The most common ways to ensure stability of umbilicals are post-lay trenching and rock-dumping. For a number of reasons, exposed sections of the umbilical on seabed may sometimes be part of the design, e.g. as a temporary condition prior to finalizing intervention measures. Light umbilicals in shallow waters may be subject to hydrodynamic forces resulting in continuing dynamic motions of the exposed sections. If the sea state is sufficiently severe, the umbilical may experience a break-out effect and gradually displace out of the rock-berm over time. Such a gradual degradation of effective rock-berm length will increase the projected drag area of the umbilical. This will in turn render the system susceptible for continued break-out at less severe sea states. The potential consequences of rock-berm failure are damage to the umbilical as well as other assets in close proximity.
This paper presents an approach to evaluate the required length of rock-berm to avoid initiation of the break-out effect. A dynamic FE model is established, using time series of water particle velocities for load calculations. The time series correspond to full 3 hr sea states of irregular waves for a range of significant wave heights. Non-linear lateral resistance curves are applied for the sections of umbilical within a rock-berm, including the hysteretic effect of displacement. The ambition of this paper is to facilitate evaluation of intervention measures for the benefit of the industry, providing a design methodology in accordance with the DNV rules system.