Powering platforms

The power demand of an offshore installation – such as an oil or gas platform – is substantial. Depending on the installation’s process and equipment, the required power may be from a few megawatts to hundreds of megawatts.

The operator of a platform has two options to power all the local machinery: generate electricity on site with gas turbines that drive generators or get electricity from shore via subsea cables.

While offshore gas turbines seem to be a natural choice, in reality they are not as advantageous as power-from-shore.

Offshore gas turbines operate at 35-40% efficiency while onshore gas turbines operate at greater than 80% efficiency with waste heat recovery.

Offshore gas turbines consume large volumes of gas that could otherwise be sold, produce large quantities of CO2 emissions, have significant and costly space and weight requirements, and require regular maintenance.

In addition to better energy efficiency and environmental benefits, transmission of electrical energy from shore involves less maintenance, longer lifetime and higher availability than gas or diesel-driven local power generation.

Where installations are located close to shore, electrical power can be transmitted from shore using alternating current (AC) cables.

In 2010 ABB delivered the world’s first power-from-shore dynamic AC cable connection.

It provides Statoil’s Gjøa floating oil and gas platform in the Norwegian North Sea with 40MW of electricity at 115kV AC along a 101km cable from the Norwegian power grid.

The solution connects, for the first time, an offshore floating platform to the onshore power network.

Goliat FPSO platform is also being powered from the Norwegian power grid via a three-core static AC cable that will run 105 km along the seabed.

A 1.5km dynamic AC cable connects the platform to the static AC cable, 350m below the surface.

Eni Norge chose this option as it would enable them to use renewable hydropower from the Norwegian power grid and reduce the platform’s carbon dioxide emissions by an estimated 50 percent.

However, an AC power supply becomes impracticable for longer distances and high power demands.

Dynamic issues, capacitive losses, amplification of disturbances and the like are associated with AC power supply and need to evaluated and mitigated.

High voltage direct current (HVDC)

HVDC transmission requires an AC to direct current (DC) converter onshore to convert grid AC power to DC power for the purpose of transmission, and a DC-to-AC converter at the platform to change it back.

While the converters increase the cost of the DC system, the number of required cables is reduced from three for the AC system to two for the DC system.

This reduction, combined with the reduced DC cable size due to inherently higher utilisation efficiency, results in cable cost savings that could more than compensate for the converter cost as the cable distances increase.

The use of a conventional, line commutated HVDC transmission system has been considered for offshore platforms.

However, the size and weight of the HVDC station with its filters and synchronous condenser has, together with the complexity of control, prohibited its use on offshore installations.

Increasingly, phase-commutated or loadcommutated converter technology – such as that used in classic HVDC – is being replaced by voltage source converter (VSC) technology.

An inverter based on VSC technology is able to feed into an otherwise passive network, without the need for synchronous condensers or other forms of reactive compensation.

HVDC Light is a cable transmission system based on ABB’s VSC-based HVDC technology.

Space and weight are scarce resources on offshore installations, and since the filters are small and synchronous condensers are not required, HVDC Light can be made compact and lightweight compared to classic HVDC.

The first offshore version of HVDC Light went into operation in the North Sea in 2005 at Statoil’s giant Troll-A gas platform, followed by BP’s Valhall field, also in the North Sea.

HVDC Light can be used for multi-terminal operation, connecting together various platforms with one transmission link.

With HVDC Light the voltage variation in the cable is smaller, and the inverter at the receiving end is able to compensate for this effect so that the AC voltage at the receiving end remains constant.

There are, ABB claims, no technical limitations to transmission lengths for HVDC cables.

People will argue whether there is a strong case for power from shore.

However, increasing numbers of projects coming online worldwide using either dynamic AC or HVDC confirm the advantages, reliability and flexibility of the technology.