Pipelines on an erodible seabed are prone to develop free spans, caused by seabed scour around the pipeline and autonomous seabed evolution (e.g. coastal dynamics or sand wave migration).
Exposure of pipelines leads to risk of damage by fishing gear, dropped or dragging anchors and by environmental forces. Free spans cause a risk of loading due to waves and current possibly resulting in pipeline failure due to maximum stress or fatigue. A long, static free span can suffer both from the exceedance of design stress during a storm and from the build-up of fatigue. On the other hand, if the free span is dynamic, there is a high chance of the pipeline getting re-buried without an unacceptable build-up of fatigue. Information on free span evolution is therefore essential in deciding on pipeline stabilisation strategy.
Standard acceptance criteria for scour induced free spans that assume the free spans are static may lead to unnecessary remediation work being undertaken. BMT Ship & Coastal Dynamics has developed analysis techniques which would allow predicting scour and span evolution in time, which results in reduced amount of offshore intervention work required to maintain the pipeline in an acceptable condition. see “Managing Free Spans on Dynamic Seabed. Krawczyk, Romke Bijker, Zhiwen Chen (now at BMT Ship & Coastal Dynamics) and Hans Boersma, Proceedings Offshore Pipeline Technology, Amsterdam, 2013.
BMT is at the forefront of analysis supporting development of both remedial measures and route/burial planning that account for free span dynamics. This is based on our expertise in:
- seabed dynamics and coastal sediment movements and
- the interactions between offshore structures, pipelines and cables and the seabed;
We apply this experience to:
- Predict self-lowering and free span evolution of sub-sea pipelines and cables;
- Define pipeline and cable burial requirements on the seabed and at landfall
- Route pipelines and cables to avoid sand waves and minimise burial depth
- Identify and designing pipeline stabilisation measures
We apply a variety of models and tools for predicting coastal evolution and free span behaviour (e.g. PIPESIN, CROSMOR and LONGMOR). Using the outputs from the models, in conjunction with analysis of wave, current, sediment composition and historical depth data, we can predict the autonomous sea-bed development and how it is likely to interact with structures and pipelines.