Torsion instability of Flexible Pipes at the TDP - Torsjonsinstabilitet av fleksible rør med sjøbunnskontakt

Dynamic flexible risers are a common term for cables, umbilicals and flexible pipes,that are freely or semi-freely suspended between two points (offshore). Flexible pipes are mostly used to transport fluids between platforms/ships etc. to a flow-line/sea floor installation or similar. The safe operating window for the installation is limited by whether induced motion of the installation vessel giving rise to dynamic tension and curvature at the touch-down-zone (TDZ). By investigation and establishing of calculative methods to determine the actual limits of installation and operation, this paper wishes to further the knowledge of global torsion instability of flexible dynamical pipes in catenary configuration at the Touch-down-point (TDP). Today's methods are conservative and an expansion of the operating/installation widow will have direct economical benefits for the industry. As the filed of global torsion instability in flexible risers have already been touched upon before, this work centres around furthering this knowledge with special wight on larger cross-section diameter as found in flexible pipes. Trough analysis have been made following already established methodologies, as well as a new method made in order to linking local effects and failure modes, to global torsion problems. The results show that not all prediction methods established for offshore cables are suitable for pipes. In addition, it is seen that the most critical waves are not always the larger ones when it comes to the coupling between dynamic heave motion and torsion instability. Lastly it is found from the analysis, that dynamic flexible pipes may very well resist global torsion buckling, even when there is compression at the TDP. On top of this, dynamic flexible pipes are also very good at resisting global torsion failure, while under the influence of other failures, such as flooding of the annulus and local lateral buckling of the tensile armour. The final conclusion is that the presented opinion in newer studies on torsion instability, about the practises of today being to conservative, has been further strengthened and the limits are much larger than first anticipated