Flameproof motors in the offshore environment
31 October 2013
Drilling for oil and gas offshore is without doubt one of most dangerous professions in the world. The risks are unavoidable. Workers are on shift for an average of 12-hours a day dealing with highly combustible materials on a platform surrounded by heavy equipment, often isolated hundreds of miles off the coast.
The harsh offshore environment poses unique challenges to the engineer and the equipment used. Heavy seas, severe weather, ice and storms put men and machines under extreme stress. Corrosion caused by salt can cause serious damage to equipment.
Limited workspace also increases the risk of accidents. Access to equipment is often difficult and there can be a temptation to cut corners on maintenance. When something goes wrong, lives can be put at risk.
On offshore platforms, the risk of explosion and fire is well recognised, but accidents continue to happen. In this environment, the latest energy-efficient flameproof electric motors have a part to play in maintaining safety and improving energy efficiency.
Two thirds of electric motors used in the offshore industry power centrifugal pump systems, dealing mostly with water and oil, and most of the remainder are used in cooling systems.
For a platform operator, electrical energy represents by far the largest part of the total cost in the life-cycle of a flameproof electric motor.
IE2 Energy Efficiency
Since June 2011, EU law has required that electric motors of between 0.75 kW and 375 kW output should conform to IE2 energy efficiency standards. However, an exception has up until now been made for motors for use in explosive atmospheres as defined by ATEX Directive 94/9/EC.
Under the regulations, the final decision rests with the operator. But with energy use such a critical issue, many will look to install ATEX-certified flameproof models that conform to IE2 energy efficiency levels.
Choosing IE2-rated electric motors makes good sense for the customer. There are two reasons for this. Firstly there are the energy saving benefits. IE2 motors will reduce the operator’s energy bills by up to 4%, a considerable cost saving on an offshore platform.
Secondly, more and more operators want to be seen to be environmentally friendly. So buying and using an IE2 efficiency level motor highlights their willingness to reduce energy use, CO2 emissions and other impacts on the environment.
In the field, an increasing number of offshore users are insisting that equipment suppliers provide IE2-rated motors across their product ranges.
When the ATEX Directive was first introduced in 2003, the use of variable speed motors was very limited. This has now changed and variable speed applications can now be found in the majority of sectors, including the refinery and petrochemical sectors.
Variable speed is a key method of reducing energy use and extending the life of the equipment. It means that the machine only uses the amount of energy it needs at that particular time – and is not running flat-out during periods of low use.
Offshore, most equipment is used in an explosive atmosphere where there is potential danger (ATEX Gas Zone 1) and so Ex d or Ex de flameproof motors are employed. In occasional danger zones (ATEX Gas Zone 2), the use of Ex n non-sparking motors is mandatory.
Key to reducing the risk of explosions and fire offshore is maintenance. Technical advances have extended the intervals between motor re-lubrication substantially in recent years, with periods of three years between bearing re-greasings now a realistic possibility.
On offshore platforms most motors operate in pairs – with one in service and one on standby. A modern IE2 efficiency standard 37-45 kW electric motor running at 1500 rpm at an ambient temperature of 25oC can now go 20,000 hours without the need of a service.
With platforms operating 24 hours a day, seven days a week, this equates to more than 8,000 hours of use a year. The last decision operators want to make is to shut down for maintenance – any halt in production is very expensive. So increasing the period between servicing is a key issue for electric motor manufacturers, and I have no doubt that maintenance times will be extended as the machines are developed further.
One method of improving longevity and maintenance costs is to reduce the heat on electric motor windings. Research and development shows that every 10oC reduction in the temperature of the windings increases the lifespan of the electric motor two-fold. There are three advantages with this:
* longer winding service life
* increased operating range on the drive
* a greater ability to withstand load impact.
With modern electric motors, the lifespan of bearings has increased from 25,000 hours to 40,000 hours. There is now an extended lifespan for machines running at high temperatures up to 55°C. So component design is very important. Low-loss magnetic laminations are employed and the end shields are ribbed to ensure maximum heat dissipation. The profile of the housing cooling fins has also been improved, so reducing the power absorbed by the cooling fans. This reduction in temperature has increased the life of the windings (more than 10,000 hours on average) and reinforces the capacity to accept brief overloads.
Cutting edge cover and fan design also reduces the noise levels – 5 dBA for example on a 30 kW motor running at 3000 rpm.
The latest high efficiency three-phase motors have a range of special surface finishes to protect them from the effects of harmful atmospheres and corrosion.
Three 110kW flameproof motors were installed on the Total Anguille platform
Equipment Protection Levels (EPLs)
Elsewhere, the IEC EN 6000 regulations governing the use of electrical equipment in explosive atmospheres continue to evolve. Recent developments include new revisions allowing risk assessment to be applied in selecting equipment for explosive atmospheres and the introduction of Equipment Protection Levels (EPLs).
These have been introduced to make risk assessment and equipment selection easier. EPLs identify and mark all products according to their ignition risk. So a motor’s EPL will indicate its inherent ignition risk regardless of its protection level.
The standards governing motors for use in ATEX gas and dust zones are continually evolving too. Since June 2012, users will have noticed modifications relating to how motors are marked.
Case Study: Anguille Field, Gabon
Leroy-Somer has a dedicated oil and gas industry team based in France which supports oil and gas industry infrastructure projects throughout the world. In 2011, Leroy-Somer took part in the retrofit of a Total platform in the Anguille Field, off the coast of Gabon in West Africa.
One of the most difficult problems to solve was the replacement of three centrifugal pumps. Newer more efficient pumps were needed to help increase oil production on the platform but at the same time it was necessary to increase the power of the drive motors from 90 kW to 110 kW.
In partnership with Sulzer Pump, three new Leroy-Somer 110kW flameproof motors were chosen for the platform. They had the same inrush current as the existing 90kW motors and this meant there was no need to replace the existing cables and switchboards – making the job easier and saving time with the retrofit.
Voltage drop on the platform was also a problem. When installing a flameproof motor with a starting current < 5.5 it is difficult to ensure enough starting torque if machines are required to start pumps with a voltage drop of as much as 20%. But the versatility of the Leroy-Somer flameproof motors allow the pumps to continue to operate despite these challenges.
The paint finish on the electric motors was also a deciding factor in the platform operator choosing Leroy-Somer. In the harsh offshore environment, electric motors have to be tough enough to withstand high levels of corrosion.
Under the requirements of the ATEX Regulations, Leroy-Somer flameproof electric motors are protected by a range of surface finishes. This includes end-shields, terminal boxes, housings, fan covers, ventilation grilles and accessories.
Most of Leroy-Somer’s ATEX ranges are now available in a flameproof version manufactured to IE2 efficiency levels, offering operators both improved safety and energy efficiency.
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