The Magazine for Underwater Professionals
Floating tidal turbines on the menu at learn and lunch event
The presentation Floating Tidal: Ahead of the Curve was given by Andrew Scott, the chief executive officer of Scotrenewables, at the SUT’s recent Lunch and Learn lecture in London.
The lecture was opened by Keith Broughton on behalf of the SUT who reminded host organisations that their generosity would be rewarded with a free personal membership for a year before introducing the chair, Joe Hulm, who orchestrated the session on behalf of the hosts, Lloyd Warick International at their Lime street office, in the City of London.
Andrew began with a bird’s eye video of a yellow, pencil-shaped floating tidal turbine in action. This was the 440-ton SR1-2000 which was launched in May 2016 and has been undergoing sea trials at the European Marine Energy Centre (EMEC) marine energy converter site in Orkney. The turbine has two propellers, each capable of producing more than one megawatt of power during a spring tide with 16-metre blades making it the most powerful tidal turbine in the world. The 2.2 megawatts generated at full capacity equates to 7% of the entire Orkney demand.
With a video of the product presented on the screen in front of us, Andrew provided a bit of background. The current model was a result of 15 years of research and innovation. It is a second-generation turbine that has attracted a great deal of interest and investment following the success of pioneering prototypes. The concept differed from many of the initial tidal turbines in that it floats on the surface of the water rather than being fixed to the seabed. An immediate consequence of this is the reduction in cost. This is seen during the deployment and the maintenance of the turbine. Deployment involves the craning of the unit into the water followed by a tow to the site where it is linked to moorings. The limited logistical requirements mean that its launch and mobilisation is in the order of £3000, making it around 100 times cheaper than fixing a turbine to the seabed.
The SR1-2000 is constructed in modules so the units can be readily transported from the factory and reassembled onsite before deployment. Once at sea, trials have demonstrated that it can be accessed by rigid inflatable in swells of up to two metres, which at this site is 90% of the year. Unlike submerged turbines, repairs can be executed without the need to recover the unit. To date, there have been failures in some of the subsystems. However, these have been relatively easily remedied. For instance, an inverter module failure takes around 24 hours to repair while common electrical faults can be rectified within around 12 hours.
One reason these faults can be dealt with so efficiently is that the turbines will invariably be stationed near land where tidal currents are strongest. These environments will generally provide shelter from ocean storms but even so, the threats from dynamic wave forces need to be considered. The trials have shown that storm waves have not had an impact on the integrity of the unit while they exert around a 100-ton load on moorings. Although this is a large force, it is less than the forces exerted during a spring tide, when the drag from the turbines can create around 160-170 tons of load on the moorings, and as such it is easily managed. Even so, if necessary that the turbines can be slowed to reduce the load.
Regarding performance, as previously stated, a maximum of 2.2 megawatts can be generated on the springiest tides with an average of 20 megawatts in 24 hours, giving a 41% capacity factor, and 125 megawatts in seven days, giving a 37% capacity factor. The unit produces an average of one megawatt per hour during neaps.
It was clear there was a viable future for floating tidal turbines.
The London evening meeting on subsea boosting systems was an overview of advances in subsea pumps especially multiphase units. Chairman Iain Knight introduced Marco Gabelloni, the business development manager of AkerSolutions. Marco, no stranger to the SUT having spoken to members twice before, including sharing lessons learnt from development, technology qualification and operational phases of the Asgaard Subsea compression project, outlined two new pump development programmes and how these products fitted within the wider context of subsea production architectures.
Marco began by reminding attendees of the typical range of subsea processing building blocks for processing, injection and boosting and just how pivotal a component reliable subsea pumps were in some form for most of these applications. Building upon their solid operational experience across several major North Sea projects, AkerSolutions has engineered and extensively tested two new ranges of subsea pumps to widen the range of available, proven solutions termed Multibooster and PowerJump.
Multibooster was intended for maximising production, whether in greenfield or brownfield applications, through a system centred on a superior high-power multiphase booster pump technology which was robust, scalable and modular yielding up to 230 bar pressure uplift. Key technologies facilitating this were:
These technologies had been progressively tested within a new state-of-the-art test facility that was capable of delivering representative flow rates and pressures. Evaluation programmes had included both a range of high-speed motor tests and operator-funded complete pump system integration testing after which the product was qualified as 100% gas volume fraction tolerant.
The second product Aker revealed was designed to enhance production from existing fields with minimal impact and reduced opex – termed PowerJump. Essentially, this comprised enhanced packaging of previous ESP submersible pump technology with additional upstream processing to confer better handling of higher gas volume fractions. PowerJump was engineered for lightweight installation and retrieval to the mudline with conventional procedures at a reasonable cost. This was facilitated by adopting communications on power permitting use of a simplified power-only umbilical. Progressive testing has now reached TRL 4 status through full-scale trials of a 600kW motor 48-stage pump unit and validation of associated modelling capabilities.
Questioning followed from the attendees regarding many aspects of the presentation ranging from sand handling, reasons for market reluctance, status of subsea VSDs (pressure compensated VSDs under test), possible future subsea projects, limiting factors on compressor size and likely developments in subsea controls. The answers to this barrage yielded much knowledge transfer to the audience such as that one operator had “recovered their capex on a pump installation after six months of operation” before Iain thanked Marco for his interesting contribution to the SUT’s subsea engineering understandings despite the difficulty of presenting with a broken foot.