Parametric CAD and Fea Model of a Saddle Tapping Tee

 Often it is necessary to branch of a pipe section on an oilrig. This operation is often performed by making a so-called "Hot Tapping", which involves welding a pipe and a flange on to the pipe section. A spherical valve and a gate are mounted on to the flange, i.e. weld-o-let. In order to perform the welding operations a so-called habitat must be constructed. This habitat encapsulates the "Hot Tapping" spot and is relatively costly. Thus, to avoid weld operations on to the pipeline, a solution with clamps has been developed, i.e. a Saddle Tapping Tee. The Saddle Tapping Tee is clamped on the pipe section applying a gasket. The system is designed for an allowable pressure of 150[lbs] and the service temperature varies between -40-38[C], i.e. an allowable pressure of 19.7[bar]. However, the system must be able to withstand a test-pressure of 150% of the allowable non-shock pressure, i.e. the design pressure 30[bar].

A parametric verification model is established based on considerations regarding the compaction of the gasket and the stress level in the brackets of the clamp.

The required minimum compaction pressure in the gasket is 1.4[N/mm] according to the European Norm for pressure vessels DS EN13445. The pressure acting on the gasket is governed by the pretension of the clamp, the position of the gasket, and the deformation of the clamp.

The steel applied is of the type ASTM A-420 GR 6 and welding is performed using longitudinal seamless groove welds. The allowable stress in the weld is determined from paragraph K302.3.2 in ASME B31.3.

A full parametric 3D CAD model of the Saddle Tapping Tee is developed where a number of user-defined parameters are controlled from an Excel spreadsheet allowing parameter studies and technical documentation to be generated effectively. The same Excel spread-sheet control a full 3D parametric FEA model which is automatically updated from the user-defined parameters set for the CAD-model.

As the gasket is subjected to a contact pressure arising from the clamping force acting on the pipe section, a FE contact analysis is carried out. This contact analysis is set up using the "contact wizard tool" in ANSYS 10.0.

The performed FE contact analysis has enabled inspection of deformations of bodies in physical contact, of stress levels in critical areas, of perpendicular stresses in the gasket, and friction.

 

The developed procedure allows the engineer to modify any required and relevant user-defined parameter on the Saddle Tapping Tee from a single Excel spreadsheet in both the CAD model and the FE model. A full 3D CAD model is effectively generated for dimensions in the range from NPS ½ to NPS 24 and FEA can be performed to provide documentation of the behaviour of a given Saddle Tapping Tee. This documentation must be provided for verification by the Det Norske Veritas (DNV).