General engineering analysis

Case Study

Offshore Centraliser Design

UWG Ltd provides a consulting engineering design, fabrication and offshore service for the oil and gas industry. LUSAS Analyst was used to carry out detailed analyses of centraliser designs to investigate their ultimate strength and fatigue life.

Oil from a sub-sea well is conducted to the surface through a set of concentric tubes called risers. Centralisers are used to locate these risers in guides that form part of the production platform support structure. Centralisers are subject to lateral loads caused by wave action on the riser and may also undergo axial loading if the centraliser catches on a guide during installation. Two types of centraliser were examined. The Type l centraliser is bolted around the riser without clamping it and is held in position by collars welded to the riser. The Type 2 device is bolted tight to the riser and deforms to the riser pipe shape.

The Type l centraliser was modelled using 3D shells with joint elements representing bolts. The design wave load was applied radially along a single line on the inside of the centraliser. Stresses were obtained for a number of different incident angles of design wave loading and compared with material properties to assess the strength of the design. A number of different designs are being investigated with variations in material thickness and number and depth of stiffening rings. Stresses and forces due to axial loading are currently being investigated and this is to be followed by an analysis of the stresses induced in the riser.

Stresses in Type 2 Centraliser

In the Type 2 centraliser a 2D nonlinear plane stress model is being used prior to a full 3D analysis being carried-out. Use of the LUSAS slideline facility simplifies modelling the contact between the centraliser body and the riser. Bolt loadings to clamp the centraliser to the riser were applied iteratively and the design wave loading was applied in a direction to give the highest stresses. A number of different models have been used to investigate the effect of varying the centraliser’s material thickness. Graphs of centraliser stress versus centraliser thickness and riser stress versus centraliser thickness have been obtained and work is in hand to check whether using a finer mesh affects results. Future work will extend the model to include 3D effects.



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