ISGCUBIP: Investigation of the critical SG for cables and umbilicals during various backfilling and installation processes

  •     Supervisors: Dr Michael Brown: (University of Dundee), Prof Howard Chandler: (University of Aberdeen)
  •     Sponsor Company: Subsea 7
  •     PhD Student: Tommaso Bizzotto

Export and inter-array cables are exposed to a hostile, high-risk environment during a wind farm’s operational life. A wide range of variables can affect the lifecycle and reliability of subsea cabling, with resulting impact on operational expenditure. Seabed conditions, wave action and tidal effects, coupled with ship movement/anchoring and fishing activity may cause a threat to submarine cables. As such mitigating exposure of cables can be a critical design driver. One obvious solution to reduce the risk to cabling is to bury them below the seabed through either ploughing, jetting or plough and backfill. Threats to product (or the cable in this case) is not a problem faced only by the renewables sector. Oil & Gas (O&G) pipelines often have to be buried for similar reason but also thermal insulation of hot product may be a factor in choice of installation technique and pipeline coatings.

In order to minimise costs most efficient design and installation is required. This results in minimising burial depth (reduced ploughing tow forces and vessel time/fuel), increased speed of installation, reduced cost and weight of the installation (product) in a wide variety of seabed soils that may be encountered over the product’s chosen route. Reducing or optimising the specific gravity (SG) of the product has obvious savings in terms of cost (which can be very considerable on long routes) but can also increase ease of handling and installation (more flexible/reduced weight). The drawback of reduced specific gravity is that buoyancy effects may results in product movement or flotation (ie the final position/depth is not as anticipated) or complete de-burial. This will vary from soil to soil and maybe influenced by the speed of backfilling where higher speeds are preferred to minimise time on site. If movement does occur the financial risks may be significant resulting in reburial, or costly post lay rock dumping.

The objective of the project is to develop a greater understanding of the controls on the behaviour of cables/umbilicals and pipelines during subsea bed burial. The outcome of the research will be to develop guidance on the most appropriate burial techniques to adopt for product burial where both seabed and product specific gravity (SG) are varied. This will be further extended to look at minimum burial depths and the effects of co-burial (ie cable(s) and pipelines in the same trench, piggybacks). This will lead to less conservative design, reduced risk for the party installing with benefits to the offshore renewables and O&G sectors.