Peritus has performed global dynamic analysis of two flexible risers as part of detailed design for tie-back to a spar platform.

The spar platform is located in the Mississippi Canyon region of the Gulf of Mexico in a water depth of 2200ft.  Peritus’ scope for the riser analysis included:

• Design Premise and Analysis Summary Report
• Dynamic strength analysis
• Wave loading and VIM fatigue analysis
• Interference analysis to assess the potential of riser clashing with adjacent risers, umbilical and top tension risers
• Sensitivity studies to investigate the effects of riser design parameters on the dynamic riser responses

Early work indicated that riser interference was likely to be a design issue, whilst riser strength and fatigue performances were expected to be satisfactory.  Therefore, considerable effort was spent within the project to establish riser headings and departure angles to meet riser interference criteria set by the client. The riser layout arrangement was constrained by the narrow corridor bounded by an existing riser on one side and an adjacent mooring line on the other, and by the requirement to maintain curve stability for the seabed sections of the project risers and umbilical. The situation was further exacerbated by the discovery of the potential for the project risers and umbilical to interfere with each other on the seabed as well as in the water column. Many combinations of riser headings and azimuths were evaluated, and the best solution was proposed to the client and accepted. This allows the subsequent riser global strength and fatigue analyses be progressed to meet project schedule.

The riser layout arrangement adopted by the project was able to prevent interference between the project risers, existing riser, mooring lines and top-tensioned risers. However, potential for interference between the project risers and the project umbilical was still predicted under the loading of extreme currents. This is due to the fact that the umbilical is much lighter than the risers, therefore deflects much more under current loading. The project made a decision to accept interference between the risers and umbilical provided it is demonstrated that the risers and umbilical can withstand the associated impact energy and maintain structure integrity. Furthermore, the project requires that riser VIV motions be taken into account when predicting the impact energy between the members interfering with each other. A new OrcaFlex model was developed to simulate the effects of VIV motions and the corresponding impact energy.

Some other new tools had to be developed to suit the design data and analysis methodology prescribed by the project. For instance, the irregular wave approach was required to be used for wave loading fatigue analysis. In addition, the low-frequency motions of the Spar platform were defined by white noise response with a limited bandwidth.