Jennifer Pallanich Hull
Special Reports Editor
ABB will install the first offshore High Voltage Direct Current Light systems on the Troll platform in 2004. The two systems in this project, awarded in mid 2002, are slated to begin running in 2005, said Nils Hörle, area manager of HVDC marketing power systems. ABB said the system extends the economical power range of HVDC transmission down to just a few tens of mega-watts.
The work for Troll A, which has the capacity to deliver 100 MMcm/d of gas to Kollsnes from all the Troll field platforms in the Norwegian sector of the North Sea, will feature two HVDC Light transmissions with 67-km sea cables and two 40-Mkw modules on Troll A.
"Power transmission is the backbone of the operation," said Stefan Rinaldo, ABB's vice president of operations and power systems.
The module for a 250-MW station would be about 40 m long, 30 m wide, and 20 m high. The station module would be enclosed in a metallic container to protect it from the humid and salty marine environment.
ABB said it made the HVDC Light modules for Troll more compact than its seven onshore installations.
Taking the electrical energy from land means using less polluting energy. HVDC Light offers improved efficiency, reduced CO2, SOx, and NOx, increased power equipment lifetime, and higher availability of electrical power than gas turbines, Hörle said. Suitable for medium- to small-scale power generation and transmission applications, expectations of an HVDC Light system also include increased production, reduced life cycle costs, and reduced fuel consumption. Removing onsite generation lowers maintenance costs on the offshore platform.
Supplying electric power from shore eliminates greenhouse gas emissions from offshore installations, leading to a significant cost saving for oil companies. One oil company has indicated a yearly savings potential of $13 million in CO2 taxes if it reduces CO2 emissions on the Norwegian shelf by 80% to 90%, ABB said. The greenhouse gas emissions trading system, supported by the Kyoto Protocol, also means cost savings can be realized in areas without CO2 taxation.
HVDC Light can connect a platform to shore, multiple platforms, or shore to platform for energy supply.
ABB said the HVDC Light concept offers advantages in meeting space and weight requirements common to offshore. Because the filters are small and synchronous condensers are not required, HVDC Light can be made compact and lightweight compared to classic HVDC.
The units are quite a step from the days of gas turbines, Hörle said of the benefits his company's technology offers.
"With the ambitions we have, it's going to be hard for the industry not to see this as a real alternative," Rinaldo said.
The HDVC Light concept draws from conventional HVDC technology, which has been around for more than half a century, he said.
"It's a simple concept," Rinaldo said of the technology the company unveiled in 1997. HVDC Light "is pretty advanced technology, but it's still fairly young. There are opportunities we've yet to tap into."
He said ABB has presently stopped the system at 350 MW capability but that the system could increase in capacity by using multiple units.
HVDC Light units can power large electric motors, compressor drivers, and other loads that typically compose an offshore production facility's power demand.
Conventional HVDC technology transmits electricity over long distances via overhead transmission lines or submarine cables. It also interconnects separate power systems where traditional AC connections cannot be used.
ABB said HVDC Light is based on voltage source converter technology, which has come to dominate in electrical drives. Being considerably more compact and lightweight than classic HVDC, HVDC Light enables transmission of electrical power to, from, and between offshore installations where distances prohibit AC transmission.
HVDC Light uses Pulse Width Modulation to generate the fundamental voltage. PWM controls the magnitude and phase of the voltage freely and almost instantaneously. PWM allows independent and very fast control of active and reactive power flows. PWM voltage source converter does not contribute to the short-circuit power, as the AC current can be controlled by the converter valve.
HVDC Light needs no synchronous condenser or local generation, is smaller than conventional HVDC, and is adaptable to multi-terminal systems. Like classic HVDC, it features a better than 99% uptime for single transmission with a scheduled two-day maintenance shutdown once every two years, the company said.
The HVDC Light cable, an extruded polymer cable for HVDC transmissions, was developed along with the HVDC Light concept. The flexible and cost-effective extruded HVDC cable can work in deepwater or be used with dynamic floating platforms. There are no technical limitations to transmission lengths for HVDC cables, ABB said.
HVDC Light can connect various platforms with one transmission link. The basic configuration of shore-to-platform provides a general power supply to a single platform or clusters.
In a platform-to-platform design, Hörle said, platforms of different frequencies, e.g., 50 Hz and 60 Hz, can be combined. Combining these frequencies optimizes operations. Platforms can use Powerformer Light for the generator and reduce – centralize – installed capacity of generators for reduced investment, he said. With the HVDC Light converter, the polarity does not change when the power flow changes, according to ABB. Instead, the current direction changes, which ABB said makes it easy to use a converter as a building block in a multi-terminal system.
A platform-to-shore setup offers the opportunity to reduce flaring or construct an offshore power plant.
Locating transmission equipment on decommissioned offshore installations could also allow postponing the cost of installation removal, according to ABB.
Economics of the system
Using HVDC Light on greenfield projects is one of the most economical ways to harness the power of the technology. The operator must consider a series of economic aspects to fully compare the upside of supplying electrical power from shore to offshore installations with that of local power generation. Consumption of fuel for local generation reduces the amount of fuel sold. On installations with no gas export possibilities, however, gas may be available as a byproduct from oil production.
For greenfield projects, the net present value indicates how much greater the return on investment can be for an electrical power supply than for local generation, ABB said. For modification projects, in which local generation and its infrastructure already exists, it is more difficult to justify use of power from shore, as investments in local generation have already been made. Restrictions on greenhouse gas emission levels, however, may prompt the operators to modify the power generation plan.