IDAC recently evaluated the structural integrity of the external components of an open water lubricator valve, under extreme operational and testing loads, in accordance with the ISO/DIS 13628-7 standard. Based on Expro’s innovative ball-valve technology, open water lubricator valves (OWLVs) are used when running subsea trees on drill pipes in open water without a marine riser.
The OWLV was subjected to different thermal and structural loads in operation and testing. As well as structural integrity the stress concentration factor (SCF) around a scallop port on the main housing was also evaluated.
The OWLV consisted of two end cap connections that were bolted to a main housing. The internal components namely a moveable cage, internal sleeves and a rotating ball were not included in the model but the loads on these parts were applied to the appropriate interfaces on the end cap connections and main housing as pressure or shear loads.
The analyses were performed using the general purpose finite element software ANSYS.
An 18° sector of the three-dimensional model was created from the two-dimensional line drawings for the thermal analysis and the subsequent operational and testing load cases.
For the three-dimensional analyses the 18° sector of the solid model was first meshed with a higher order element with thermal capability. A thermal analysis was conducted to evaluate the temperature distribution in the bodies, as shown in the figure on the left. The elements were then changed to the counterpart structural elements for the structural load cases, taking the thermal expansion or contraction into consideration.
The free end of one end cap connection was constrained in the axial direction. The nodes at the free end of the other end cap connection were coupled in the axial direction while the end cap pressure and additional tensile loads were applied to the master node.
Symmetric boundary condition was applied to the 0° and 18° cut planes of the model. Different pressure loadings were applied to internal surfaces to represent the internal fluid pressure, control line pressure and reactions from internal components. Special post-processing macros were written to perform the specific code checking automatically.
Stress concentration factor (SCF) analysis
A higher order quadrilateral structural element, with an axisymmetric option, was used for the geometry containing a scallop port. Bonded contact was used to represent the threaded connection between the housing and the stud. The lower part of the housing was constrained in the axial direction, while the nodes at the free end of the end cap connection were coupled. A unit load was applied to the coupled end and the nominal stress, as well as the local stress were used to compute the stress concentration factors around the scallop port and other geometric features.
By using results from finite element analyses, Expro North Sea was able to evaluate the OWLV design in accordance with the required design standard on computer rather than by physical testing. Physical testing is not only time consuming and expensive in comparison to numerical modelling, it is also impractical to conduct load-to-failure test in the laboratory.
The Expro design engineer who assisted with the FEA section of the project at Expro, commented: “The speed of the response, the helpful and professional attitude shown by IDAC on this project was extremely appreciated by ourselves and our client. The results that were produced by performing this analysis saved months of physical testing to ‘fine-tune’ the design in order to satisfy the needs of our client. Subsequent testing utilising strain gauges provided results that correlated very closely with the analysis.”