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Flame-proof motors offshore

Total Anguille

Leroy-Somer’s Charles Notte explains what’s needed to ensure high levels of energy efficiency and equipment use in the harshest of environments

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, isolated hundreds of miles off coast.

The harsh offshore environment poses unique challenges to the engineer and the equipment used. Severe weather, ice and storms put men and machines under extreme stress. Corrosion caused by salt can cause untold damage to equipment.

Limited workspace also increases the risk of accidents. Access to equipment is poor and there is a temptation to ignore maintenance. When something goes wrong, it usually goes badly wrong and lives are put at risk.
On offshore platforms, the risk of explosion and subsequent fire is well known but this still doesn’t prevent accidents happening as the Piper Alpha and Deep Water tragedies testify.
The risk of explosion and fire offshore is permanent. Energy-efficient flameproof electric motors can play a part in maintaining safety and encouraging energy efficiency offshore.

Two thirds of electric motors used in the offshore industry work form part of centrifugal pumps systems – dealing mostly with water and oil. The remainder are employed in cooling systems, although there are some exceptions.

For a platform operator electrical energy use represents by far the largest part of the total cost calculated on the life-cycle of a flameproof electric motor.

IE2 Energy Efficiency
Since June 2011 European law has required that electric motors with power between 0.75 kW and 375 kW should be of IE2 energy efficiency standards. But in some cases, these regulations do not require motors – including those that are concerned by Directive 94/9/EC – to meet these standards.

Under the regulations the decision rests with the operator but with energy use such a critical issue, Leroy-Somer has taken the decision to make the majority of its ATEX ranges available in a flameproof version manufactured to IE2 efficiency levels.

Choosing energy efficient IE2 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 three to four per cent and this is 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 and the impact on the environment (CO2 emissions).

In the field, an increasing number of users are insisting that their equipment suppliers integrate IE2 motors on all the machines they supply.

Variable speed
When the ATEX Directive was first introduced in 2003, the use of motors with variable speed was very limited. This has now changed and variable speed applications can now be found in the majority of sectors including the refinery and petro-chemical sectors.

Variable speed is a key method of reducing energy use and the life of 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. All Leroy-Somer flameproof motors have been tested and approved on variable speed inverters.

Offshore most equipment is used in an explosive atmosphere and represents a potential danger (ATEX Gas Zone 1) and so Ex d or Ex de flameproof motors are employed.

Non-sparking motors
In the ATEX Gas Zone 2 (minimal danger), the use of Ex n non-sparking motors is mandatory. Leroy-Somer high-efficiency electric motors give customers this protection. Motor ranges for potentially explosive dust atmospheres are also available in an IE2 version.

Maintenance offshore
Key to reducing the risk of explosions and fire offshore is maintenance. Re-greasing times for electric motors have increased substantially in recent years. We are getting closer to the maintenance dream of extending the re-greasing time for bearings in electric motors to every three years.

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 8000 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 services 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. Leroy-Somer has recognised this as an important step to reducing wear and tear. Research and development shows that every 10 degrees reduction in the heat on 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.

Component design
With modern electric motors, the lifespan of the 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 been improved, so reducing the power absorbed by the cooling fans. This reduction in temperature rise 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.

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. For more information see Fig 1.

Surface finishes
The environment on a rig platform provides the toughest of tests for electrical equipment. 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.