On June 19, the cruise liner Queen Mary II visited the Scottish Highlands, berthing at Invergordon on the Cromarty Firth. The week running up to the arrival of the QM2 was a time of excited anticipation by the townsfolk and fevered activity by the port authority. At 345m long, towering 60m above the waves and drawing 10m of water, the QM2 is the largest liner ever to sail the seas. Passing between the lines of oil rigs laying idle in the tranquil waters, the QM2 was also the largest floating object ever to venture into the chilly waters of Cromarty Firth.
Months before the liner was due, Fathoms Ltd had conducted a pre-dredging multibeam swath survey of the entire port area for the Cromarty Harbour Authority. The quality of the data and the precision of presentation were such that, when it came to checking the seabed for the imminent arrival of the great leviathan, Fathoms was the contractor of choice.
The objective of the new survey for Bannerman Ltd, the operators of Invergordon’s Admiralty Pier, was to ascertain the extent of the depth of water alongside the Admiralty Pier and to check for any hazards to ensure a safe the berth for the Queen Mary II. The imminent arrival of the ship meant the preliminary findings had to be presented to the client and the port authority immediately on completion of field work.
The necessary tools
From previous experience at the site, Fathoms’ chief surveyor, Chris Harper, chose the Reson SeaBat 8124 multibeam system as the best instrument. The 8124 was integrated along with a IXSEA Octans 3000 motion sensor and gyrocompass into the on-line system. For speed of sound measurements, a Valeport 650 sound velocity probe was used and for position, a Trimble AG132 Differential GPS system was employed utilising differential corrections from the IALA beacon broadcast network. The DGPS system was checked against a previously established control station and was shown to be within 1.5m.
The survey was to be conducted and recorded in WGS 84 terms with coordinates quoted in grid values on the UTM zone 30N projection. For vertical control, the soundings were reduced to Invergordon’s Chart Datum, 2.10m below Ordnance Datum, Newlyn. Tidal readings were automatically recorded at 15 minute intervals at the Cromarty Port Authority’s official tide gauge. For the earlier pre-dredging survey, Fathoms had used the port authority’s own survey vessel. On this occasion, it was decided to use Fathoms’ survey catamaran Elliann. This small inshore vessel has a moonpool through which multibeam sounders and other systems can be installed.
With just three days to the arrival of the QM2, the Elliann was quickly mobilised and the survey began. The gyrocompass alignment was established using a RegElta Total Station at a known point and reference object on shore. The little vessel’s heading was then calculated by observing angles and distances to the bow and stern while simultaneously recording the gyro’s output. The next task was to perform a ‘patch test’ to calibrate the SeaBat 8124. All swath sounders have to undergo this vital calibration, which comprises of a series of survey lines to establish the roll, pitch, yaw ‘offsets’ between the different components, to remove any residual biases that exist. Fathoms insists on a patch test at the start and end of every job and, if the survey takes more than a week, further tests are made.
For the survey programme, lines were run parallel to the Admiralty Pier jetty and extended out past the dolphins to the west and east. The on-line QPS QINSy software was used to ensure that there was sufficient overlap of the lines and that adjacent lines were observed at the shallowest depths. Additional lines were then run along the jetty to saturate the critical area of the berth with redundant observations. On-line quality control was provided by the QPS QINSy software suite.