Additional Analysis Options
PipeLay is most typically used in the analysis and design of the range of conventional operations undertaken by pipe laying contractors, including normal S-lay and J-lay pipeline installation, various initiation procedures, abandonment and recovery operations, pipeline davit lift and so on. In addition to the program capabilities provided for these scenarios, a number of features have been added to PipeLay to enable it to also analyse some innovative solutions being proposed for flowline and riser (SCR) installation. One such solution is to self-install pipelines and risers from a floating production unit (FPU) using pipe laying equipment that would normally be used on lay vessels. In one such concept, a J-lay tower is mounted on the deck of an FPU, and this can deploy a pipe string that is pulled out by a dynamically positioned tug to its full required length. PipeLay contains two additional sets of features that enable it to be used in the analysis of such operations, as follows:
For many ‘normal’ pipe laying operations, it is not necessary to be able to simulate what is in reality a continuous process through time; it is usually sufficient to examine ‘snapshots’ at selected times throughout this continuous process. However, for the ‘self-installation’ process, this may not be sufficient, because this process involves pulling the pipeline along the sea floor. This pulling process is heavily influenced by friction, and since friction is a highly non-linear, path-dependent process, an ability to monitor the complete installation process may be required. This necessity to monitor the continuous installation process provides the rationale for a pay-out/pay-in capability. You can use this capability to simulate the continuous addition of pipeline at the FPU end, or the pull-in of a cable at the tug end.
In the area of seabed friction, PipeLay provides three additional capabilities as follows:
•A kinetic friction option for dynamic simulations.
•A facility to vary friction coefficients on the Analysis component.
When you define a seabed component, you have the option of inputting longitudinal and transverse friction coefficients. If you want to look at the effect of variations in these parameters, you can of course define a number of different seabeds, and examine the effect of each in turn on your simulation. However a self-installation analysis will typically consist of many steps, the first few of which may not be influenced by friction. It would be tedious to have to repeat all steps of a multi-step procedure with a range of different seabeds if only some stages are influenced by the seabed, so that is why the option to vary friction coefficients is provided as part of an analysis specification.
•A ‘friction sensitivity’ analysis capability.
This is mainly intended for real-time tuning of installation analyses. Obviously any pipe laying operation is typically analysed onshore in advance. In the case of a self-installation process this will generally use assumed friction coefficients. During the actual installation, monitoring of the process might show that actual pipeline motions differ from those predicted by PipeLay because the assumed friction coefficients were inaccurate. In this event, PipeLay provides the option to specify the actual observed pipeline motion, from which the program will calculate by interpolation the friction coefficients corresponding to this motion.
It is important to stress that any or all of the capabilities provided for the analysis of pipeline self-installation from an FPU can be used in the analysis of any of the more ‘normal’ simulations typically performed with PipeLay. These capabilities are described in full in their own appropriate articles, although in most cases a brief comment will alert you to the fact that they are provided for the most part for this specialised application.