Free Span is a condition where the pipe does not have a supporting foundation so that the condition of the pipe hangs freely. Free span conditions are common for underwater pipelines. This can occur due to several factors, i.e. irregular seabed, changes in the bathymetry of the seabed (due to scouring), and artificial supports/rock blocks. In addition, the free span can also occur if the pipeline route has an intersection (crossing) with other pipes or cables under the sea.
Subsea pipelines are used to transport commodities such as oil and gas. The condition of the pipe that has a free span can be buckled or known as local buckling, in the worst condition local buckling can result in the collapse of the pipe and a full stop of production.
Information on the height and length of the span is needed to analyze whether the subsea pipeline is within safety limits or whether mitigation efforts are needed to avoid possible problems. This information can be obtained through a side-scan sonar survey by analyzing the shadows formed or through a multibeam echosounder.
The following is an example of a side scan sonar image showing free-span conditions on a subsea pipeline, Figure 4(b) is a waterfall image while Figure 5 is an SSS image that has been plotted according to coordinates. Figure 4(b) and Figure 5 was acquired using a starfish 452F side scan sonar last November in Balikpapan.
- Fernandes et al. 2021. Semi-automatic identification of free span in underwater pipeline from data acquired with AUV – Case study. Applied Ocean Research: Volume 115, 2021, 102842. https://doi.org/10.1016/j.apor.2021.102842
- Pratomo et al. 2021. Identification of Free Span on Underwater Pipeline using Side Scan Sonar and Dual-head Scanning Profiler. IOP Conf. Ser.: Earth Environ. Sci. 698 012035. doi:10.1088/1755-1315/698/1/012035
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