Resulting from work established through their worldwide agents Offspring International, Quintas and Quintas Offshore required IDAC to conduct a finite element analysis on a rope spool assembly, to study its structural integrity under a load case.
A fatigue assessment was also required in accordance with the DNV standards document, DNV-RP-203 (Fatigue Design of Offshore Steel Structures August 2005), using the load spectrum supplied by Quintas and Quintas Offshore.
The project was divided into the following phases:
- Generation of a 3D model within ANSYS DesignModeler
- Generation of the finite element model, including material definition and contact definition into ANSYS Workbench
- Non-linear finite element analysis of the model
The entire rope spool assembly consisted of a spool, a pin and a rope. A quarter symmetry model was created using the engineering drawings provided by Quintas and Quintas Offshore. The rope spool assembly model was imported into ANSYS Workbench Simulation 11.0 for the structural analysis.
The rope and the pin were meshed using higher-order hexahedral solid elements. Due to the irregular shape, the spool was meshed using higher-order tetrahedral solid element. The mesh in the area of high stresses (at the contact corner of the pin and spool) was refined. Contact and target elements respectively were defined at the contact interfaces.
A three-dimensional, nonlinear structural analysis was considered for the study to account for the large deformation of the polyester rope and the frictional contact between the spool rope and spool pin.
The objective of the finite element analysis was to investigate the structural response of the assembly under the minimum breaking load (MBL) of the rope. This allowed the most critical locations, stresses and strains in the spool to be identified for the structural and fatigue assessments.
The MBL and the maximum tensile principal stress were used as reference load and maximum principal stress respectively in the fatigue assessment.
The high stresses beyond yield in the spool were found in and beneath the contact area. The level of stress in this area was unrealistic as the material used in the analysis was assumed to remain elastic. In reality the material would yield plastically and the stresses and the extent of the plastic zone would come down significantly as plasticity is a mechanism to absorb mechanical energy.
Apart from the expected high stresses in the contact zone, stresses elsewhere remained below the ultimate tensile strength of steel even with the assumed elastic material properties, and it was concluded that the design was not likely to fail at the specified maximum break load.
The fatigue calculations were conducted using the fatigue load data as provided by Quintas and Quintas Offshore in accordance with DNV-RP-203.
The MBL of 7,700kN and the maximum tensile principal stress of 554.3MPa were used as reference load and maximum principal stress respectively, in the fatigue assessment.
The fatigue assessment over the design life of ten years with a factor of safety of ten showed that the spool passed the fatigue assessment as required by Quintas and Quintas Offshore.
The finite element analysis/fatigue assessment conducted by IDAC enabled Quintas and Quintas Offshore to validate the rope spool design prior to test and installation.