The braking resistors that will not break

20th July 2018
Alex Lynn

One of the most popular works of Renaissance sculpture is Michelangelo’s David, a piece that has become synonymous with human strength and beauty. Despite its seemingly perfect proportions, a basic design flaw of the statue has recently been brought to light. Recurring micro-fractures in the ankles might cause the statue to collapse under its own weight.

Author: David Atkins, Projects Director at Cressall Resistors

With the rise of electrical drive systems, power resistors are now commonly used in offshore drilling, particularly on mobile rigs and marine vessels of all types. When electric motor driven machinery is stopped, the braking energy needs to either be put back into the power supply or dissipated as heat. However, the sheer size and force of drilling in marine applications means power resistors need to be extremely reliable, rugged and durable.

Drilling applications

Power resistors can be used on oil rigs in the main drives that move the drill up and down. In an environment where space is limited and time is money, the rapid deceleration of the drill is essential and drive overvoltage protection is a key part of the process.

A couple of years ago, Cressall started work on an ENSCO 120 project in the North Sea. The company supplied four forced air cooled braking resistors for the drives system of the rig. Each resistor has a 1200kW capacity and the enclosures, fittings and fan are constructed from type-316 stainless steel, which is perfectly suited to outdoor use. As is the case with all Cressall fan cooled resistors, the cable box is IP56 rated and comes complete with an automatic anti-condensation heater, a PT100 temperature sensor and an airflow sensing switch.

Offshore support vessels

Water cooled resistors are sometimes preferred when an existing water supply and water system are already in place. This was the case with a project that involved several BOA SUB C vessels, for which Cressall provided braking resistors to be used in winches and cranes.

The Offshore Subsea Construction Vessels (OSCV) in question had a control system upgrade and required a new range of resistors. Cressall supplied sea water cooled braking resistors, which were housed on deck in a small fabricated building. The capacity varied between 300 and 400kW and fitted in perfectly with the existing electrical drive system, replacing the less reliable gearbox system that had been in placed previously.

Pipelaying ships

Enormous maritime vessels fitted with heavy lift cranes are used to construct subsea infrastructure that connects oil production platforms to onshore refineries. Cressall has provided several air cooled power resistors for the tensioner systems on the decks of pipe laying vessels.

The client had previously had negative experiences with both air cooled and water cooled resistors, and decided to switch to custom design forced air cooled products, which were fitted in an enclosure on deck. Cressall recommended a 700 to 800kW continuously cooled, type-316 stainless steel power resistor, which fitted and functioned perfectly within the tensioner system and suited the application perfectly.

One of the safest ways to save on design and production costs when purchasing a power resistor is to accurately communicate the product requirements to the supplier. Clear explanations regarding the system layout and specifications, the location of the resistor, the power requirements and footprint can help avoid unnecessary design delays or errors.

After all, unlike sculptures of biblical figures with weak ankles, power resistor design is not an art, but a precise process that results in rugged, reliable products that function perfectly in the harshest environments.

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