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Rugged resistor more than a pipe dream

Cressall Resistors and Control Techniques combine to produce a rugged braking solution for pipe laying vessels

Ruggedness is a much over used word in industry. As a result, in applications where a control system is required to truly withstand the rigours of an unforgiving environment, lots of supposedly ‘rugged’ products fall by the wayside. However, this can’t be said of an ongoing project in which Control Techniques uses Cressall Resistors’ brake resistors on the deck of pipe laying vessels operating in some of the world’s harshest seas.

Amongst its huge range of products, Control Techniques manufactures a motor and inverter combination, which is predominantly used in marine applications. It finds a home onboard pipe and cable laying vessels, including the Caesar, owned by Helix, which is due for launch in 2010. However, one of the key features of the unit is its mobility and, as a result, the same piece of equipment can be used on multiple ships and often needs to be moved quickly from one to another.

pipe laying vesselControl Techniques needed a practical and mobile solution to dissipate the excess braking energy during the cable laying process. Cressall’s solution was to offer an eight section resistor using titanium-sheathed elements, which is completely corrosion resistant. The rest of the unit is made of stainless steel, with a supporting framework that features anti-vibration mounts. Prior to working with Cressall, Control Techniques had sourced resistors from another company, but found that problems continually arose with corrosion on the resistor elements, which reduces the insulation resistance.

Teun van der Heiden, Control Techniques’ account manager on the project, explains how the pipe-laying units work on board ship, “The vessels connect and lay sections of twelve metre pipe to form the overall structure. Clearly, because the pipe is constructed from multiple sections, this results in a lot of braking. On the deck of the ship, there is a tensioner holding the pipe, which is being fed in through a huge system of conveyors.”

He continues, “The pipe is brought out from the stern of the ship and is laid out in an S shape. It’s then lowered 1000 metres to the bed of the sea, braking with our motors regularly. During this process, 80% of the energy that is generated has to be taken into the braking resistor and divided across the eight sections within it. The result is a 700V DC braking voltage which, when combined with the atmospheric problems, such as salt, water ingress and a temperature of up to 300º Celsius, creates a pretty tough working environment.”

Insulation resistance also presents an unusual problem onboard a cable laying ship, because you are not allowed to connect the DC voltage of the unit to the vessel. This means you can’t use any of the re-generative capacities of a motor to produce energy to power the ship, hence the need for such a large resistor. Furthermore, the insulation capabilities of the resistor itself needs to be pretty impressive, with 20 megaohms the standard on the Control Techniques/Cressall unit.

Another design issue arose when initially fitting the resistors, because they produced an unacceptable level of EMC noise on the cable. As a result, EMC cable glands, busbars and more effective connection system were fitted to work together with the resistors, reducing the noise to an acceptable level.

Given the demanding environment in which the equipment is expected to operate, it’s no surprise that steps have been taken to mitigate for failure. There is a spare resistor bank already connected inside the unit, so it’s a simple task for the superintendent onboard ship to change over from a damaged or defective one and use the backup instead.

“The biggest challenge the resistor could face would be a situation in which the crew had to abandon ship,” explained van der Heiden. “In this kind of situation the open end of the pipe would be sealed and it would be winched down to the bottom of the ocean. This would mean constant activity for the resistor until the pipe reached the sea bed. As a result, it has to have the capability to work on a constant regenerative load for an extended period of time.”

As one would expect, safety was also key consideration during the design process. To prevent an operator touching the ultra hot resistor elements when opening the terminal box, a second door, made of Perspex, was installed. As a result, you can easily look inside the unit, without fear of touching the DC voltage. This is a crucial safety precaution on board ship, because the user is less likely to be stable here than on land.

The existing unit doesn’t necessarily represent a fait accompli because the design process is ongoing, as van der Heiden explains, “I think our two companies have found out that we can learn from each other while building these units and, during the last ten projects or so, we have evaluated our results every time. This has resulted in improvements, such as improving the layout of the connection box and making it easier for the cabling companies to connect the unit while onboard ship. We’ve also been able to make the unit more solid and impervious to damage.”

At present, these installations are not approved by Lloyds DNV, or any similar underwriting body, so the units can’t be connected to the ship to provide usable energy. However, both Cressall and Control Techniques are very aware of the energy saving possibilities and are working on ways to connect the systems together on board the ship. When this becomes possible, the energy produced during the braking process could be used to power anything from anchor winches or lighting to propulsion.

“While it sounds illogical for a resistor business to work on ways of replacing the resistors there is method behind this madness,” explained Peter Duncan, Cressall’s managing director. “Large resistors would still be required for emergency stops or in situations where more energy is being produced than could be practically stored. The end result would be a lower energy consumption on the vessel overall. This is something we care about across the board, from working with the Carbon Trust to reduce our own footprint to developing products to help electric vehicles re-use the energy produced in braking for power. It’s a process of continual improvement.”

The launch of the Caesar will certainly come too soon for it to take advantage of the energy saving potential of such future devices. However, it will benefit from the safety features and durability built into the unit overall. So next time you consider describing a product as ‘rugged’, consider the context and if it isn’t operating at temperatures of at least 300º in a salt rich environment, while also coping with the potential for water ingress, think twice!

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