Powering an oil rig: choosing the right genset

Installing an engine generator that can handle all your foreseen needs – and be easily expanded above those when the time comes – is probably the most critical part of the whole design and purchase operation.

This applies equally whether it's prime (permanent) or standby (backup) power that you want. If the individual set or matched bank of prime movers (common with rentable low-cost diesels) won't meet all of today's requirements, and next week's too, then it will do more harm than good, simply because inadequate power from the engine/alternator combination will introduce stress at the supply end and cause malfunction, or even physical damage, down the line.

Rule one

Making the necessary calculations and allowing for expansion later on is not difficult; all that's needed being an inventory and an annotated shopping list. Rule number one is always to install generating equipment in a special plant room with plenty of space for putting in, synchronising and maintaining new units later; double the required capacity you calculate right now and then add some is the general rule. Rule two is to have a qualified electrician with you when you add it all up and the salesman calls. And number three is to always buy the best you can afford, a reconditioned known brand (available all over Africa wherever oil is drilled) being always better than an unknown one.

Industrial gensets are readily available from a wide range of sources in sizes from 50kW up; anything smaller is usually called a domestic or pickup-carried model (very useful in logging operations nevertheless). The usual practice with diesel sets (safe, reliable and involving far less capital outlay than a beefier dual-fuel gas turbine) is to install a series of gensets in a side-by-side bank, so that reliable baseload output can be available all the time from a selection of these with additional matching sets switched in and out (via a complicated synchronising process which is usually automated these days; don't try to do this yourself) as the load varies, all the way up to 2MW or more.

Remember that 'starting' load for any piece of rotating machinery is much larger than running demand; this should be specified in the manual. A typical pressure washer or circular saw only needs about 1200W to operate under normal conditions, but when starting from cold 2-4kW will be needed on tap. A standard water pump consumer three times as much power when starting up compared with running conditions. And an air compressor runs on a ratio of about 1:2. Only a few machines like chain saws operate at 1:1.

Never rely on a small portable set to supply standby power when demand tops out, except when the application is a completely off-circuit 'island' one. This will be very difficult to synchronise (match up frequencies) with the main (not mains!) supply and damage to attachments like tools can result.

The inventory-making process is as follows:

• Walk round the whole plant, via all perimeters, noting down every single electrical item that uses power. Don't forget the access control equipment, and the security lights that come on automatically.

• Against each device record separately its power starting and running requirements, in common units. These are nearly always stated on a fireproof specification plate attached to the body, but it will be in the manual too. Some annoying standardisation of units may be needed if the equipment was sourced in an ad hoc way.

• Add these up in either KVA or KW terms.

• Add on the contingency factor recommended by your trusted electrician, in consultation with head office who may know more than you do about site development plans.

• Beware of 'mission creep' – the unwritten rule that says whenever electric power is available someone plugs in something extra. Electric cooking rings are the most common cause of trouble.

• Inform staff after maintenance to start up heavy-load machinery in sequence so that the high initial loads are spread across the whole bank. Never turn individual items on until the anticipated required supply for the next few hours is available, with all prime movers warmed up; this is particularly important at change-of-shift time. If not fully lit up constantly most rigs operate to a scheduled start-up/shut-down procedure, which may be automated.

Apart from piece of mind the benefits of all this calculation and planning will include extended life for all the system components, easing of the regular maintenance schedule, seam-free power supply whenever it is needed, a complete absence of system shutdowns due to overloading, achieving of performance expectations of all powered plant, enhanced safety, and a reduced risk of power surge/dip damage to any of the equipment on board.

But having said all this a completely independent emergency lighting and audible circuit should always be built in, powered to LV wiring standards by lead-acid batteries which are kept constantly at full charge. Both genset and battery circuits need to be tested regularly.