The Offshore Operators Committee (OOC) is a consortium of entitities owning offshore leases and/or engaged in offshore activity as drilling contractors, service companies, or suppliers. Member companies are required by the National Pollutant Discharge Elimination System (NPDES) permit for the Western and Central Portions of the Gulf of Mexico to monitor entrainment of passive lifestages (i.e., eggs and some larvae) in cooling water intakes at certain offshore facilities. At offshore platforms, seawater is withdrawn for a number of reasons, including for equipment cooling needs, injection to maintain well pressure, for heating, ventiliation, and air conditioning, to supply the on-board desalination system that provides the crew with potable water, and for the fire suppression system. The NPDES permit requires that if the intake draws more than 2 million gallons per day (MGD) of seawater with more than 25% of that used for cooling, then the operator must monitor for entrainment. The magnitude of entrainment at such intakes can be impacted by the characteristics of the flow field near the intake. In particular, the location of passive lifestages in the water column relative to the hydraulic zone of influence (HZI) of the intake determines the probability of entrainment. It is generally understood that designing intakes with lower through-screen velocities reduces the HZI. Describing the extent of the HZI graphically is often quite useful for illustrating the potential for an intake to entrain organisms.
The OOC retained Alden to conduct a numerical simulation, using Computational Fluid Dynamics (CFD), to describe the flow field around a generic offshore platform cooling water intake structure. The simulation was carried out to visualize the extent of the water column near the intake that is influenced by the withdrawn flow. The simulation was designed to model typical intake flows and ambient ocean currents. The numerical model simulated the intake of 5 MGD at an intake face velocity of 0.5 ft/sec (the NPDES permit regulatory limit on face velocites based on an EPA-defined threshold above which impingement may become a concern). The model simulated an intake consisting of a single cylindrical pipe (53-inch diameter) oriented vertically downward in an ambient current of 0.33 ft/sec (EPA’s generic assumption for current speed for effluent discharge modeling in the Gulf of Mexico). It was assumed that there were no other intakes or structural members close to the intake of interest.
Results of the simulation demonstrate that the HZI of an intake operating under these conditions dissipates rapidly with distance from the intake face. In fact, the region where the 5 MGD flow creates a vertical velocity greater than 0.05 ft/sec extends less than one pipe diameter (i.e., 53 inches) below the intake face. The results show that only eggs and larvae that happen to pass by the intake within a small distance of the intake mouth would be at risk of entrainment, as opposed to the much larger volume of water beneath an entire platform.