Page 468 - Pipelines and Risers

Piping Systems 435

Membrane + Bending Stress;
Total stress.

These stress categories are given different allowable values, based upon the implication for
structural integrity of each stress classification. The allowable stress values are listed in Table
23.1.

The stress components can be automatically obtained from ABAQUS finite element program.

It should be noted that the BS5500 code only considers the "design" load condition.

The allowable stress values given by the BS5500 code are based on elastic stress theory. The
recommended BS5500 limiting stress values that can be higher than the material yield
strength, are to be interpreted as pseudo-elastic values and are to be compared with results
based on linear-elastic material simulation.

23.2.2 Allowable Stress/Strain Levels
Due to the unusual nature of its shape and purpose, the tee connection does not readily
conform to all the requirements in BS5500, and therefore allowable stress/strain levels will be
established for each load condition, based on the above mentioned codes and in-house
expertise.

For the general body of the tee, which is the area of the tee geometry not influenced directly
by the gross structural discontinuity in the extruded outlet area, the stress pattern is almost
equal to that if a pipe, with hoop stress as the governing component.

The BS5500 code accounts for the complex stress/strain pattern due to discontinuities, as
presented in Section 22.2.3, and allows the total equivalent stress to become 1.5 times higher
(678 MPa) than the yield strength of the material (450 ma). The allowable stress values
given by the BS5500 code are based on elastic stress theory, and the recommended limiting
stress values that can bc higher than the materia1 yield strength, are to be interpreted as
pseudo-elastic values and are therefore to be compared with results based on linear-elastic
material simulation. Since we are using an elasto-plastic material model the allowable stresses
in BS5500 can not be used as a design criterion.

The high calculated stresses in the extruded outlet area are peak stresses and are not critical as
the plastic zone are very limited.

The von Mises equivalent stress is calculated by ABAQUS as part of the solution routine. The
numerically calculated equivalent stress will be compared with the allowable limits.