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Mechanical Engineering

The Mechanical Engineering group designs and provides all the structural elements needed to support instruments at sea. Under the Head of Mechanical Engineering (HoME), it designs and manufactures components which range from the highest-precision parts for instrument-cases and mechanisms to simple frames made out of scaffolding-pole.

Four staff work under the HoME. There are two skilled Workshop technicians, a mechanical engineering apprentice and a marine technician. The Workshop staff manufacture precision parts (unless they are too big for the machines) and the marine technician maintains major sea-going items such as the lander-frames and surface marker-buoys. All Mechanical Engineering staff install equipment in the sea from research ships or charter-vessels or at coastal measurement sites for groups like the Tide Gauge Inspectorate.

Scientists discuss their requirements with the HoME, who drafts a preliminary design. This is refined as necessary and then detailed drawings are prepared using CAD systems. Most parts are made in the workshops in the Kempston Street facility (see the Facilities page). Any special surface finishes (e.g., hard anodising) are applied by specialist contractors and larger parts are made at local engineering companies, e.g., the frame and legs for STABLE III were made by a Liverpool fabricator.

Picture 1 - An experimental framework built for a large-scale collaborative experiment in the Delta Flume in Holland. It is built out of scaffold-tube, has heavy lead feet and sits on a compacted bed of fine sand. The instruments measure wave-effects generated in the flume and the resulting suspension and movement of the sand from the bed.	Picture 2 - A Computer-Aided Design (CAD) drawing of a lander to carry a large Acoustic Doppler Current Profiler (ADCP) down to the sea-bed and back to the surface at recovery. Drawings like this represent the final stage before the components are manufactured in the POL workshop or by outside contractors.
	Picture 3 - The completed ADCP frame. This is the buoyant part of the frame which brings the ADCP back to the surface at the end of a deployment. The heavy bed-frame, which takes the instrument to the sea-bed at the beginning of the deployment, is not shown.

Factors which must be considered when designing oceanographic equipment include:

• Function: What task does it have to perform? Are there implications for size, shape or weight, etc.? Size: How big or how small should it be for deployment and recovery?
• Weight: How heavy can it be before it becomes impossible to deploy and recover?
• Depth: How deep is the working-site? Will the instrument work on the surface, on the sea-bed or in mid-water?
• Position: Will the instrument be moored in one place, or can it drift?
• Mobility: Does the instrument have the ability to maintain or change its position?
• Waves: Will the instrument be subject to wave-action?
• Currents: Will the instrument be affected by currents? Are those currents steady (eg., major ocean currents such as the Gulf Stream, do they vary slowly and change direction (eg., tides), or do they vary quickly and change direction (eg., wave-induced currents)?
• Stability: If the instrument moves, will it affect the data?
• Flow-disturbance: Is it important that the instrument does not produce unacceptable flow-disturbance around, under or over itself?
• Measurement-volume: How big is the measurement-volume? Will the required sensors fit into it?
• Endurance: How long does the instrument have to work for (days or years)?
• Resolution: How big or how small are the effects being measured?
• Accuracy: How accurately should the relevant parameters be measured?
• Interference: Will instruments in the measurement-volume interfere with each other (physically, hydrodynamically, electrically or acoustically)?
• Disposition of instruments: Should instruments occupy specific positions or orientations relative to each other?
• Chemistry: Will corrosion affect the parameters being measured?
• Surface finishes: What finish is acceptable (grease, paint, anodising, plating or none)?
• Cost: What is the budget for the project?
• Delivery: How long before the apparatus is required?
• Life: Will the apparatus be used repeatedly over an extended period, or is it expendable after recovery of the data?

Some of these considerations may be impractical and compromises must be agreed with the users before a design is developed.

A Brief History of the Design and Development of a Major Item of Marine Hardware

The biggest and most complex apparatus designed and built by the Mechanical Engineering sub-group is the STABLE III equipment. STABLE stands for Sediment Transport And Boundary Layer Equipment and STABLE III is the third generation of this apparatus which was conceived originally in 1981. It measures the interactions between turbulent currents and sediments at the sea-bed. It is a tripod standing about 2.5m high and the feet occupy a circle about 3.5m in diameter: it weighs about 2,500kg. Total cost is about £ 250k.

Before committing to manufacture, an initial 3D CAD concept-model was subjected to CFD analysis (Computational Fluid Dynamics). We needed to know if the rig would fall over or move in strong currents and whether the presence of the frame and measuring-instruments would interfere unacceptably with the parameters being measured. The analysis was performed by CFD Solutions Ltd. under a NERC-funded contract.

Picture 4 - A 3D CAD concept of STABLE III, prepared for CFD analysis.	Picture 5 - Wake effects of whole frame.
Picture 6 - Flow accelerations under STABLE III inst platform.	Picture 7 - Shadow-effects on the current sensors in reversing flows.

The rig is a classic engineering compromise. A large number of instruments are required to measure the parameters needed for scientific analysis, and these must be held as far above the bed as possible. A large framework is needed to house the instruments, but this is directly above the measurement-volume between the legs: the legs offer physical protection to some of the sensors. Against this, the apparatus must be small and light enough that it can be handled by a coastal research ship just 35m long. Sediment-resuspension is caused primarily by vertical currents and these will be affected directly by the presence of the instrument-platform above. The CFD analysis would show us whether the prototype design would perturb measurements to an unacceptable degree.

Working backwards from the wake-effects calculations, the true drag coefficient of the frame was calculated and a better estimate of the toppling current was obtained. As a result, the weight of the STABLE III feet was increased to improve stability in strong currents.

It can be seen that the flow varies considerably with height under the frame. In particular, the upper current meter position is very close to a transition-zone between accelerated and decelerated flows. This indicated that the instrument platform had to be raised to record less-perturbed flows under the rig.

CFD analysis of reversing flows around the current sensors showed that considerable turbulence was produced by the sensors themselves, their connecting cables and their supports. To reduce this, short, smooth, sausage-shaped shrouds were used to ease the flow round these packages. The shrouds were designed in 3D CAD and the drawings were sent to a supplier who used CAM (Computer-Aided Manufacture) to produce the rounded fairings.

Picture 8 The current meter assemblies as built for STABLE III. The CFD analysis showed that currents would not be affected unacceptably except in an arc of +/-45° about the centre-line, flowing from behind.	Picture 9 - The complex instrument platform of STABLE III. The platform, made of aluminium tube, is very strongly braced to resist the rigours of deployment and recovery. No instrument hangs outside the frame, except underneath, where they are protected by the legs. The frame was designed to be as small as possible, with the instrument-tubes packed as densely as possible: this means that the platform offers a considerable barrier to lateral currents. The platform is at the top of the rig, where overturning moments are at a maximum: STABLE III's legs are splayed by 20° and the lead feet weigh 450kg each in an attempt to counteract the toppling forces
Picture 10 - STABLE III awaits deployment on the after-deck of the Prince Madog, a small coastal research vessel. Also shown are a solar-powered Triaxys wave-measurement buoy and the yellow, wedge-shaped float of the spar-buoy which was used to mark STABLE III during its first deployment.	Picture 11 - STABLE III enters the water for the first time on Thursday, 15th March, 2007. It was deployed in about 15m of water in the channel to the West of Hilbre Island at the mouth of the Dee estuary, between England and North Wales. It was recovered on Tuesday, 17th April without mishap, and was taken back to the docks at Birkenhead.
Picture 12 - STABLE III is recovered for the first time on Tuesday, 17th April, 2007. There was no damage to any instrument and there was only minor fouling by weed of one of the current meters.