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Rugged resistor is more than a pipe dream

09 November 2014

Cressall Resistors and Control Techniques collaborate on a project to produce a rugged braking system 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 marine environments. 

Among the many products it offers, 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 Helix owned Caesar, which was launched 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. 

Control Techniques needed a practical and mobile method of dissipating the excess braking energy during the cable laying process. Cressall proposed an eight section resistor using corrosion-resistant, titanium-sheathed elements. 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 obtained resistors from another source, but these suffered from corrosion which reduce the effectiveness of the insulation. Control Techniques’ Teun van der Heiden takes up the story:

“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.

“The pipe is brought out from the stern of the ship and is laid out in an S shape. It’s then lowered 1,000 metres to the bed of the sea, braking with our motors regularly. During this process, 80 percent 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 700Vdc braking voltage which, when combined with atmospheric problems, such as salt, water ingress and a temperature of up to 300°C, creates a pretty tough working environment.”

Insulation resistance also presents an unusual problem onboard a cable laying ship. The dc voltage of the unit cannot be connected to the vessel, which means that any of the re-generative capacities of a motor cannot be used to power the ship - hence the need for such a large resistor. Furthermore, the resistor insulation must be high - 20MO is the standard on the Control Techniques/Cressall unit. 

Another design issue that arose when initially fitting the resistors, was 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. van der Heiden again:

“The biggest challenge the resistor could face would be a situation in which the crew had to abandon ship. 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.

“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 and 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,” says Cressall managing director, Cy Wilkinson. “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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