Need:

As part of the DNV qualification of Lift Caps used for Intervention Riser system, Pythagoras Engineering Pvt Ltd were tasked with stress analysis of Lift Cap, for Maximum Gross Weight of 130,000 lbf, as per DNVGL-OS-E101. The FEA analysis incorporated assessments of the locking bolts and Lift cap body.

Scope:

Objective of this work is to evaluate the structural integrity of Lift cap used in offshore lifting and design qualification as per DNVGL-OS-E101.

Methodology:

 Pythagoras Engineering Ltd was contracted by Worldwide Oilfield Machine (WOM) Ltd to evaluate the structural integrity of Lift caps through FEA. The purpose of the study was to demonstrate that the Lift cap is suitably designed to withstand the loads it will be subject to while offshore lifting, as per DNVGL-OS-E101. The study was carried out using Finite Element Analysis (FEA) methods. A linear elastic solid model was used to assess the stresses in the lift cap.

The Finite element model was generated using SOLID 185 and SOLID 186 higher order tetrahedron and hexahedral elements. Chamfers, fillets, bolt holes and other features which does not affect the FE results are suppressed to reduce the mesh count.  Fine mesh was generated at critical regions and on locking bolts.

Structural design evaluation was carried out on lift cap when subjected to design load of 650,000 lbf. The design load of 650,000 lbf was applied as a concentrated point load at the bottom face of the dummy block. Fixed boundary condition was applied on the shackle

Acceptance Criteria:

 As per DNVGL-OS-E101, section 1.7.5, design factor of 5 should be used for single point lift calculation. Lift Cap should be designed for 130,000 lbf lifting application, thus 130,000 x 5 = 650,000 lbf was considered for the FEA.  Stress evaluation was performed as per DNVGL-ST-E273.

As per DNVGL-ST-E273, section 3.4.3, the allowable von mises stress of the lift cap is 0.86 x yield strength.

Results Outcome:

From the FE results presented, it can be seen that the stress on the lift cap are within the allowable limit for a SWL of 130,000 lbf and the design qualifies the requirement as per DNVGL-OS-E101.