Pipeline towing method is useful for bundled pipelines where several pipelines with different functions are packed together inside a large carrier pipe. The pipeline is constructed in a designed length onshore and towed into the sea.
Since there are many pipelines bundled together inside a big carrier pipe, it is imperative to get everything right prior to installation. Due to this reason, this installation technique allows the bundled pipeline to be welded, inspected and tested onshore prior to installation in order to minimize failure.
Four categories for the pipeline towing method are as follows;
Surface and Near Surface Tow
For the surface tow, buoyancy modules are installed at designed intervals so that the pipeline is floated and the top of the pipe just breaks the surface. For the near surface tow, this is similar to the surface tow. The only different is the pipeline is usually suspended below buoyancy modules. Two towing vessels are used to tow the pipeline. One is used to pull and the another one is used to hold back, therefore the pipeline can be transported in a controlled manner. Once the pipeline is towed to the desired location, the pipeline is properly flooded by a specific procedure in order to safely lower the pipeline onto the seabed. This technique is vulnerable to weather deterioration. Strong weather conditions can damage the pipeline while it is being transported. Additionally, in a strong current condition, it is extremely difficult to accurately position the pipeline.
Controlled Depth Tow (Mid Depth Tow or Catenary Tow)
The pipeline is not floated with this technique and it submerses due to its weight or hanging chains on the pipeline at particular intervals. While the pipeline is being towed, the pipeline is suspended in a flat catenary between two vessels and proper tension to the pipeline must be maintained. One of the most critical parts is control of the submersed weight of the pipeline, which is roughly inversely proportional to the square of length of pipeline. Maximum length is about 5 km.
The off-bottom tow method has a similar configuration to the controlled depth tow; however, the pipeline is held down to float about 1 – 2 meters off sea bed by chains hanging from the pipeline and dragging on the sea bed. The risk of pipe damage due to abrasion is eliminated because the pipeline does not contact with the sea bed.
The pipeline is dragged on the bottom of sea so the pipeline is not affected by currents. If the sea state is too bad for the tow vessel to operate, the pipeline can be simply left on the bottom and retrieved later when the weather permits. Before selecting this towing method, the sea bed must be seriously surveyed in order to confirm the bottom area is suitable for towing. During the tow, it is very important to have survey as precisely as possible. Since the bottom part of the pipe will touch the seabed all the time, the outer protection of the pipeline can be damaged in some extent. Therefore, buoyancy modules can be installed in order to reduce submerged weight and friction force acting on the body of the pipeline.
On the other hand, if the over-bend is excessive, localized transverse buckle can be occurred. This will result in reduction of collapse resistance of pipe in the buckled zones. When the pipe is lowering down into the sea, hydrostatic pressure increases as it gets deeper. If collapse resistance at the buckle is exceeded, transverse buckle can quickly propagate along the pipeline. Therefore, a large area of pipe can be buckled and damaged.
It is imperative that in some areas where high potential buckling can be seen, it is recommended to install buckle arrestors at optimum points in the pipeline where water depth imposes high collapse pressures.
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