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Third generation of STABLE close to completion

10 Feb 2007

The third generation of STABLE (Sediment Transport And Boundary Layer Equipment) is nearing completion.

After much discussion between scientists and engineers, a new design for the new instrument was draftred about a year ago. It would have a similar form to earlier versions, but would have an enhanced instrument suite and better temporal and spatial resolution, achieved by using a large number of commercial instruments. The enlarged instrument-volume and weight meant that the rig would have to be bigger and stronger. Would this degrade performance and interfere with the currents being measured to an unacceptable degree?

Before committing POL to the design, the project team decided to subject the design arrangement to an analysis using Computational Fluid Dynamics (CFD). This would test rig-stability and measurement performance in steady (tidal) flows and in oscillatory (waveinduced) flows.

Flow around and over the rig were modelled, yielding drag-figures and topplingcurrents, turbulencegeneration behind the instrument-platform and current-acceleration in the measurement-volume underneath. Attempts to streamline the whole rig using a large shroud were abandoned when CFD indicated a tendency to fly in strong currents (a tripod-rig 3.5m wide and 3m high, weighing about 2.5 tonnes flying). The study did, however, demonstrate the advantage of using smaller shrouds round the sensorbodies to reduce turbulence caused by the instruments protruding into the measurement-volume, especially during reversing flows.

The frame and legs were built by of aluminium and stainless steel respectively. The lead feet are on strong hinges and held horizontal by weak wires: these wires will break to ease recovery if the rig sinks into the bed-material. Strong braces between frame and legs are made of titanium. We considered making the whole frame of titanium to avoid corrosion and strength problems, but the cost would have been enormous! As it is, four different metals in seawater could set up severe corrosion conditions.

STABLE III will measure highfrequency turbulent- and mean-current profiles, pressure (depth and waveheight), conductivity, temperature, suspended sediment-concentration profiles and grain-size distributions, bed-ripple size, orientation and advection, and rig orientation (pitch, roll and compass). It will carry a suspended sediment sampler, an acoustic location-beacon and batteries and memory to yield about a month's worth of data. STABLE III should provide quality information on the bottom boundary layer for the use of modellers and sedimentologists. The new instrument will be used for the first time in the River Dee in March.

Useful links

Ocean Engineering and Technology Group

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Notes

The Proudman Oceanographic (POL) scientific research focuses on oceanography encompassing global sea-levels and geodesy, numerical modelling of continental shelf seas and coastal sediment processes. This research alongside activities of surveying, monitoring, data management and forecasting provides strategic support for the wider mission of the Natural Environment Research Council.

As a public funded body it is part of our remit to inform the public of the science and research undertaken at the laboratory. Attending events like the 'Ocean Awareness Weekend' at the Blue Planet Aquarium offers the opportunity for our scientists to meet members of the public and present the laboratory's work.

The Natural Environment Research Council is one of the UK's eight Research Councils. It uses a budget of about £ 350m a year to fund and carry out impartial scientific research in the sciences of the environment. NERC trains the next generation of independent environmental scientists. It is addressing some of the key questions facing mankind, such as global warming, renewable energy and sustainable economic development.