A Deployment Skid is a frame which is used to lift equipment from a vessel onto 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 hence evaluate, two designs of the skid; a base model (as shown in the graphic to the left) and a braced model incorporating additional braces on the top and sides, and a modified pad-eye design (below left), 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 4 outer pad-eyes
1.5 x MGW using 2 outer pad-eyes
3.0 x MGW using 3 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 2000mm, and was positioned to impact the midpoint of the long side of the skid. For the skid to skid impact the corner of the moving skid hit the midpoint of the long side of the stationary skid as shown in the graphic below.
The geometry of each of the skid components was determined from drawings supplied by the customer and using these, a model was created by IDAC within ANSYS Workbench DesignModeler. All the skid components were made from S355 J2 Steel; this material was modelled as elastic in the static analysis and elastic-plastic in the Dynamic (Impact) analysis. The analyses were carried out using ANSYS (static) and ANSYS LS-DYNA (Impact).
The pad-eyes, SCMs and MRT were meshed using higher order 3D hexahedral elements with all the other components being meshed using higher order 3D shell elements. The skid components were connected using bonded contacts and frictionless contact was used between the pad-eyes and the shackle bolts. Frictionless contact elements were also used between the SCMs and the container base plate. The centre-points of each pair of SCM’s and base plate were rigidly fixed together. However, a coefficient of friction of 0.3 was used for the contact between the two skids in the dynamic analysis.