Paper Review: "Whole-Life Assessment of Subsea Shallow Foundation Capacity"

Recent papers from UWA/COFS have presented cutting edge developments in subsea foundation design which can allow significant optimisation of foundation sizes.  This has direct practical benefits that have been seen in practise.  As such, these papers have been of great use to the offshore industry and they are well summarised in this paper. Starting from the codified approaches of developing VHM interaction envelopes, how these have been improved upon to generate refined VHMT envelopes, how further gains can be made by accounting for consolidation from sustained vertical and lateral loading and then moving onto their work on sliding foundations.  A similar publication was also made at OTC (OTC-27703​)

​One of the critical aspects in subsea shallow foundation is the ability to assess them differently to fixed offshore installations which is typically the focal point of design codes). The figures below demonstrate the difference in typical ​loading regime and how this may impact the respective soil strength over time from foundation design perspective.

In the final section of the paper a worked example of presented showing that for a particular PLET it may be possible to reduce the foundation size by 75% using the design methods provided, in the end moving from a fixed foundation with a structural sliding mechanism to accommodate pipeline expansion to a foundation which is designed to slide upon the seabed.

The following observations are made:

• ​The foundation size of the PLET may be governed by many things other than the foundation capacity, such as the length of piping/piping arrangement, magnitude of pipeline expansion, spool/jumper limitations, connector type, vessel limitations etc....
• The loading regime for a sliding PLET would be quite different than that of a PLET with a fixed foundation and sliding mechanism.  As such, it would be extremely unusual to have a PLET with a fixed foundation to be designed to resist a lateral load of 320 kN, the PLET would be designed such that it never comes to the end of the sliding rails and therefore the lateral load (assuming negligible environmental loads) all comes from friction on the rails from the weight of the structure (say 10% to 20% of its weight) which would therefore have to be >3200 kN in this example.  It is usually vertical load and overturning moments which governs PLET design.
• It is not uncommon for PLET foundation sizing to be governed by short term loading (often they require significant equipment weight to facilitate hydrotest, spool tie-in or even the change of valve seal which could happen at anytime during its life) which would generally preclude the advantages of consolidation from being adopted.​
  

As such, the paper summarises some excellent research which allows industry to optimise the way that shallow foundations can be designed.  However, the worked example that is provided does not best reflect how it can be used to optimise PLET foundation design.