IDAC Assesses Connector For First Subsea
IDAC has carried out analyses on a mandrel-receptacle mooring-line connector that First Subsea designed for the Thunder Hawk Regional Development project. These analyses included: analysing the structural response of the mandrel-receptacle assembly; assessing the fatigue responses of the mandrel and receptacle in accordance with the DNV standards document, DNV-RP-203 (Fatigue Design of Offshore Steel Structures - August 2005) and using the fatigue load data as supplied by First Subsea.
The solid geometries of the mandrel, receptacle, pins and shackles were supplied to IDAC in Autodesk Inventor format. The assembly was then imported into Ansys Designmodeler and reduced to a quarter symmetry model, allowing a finer mesh density to be used in the analysis. The assembly was imported into Ansys Workbench Simulation, where a mixed hexahedral-tetrahedral mesh was generated. A nonlinear finite-element analysis was carried out to investigate the structural response of the mandrel-receptacle assembly under two load steps: the peak cable load and the design load at the quoted minimum breaking load (MBL); this allowed the most critical locations, stresses and strains in the mandrel and receptacle to be identified.
The nonlinear characteristics in the analysis were large deflection effects, nonlinear material behaviour (metal plasticity) in the mandrel and receptacle, and also nonlinear contact (with opening and closing behaviour) between the different parts, except those at the ball and taper connection, where bonded contact was used. The fatigue life of the entire assembly was calculated using the most critical component, which was found to be the receptacle. The maximum absolute values of the principal stresses were used for the actual fatigue calculations.
The fatigue damage calculations were performed using the S-N curves derived from Equation 2.4.3 of DNV-RP-C203. Based on the S-N curve derived from this equation, the predicted number of cycles to failure for all events could be evaluated at the selected locations. It is a requirement of First Subsea that the design life of the components is 20 years and a safety factor of 10 is applied to the calculated fatigue life, which corresponds to a minimum fatigue life of 200 years when the fatigue limit of one is reached. It was concluded from the analysis that the receptacle, being the most critical component of the two, would first reach a fatigue damage of 1.0 after an estimated period of 9,735 years. Stress linearisation was carried out at locations identified as critical sections, based on stresses obtained.
The stresses at these locations were linearised, so that the membrane and bending stresses could be derived. The linearised stresses could then be compared against the allowable design stress intensities for the particular material and a safety factor derived. IDAC and First Subsea have worked closely together on a number of projects covering a range of designs and load cases. The use of FEA allows First Subsea to not only optimise their designs but to also prove their load-carrying capacity and fatigue life. These FEA results have been confirmed by in-house physical testing of the connectors.
The solid geometries of the mandrel, receptacle, pins and shackles were supplied to IDAC in Autodesk Inventor format. The assembly was then imported into Ansys Designmodeler and reduced to a quarter symmetry model, allowing a finer mesh density to be used in the analysis. The assembly was imported into Ansys Workbench Simulation, where a mixed hexahedral-tetrahedral mesh was generated. A nonlinear finite-element analysis was carried out to investigate the structural response of the mandrel-receptacle assembly under two load steps: the peak cable load and the design load at the quoted minimum breaking load (MBL); this allowed the most critical locations, stresses and strains in the mandrel and receptacle to be identified.
The nonlinear characteristics in the analysis were large deflection effects, nonlinear material behaviour (metal plasticity) in the mandrel and receptacle, and also nonlinear contact (with opening and closing behaviour) between the different parts, except those at the ball and taper connection, where bonded contact was used. The fatigue life of the entire assembly was calculated using the most critical component, which was found to be the receptacle. The maximum absolute values of the principal stresses were used for the actual fatigue calculations.
The fatigue damage calculations were performed using the S-N curves derived from Equation 2.4.3 of DNV-RP-C203. Based on the S-N curve derived from this equation, the predicted number of cycles to failure for all events could be evaluated at the selected locations. It is a requirement of First Subsea that the design life of the components is 20 years and a safety factor of 10 is applied to the calculated fatigue life, which corresponds to a minimum fatigue life of 200 years when the fatigue limit of one is reached. It was concluded from the analysis that the receptacle, being the most critical component of the two, would first reach a fatigue damage of 1.0 after an estimated period of 9,735 years. Stress linearisation was carried out at locations identified as critical sections, based on stresses obtained.
The stresses at these locations were linearised, so that the membrane and bending stresses could be derived. The linearised stresses could then be compared against the allowable design stress intensities for the particular material and a safety factor derived. IDAC and First Subsea have worked closely together on a number of projects covering a range of designs and load cases. The use of FEA allows First Subsea to not only optimise their designs but to also prove their load-carrying capacity and fatigue life. These FEA results have been confirmed by in-house physical testing of the connectors.
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