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Expro North Sea Ltd. is part of The Expro Group, a leading international oilfield service company focused on providing well performance and production optimisation services to the global oil and gas industry. Employing over 2,000 people in 40 countries Expro is committed to providing clients with outstanding service, quality and customer focused solutions in a safe and environmentally friendly way.
The Expro Group provides a closely related range of specialised products and services to companies engaged in oil and gas exploration and production. These are fundamental to the safe, efficient and economic exploitation of hydrocarbon reserves. The delivery of Expro products and services covers the four geographical regions, namely Europe, Africa/FSU, Asia Pacific and the Americas.
Description of Work
The subject of this work was to evaluate 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 picture to the right shows one type OWLV design.
The Open Water Lubricator Valve was subjected to different thermal and structural loads in operation and testing. Expro North Sea Limited required CADFEM UK CAE Ltd. to assess the structural integrity of the design based on finite element analysis, in accordance with the ISO/DIS 13628-7 standards. Stress concentration factor (SCF) around a scallop port on the main housing was also evaluated. In addition CADFEM UK CAE Ltd. were commissioned to carry out a finite element analysis on the Riser Pipe to determine if the Open Water Lubricator Valve was stronger than the pipe under the same thermal and structural loads.
Two-dimensional drawings of the End Cap Connections, housing, studs and nuts were supplied to CADFEM UK CAE Ltd. in AutoCAD format. The 2D geometry was then imported into ANSYS to generate the three dimensional (3D) solid model for the main part of the analysis work. The assessment of the stress concentration factor (SCF) and KOP pipe was performed in two dimensional (2D).
The Open Water Lubricator Valve consists of two End Cap Connections that are 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-degree 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-degree 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.
For the structural analyses, 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-degree and 18- degree 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.
For the two dimensional stress concentration factor (SCF) analysis a higher order quadrilateral structural element with an axisymmetric option, was used for the geometry containing a scallop port, as shown in the top right picture. 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.
The materials were assumed to be isotropic and homogeneous. Bilinear kinematic hardening was assumed for the plasticity model.
The main benefits that accrue to Expro North Sea from this form of modelling are the abilities to evaluate the Open Water Lubricator Valve 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 lab.
Jamie Walker, 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 CADFEM UK CAE Ltd. 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."