It’s very difficult to lay oil and gas pipelines under the sea!
Firstly, the working environment is in the ocean, not only the working space is seriously limited, but also the complex and changeable sea conditions will bring great challenges to the normal construction.
The seabed environment is complex, because the seabed is not flat, it is composed of various landforms, such as: high seamounts, undulating seamounts, long ridges, deep trenches and so on. We can’t reconstruct the seafloor topography like laying pipelines on land, so in the process of design and construction, the factors of laying submarine pipelines are very complex.
Problems such as low temperature, high pressure, high salinity, undercurrent surge, hydrate blocking, pipeline corrosion and impact loss will continue to exist. At the same time, these problems make the laying of subsea oil and gas pipeline not only an engineering problem, but also a scientific problem.
For operation, water depth is the biggest influence factor, which is different from pipe laying operation according to water depth. Offshore near, shallow places, you can use winch directly to pull the sea pipe to land.
In the shallow sea area and far away from the shore, the pipe laying can be carried out by lowering the pipe laying ship and welding at the same time.
S-lay: the submarine pipeline enters the water through the stinger at the stern to form an S-shaped curve and pave into the seabed.
To complete the pipe laying operation, there are several magic tools on board: tensioner, stinger and welding line.
In addition, due to the limited working space at sea, the laying of subsea pipelines needs a professional fleet to cooperate with each other, and the cost is 1-2 times higher than that of land in the same situation.
In the deep water area, the water depth increases, which leads to the exponential increase of construction difficulty. As the length of the subsea pipe increases, the weight increases, and the laying vessel needs to bear more tension. At this time, J-lay method is used for construction.
J-lay (J-lay method): the subsea pipe leaves the laying vessel in a nearly vertical form and goes down the vertical bending section until it is laid into the seabed. The pipeline is in the “J” shape as a whole, which is suitable for the deep sea area of hundreds to thousands of meters.
Of course, with the continuous development of new technology, the offshore pipe laying operation form also continues to progress. For the offshore pipes with small pipe diameter and weak strength, they can be manufactured on land and rolled directly on the drum, and then transported to the sea for laying by the pipe laying ship. This operation mode is called reel lay.
At present, the coiled pipe laying vessel can be divided into two types: horizontal type and vertical type. For horizontal pipe laying vessel, its rotation axis is perpendicular to the deck of the laying vessel, and S-type laying is adopted (commonly used in shallow water); for vertical pipe laying vessel, its rotation axis is horizontal with the deck of the laying vessel, and J-type laying is adopted (commonly used in deep water).
Of course, the final choice of operation mode should not only consider the depth of water, but also combine the operation cycle, operation cost and other comprehensive factors.