A title seabed challenge

The Meygen project in the Pentland Firth is planned to be the UK's first commercial tidal project. Images from Xodus group.

Extreme environmental conditions and complex geotechnical challenges make tidal energy projects a huge undertaking. Andrew Small and Greg Cook explain how some of these challenges can be assessed and mitigated.

Though still very much in its infancy, tidal energy is gaining momentum. In 2006, the first test rig turbine was installed and generated electricity to the UK grid, using OpenHydro Technology’s prototype. Seven years on, the Meygen project in the Pentland Firth is planned to be the UK’s first commercial tidal project, initially comprising six turbines with a total installed power of 9MW, with planned further expansion to 86MW.

Unlike offshore wind, which has seen huge investment from major energy companies, smaller developers and intelligence investors have been the key players in this new and more uncertain market. Despite limited capital, a number of tidal energy converter (TEC) devices have now been installed at several loca- tions around the UK.

The high tidal energy environment required to drive tidal devices presents a combination of design and installation challenges, particularly geotechnical uncertainties, where novel solutions are required.

Challenging Environment

The bedrock geology of the UK coastline is complex and varies significantly. From Precambrian marine sedimentary rocks, including sandstones, limestones, and shales in the West of Scotland, and slates and schists around the Welsh coastline, a variety of base- ment materials need to be considered for foundation design. In addition, a thin veneer of unconsolidated coarse sedi- ments often blankets the bedrock.

Tidal turbine site investigation can be extremely difficult and challenging and geotechnical design and installation elements pose a major risk to the project, unless properly considered and effectively managed. Jagged, hard substrates often present challenging seabed profiles (Fig.1) to design around and typical seabed preparation techniques, such as rock carpeting or dredging, may not be practical or feasible.

Data Acquisition

In order to design efficiently, a comprehensive site investigation program should be developed, beginning with a detailed desktop study, shore site walkover, and geophysical and visual camera survey. This is followed by more complex and costly geotechnical surveys, potentially comprising onshore, nearshore, and offshore drilling.

A detailed geophysical survey using surface and subsurface techniques, such as multi-beam echo sounder, side-scan sonar, and subbottom profilers, provides data that are essential to understand the topography of the development area. The profiles are used to select and optimize the location of the TECs and the lay routes of interarray and export cables. The aim of the survey is to identify hazards and determine where potential mitigation measures may be needed.

Geotechnical data may be required to 20-30m below the seabed for detailed foundation design, to determine foundation capacity, soil-structure interaction, and install ability.

While offshore geotechnical drilling can seem expensive, the data can be supplemented by data from onshore drilling, and additional laboratory tests could provide a better understanding of the variation in bedrock properties and their geotechnical characteristics, at a fraction of the cost of marine investigation.

Additionally, material-specific research on soil-material interface friction may also promote efficient foundation designs. Acquiring appropriate data to coincide with project milestones will culminate in a comprehensive under- standing of the geological features and geotechnical conditions relevant to design.

Typical Horizontal Axis Tidal Turbine (HATT).

Design considerations

Initial design evaluations typically use geotechnical parameters derived from qualitative descriptions of shore geology and from camera surveys, often resulting in very conservative foundation designs with safely factors calculated to accommodate the uncertainty in site conditions and seabed materials. As a result, base structures may grow significantly in size and cost. While this may be acceptable for front-end engineering design considerations, it may lead to issues with certification during detailed design, where optimizations are actively investigated.

The design of each tidal turbine varies significantly between manufacturers and a wide variety of foundation systems have been installed, including drilled and grouted monopiles, pin piles, and gravity-based solutions.

The foundation solution is typically governed by seabed conditions and the tidal energy converter design, which may limit potential foundation solutions. Selecting the appropriate TEC for the seabed and tidal conditions is a key to success.

Tidal power and TEC development

Together with smaller demonstrator sites, the European Marine Energy Centre (EMEC) test site off the coast of the Orkney Islands, northern Scotland, has provided a catalyst for innovation into wave and tidal energy research. EMEC is near the treacherous Pentland Firth, with high-velocity marine currents and challenging geomorphology. Reports suggest that the Pentland Firth area alone could theoretically generate around 1.9GW tidal power.

One e of the demonstration sites in the UK includes Marine Current Turbines’ (MCT) SeaGen device, installed in Northern Ireland’s Strangford Lough in 2008, which used a novel foundation installation approach. A quadrapod-design base was temporarily ballasted for stability. A temporary work deck was installed on top of the turbine structure, above water level. In a similar manner to the OpenHydro device, this allows the turbine assembly to be raised above the sea surface to realize reduced maintenance costs.

In mid-2011, Germany’s Voith installed the foundations for its prototype HyTide 1MW device at the EMEC test site.

Unlike SeaGen and OpenHydro devices, which protrude above the sea surface, the Voith device is completely submerged.

Possibly the most conventional TEC designs are those featuring large gravity-based or piled tripod foundation configurations, similar to those some- times used at German windfarm sites. A prototypical horizontal axis tidal turbine with a gravity-based foundation solution is shown in Fig. 3.

Cable protection

In shallow seas (<200m water depth), pipelines and cables laid across the sea- bed are protected by being placed into a trench or by being covered with crushed graded rock, or using cable casings. These systems protect the pipeline or cable from fishing gear, dropped objects, anchoring, and also ensures on-bottom stability from hydrodynamic forces. The amount of protection is used will likely be selected based on protection philosophy, the requirements of other sea users, and the level of risk the developer is will- ing to accept.

Decommissioning

Decommissioning is a key design consideration for new offshore developments and should be considered early in the design phase to minimize future environ- mental impact, offshore activities, and associated costs.

As tidal turbine installation is typically remote, it is presumed that decommissioning will take place remotely. In 2011, the UK Department of Energy and Climate Change (DECC) published guidance notes on decommissioning renewable energy installations, including the removal of foundation elements and cable deburial.

Assessing the recovery loads from a structure placed on the seabed for a typical 20-year design life requires careful determination of the additional loads arising from marine growth, seabed suction, and potentially cementation. It is also necessary to consider the capacity of existing lifting points intended for re-use. At present, there are few recorded cases of recovery of large gravity-based subsea structures after long periods of use. However, it is likely that as North Sea decommissioning activity increases, these will be better understood.

Harnessing tidal power

Extreme environmental conditions and complex geotechnical challenges make tidal energy projects a huge undertaking. The ambition for tidal to become a viable and reliable source of renewable energy for the future continues to require significant financial and intellectual input. However, the challenges are being overcome and solutions found. It is important that seabed conditions and materials are adequately investigated prior to deployment to minimize project risk.

While the design of tidal devices does vary, each has the same fundamental issues to resolve, from concept to project sanction, through asset integrity to end of life.

Andy Small is a senior geotechnical engineer for Xodus Group based in Aberdeen Before taking his current position in 2013, he worked for offshore EPIC Contractors and specialist offshore geotechnical consultancies. He holds a MEng in Civil Engineering Design and Management from the University of Dundee, UK.

Greg Cook is a Consultant Engineer contracted to Xodus Group in Aberdeen. He holds an MSc in Engineering Geology from the University of Canterbury, New Zealand, and worked in onshore civil consultancy prior to coming to the UK. For the past 10 years his work has been focussed on marine geotechnics within the oil and gas and renewables sectors.