ELECTRICAL INSTALLATIONS AT OFFSHORE
OIL & GAS PLANTS
A number of E&P (Exploration and Production) companies are engaged in the recovery of crude oil and natural gas from the offshore reservoirs and hence offshore oil and gas processing plants are quite common all over the world.The incoming oil and gas from the oil wells are separated at these offshore plants and then transported to shore based facilities for recovery of various petroleum products.
In India the main offshore plants are in Mumbai High oil field in the Arabian Sea off Mumbai coast.These plants are the backbone of Indian economy.
This article is an attempt to provide an overview of the electrical systems of these offshore installations.
API RECOMMENDED PRACTICE 14 F PROVIDES ESSENTIAL GUIDELINES FOR DESIGN AND INSTALLATION OF ELECTRICAL SYSTEMS FOR OFFSHORE PETROLEUM FACILITIES.
The offshore plant has to be self-sufficient in power generation capacity as there is no external source of power ( eg. Power from electricity board ).Following are the power generation equipments :
Since natural gas is already available at the plant,it is used as fuel gas for the gas turbines.The whole power generation package (turbine + alternator) is installed within acoustic enclosures.The combustion air and ventilation air is derived through air intake filteration system.The gas turbine and alternator are within separate compartments.Normally, the turbine compartment is maintained at a slightly lower pressure than the atmosphere in order to contain any gas leakages,whereas the alternator compartment is maintained at a slightly higher pressure than the atmosphere to prevent ingress of any gas in case of leaks.
The turbine controls,generator controls and protection panels are housed in a separateTurbine Control Room.The generators normally operate in synchronized mode and the common generation voltages are 11 KV or 6.6 KV ,3 phase,50 Hz.
For initial plant startup and starting turbo-alternators after blackouts,diesel engine driven alternators are installed.These are sized to meet only the emergency and critical loads and are much smaller in rating compared to the main turbo-generators.
The main components of power distribution are as follows :
As the voltage generation is at 11 or 6.6 KV,power distribution transformers are installed for stepping down the voltage to 415 V,3 phase,50 Hz for catering to various process and utility loads (mainly consisting of 3 phase AC induction motors).These transformers are normally oil filled type.To reduce the fire hazard only synthetic transformer oil is used at offshore oil and gas plants.However,the present trend is shifting towards Dry Type Cast Resin Transformers because of there maintenance free nature.
One or many HV(High Voltage) and MV(Medium Voltage) switchboards are installed for receiving power supply from generators and transformers and distribution to various AC loads.These switchboards are housed in switchgear rooms serviced by HVAC systems.For MV Air-break circuit breakers and MCCBs are commonly used.For HV earlier MOCBs were in use but now SF6/Vacuum circuit breakers are being used.
Armoured cables with copper conductors are used in offshore oil and gas plants.These cables are laid in FRP/metallic cable trays in accordance with international standards.In view of the high risk of fire at offshore oil and gas plants ,the present trend is to use FRLS (Fire Retardant Low Smoke) cables for normal operations and FS (Fire Survival) cables for emergency applications.These help in maintaining emergency services (eg. Emergency lighting, communications etc.) even in case of fire.
Lighting at offshore plants can be divided into two categories :
API RP 14 F provides the recommended illumination levels.
Normal plant lighting is the same as in any other plant.It mainly consists of HPMV lamps and fluorescent lamps.
Emergency lighting is required during blackouts or fire shutdowns.It is required to restart the plant safely and in the worst case to abandon the platform if the situation gets out of hand.These are mainly incandescent lamps with centallised battery banks.Nowadays,fluorescent lamp fixtures with individual battery backed inverters are also being used.These lights come on automatically on failure of normal mains power.
At offshore plants , battery banks are installed to keep alive emergency and safety systems during power failures.A few typical systems are listed below :
NiCad (Nickel-Cadmium) batteries have in use for quite a long time because of their long life and superior qualities.However,the latest trend is towards VRSLA (Valve Regulated Sealed Lead Acid) batteries.
These battery banks are connected to suitable chargers.The chargers have float and boost charging modes.During normal operation,the battery chargers float charge the batteries while supplying the load.If normal power fails battery banks immediately take-over the load.
Navigational warning lights emitting morse code ‘U’ signals are installed on all corners of an offshore oil and gas plant as per IALA recommendations.The usual range is two nautical miles.Also,in areas like north sea,fog horns are installed to warn the ships/boats against possible collision with the offshore structure.
Each offshore platform is equipped with a helipad as helicopters are a primary means of logistics support and emergency evacuations.These helipads are provided with blue and amber periphery marker lights.In addition,red aviation warning lights are provided on crane pedestal tops and other high structure points.The guidelines of ICAO (International Civil Aviation Organisation) are followed in this regard.
As in most of the industrial plants,mostly 3 phase induction motors are in use for various applications.Flameproof/Increased Safety motors are used in hazardous areas in accordance with international standards.For long life,motors having insulation class F but temperature rise limited to class B are commonly used.In outdoor locations , enclosure type IP55 is used as a minimum.
The manned offshore platforms have living rooms for the operating personnel.These living quarters alongwith control rooms,switchgear rooms etc. are all serviced by HVAC(Heat Ventilation and Air Conditioning ) systems.These mainly consist of the following major elements :
The HVAC systems control the temperature and humidity of the enclosed areas and adhering to safety norms also keep the areas slightly pressurized with respect to the outside atmosphere to prevent any gas ingress from outside in case of any leak etc.
Any description of an electrical installation is incomplete if we do not talk about the earthing system.The offshore structure stands in the sea on metallic jackets.This structure with sea water all around is used as the main earth sink.This is different from onshore installations where earth pits are used to conduct the leakage and earth fault currents safely to the ground.
At offshore plants, electrical equipments as well as oil and gas vessels are grounded with the help of copper earthing cables and the network is connected to the leg jackets.Proper and adequate earthing is of extreme importance at offshore installations as the metallic decks pose great shock hazard to operating personnel in case of inadequate earthing.Also, electrostatic discharges from vessels may lead to sparks which may prove to be disastrous in the explosive atmospheres.