Torsion instability of Dynamic Cables during Installation

The oil and gas industry today is a heavily regulated segment, and current standard hasestablished restrictions which yields a very limited weather window for offshore cable installations. This is due to experience with cable failure in harsh weather. A limiting factor in current practice is the design criterion for the minimum allowable radius of curvature in the touchdown point. In addition, current practice does not allow for occurrence of compression in the touch down zone, since this is associated with loop formation and loss of the functionality of the cable. The main purpose of this thesis is to investigate the validity and conservatism of current practice, by investigating the issues related to torsion instability through a proposed design procedure. The scope of the thesis comprehends numerical analyses using the SINTEF OCEAN developed software SIMLA. The numerical studies concern a J-lay installation scenario at 100 meters water depth with an implemented cable laying vessel with realistic RAO properties. Two real-life cable cross-sections are evaluated; a single-layered cable without armoring (umbilical1) and a torsional balanced cable with double-armoring (umbilical 2). The mechanical properties of these are calculated analytically, where the main differences lay in the torsional stiffness of the cross-sections. The effect of installation route is evaluated, and two scenarios are simulated; an end cap turn and a curved route to avoid obstacles. The results are obtained in two steps. Primarily, the critical values of torsion momentand resultant curvature with respect to loop formation are established. These parameters are determined for both a linear and a non-linear pipe material model. Subsequently a range of dynamic analyses with irregular waves are carried out. The irregular waves are applied using a Pierson-Moskowitz wave spectrum with Hs= 2−3mandTp=7−10s with a one hour simulation time. In the dynamic analyses, the torsion moment and curvature in the cable are analyzed and compared with the capacity parameters for each scenario. For umbilical 1 it is found that the maximum resultant curvature for all sea states exceeds the critical API curvature, while the torsion moment is below the critical torsion moment. The sea states can, due to excess of the API criterion, not be classified as acceptable. It is also concluded that a contributing factor is the low magnitude of the horizontal bottom tension. The maximum resultant curvature for umbilica