Seabed Intervention

“Facts About Seabed Intervention” describes how dredging performs the crucial task of preparing the seabed for offshore installations and pipelines.

To place a structure on the seabed securely, the seabed must be as flat and regular as possible. If the seabed is irregular or undulating, the structure, for instance, a pipeline or cable or offshore platform, will suffer the risk of spanning and overstressing. To avoid these and other risks, dredging contractors are asked to intervene to make the seabed flatter or to cover the structure that is being placed to protect it. Offshore infrastructure installations are often exposed to high external pressures and cold temperatures. They are subject to tidal movements, currents and scour unless buried or trenched in the seabed. Preparation of the seabed is therefore of crucial importance. The need for seabed intervention for the gas and oil industry has continued to grow as mining is taking place at ever greater depths, often in remote areas. These resources have to be brought on shore and this is frequently done through pipelines laid upon the seabed. These pipelines must often traverse long distances over rough seabed terrain to land-based sites. To be secure they must lay flat on the seabed and be protected. When this is not the case, free spans – sections of the pipeline that are not touching the seabed – can occur. These can be caused by the irregularity of an uneven seabed created by the turbulence or scouring action of water and sand. They are dangerous because they can result in pipeline fatigue when the unsupported pipeline sections are subject to currents and vibrations. Sand waves caused by relatively high velocity water currents, outcrops of rocks or coral reefs and pockmarks, which are craters in the seabed caused by gasses and liquids erupting and streaming through the sediments are amongst the causes of unevenness that can result in free spans. Pre-sweeping, post-trenching, high pressure jetting and subsea rock installation are all activities that require dredging expertise and that have resulted in specialised equipment such as flexible fall pipe vessels. “Facts About Seabed Intervention” answers essential questions such as:
  • Why is seabed intervention necessary?
  • How often does seabed intervention take place?
  • Are uneven seabeds a common occurrence?
  • How do free spans affect pipelines?
  • What other factors influence preparing the seabed?
  • What are sand waves?
  • What are outcrops?
  • What are pockmarks?
  • How does weather influence seabed intervention?
  • How is pipe-laying achieved in subsea circumstances?
  • What factors influence the choice of equipment?
  • What is pre-sweeping?
  • What is a mass flow excavator (mfe)?
  • What if pipeline protection is necessary?
  • What is high pressure jetting?
  • What about post-trenching?
  • How can rock installation support offshore pipelines?
  • What environmental and safety issues may arise?