Thermal Discharge Recovery For Offshore Lng Port Pelican Terminal
Authors: Collins Michael A. , Borey Roland B. 
Published: October 2008

The proposed Port Pelican deepwater port will be an offshore Gulf of Mexico (GOM) facility located in about 80 feet of water approximately 40 miles off the Louisiana coast to receive and then gasify liquefied natural gas (LNG) for delivery via pipeline to onshore facilities. Seawater will be withdrawn from about a 60-foot depth, passed through heat exchangers to warm the LNG to a gas, and then discharged at a reduced temperature to the GOM. The US EPA's 3- dimensional jet merging model "Visual Plumes" was used as the basic modeling tool to examine the mixing between the cooled "once-through" discharged seawater and ambient receiving waters. Supplemental methodologies were used to address model limitations arising from sea bottom boundary effects. The modeling examined summer and winter temperature stratification conditions for two current conditions and different discharge manifold designs, and defined a "region of influence" for a specified temperature differential between discharged and ambient waters.

Introduction

Port Pelican Terminal is an LNG receiving, storage, and gasification facility proposed for construction approximately 40 miles from the coast of Louisiana in the GOM south of Vermilion Bay. The primary feature of the facility is a large concrete-based structure (GBS) that will rest on the sea bottom. The GBS will house and support unloading, storage, ORV process equipment, transfer equipment, and pumping facilities. LNG carriers will bring LNG to the facility where the LNG will be vaporized before delivery via pipeline to onshore distribution facilities. See Figure 1. An open rack vaporizer (ORV) process system will use the ambient heat in the seawater to vaporize the LNG. Seawater will be drawn into the facility and passed through the vaporizer system, with a resulting lowering of the temperature of the seawater returned to the GOM. The result will be somewhat analogous to the heated water discharge from a power plant, except the temperature differentials are reversed. Cool, rather than warm, water will be discharged to the ambient receiving waters [EIS, 2003].

To assess potential environmental impacts, the mixing of the discharged cool water with the warmer receiving waters was examined, pursuant to National Pollutant Discharge Elimination System permit application, to determine the region over which the mixing will occur and the temperature conditions in this mixing region. Numerical modeling was used to evaluate the mixing behavior. This discussion describes the thermal discharge modeling and highlights some of the modeling results.

Setting

The cool water discharge is expected to occur over a water column about 80 feet deep at temperatures as great as 20°F below the ambient receiving seawater. Discharged waters will be the same salinity as the receiving waters. The ambient source and receiving seawaters will have a seasonally dependent stratification ranging from a very small temperature gradient (approximated as a zero gradient) to an approximately linear gradient of some 25°F in 70 feet. Currents in the vicinity of the terminal approach 0.5 feet per second.