Flexible Joint Stiffness Modelling Improving Ultra Deep Water Rigid Risers Fatigue Damage

Abstract

Fatigue is one of the key governing conditions in the design of rigid risers, in particular those in ultra-deep water. One effective way of improving fatigue is to adopt a lazy wave configuration, rather than a simple catenary. Steel Lazy Wave Risers (SLWR) have been successfully used offshore Brazil (Hoffman et al. 2010, Oliveira et al. 2017) and in the Gulf of Mexico (Beattie et al. 2013), and have been considered for the North Sea (Felista et al. 2015) and offshore Australia (Vijayaraghavan et al. 2015). Yet, it is probably the most computational-intensive aspect of it. Fatigue analyses require a very large number of load cases to be run, on complex, non-linear models. Methods for simplifying aspects of the analysis are highly desirable, but they must be weighed to provide the required safety levels whilst not introducing uneconomical, overconservative assumptions. The top first weld is a crucial hotspot, in particular for production SLWRs (Senra et al. 2011). These typically adopt flexible joints (FJ) at the connection to the vessel/platform, and linearization of the FJ stiffness is one of these key simplifications that bring significant value in reducing analysis cost. This paper describes a method for estimating the characteristic angle used for the linearization, which results in significant stiffness reduction in contrast with the usual, simpler method. Non-linear FJ stiffness curves are usually available, and they provide stiffness associated to the FJ absolute angle. The FJ stiffness significantly reduces with the angle of rotation. The conventional method adopts the stiffness corresponding to the most likely riser angle – absolute value measured from the static configuration. Conversely, the proposed methodology for estimating the most likely change in angle. As the angles often turn up in alternate angles, the proposed method results in much higher characteristic angle, and hence much lower FJ stiffness. The outcome is significantly less conservative designs, whilst still meeting the same required safety margins.