Page 471 - Pipelines and Risers

438 Chapter 23

The global shell model, built up by 8-node shells has been extended far enough to avoid
influence based on rigid constraints at the pipe ends. The header pipe is modeled with a length
of 12.2 m with the tee-piece in the middle.

The local 3-D model is based on 20-node bricks and a fine mesh is specified at the branch
outlet area where high stresses are expected.

For the operational load cases 3.0 mm of the wall thickness is removed to account for the
internal pipeline corrosion.

Material
Both linear elastic and elasto-plastic material are specified for the FE models. In case of the
non-linear material properties an elasto-plastic material model is calibrated from the
Ramberg-Osgood equation.

Boundary Conditions and Loading
The shell model has one end constrained for all degree of freedoms, except the rotation
around z-axis. The other end is constrained for translation in y- and z-direction. All open ends
in the shell model is prescribed to behave as a rigid ring ( however the shell should be long
enough to avoid any effects of this boundary condition).

The solid model (local model) have prescribed displacements along the external boundary
(the solution obtained from global shell model is used to derive the boundary). The model has
one plane of symmetry (both loads and geometry) which is utilized in the local 3-D model.

External loading is applied as nodal forces at the header and branch ends of the global shell
model.

Selfweight effects will be negligible compared to the loading caused by pressure and external
loading. Hence, no selfweight is applied in the E-model.

Numerical Analysis
The global shell model, built up by 8-node shells has been extended far enough to avoid
influence based on rigid constraints at the pipe ends. The pipe consists of parts with different
thickness.

The 3-D model is based on 20-node bricks, and is fine enough for calculation of stresses at the
intersection between header and branch.

The analysis has been performed in a two-step approach with the global model based on shell
modeling followed by a 3-D local model where the stress concentrations in the intersection