Residual Curvature for Freespan Mitigation

Written by Martin Teigen.

Ever since I was introduced to RCM in 2012, I have run a lot of analyses. I had a particular productive period while I rented SIMLA in 2015 as I had to make the most out of the license. At the time I read a paper [Ref. 1] proposing RCM for freespan mitigation I also ran some analysis to have a closer look at the feasibility. Today I flicked through my old folders and came upon this illustration and thought I would share it. This is one of many case studies I performed but that I never published.

What we see in the figure below is a 90 m wide and 10 m deep seabed scar. The pipe is installed with a combination of under- and over-straightening [Ref. 1].

The advantage of installing an RC over a freespan is to reduce the gap between the pipe and the seabed. However, I experienced that the bending moment at the span shoulders was very sensitive to the shape of the RC. Only a tiny change in the RC length could be detrimental to the pipe integrity. As such, the installation tolerances would need to be very tight, which could render the method unfeasible.

The reason to this high bending moment at the span shoulders is that the pipe need to be under-straightened at this location. The residual lay tension in combination with the under-straightening induces an initial bending moment which typically is beneficial to the pipe integrity in lateral buckling. In contrast, for freespan mitigation the bending moment caused by the span shoulders would add to this initial bending moment, and pipe integrity could be compromised.

Nevertheless, the application is interesting and I would like to have a closer look into this at some stage, but probably in a project and not as a case study because I think the feasibility depends very much on the span under consideration.

Figure – Freespan Mitigation. 90 m wide and 10 m deep seabed scar.

References:

  1. Endal and Nystrom, “Benefits of generating pipeline local residual curvature during reel– and S–lay installation”, OPT2015.