Installing a subsea flowline is challenging. Subsea flowline installation must be conducted by skilled engineers and operators who know best how to approach each situation. They must be equipped to handle the difficulties that can arise with different types of installation as well as at various depths. There are three main ways that subsea flowlines are installed:
S-Lay Flowline Installation
The S-lay installation method gets its name from the shape the pipe makes in the water as it is being maneuvered to its touchdown point. For S-lay subsea flowline installation, the pipe slides off the stern of the pipelay vessel as the boat moves forward. The flowline curves downward from the stern until it reaches the sea floor. As more pipe is welded and maneuvered off the boat, it will form an S in the water.
J-Lay Flowline Installation
J-lay installation can cause less stress on the flowline than S-lay installation. In this method, the pipe is maneuvered into the water in an almost vertical position. It is lifted by a crane or a tall tower and put into the sea. The pipe only curves once when it hits the sea floor. This gives the flowline the appearance of a J, hence its name.
In this method of subsea flowline installation, the installers work with the pipe’s buoyancy in order to suspend it in the water and then one or two tugboats tow the flowline to its place. Once the pipe arrives at its dedicated location, buoyancy is removed or the flowline is flooded with water, and the pipe will slowly sink to the sea floor. Tow-in installation exists in four main forms:
1. Surface tow, which involves towing the flowline on the surface of the water
2. Mid-depth tow, which uses the speed of the tugboat to keep the flowline submerged
3. Off-bottom tow, which uses buoyancy models and chains to keep the flowline just above the sea floor
4. Bottom tow, which drags the flowline along the sea floor
Dealing with Buoyancy
Buoyancy affects laying flowlines in both positive and negative ways. While in the water, the flowline weighs less if it is filled with air, which reduces the stress placed on the barge. However, this serves as a disadvantage once the flowline is on the sea floor. At this point, weighing it down can become problematic. Downward force is necessary for the flowline to remain in place. This force can come in the form of oil within the flowline, but gas does not weigh enough to keep the flowline in place in shallow water. In this situation, concrete is often poured over the flowline to hold it down; however, in deep water, the thickness of material required to ward off hydrostatic pressure and the amount of insulation used are often enough to keep the line in place.
No matter what type of installation is used, engineers and operators need to be knowledgeable and skilled enough to choose the right approach for the flowline’s location and their equipment.