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Quintas & Quintas Offshore, now part of Lankhorst Ropes, is a member of the Royal Lankhorst Euronete Group. With more than 200 years of experience and 1,200 employees they are a worldwide innovative group with fully equipped production sites and R&D departments, located in The Netherlands, Portugal and Greece, for the production of a wide range of products in maritime and offshore ropes, technical yarns, fishing/yachting gear, pure composites, recycling and moulding material.
Resulting from work established through their worldwide agents Offspring International Ltd, Quintas and Quintas Offshore required a finite element analysis be conducted 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:
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 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 graphic to overleaf depicts the finite element mesh in different parts of the assembly. The mesh in the area of high stresses (at the contact corner of the pin and spool) were refined as can also be seen in the graphic overleaf. Contact and target elements respectively, were defined at the contact interfaces.
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 graphic to overleaf depicts the finite element mesh in different parts of the assembly. The mesh in the area of high stresses (at the contact corner of the pin and spool) were refined as can also be seen in the graphic overleaf. 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 the contact area and beneath as can be seen in the graphic to the right. 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 (seen in the graphic to the right), stresses elsewhere remained below the Ultimate Tensile Strength of steel even with the assumed elastic material properties and it was concluded therefore 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 minimum breaking load (MBL) of 7700 kN and the maximum tensile principal stress of 554.3 MPa, were used as reference load and maximum principal stress respectively, in the fatigue assessment. The graphic to the left shows a contour plot of the maximum principal stress. The fatigue assessment over the design life of 10 years with a factor of safety of 10 showed that the spool passed the fatigue assessment as required by Quintas and Quintas Offshore.
The finite element analysis/fatigue assessment conducted by CADFEM UK CAE Ltd. enabled Quintas and Quintas Offshore to validate the rope spool design prior to test and installation.
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