Special Report: A no-emissions offshore solution

Power over long distances is just one of the advantages of using HVDC, writes ABB group senior vice president and chief technology officer, industrial automation division, Bernhard Eschermann

HVDC is ideally suited to deliver electric power via high performance cables to remote platforms.
HVDC is ideally suited to deliver electric power via high performance cables to remote platforms.

Some of the world’s largest cities receive huge volumes of electric power, often transported over thousands of kilometres. National and regional power grids throughout Europe, the Americas and Asia interconnect their power grids and trade electricity with one another in real time. A crucial technology that has changed the way that electric power is transported is HVDC (High voltage direct current), which results in extremely low transmission losses.

Another benefit of HVDC, particularly with ABB’s HVDC Light technology, is that it can bridge larger distances with high-performance and environmentally friendly cable connections. This makes power transmission through the ocean possible, be it for sourcing power from large off-shore windfarms or for bringing power to offshore oil and gas platforms. The world’s first such power-from-shore solution was the Troll A project in the North Sea.

ABB installed two compressors on Troll A and provided the devices with an electrical drive system solution that brings together two ground breaking technologies: the first power-from-shore solution and the first commercial installation of very high voltage synchronous motors, each with a power rating of 40 megawatt (MW).

The solution takes 132 kilovolt (kV) power from the grid and turns it into DC at the converter station on the mainland. Two of ABB’s HVDC Light power cables, each with a capacity of 40 MW then transmit the power to 60kV to Troll A, averaging a distance of 70 kilometres. The power is turned back into AC by a compact lightweight converter station and fed to the two ABB motors that drive the compressors. Bringing power to the compressors provided a huge benefit compared with installing gas turbines on the platform – the conventional solution - because the capital, operating and maintenance costs were all significantly reduced. The environmental impact of running the compressors decreased by 230,000 tonnes of carbon dioxide a year up to the expected end of the productive life of the field in 2063.

The offshore industry’s main requirement of any power supply solution is high availability, since an emergency shutdown means loss of production capacity and income. An HVDC Light installation typically has 99% availability per year, taking into account both scheduled and forced outages. The lifetime is, on average, 30 to 40 years, which is higher than a local generation solution. Efficiency rates vary depending on the transmission distance and cable type. With an HVDC Light transmission system there is often more than a 90% efficiency rate.

Coupled with the requirement of high availability is the cost-effectiveness of the transmission system. Each potential user of HVDC Light for offshore power is unique in terms of power rating, distance from the platform to the mainland, sea depth etc. Initially the equipment cost to install generation on the platform can be lower than the cost to install a power-from-shore system, especially when power ratings are low, the sea route is deep and the distance from the mainland is long. However, by using an electricity supply from the mainland, weight and volume on the platform are greatly reduced, enabling significant savings. A small and compact HVDC Light solution may also enable a complete onshore assembly, with shorter installation and commissioning time, thus further reducing costs.

In terms of operating costs, power-from- shore alternatives require less maintenance and shorter maintenance shutdowns. Power-from-shore solutions produce no emissions, so there is no risk of emissions taxes being levied. From a health and safety perspective, power-from-shore eliminates the hazards associated with gas-fired rotating equipment operating in the vicinity of platform workers.

Technical background

Initially power-from-shore solutions were limited to AC cable systems over short distances, or conventional line commutated HVDC systems. The introduction of HVDC Light with voltage source converter (VSC) technology in the late 1990s opened up the market segment, because VSC technology enables a cost-effective supply of large amounts of power over long distances using robust, lightweight, oil-free cables.

Power from shore can be supplied via an alternating current (AC) system as well. The Al Nasr Full Field Development Project located off the UAE coast includes a 132 kV AC subsea ring cable from Das Island, based on ABB’s System 800xA Extended Automation control system. AC is simpler and cheaper to install, but it is not as effective as DC over long distances due to high cable losses. HVDC systems take alternating current from the land grid and convert it to direct current at a land installation before transmitting the electricity in cables out to the platform. The power is converted there back to alternating current which can be used by the offshore systems. A direct current system is preferred where there is a need to transmit large amounts of power with cables longer than 40-50 kilometres.

HVDC Light does not need any short circuit power to operate, which makes it an ideal technology to start up and energise offshore platforms and allows for fully independent control of both active and reactive power.

During a black start, active and reactive power is automatically adjusted according to the ongoing power need of the platform. The HVDC Light link will balance the load generation automatically as new loads are connected, instantly changing the transmitted power according to the size of the load.

By replacing costly onboard electrical generating systems that burn natural gas or diesel fuel, companies can increase available space and reduce weight on the platform. For a group of platforms, the converter station can be located on one unit and power can be distributed through short AC cables to several platforms.

As a key enabler in the future energy system, HVDC Light is truly shaping how oil and gas companies are powering their plants regardless of geographic location.

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