A strong Loop Current or an Eddy (LCE) persisted in the Gulf of Mexico through the peak hurricane season of 2015 and impacted several BP floating facilities. It was a concern that if a hurricane passed near one of the floating facilities in the presence of LCE, the combined loading may increase the probability of exceeding the mooring system design capacity. As a result, this type of combined event could be more severe than the original hurricane or LCE only design case. Therefore, a long term combined hurricane and LCE risk was evaluated. Since metocean criteria for this type of combined event is not readily available, two approaches were compared: 1) combining observed average LCE speed at the site with the N-year return period hurricane only metocean; 2) reconstructing a long time series of combined LCE and hurricane data and developing N-year return period criteria from this.
The first approach is straightforward; however, it proved to be too conservative, while the second approach reflects a more realistic scenario. Both approaches were applied to evaluate the long term mooring line risk. The critical environmental direction and tension levels were identified to determine minimum factors of safety on mooring components breaking strengths. The results indicated that the mooring line risk of a 1000-year return period combined event is equivalent to the risk of a 100-year hurricane only event.
The other aspect of Loop Current risk is the potential inability of the platform to get back to the nominal position if it is at an unfavorable position during a LCE event. It is possible that the mooring tension could be above the chain jack operational limit due to increase loading from the LCE while also offset from the neutral position, thus increasing the risk of mooring or riser overloading. Different mooring operational options were compared to select the proper strategy to reduce the operational risks. The final operational recommendations were tested in the field to confirm fitness for operation.
