By Tjarda den Dunnen, Carel Aeijelts Averink, Hatenboer-Water BV
There is an increasing awareness in the offshore drilling industry that safe and potable water is an important factor for good health onboard the rig. Many offshore platforms use bottled water for human consumption to ensure quality. Yet, bottled water cannot be used for every purpose. What about showering? Contamination by the Legionella bacteria can occur if a person inhales water droplets. The logistics and costs for bottled water are also unfavorable. Implementing a good water safety plan would be less hazardous and more efficient.
Bacteriological outbreaks from water can result in commotion and even panic among the crew. In fact, there are known cases where a Legionella contamination led to a rig shut-down. Additionally, corrosion and other deposits can give color to water, making it undesirable to consume.
A range of water-processing and treatment technologies are available to ensure safe and quality drinking water on rigs. With the right materials and installations to purify and condition the water, combined with a proper water management plan, maintenance and control of the water quality from supply to tap points can be guaranteed.
Potable water on offshore rigs can be obtained by bunkering or desalination of sea water. When bunkering water, attention must be paid to the quality of the water provided by the supplier. Always take water samples and check the water on the basic parameters. Furthermore, all necessary steps to ensure safe and potable water is bunkered must be described in a bunkering procedure. For example, flushing of the bunker hose prior to bunkering should be mentioned in this procedure.
Desalination of sea water is possible by a reverse osmosis system (RO) or by an evaporator. Both techniques will produce fresh water, but not all parameters are within the guidelines for drinking water. Due to the desalination process, the produced water has a low pH and no hardness. The produced water should, therefore, be treated to correct the pH value to prevent corrosion in the distribution system, and to provide a better taste.
This can be done by dosing a mineral solution or by installing a mineralizing filter. In the first option, the minerals are directly added to the water by a dosing pump. The more commonly used technique is a neutralizing or mineralizing filter. In the mineralizing filter, the water flows through a filter bed containing mineral grains, and the minerals dissolve slowly in the water.
STORAGE & DISTRIBUTION
After bunkering or production, the potable water must be stored and distributed to the tap points. The storage and distribution system will affect water quality. First, the capacity and material of the potable water tank is important. If the water tank is too big, the residence time of the water will be too long and its quality can be affected.
From the tanks, the water is pressurized by the hydrophore. This is formed by a system of pumps, hydrophore tanks and pipe work. The most commonly used tank is a galvanized tank with air/water, which is vulnerable to corrosion and sediments. A membrane tank would be better, as would a flow-through tank in order to prevent dead ends.
The main piping between the tanks and the accommodation is often made of black or galvanized steel. Steel piping is economically attractive but will corrode in the course of time. This corrosion causes rust water and is a breeding ground for bacteria. Using stainless steel can prevent this. Other options are copper and plastic.
As mentioned earlier, drinking water can be obtained by bunkering or onboard desalination. Normally, the water meets up to the norms for safe drinking water:
A) When bunkered, the water supply company has to follow strict rules on water quality.
B) When produced on a rig, proper water treatment will determine that quality water is produced.
This ensures that the physical/chemical composition of the water will not be a problem. However, the bacteriological quality of the bunkered or desalinated water is also important.
The bunkered or desalinated water, and the biofilm in the piping system, contains bacteria and is vulnerable to decay. If insufficient disinfection is used, bacteriological growth will occur.
Bottled water has no problems with bacteriological growth and seems safer to use on rigs. But what about other bacteria like Legionella, which are not obtained by consumption but by inhaling aerosols? Consumption of bottled water does not reduce this risk.
In water systems, the Legionella bacteria has been a cause of great concern in recent years, particularly around risk areas such as showers, whirlpools, etc. The drinking water system must be periodically maintained, such as, cleaning the tanks, appliances and pipe work. Disinfection techniques, such as ultraviolet disinfection and the dosing of sodium hypochlorite, are also good preventive measures. UV radiation for disinfection is applied in the drinking water system after the hydrophore unit. With UV, no chemicals are introduced into the system. The disinfection effect is caused by damaging the DNA/RNA of the bacteria so they cannot multiply.
As described, the rig’s drinking water system can be complex, consisting of items such as tanks, piping, filters, etc. A risk assessment of the complete system can determine whether the supply can deliver water that meets health-based targets. Assessment of the drinking water system should aim to determine whether the final quality of water delivered to the consumer will routinely meet established health-based targets.
Good water management, combined with education and communication, are critical to preventing problems, and can help save costs on rigs. It also puts the control of water quality in your own hands.