Quantification of rockfall risk for submarine pipelines and umbilical lines

Much like their onshore counterpart, offshore pipelines and flowlines can be subject to rockfall impact when crossing nearly vertical slopes such as shelf-edge carbonate cliffs. In subaqueous environments, quantification of the risk is often limited by lack of direct observations of historical events and by an incomplete characterization of the fracture properties of the rock-mass. Here, a statistically robust methodology is presented, aimed to overcome the above limitations and estimate the probability of damage by rockfall impact on pipelines and umbilical lines, with examples from a project offshore Mozambique. Ultra-high resolution bathymetric datasets suitable for high-gradient cliffs were used to identify rockfall source areas and map individual boulders at the seabed. Hundreds of thousands rockfall events have then been simulated; simulation results have been fed to a dedicated FEM model aimed to derive the critical value of kinetic energy necessary to induce non-acceptable damage to steel pipelines and umbilical lines. Age dating techniques were used to constrain the temporal frequency of rockfalls and finally compute the joint annual probability of damage for several line configurations. Results allow a risk-based comparison of several scenarios and line configurations and inform the design of appropriate mitigation measures such as active and passive berms.