See excerpt from section 5.3.1 of the version 4.0’s user manual:
Preliminary stability analyses should be completed using Level 2 or the ASM module, so as to have a good starting point for a Level 3 stability analysis.
- For each pipe outer diameter and weight, sufficient runs need to be made with different “seeds”. Random seeds are numbers used as internal input to generate random irregular sea states, with two different seed values resulting in two different sea states. Performing the analysis with a sufficient number of storm simulation seeds provides a reliable indication of the range of results which may be obtained for the specified wave spectrum parameters.
Note: this may need to be repeated at other water depths and wave conditions along the pipeline route.
- Once a satisfactory pipe weight and outer diameter have been determined for one condition, say the maximum significant wave height (Hs) case, other wave conditions need to be checked e.g. storms with lower Hs but greater peak period and/or different peakedness. Add some additional cases to check the effects of currents. These all need to be run with sufficient different seeds. o Note: this may need to be repeated at other water depths and wave conditions along the pipeline route. • Instead, the criteria for pipeline stability need to be established. With Level 3 the instantaneous factors of safety computed are not meaningful. The following questions need to be answered:
- What displacement is acceptable?
- If a single 3-hour storm produces displacements greater than a minor fraction of the pipeline diameter, what will be the total displacement (and resulting stresses) of the pipeline during a lifetime of storms and how can that be determined?
- Level 3 reflects many kinds of nonlinear behavior as discussed further below. It is not rare for this to produce unexpected results which then require further evaluation and time.
- Due to Level 3’s greater complexity, there is a larger potential for errors in the usage of Level 3 as compared to the other levels, as well as greater difficulty in identifying the errors. In summary, the stability analysis using Level 3 will require additional time and effort compared to that using Levels 1, 2 or ASM. Additionally, Level 3 assessment is more suitable for experienced users who are able to provide at least engineering judgement on the results. However, this may be beneficial depending on the case considered, with the decision of design approach left up to the designer.