Material Best Practice
1.The choice between a linear or non-linear material should be based on the maximum level of deformation expected along the pipeline, which depends on the geometry of the cross section and the lay method. A simple hand calculation should establish whether the linear limit (often ~ 0.2% strain) is to be exceeded or not and so help to decide the appropriate material type. A linear material can result in quicker run times and easier convergence so it is worth considering where possible. However, if the deformation is above the linear limit then a non-linear material is needed to ensure valid results.
2.Given the inherent flexibility of cables, regardless of their metal material properties, it is best to use a linear material type on Cable components. This provides a more stable solution, which should allow for quicker run times and less convergence issues.
3.The validity of the non-linear material curves should be checked using the provided user interface graph. Curves for steel materials should be monotonic and have a smooth profile, as per the example above. Curves for coating do not necessarily have to adhere to the same rules.
4.Try to use the Stress/Strain definition for non-linear materials as this has a number of benefits over the other definition types, as subsequently listed. Note that for more information on how PipeLay handles Stress/Strain materials, please refer to Technical Note 7.
i.You only need to define the stress-strain curve once and then you can just adjust the expected tension thereafter from one installation operation to the next. Variations of the expected tension can be automated by selecting to use the maximum specified criteria tension from the corresponding analyses using the material. All other non-linear material specifications require the curves to be re-defined for each change in cross section and tension.
ii.It is easy to create a database of different material grades (e.g. steel X65, X70, etc.) as the stress-strain properties do not typically change.
iii.The coating stiffness capability requires stress-strain material definitions. Should you need to use this feature in the future, then it is best to have your existing materials specified in the same way.
5.All non-linear materials, regardless of definition, are internally combined with cross section properties to calculate a set of stiffness curves for the various elements. These curves include axial force-displacement, torque moment-twist per unit length and bending moment-curvature. You can review these curves using the Input Echo in the Detailed Output file, just search for the NONLINEAR MATERIAL TABLES section as per the following sample. In the case of a stress-strain material, you may wish to consider using a linear axial stiffness rather than a non-linear one (default), the reason being that generally during installation the purely axial deformation is in the linear range (deformation is largely bending induced) and so a non-linear axial stiffness curve is an unwanted/unneeded complexity.