FRV Alba Na Mara 1114A
|Ship name (ship code)||FRV Alba Na Mara (74NM)|
|Cruise period||2014-06-27 — 2014-07-04|
|Port of departure||Fraserburgh, United Kingdom|
|Port of return||Fraserburgh, United Kingdom|
1. To estimate horizontal drag forces (hydrodynamic and geotechnical drag) for ground gear shaped objects towed on the sea bed, with different vertical loads.
2. To measure the quantity of sediment remobilised by ground gear shaped objects.
3. Test new water sampling equipment to determine if it will be possible to obtain samples from the sediment plume generated by the ground gear shaped objects.
Alba na Mara will leave Fraserburgh on 27 June and steam towards the work site between Nairn and Burghead. Sediment samples will be taken with the day grab at the work site. The rest of the survey will be dedicated to sledge sampling experiments. Alba na Mara will return to Fraserburgh on 3 July and the scientific personnel and equipment will be unloaded on the 4 July.
The BIGG Sledge and Ground Gear Components
The BIGG sledge has been designed to allow the horizontal drag of the ground gear (hydrodynamic and pelodynamic drag) to be measured via load cells mounted within the framework. The vertical force applied to the gear shapes can be altered via weights (0 to 120 kg) mounted in the side frame.
There are four ground gear shapes to be tested on the sledge (200, 300, 400 mm diameter discs and NACA0025 ("Hydro-Hopper")). These will be arranged into three configurations during the survey (six objects "spaced", six objects together in a "block", 12 objects in a "long block"). Three flat rectangular shapes measuring 600 mm wide and 200, 300 and 400 mm high will also be tested. All the shapes are made out of high density polyethylene (HDPE).
The sledge will have a LISST 100X mounted 1.9 m behind the ground gear. This will enable particle size and quantity to be measured within the sediment plume created by the gear shapes. A programmable water sampler will be mounted alongside the LISST for some of the tows. This will attempt to take six water samples during each tow, three within the plume and three outside the plume/control samples. Two video cameras will be mounted on the framework of the sledge, one will be angled to verify the ground gear is in contact with the seabed, and the other will show if the LISST and water sampler are within the sediment plume.
The sledge will be towed off the central warp, with a wire bridle (Figure 2). A dyneema rope will lift the sledge in and out of the water by the deck winch through a block on the gamma frame. Each tow will last 30-40 minutes, during the tow the speed will be altered at 10 minute intervals (2.5, 3.0 and 3.5 knots - dependent on tide, swell and windage). Once the sledge is back aboard, a quick alteration to either the ground gear or weights will be carried out, before redeploying.
|Chief scientist||Keith Summerbell (Marine Scotland Aberdeen Marine Laboratory)|
|Track charts||(00 MB)|