The Port of Sohar (Oman) is protected by two breakwaters which were damaged during the 2007 tropical cyclone “Gonu”. Extensive surveys by third parties have been carried out in the past to determine the extent of the damage and studies were made for repair and upgrade designs. However, the costs for the proposed repairs showed so large that in 2016 repairs still had not been carried out.
When asked for a its opinion, Royal HaskoningDHV advised the port that in the decision making process to repair the breakwaters an important item was overlooked, i.e. that also the repaired breakwaters would have a certain risk to be damaged. Consequently, the repair should only be made in case the economic risk without repair would be (much?) larger than the repair costs + economic risk after the repair, i.e.:
If Present Risk >> Repair + Future Risk, then do the repair
The present risk looks at the probability of failure and the economic consequences of such failure. This takes into account the current (damaged) status of breakwater cross sections. The repair costs are affected by the method of repair. Hence prior to deciding to repair, one needs to have an impression of the costs of such repair. The future risk looks at the probability of failure after a repair and the economic consequences of such failure.
Following these observations, Royal HaskoningDHV was requested to quantify the above evaluation in a Cost Based Risk Assessment. An important element in this quantification was formed by finding a relationship between the damage to the concrete armour units (predominantly showing settlement) and the occurring wave conditions. Such relationship was established based on the model tests carried out in the past and relates the number of settling armour units (Nod) to the incident wave conditions (Hm0, Tp), the packing density of the armour units (ϕ) and the number of waves in a storm (Nw). Using this established relationship the present and future risk of the breakwaters could be quantified, taking into account the probability of occurring wave conditions. The approach to the cost based risk assessment, the damage function and the results of the analysis will be presented in the paper.