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Thursday, May 4, 2006

Hybrids from Holland

In the southern North Sea, Shell has equipped two unmanned platforms with combined solar and wind power - but no backup energy system. Terry Knott reports.

Innovation has been in no short supply for Shell's Cutter and K17 platforms. The two unmanned gas installations, Cutter in the UK sector and K17 over the median line in Dutch waters, are based on Shell's Trident Monotower concept for delivering low cost, low maintenance solutions for marginal gas fields (Booth 1141). Installation of these shallow water platforms by main EPCI contractor Genius Vos (Booth 2629), and assisted by hydraulics specialist Hytop (Booth 2523), was completed last year using a novel installation technique which dispensed with the use of a heavy lift vessel.

But innovation did not stop with the installation of the platforms 260t monotower substructures and 150t five-deck, four-wellslot topsides. Following subsequent drilling and hookup operations, additional equipment skids have been added to the top level of the decks to provide the two platforms with power. And these are skids with a different look and purpose from those normally encountered in the North Sea environment.

The four identical skids - two on each platform - each house a wind turbine and a bank of solar panels, capable of supplying the platform with 1.2kW of continuous electricity for equipment operation.

'Many buoys and small platforms have renewable energy systems, typically solar panels, for operation of lower energy devices such as navigation aids,' says Remco de Vries of Dutch renewables specialist Tss4U, designer of the Cutter and K17 systems. 'But for these two platforms the challenge was to deliver a main 24 volt DC power supply for operating wellhead equipment, hydraulic and emergency shutdown systems as well as navigation aids, throughout the year, whatever the weather in the North Sea, and without a backup supply.'

Tss4U - an acronym for 'The solar solutions for you' - was founded in 2003 by a small group of former employees of Shell Solar, when the latter closed down its operation in Holland. De Vries, with 20 years' experience in the solar power market for Shell, is now in charge of R&D and engineering for Tss4U.

'Solar panels alone could not meet this challenge,' he explains. 'Although 1.2kW may seem small in energy terms, to achieve this output continuously through the winter too would require enough solar panels to cover half a football pitch. Clearly not an option on these small, unmanned installations. Another source of renewable energy was required, namely wind power.'

The background to the project goes back about six years, says de Vries, when Shell UK was looking at hybrid renewable energy systems that bring together different means of generating electricity, as an alternative to diesel generators or subsea cables, the more traditional routes for powering unmanned installations. A trial on Shell's Indefatigable K platform in the southern North Sea was initiated to investigate the combination of wind turbines and solar panels - de Vries and his colleagues were involved from the outset and took over supervision of the trial in 2003 as Tss4U. Two turbines, preselected by Shell from Scottish supplier Proven Energy, and 68 Shell Solar photovoltaic panels were put through their paces over a two year period. According to de Vries, the trials were successful in demonstrating the concept for offshore applications, and led to some modification of the wind turbines to enhance reliability.

The trial was followed by a contract for Tss4U to design two hybrid renewable energy systems for Shell's Cutter and K17 platforms, designed to enable the installations to operate without intervention for two years.

Each system on a platform consists of two 12m high, 2.5kW wind turbines with 3.5m diameter blades from Proven Energy, plus 72 solar panels from Photowatt of France, giving a total panel area of 50m2 with a peak power rating of 6.12kW. The semicrystalline silicon solar cells - housed in glass modules - were delivered to Tss4U in 'raw form', allowing the Dutch company, with the assistance of Orga in Schiedam, to assemble them into explosionproof units, certified for offshore use.

But why a combined power rating well above the required 1.2kW?

'This is where our expertise comes in,' answers de Vries. 'We have supplied around 40 solar systems for offshore applications in the Far East, Middle East and Nigeria, and have now developed mathematical models that tell us what energy you really get out of a solar panel and wind turbines. The results of the Inde K trial have also been fed into this for North Sea conditions. So we can calculate the practical output power for the range of cloud and wind conditions that you can expect in the North Sea, and allowing for the worst case weather scenarios, we know how much theoretical power generation capacity to install.'

He points out that while the wind turbines may be rated at 2.5kW, to achieve this level of output they need constant, strong winds. 'On average, you might get 600 watts or less,' he adds. Similarly, the 85 watts peak power for a solar call is also theoretical, with the practical output being much less than this - heavy cloud cover reduces their efficiency, and they do not operate at night. Hence 'oversizing' to handle fluctuations in the local environment is a necessity.

In operation, he expects that over the course of a year the solar panels and wind turbines will each generate on average about half of the electricity used, but in winter months or at night, the turbines are likely to generate 70-90% of the total, while the solar cells take over more in the summer. On each platform, both skids will be in operation all of the time, although on average, one skid could power the platform. Even in windless, low sun conditions, the panels will give enough power to keep the navigation aids running, an essential safety feature.

Acknowledging that there may be days when it is neither sufficiently windy nor sunny to generate enough electricity for the other equipment, the skids also contain two banks of batteries rated at approximately 7000 amperehours which could power a platform at full load for five days - a period expected not to be exceeded given North Sea conditions. The batteries are charged by the solar cells and turbines.

'These batteries are sealed, valve regulated lead acid units, designed for renewables use,' he explains. 'Unlike car batteries they are not cycled daily - that is, charged and discharged. A major factor in aging of batteries is cycling, hence this type will last for at least six years in offshore use.'

Also mounted on the explosion-proof skids, which measure 8m by 3m and weigh around 10t each, is a range of instrumentation to monitor and record the performance of the power equipment, for example wind speeds, solar radiation, temperature, battery voltages plus overall power output. The information is held in a dedicated onboard data logger which can be interrogated remotely from shore or other platforms. Of the 36 solar panels on each skid - which operate in pairs - 34 generate electricity, one measures solar radiation, and one is 'spare'.

'Although there many platforms with solar or wind generators onboard,' notes de Vries, 'we believe these hybrid combinations without backup are the first of their kind in the world.'

In terms of reliability, the turbines are expected to run maintenance-free for two years and the solar panels are guaranteed for at least 10 years. And compared to the alternative power options of a diesel generator or subsea cable, Tss4U is confident the hybrid solution is far cheaper for unmanned, marginal fields, certainly over several years of operation.

The company recently delivered another hybrid solar-wind package to Total in Nigeria for the AMD3 platform in the Amanam-Kpono development, and is currently at the factory acceptance testing stage of a similar hybrid package - but with diesel generator backup - for Newfield Exploration's Grove field in the southern North Sea. Each package is purposedesigned to suit to local requirements and conditions, says Tss4U.

Renewable hybrids would appear to have a sunny - or windy - future.