pipe storage capacity and are manned and equipped to lay pipes 24 hours per day providing weather conditions permit. Before they are released for welding, the pipes will be inspected on board for potential damage incurred during transport.

Upon completion of a weld,  it will be carefully inspected  to identify possible welding defects. Acceptable welds will be covered with an anti-corrosion coating. The pipeline will then be lowered from the vessel into the water via the “stinger,” which is attached to the stern of the pipelay vessel and supports the pipeline. This way, stress on the pipeline during laying is controlled and remains within acceptable limits.

Pipe-laying Sequence

In detail, the pipe-laying sequence is as follows:

• First, two 12-metre pipe joints are welded together in a “double-joint welding station” to form a 24-metre long “double joint” with both an interior and exterior welding pass.
• After “double-joint welding,” the weld is inspected, and if found acceptable, the double joint is then released to the firing line. All double-joint welds undergo a very precise additional test. This allows even the smallest welding defects to be detected.
• The pre-fabricated 24-metre long double joint is then conveyed into the central assembly line, known as the “firing line.” Here, the double joint is connected to the pipeline end and welded together with semi-automatic machines. Welding of the numerous girth weld layers takes place in five to six welding stations in order to achieve high productivity.
• The “firing-line” welds also undergo a precise inspection to ensure defect-free welds. After acceptance of the weld, it is coated and the next 24-metre length is ready to be lowered to the seabed. This is done by moving the pipelay vessel 24 metres forward while keeping the pipeline under tension over the stinger.
• Using this method, over three kilometres of pipes can be laid per day. The pipeline will rest on the seabed for most of the offshore route. In some areas, such as in the vicinity of the landfalls, the pipeline must be buried and backfilled with sand in order to assure sufficient stability. In areas with frequent marine traffic, the pipeline may also need to be buried and backfilled in order to protect it from anchor impacts.

Smaller pipelay vessels will use the same procedures in shallow water between 5 and 15 metres deep, which is inaccessible for the large pipelay vessels.

Final Tests

After construction is completed, the pipeline will be filled with water and pressure-tested for at least 24 hours at a pressure higher than the maximum gas pressure during operation. This is done as a final test before starting operations to demonstrate that the pipeline is 100% impermeable and is sufficiently durable to transport gas at high pressures. After this test, the pipeline is emptied of water before the first gas is introduced.