Prior to laying the pipeline it is necessary to prepare the route. This involves removing obstructions by either filling or excavating. If there is existing infrastructure such as pipelines or cables in the route path, supports must be pre-installed on either side of the path ensuring no contact between the two lines. The supports must be as wide as the laying tolerance for the pipeline – approximately six metres or twenty feet.

Sandwave removal must be undertaken in certain areas to prevent unwanted spans. It is carried out just a few weeks before the pipeline is laid since sandwaves may reform very quickly in strong tidal currents. Sandwaves may be static or mobile and it is the mobile variants which cause the greatest concern. Modelling can be undertaken to determine the size of spans that will remain after installation. This will inform the level of sandwave removal that must be undertaken to be left with acceptable spans. Once the pipeline is installed, prompt trenching is often used to lower the line well below the seafloor.

Work continues after the pipeline has been installed. Though most of the pipeline may be trenched, at either end mattresses or rock dump are provided for dropped object protection. Any exposed sections of the pipeline – especially at the crossings – may also be rock dumped.

This is a snippet of content covered by our Construction of Subsea Pipelines course. To read more about this course visit the course information page.

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