Cooling subsea power

Siemens' subsea power grid visualization. image source: Siemens.

Siemens says its subsea power grid aims to be the first at water depths down to 3000m at long step-outs. Elaine Maslin went to find out more.

At a research center in Trondheim, on the northern coast of Norway, Siemens is close to completing a new system to help power the subsea factories of the future.

Siemens’ says its subsea power grid will negate the need for multiple power cables from power suppliers to power consumers on the seafloor, reducing or eliminating the need for equipment topside.

It will also enable longer step-outs, in harsher environments, and increase oil recovery by enabling more subsea boosting and gas compression facilities. Key to Siemens’ system is containing nearly all of it in balanced pressure environments, including heat-producing components.

By using oil to fill the modules, instead of gases, they will be at the same, or absolute, pressure to the surrounding seawater, reducing the pressure resistance requirements on the vessels they are encased in, and therefore the system’s scale and weight.

The individual modules will be connected by Siemen’s 36kN subsea connector, with power supplied via one HV cable, containing the 36-145kV power, control, and signals cables, with monitoring, and firmware upgrades able to be performed remotely, potentially from onshore, up to 200km away.

Pressure-compensated equipment subsea is well established in the industry. But Siemens is aiming to put nearly the entire system, including electrical equipment and power electronics, in this environment. This means those components that produce heat need to be naturally cooled by convection to the surrounding seawater.

“This is a major technology challenge,” says Jan Erik Lystad, head of Siemens’ Subsea Technology Centre, Trondheim. “I think this will be the state of the art in 10-20 years’ time.”

The 155-tonne 6Mw system, which comprises a 25-tonne transformer, 30-tonne switchgear, and 100-tonne variable speed drive (VSD), or frequency converter unit, has been in development and testing since 2009. A prototype is under construction, with the first transformer complete and in storage, following shallow water testing in 2012. The subsea switchgear is in assembly, and the VSDs completing final testing and qualification to 3000m water depth before assembly and testing in 2013-14.

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The subsea power grid switchgear. image source: Siemens.

Origins

The origin of the system was a project with Norway’s EMGS AS. Siemens was asked to provide a power converter for a towed electromagnetic seabed logger, used for performing electromagnetic surveys offshore.

In the initial version, the power system was contained at atmospheric pressure in a cylindrical pressure vessel. The differential pressure created at depth caused leakage, so alternative means of containing the power systems were considered.

In 2004, oil was used, creating the start of Siemens’ balanced pressure system. It was tested in water in 2005, and qualified to 3500m water depth, or 400 bar. Due to the balanced-pressure system, it has a Plexiglas top. Development on this, and the grid, involved close co-operation with the Norwegian University of Science and Technology in Trondheim. Some 16 towed units have since been delivered and a sixth-generation system is under development.

“The big difference with the power grid is that the system is on the seabed for 20-30 years,” Lystad says. “We have used oil for years in transformer production. But we did not know how it would be when you put it on the seafloor—that had to be investigated and qualified by our qualification project.”

The alternative is to have atmospheric pressure vessels, which require active cooling systems, in addition to thicker walls, making systems significantly larger, with greater numbers of components.

Siemens says it set out to use already proven technology, and qualify it for the subsea environment. The key was working out the cooling systems on an oil-filled balanced pressure system. “We have to control the equipment inside, including what heat it produces,” Lystad says. “The distance between the equipment inside and the walls, and how you construct the housing is important, to create a good convection through the seawater.

Jan Erik Lystad, next to a pressure vessel used for testing components at Siemens in Trondheim. image by Elaine Maslin.

Complexity

The most complex component to design was the VSD module, accounting for about two-thirds of the research and development cost.

“This a complex unit,” Lystad says. “You have a lot of power modules inside, with many parts, and even if we have high efficiency, a lot of heat is generated and this must be removed and transported into the surrounding seawater.”

Siemens’ prototype VSD is built up by total 18 power cells (six cells per phase connected in series). All power cells operate in oil and under pressure. The power cells are cooled by the surrounding oil, which circulates inside the module by natural convection. The oil is cooled through the enclosure, which is relatively thin due to the system being at balanced pressure, by the surrounding seawater.

Since 2009, Siemens started using MIDEL 7131, a biodegradable liquid, which has good insulating properties and is often used as transformer oil where environmentally friendly solutions are required. “The main issue with the oil is when you fill the system, it has to be done very slowly so you are sure oil penetrates every hole and pocket,” Lystad says.

To control the pressure, during lowering, each module is fitted with a pressure compensator, which keeps the pressure inside the module the same as outside as it is lowered through the water column, compensating for both the compression and heat, which influences the oil’s volume.

The only component not contained within oil is the medium voltage switch, within the switchgear. None have been developed in oil to date, Siemens says, so it is contained in an atmospheric canister, which is integrated onto a pressure-compensated base module. While the

prototype is a 6Mw unit, it is based on a modular system, which could be built into larger or smaller systems. The switchgear, for example, is itself built in modular form, so that it could be scaled up or down, by stacking additional sub-assemblies.

Redundancy

The transformer undergoing shallow water testing. image source: Siemens.

The system will have inbuilt redundancy. The VSD has a three phase power circuit, built up by six power cells in each phase. The power circuit has a cell by-pass system, where a fault in a power cell can be automatically by-passed to allow the VSD to continue operation without any intervention. Several cells can be bypassed and still maintaining full power of the VSD.

Condition-monitoring will involve monitoring and analyzing temperature and several other parameters to predict when something might need replacing, in order for preventative maintenance to be carried out during planned shutdown periods.

Siemens started testing components three years ago, in air, then in oil, at pressure, and included heat and stress tests. At its laboratories in Trondheim, Siemens has 23 pressure vessels, of varying sizes, which enables it to test components up to 530 Bar. Conditions in up to 3000m depth have been simulated in the laboratory and every sub-assembly has been pressure-tested.

Components are then tested in water, to perform heat runs. The entire system will be assembled and complete a short dry heat run before it is endurance tested underwater in a former submarine station, near Trondheim, in 2015. The complete system will not be pressure-tested as a single unit until its final installation.

“The next step is how we expand the system,” Lystad says. Siemens has been working on a modular system, with standard size comprising six modules. Bigger and also smaller systems will need to be built, and the system qualified for other ocean-based power consumers or exporters, such as offshore wind turbines or tidal energy farms, but only when it becomes economical within these industries.

“When this started, I thought it would be impossible because of the issues around the liquid and cooling, and that this has to be on the seafloor for 20 years,” Lystad says. The future could see power generation on the seafloor. However, Siemens’ doesn’t think this will arrive any time soon, with public antipathy towards nuclear, which had been an option, and fuel cell technology limited by requiring very pure natural gas and being limited in scale, says Bjørn Rasch, head of Subsea Power, Siemens Oil & Gas.

Siemens’ subsea power grid project is support by equal partner Chevron, ExxonMobil, Statoil, and Petrobras, as part of a joint industry project.