The subsea factory has long been the aim of E&P companies, and step by step it is becoming a reality. One of the key components is technology for the distribution, delivery and control of the subsea power system.

That is the theme of a joint industry project (JIP) that ABB Oil and Gas has been running with Statoil, Total and Chevron with an aim to develop technologies for subsea power transmission, distribution and conversion at greater distances, in deeper waters and in harsher environments. The project started in 2013 and targeted a 3,000-hr shallow-water system test in 2017, including the qualification of pressure-tolerant medium-voltage switchgear, medium-voltage drives, and supporting controls and auxiliary supplies.

The project budget is more than $100 million and is funded by ABB, Statoil, Total, Chevron and The Research Council of Norway. The target environment is water depths up to 3,000 m (9,843 ft), transmission distances up to 600 km (373 miles) and power levels up to 100 MW.

According to Jan Bugge, project director, there are two main challenges with providing subsea power: pressure and reliability.

“You don’t want to go down there every now and then and service the equipment,” he said. “You want a fairly long intervention cycle—years rather than months. On top of that, the systems need to operate at 300 bars [4,351 psi] pressure, so all the power electronics, all the switches, everything, has to work under those very tough conditions.”

Testing times

The project was scheduled to reach a key milestone in late 2017 with the shallow-water test for the drive systems that were expected to start in Vaasa, Finland, in November.

Bugge said two variable speed drives in a parallel configuration will be installed with subsea switchgear and controls and operated in shallow water for 3,000 hours.

“The aim of this is to demonstrate full system function and interfaces under normal operation and fault conditions; gain reliability experience; and demonstrate the suitability of the full-scale prototypes, including thermal properties and marinization for a technology readiness level of 4+,” he said.

The technology readiness level (TRL) scale was devised by NASA researcher Stan Sadin in 1974. Each TRL represents the evolution of an idea from a thought, perhaps an initial sketch to the full deployment of a product in the marketplace. Today’s scale runs from TRL 1 to TRL 9. A technology that has achieved TRL 9 is one that has been incorporated fully into a larger system. To achieve TRL 4, a technology must be validated in laboratory conditions, while for TRL 5 it must be validated in its working environment.

“The requirements for TRL 4+ are the same as for TRL 4 with the additional requirement of demonstrating 3,000 hours of operation in shallow water. At this readiness level the equipment may be part of a real pilot installation,” Bugge said. “Naturally, the thermal performance and cooling efficiency of the subsea drives will be in focus during the test. It is essential to verify that individual material temperature limits are not violated to claim the target design life.”

For the test the units will be as close as possible in design and functionality to units that will be deployed in a future pilot installation; however, the drives will be operated in a back-to-back configuration directly with the grid without motor loads. This is a so-called “power- in-the-loop” test where only power losses need to be supplied from the grid. Testing with a high-power motor load is not done in seawater but rather as part of a factory acceptance test (FAT) before leaving the factory.

The shallow-water test consists of several stages of testing in seawater in a sheltered harbor. “Before connecting the power there will be a period of system testing that will cover various redundant communication tests, black-start sequences, verification of ride-through (energy storage), protection setting adjustments, various breaker and disconnector operations, insulation monitoring system verifications and operation of the drives with one or more cells out of service,” Bugge said. “The topside control system is not part of the qualification but is needed for control and operation of the prototypes.

“The 3,000-hr tests are the culmination of a number of interim tests of the key equipment types. These tests include the first full-scale prototype of the variable speed drive, which [was scheduled to] be tested in shallow water during 2017. In addition, all prototypes will be tested and verified in line with the design specifications during a routine test/ FAT with all control interfaces present and confirmed suitable for the shallowwater test,” he added.

The subsea power substation is a component of the subsea factory being developed as part of a JIP. (Source: ABB)


Next steps

As for bottlenecks in the process, Bugge conceded there are a lot of them.

“You need to have a very systematic approach where you really understand the physics of what is happening and to understand the limitations of the various components. To achieve this, there is a very dedicated creation process,” he said. “We need to understand carefully where the weaknesses are and how we can mitigate those weaknesses so that we get a system that has the reliability that we need, because at the end of the day the oil companies need to produce with a certain probability.

“In terms of technology, numerous advances have been made. These technology developments and the project progress rely heavily on experience, know-how and an ability to deliver, but they are also absolutely dependent on the form of collaboration—a joint industry project,” he said. “This setting gives a strong commitment, a clarity of goals and common purpose and a strong sense and anchoring of the commercial from different perspectives of the partners. We have already seen the fruits of this partnership having passed an important decision gate milestone in April 2015, having verified the technology concept as well as passing TRL 2 for a number of key components.”

Once the shallow-water test hurdle has been cleared, the next stage will be a pilot project, and Bugge confirmed the JIP has several in mind but could not confirm where and with whom.

This article first appeared in Hart Energy's Subsea Engineering News.