Holdfast Training Services Trial Blog

Alongside project delivery I am currently working as part of the mechanical discipline engineering team. The work I am doing for them is answering Design Technical Queries (DTQ) and Engineering Queries. I started trying to explain both but it became too wordy, the DTQ is explained below:

A DTQ was submitted in relation to the pipe support loads experienced during a blast event along new production flowlines that are due to be installed (some have actually already been installed). The Clair platform has legacy issues relating to blast whereby the original platform design was never designed for blast. As such, all current in-use flowlines and pre-invested (installed but not hooked up) are supported using standard U-bolt pipe supports. The U-bolts are not designed to be able to withstand a blast event since the primary load path for the bolts is vertical. The U-bolt manufacturer only specifies a max vertical load therefore a max lateral load must be assumed.

Wood Group are the contractor designing the new flowlines (to be tied into pre-invested) and are concerned that in the event of a blast a number of the pipe supports will fail due to excessive lateral loads. Their basis for this statement is a rule of thumb that the max lateral load is a nominal 30% of the max vertical load. The manufacturer stated max vertical load is based upon the yield stress of the material. They have asked if they are to replace all the pipe supports since they all fail.

Recognising there are platform wide issues relating to blast, Fraser-Nash Consultants (F-N) were contracted to conduct targeted blast analysis on a complete flowline (one designed IAW original platform design). By modelling the U-bolt failure load using a similar method as Wood Group, F-N came to the same conclusion that the lateral loads are excessive. In order to understand the actual post-yield material characteristics (ie strain hardening etc.), a nonlinear analysis was conducted based upon the U-bolt geometry and material. From this they were able to establish a plastic collapse load. When modelled in this manner all the U-bolts were found to remain within the plastic collapse load. The picture I’ve attached kind of explains this problem through the stress-strain graph.

My response to the DTQ was initially to accept the F-N analysis since the U-bolts do not collapse fully, maintaining some structural integrity. My justification for this was that the design event is blast, a one-off whereby the performance criteria is to maintain primary containment. No-one seems content with this response and I am now stuck trying to provide more justification, any ideas?