Extreme Coastal Inundation Under Different Climate Scenarios: Fourchon Junction Case Study

Abstract

Port Fourchon Junction is located within Chevron's Fourchon Terminal, just south of Port Fourchon and is operated by Shell Pipeline Company LP. This manifold metering station is a critical junction for the Mars Corridor oil, as oil production from Mars (MC-807), Ursa (MC-809), Titan (MC-941), Who Dat (MC-547), Medusa (MC-582), and Olympus (MC-807B) flows through this station via a 24" pipeline. Port Fourchon is at the edge of the Mississippi delta facing the sea, one of the world's most vulnerable low-elevation coastal zones. It is highly exposed to storm surge and wave-induced inundation under hurricanes which regularly visit the Gulf of Mexico. In addition, it experiences one of the largest rates of subsidence in the world, which combined with sea level rise, will increase the site vulnerability in the coming decades. This study assesses present and future scenarios of subsidence and sea level rise under extreme metocean conditions induced by hurricanes and their impact on Port Fourchon Junction. Local effects such as the differential settlement of the barrier beach have been also considered. Using results from the numerical model XBeach, a set of different present and future scenarios are modelled under extreme metocean conditions. These conditions and the subsequent design parameters calculated, are not obtained through traditional extreme value analysis methods, instead, they are estimated through the influence of boundary conditions forced with the corresponding return period values of the parameters. Boundary conditions for the simulations are extracted from Grand Isle and Port Fourchon sea level observations, and from FEMA and the Water Institute of the Gulf simulations. Port Fourchon site should be subject to flooding for 10-year return period conditions based on Grand Isle observations. For 5-6 years return period conditions some degree of milder partial flood should also be expected. This is well captured by the model. While the highest inundating level is mostly dependent on winds, waves and surge acting together, surge is the single most critical parameter that defines the asset's base inundation level. Design future conditions based on surge extreme from FEMA simulations are recommended over surge extremes derived from Grand Isle observations. The barrier beach and the breakwaters play a key factor in sheltering site from waves and surge. Even when submerged under extreme high return period conditions they dissipate the waves ensuring that the maximum water level (wave crest elevation) on site is lower than would otherwise be without them. It is then important to maintain them fit for purpose during the entire lifespan of the asset. Both Grand Isle and Port Fourchon subsidence scenarios yield similar results. Based on the importance of Port Fourchon Junction facilities, the design criteria obtained, and the higher subsidence level observed at Port Fourchon (compared to Grand Isle), it is recommended that a 1000-year return period and future scenario based on FEMA surge level and Port Fourchon Relative Sea Level Rise (RSLR) is adopted for design. The subsidence associated to this scenario is 9.8 mm/year. The sea level rise associated to this scenario is 2 mm/year.