On-Site Freshwater Production for Offshore Facilities

Abstract

Abstract Natural supplies of fresh water are limited while the demand for fresh water is increasing worldwide. Production of fresh water at sea is often achieved through a restricted group of conventional desalination technologies including reverse osmosis and modern versions of distillation. Two methods of unconventional water production, one of which is based on the unique set of resources available at an operating production platform, may provide solutions for provision of potable water and for industrial dewatering of process water. Introduction Fresh water is a dynamic resource produced largely by solar energy and stored in lakes, rivers, disappearing glaciers, and groundwater aquifers. Fresh water can be pumped over large distances to supply farming, industry, and population centers, although where this takes place on a large scale, the cost of infrastructure and energy can be very high. When a center of water use is far from a water source or is completely isolated, water produced on site may often be the only answer. This may be especially true where barging or bulk transport instead of pipeline transport is concerned because of the increasingly high cost of energy. Fresh water production from a more saline source water can be achieved by using two primary mechanisms. First, desalination of local saline water into a fresh water product stream and brine reject stream (Fig. 1). Typical methods for achieving desalination are reverse osmosis (RO), or any membrane-based technology, and distillation (MED, Multi-Effect Distillation; MSF, Multi-Stage Flash), which relies on vaporization and condensation. There are presently a host of hybrid techniques where different methods are used on the same water stream to magnify overall efficiency. The second general method condenses water from humid air, without first having to vaporize it. This is known as water harvesting (Max, 2004) and can be achieved on an economical scale depending on the local environmental conditions. Large scale water harvesting to support military expeditionary activity is currently an area of interest to the U.S. Army, which has developed truck mounted apparatus that produces up to 1,600 gallons per day. Additional methods for producing fresh water include solar humidification, which is a direct analog of the natural hydrologic cycle, and clathrate desalination, which is an industrial analog of desalination that takes place naturally when natural gas hydrate forms in marine sediments. Clathrate desalination (Max, 2006, Max and Pellenbarg, 1999) makes use of the tendency of some gases to form solid compounds in natural ocean water at appropriately low temperature and high pressures. Although uncontrolled clathrate formation in pipelines can lead to very costly and hazardous blockages, formation of clathrates under controlled conditions has the potential for sweetening (removal of hydrogen sulfide) and energy density enhancement (removal of CO2). Primarily, however, clathrate desalination has the potential for desalination of seawater and for industrial dewatering (Max and Osegovic, 2004).