A deployment skid is a frame which is used to lift equipment from a vessel on to a rig. The deployment skid analysed in this project was required to lift subsea control modules (SCM) and module running tools (MRT). Both the SCM and MRT contain electronics, instrumentation and hydraulics, so it is imperative that they are transported safely with minimum displacement to the skid. Both static and dynamic analyses were carried out. The dynamic analysis was an occupant safety type of analysis where the SCM and MRT modules were considered to be the occupants. This was done to ensure that the cargo did not experience any detrimental displacements.
IDAC were required to analyse, and evaluate, two designs of a deployment skid; a base model (without bracing) and a braced model incorporating additional braces on the top and sides. In addition to this a modified pad-eye design was also to be analysed, under two different analysis conditions; static and impact. For each design the loadcases that were considered are detailed as follows:
- 2.5 x max gross weight (MGW) using four outer pad-eyes
- 1.5 x MGW using two outer pad-eyes
- 3.0 x MGW using three outer pad-eyes
- Impact at 3.5m/s with a rigid cylinder
- Impact at 3.5m/s with an identical skid
The impact velocity was supplied by Whittaker, but was based on the skid unit being suspended from a Knuckle crane on a dynamically positioned support vessel. The rigid cylinder for impact was modelled as having a diameter of 200mm and a height of 2,000mm, and was positioned to impact the midpoint of the long side of the skid.
The geometry of each of the skid components was determined from drawings supplied by the customer and the analyses were carried out using ANSYS (static) and ANSYS LS-DYNA (impact).
Results – static analysis
From the static analyses it could be seen that the braced design showed a significant reduction in stresses and deflections compared to the base model in all three static load cases. The peak stresses were found to be in the base model pad-eyes for all three loadcases.
Results – dynamic analysis
The braced design showed a significant reduction in deflections compared to the base model in both the impact loadcases. The addition of bracing reduced the maximum plastic strain for both the rigid cylinder impact and the skid to skid impact although the bracing was found to have less effect in the skid to skid impact. The braced design had higher levels of plastic strain in the payload canisters. This was due to the stiffening of the frame creating greater relative motion of the payload. LS-DYNA provided a solution for occupant safety simulation, giving a realistic prediction of how the electronic modules behaved under impact.
The skids have to be designed to withstand the rigours of the offshore environment and as such the braced design of the skid was developed to compare with the existing base design. From the work carried out in the analyses it was demonstrated that the bracing had a significant effect on the behaviour of the skid in both the static and dynamic analyses. A full crash test to reproduce the dynamic conditions of a real life crash would be very expensive; however, carrying out the finite element analyses and testing the ‘virtual’ prototype meant that Whittaker did not have to manufacture and test a prototype. IDAC were able to reproduce results at a fraction of the cost of a full crash test.
Ken Whittaker says of IDAC: “Once again IDAC demonstrates that an FE model can do a better job of crash simulation than building and destroying actual hardware.”