To verify that the calculated result was reasonable, a simplified model was created to perform a static analysis of the locking mechanism. The simplified model consisted of a horizontal plate, linkages and the lower clamp segments. It was constrained in appropriate directions and a force was applied at the middle of the horizontal plate (Fig. 2.12).
Fig 2.12 The loads, fixtures and connector conditions applied on the FEA model.
SolidWorks Simulation is a user friendly add-in to the CAD software SolidWorks. Although it gives the user an easy way to carry through a static analysis, it has some limitations. For instance, contact, fixtures and applied loads needs to be placed on surfaces of the geometry, and not directly on nodes or keypoints which is common practice in other analysis programs. It does not provide the user advanced analysis tools and possibilities, but it is a handy software for this case. A curvature based solid mesh was used. The analysis did not account for friction since this was not done in the hand calculations. The analysis established 18510 nodes and 10468 elements.
Applied force (1642,850kN)
Fixed hinge Pin connector Roller/slider
Pin connector
Roller/slider
Design of Temporary Subsea Installable Pressure Cap Confidential until May 2018
36 Lars Rimmereid Spring 2013 Fig 2.13 The mesh of the SolidWorks Simulation model.
A reasonable verification of the simulation was to check the force F3 in the solved model to ensure a correct use of boundary conditions, applied load and geometry was used. The result was satisfying, the F3 was 355kN (Fig 2.14) where F3 from equation 2.1 had a force of 345kN.
Fig 2.14 An edge was created at the end of the clamp segment to constrain it in y-direction, the resultant force acting on the planar face was 355kN which gave a difference of approximately 10kN to the calculated result.
A simplified model of the lower clamp segments and the locking mechanism was also made in Ansys Classic.
Design of Temporary Subsea Installable Pressure Cap Confidential until May 2018
Lars Rimmereid Spring 2013 37 Fig 2.15 A simplified model of the cap, the model is divided into 78 elements.
Fourteen keypoints were made, and two keypoints were placed on the same location on both ends of the linkages. This was done to create linkage joints with Ansys “coupled constraint” function. The function enables two components to be connected to each other by a joint which is constrained in all directions except in the rotational direction. All of the keypoints where constrained in the z-direction. The two end-keypoints to the horizontal plate was constrained in the x-z-direction. The clamp segments hinge was constrained in all directions and rotations, except in the z-rotation. At the bottom of the two lower clamp segments, a keypoint was constrained in y-direction to obtain the desired output (Fig 2.16).
Fig 2.16 The model boundary conditions. The red arrow illustrates the applied force, the turquoise triangles shows which direction the model is constrained in. The green triangles show the pin joints and the yellow triangles shows where the model is constrained in the rotational direction.
The meshing was done with Beam189 elements, 78 elements and 161 nodes were created. As a final control to verify that the boundary conditions, applied loads and geometry, the force in y-direction was checked in the elements at the end of the clamp segment. The result was satisfying. A complete source code for the analysis can be found in appendix 1.
Design of Temporary Subsea Installable Pressure Cap Confidential until May 2018
38 Lars Rimmereid Spring 2013 Fig 2.17 The element 67 and 78 have a resultant force in the y-direction of approximately 343kN, which is a satisfying match compared with the hand calculated result.
Fig 2.18 The axial force diagram show that the largest axial force is acting on the linkage with a force of 839.7kN. This is in accordance with the hand calculated result of 840.878kN (Fn).
The two linkages cannot pick up any moment (Fig 2.19), but as figure 2.18 shows, the axial force is acting through the linkages and makes a compression stress in the beam. It will therefore be appropriate to verify the linkage beams against buckling.
Design of Temporary Subsea Installable Pressure Cap Confidential until May 2018
Lars Rimmereid Spring 2013 39 Fig 2.19 The largest moment is located at the middle of the horizontal plate where the power screw force is applied. The moment is excessive high giving a bending moment of 345 000kNm.
Fig 2.20 The shear force showed as the blue and red diagrams. The applied force on the horizontal plate gives an excessive shear force in the middle of the plate of 821.425kN.
Based on the moment diagram and shear force diagram, the mechanism weakest point is the middle of the top plate. This is the area where the construction will yield/break first.
Design of Temporary Subsea Installable Pressure Cap Confidential until May 2018
40 Lars Rimmereid Spring 2013 STRESS DETERMINATION OF THE LOCKING MECHANISM
2.2.4
The previous hand calculation indicated the magnitude of the force that was necessary to be applied by the power screw mechanism. The next step will be to determine what kind of stress the applied force causes in the horizontal plate and the linkages.
Fig 2.21 The location of the bending stress and the compression stress that occur in the horizontal plate and linkages.