Depth charger

What is believed to be the world’s most powerful subsea motor will drive seawater injection pumps to help extract oil and gas from the Norwegian Sea next year.

Hayward Tyler, a UK manufacturer of motors for the oil and gas industries, has built the first of three 2,500kW motors for the Statoil Tyrihans project.

This embraces two structures, Tyrihans South and North, where it is estimated the recoverable reserves are around 182 million barrels of oil and condensate, and 34.8 billion m3 of gas.

Extracting all this oil won’t be easy, which is why it will require powerful pumps to inject seawater into the bed and force more oil to rise to the top.

The two pole variable speed motor was designed electromagnetically to deliver 2.5MW and speeds up to 4000rpm. This was a challenge for Hayward Tyler engineers whose two pole motors were originally designed to generate power of around 1.2MW at 3600rpm.

The power output was the largest they had ever produced for a two-pole motor. ’The largest we had made previously was less than 1.2MW,’ said Peter Dutch, a design engineer involved in the Hayward project. ’It was quite a step change in size for us.’

Dutch said one of the main design challenges was cooling the fluid-filled motor. ’There is a lot of heat generated, and we were losing about 400kW,’ he said.

So the engineers developed an internal circulation system that redistributes cooling fluid so the motor’s wire and bearings work at operable temperatures.

But the lost heat does not just disappear. The Hayward team designed a way to dissipate it into the sea with an external cooling coil. Dutch could not give specific details about the design of the coil because the company is waiting for patent approval, but he said that compactness and resistance to marine fouling were some of the challenges his team faced.

’We had to prove these components would last up to 15 years in a subsea environment,’ he said.

Another major challenge was designing a method of delivering electrical supply in a high-pressure underwater environment — the Tyrihans field is located at a depth of 285m.

So the engineers developed a high-voltage penetrator — an electrical connector — that overcomes the extreme pressure and delivers power to the motor’s windings.

’We made sure every stage of this operation was well insulated,’ said Hayward’s senior electrical engineer Ayad Al-Khoury.

This is critical, because if just one drop of water meets the motor’s windings the entire unit will malfunction faster than a mobile phone being dropped in bath water.

The motor’s stator was insulated by winding it with 2km of insulated cable. ’We had to insulate for up to 11kv maximum voltage, and it had to resist a high differential pressure and survive various loadings and heat shock loadings,’ said Al-Khoury. ’This involved an extensive test programme.’

The penetrators have already been tested underwater and testing on the motor and pump is progressing at various depths.

Al-Khoury said the Hayward team believes the 30-tonne unit is unique in its size. ’There have been smaller designs but this is the largest completed,’ he said.

Rupert Knowles, Hayward’s engineering manager, said their two years of work on the design and building of the subsea motor taught them that they can deliver more power if they need to in the future.

’We think a 5.5MW motor is the next generation,’ he said. ’And with our experience we think we’re well equipped to achieve that.’