FRV Alba Na Mara 1912A
|Ship name (ship code)||FRV Alba Na Mara (74NM)|
|Cruise period||2012-10-26 — 2012-11-05|
|Port of departure||Fraserburgh, United Kingdom|
|Port of return||Fraserburgh, United Kingdom|
1. To estimate horizontal drag forces (hydrodynamic and pelodynamic drag) for ground gear shaped objects towed on the sea bed, at different towing angles and vertical loads.
2. To measure the quantity of sediment remobilised by ground gear shaped objects.
Weather permitting Alba na Mara will sail from Fraserburgh on 26 October and steam towards the site at Lossiemouth. 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. If work is not possible at the Lossiemouth area due to adverse weather, sampling may be transferred to the Dornoch area. Alba na Mara will return to Fraserburgh on 4 November with scientific personnel and equipment unloaded on 5 November.
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 sledge allows the ground gear to be angled at 30, 60 or 90 degrees to the towing direction and weights from 0 -120kg can be applied to the ground gear - altering the vertical force.
There are four ground gear shapes to be tested on the sledge (200, 300, 400mm discs and NACA0025 - "Hydro-Hopper"). The shapes are made out of high density polyethylene (HDPE) and will be arranged into three configurations during the survey (six objects "spaced", six objects together in a "block", 12 objects in a "long block").
The sledge will have a LISST 100X mounted 1.4m behind the ground gear. This will enable particle size and quantity to be measured within the sediment plume created by the ground gear. 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 is within the sediment plume.
The sledge will be towed off the central warp with a wire bridle. 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 (2.5kn 3.0kn and 3.5kn - dependent on tide, swell and windage). Once the sledge is back aboard, a quick alteration to either the ground gear angle or weights will be carried out before redeploying.
Normal contacts will be maintained with the laboratory.
|Chief scientist||Keith Summerbell (Marine Scotland Aberdeen Marine Laboratory)|
|Cruise programme||(0.11 MB)|
|Track charts||(0.05 MB)|