Water Mist Systems for Offshore Facilities a report by Larry W Owen Dooley Tackaberry, Inc., Deer Park
The use of water mist fire suppression systems for offshore drilling and production facilities continues to increase as more people realise the benefits. On offshore facilities where keeping weight loads minimal is crucial, water mist systems can result in weight savings of up to 90% compared with conventional water-based fire-protection systems. Not only are water mist distribution systems lighter but the amount of water in these distribution systems is also much lower. Water mist systems discharge smaller quantities of water so when a mist system discharge occurs, the water damage caused by the system is minor compared with a sprinkler or deluge system. The ability of a water mist system to be used in all parts of an offshore facility including engine rooms, turbine enclosures, living quarters and machinery spaces is also a major benefit of mist technology.
Diesel-driven Pump System
One concern that has arisen in the past is the amount of electrical power needed for a water mist system. Since offshore facilities have to generate their own power, every bit of energy is important. With the use of diesel-driven pumps, much of the power issue is a thing of the past. While diesel-driven water mist pumps have been offered in the past, there are now water mist pumps with a single driver that can deliver 1,600l (422 gallons) per minute at 130bar (1,885psi). Diesel pumps do still require a positive inlet pressure; however, less energy is required to produce 3bar of pressure to supply the pump than the 130bar produced from the diesel pump. The new diesel-driven pump systems utilise engines with approximately 575 horsepower (hp) and a single pump. The single-driver architecture is a departure from many electrically driven water mist systems that use multiple 27kW (36hp) motors to drive 97.4l/min pump sets. When large volumes of water are required, numerous pumps must be used, resulting in sizeable power requirements. For example, it would take sixteen 27kW motors to match a single diesel driver. The electrical energy required for an electric driver is 432kW while the diesel driver requires almost no external power.
Benefits of a Water Mist System for Offshore Facilities Water mist systems on offshore facilities distribute water for fire suppression by two methods: sprinklers and spray heads. Sprinklers are used primarily in the accommodation and service spaces and are
Since 1970, Larry W Owen has worked in a variety of fire protection roles including consulting, project management and business development. Over 33 years of Larry’s experience are directly related to the refining, petrochemical, offshore and power industries. He has a BA in Fire Prevention Technology and Fire Administration and serves as Chair of the National Fire Protection Association (NFPA) 750 Standard on Water Mist Fire Protection Systems as well as being a technical committee member of the NFPA 15 Standard For Water Spray Fixed Systems For Fire Protection.
automatically activated by high-speed, heat-sensitive glass bulbs. Spray heads (see Figure 1) are used in machinery spaces and are activated by opening a section or control valve. When the section valve is opened, all of the spray heads downstream of the section valve will discharge at the same time. Opening the section valve can be accomplished automatically or manually. Stainless steel tubing is used to distribute the water from the pumps to the sprinklers and spray nozzles.
Offshore accommodation and service areas have traditionally been protected with sprinkler systems. The old sprinkler systems used pipes and delivered quantities of water that could result in substantial water
The ability of a water mist system to be used in all parts of an offshore facility is also a major benefit of mist technology.
damage in addition to fire damage. Water mist systems use small-bore tubing and deliver approximately 10% of the water discharged from a sprinkler system. As the water is discharged in the form of clean water mist, there will be very little water damage from the mist system.
In galleys, water mist systems can also be used and special nozzles are available for deep fat fryers. The nozzles are supplied high-pressure water from the centralised pump supplying the accommodation sprinklers and other areas of the facility.
Power-generation turbines and compressors can also be protected by water mist systems. Properly designed water mist systems have been proved to be efficient at extinguishing flammable liquid fires in turbine enclosures and not causing any damage to the turbine. Tests have been conducted by various agencies to validate the extinguishing capabilities of the water mist systems and that the mist discharge will not result in damage to the turbine.
All offshore facilities have sizeable machinery spaces for ballast, process or pumping equipment. These spaces can also be protected with water mist systems. Water mist is totally harmless to humans and can be activated the moment a fire is detected. Water mist systems do not require time delays to ensure personnel are out of the protected space since the mist will not harm personnel; the fire will be attacked early on, reducing concomitant damage.
Unlike gaseous extinguishing systems, which require tightly sealed enclosures and often leak the gaseous agents, water mist systems can tolerate spaces that are not sealed as tightly. Once a gaseous
90 © TOUCH BRIEFINGS 2011
HSE
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