Work Support Vessel

The MV Flatholm from Coastline Surveys Limited was used as the work support vessel throughout the 2005 field sessions. The Flatholm is a converted harbour tug, built in Japan in 1976. The vessel has a length over all of 23.3m, a breadth over all of 8.0m and a 2.6m draft.

The Wessex Archaeology Remotely Operated Vehicle and diving operations were directed from a control room, permanently installed on deck aft of the superstructure.

Mooring System

Throughout the Remotely Operated Vehicle session Flatholm was moored on a four point mooring above the wrecks to be surveyed.

The vessel's two bow anchors were used at the front and two 200kg high power holding anchors with short lengths of chain were deployed at the stern. Two hydraulic winches were used to handle the stern mooring.

For the mooring procedure a computer file (.dxf) showing the wreck outline visible in the geophysical survey was displayed in the vessel navigation software package on the bridge. Points were taken where anchors were to be dropped, so as to avoid damage to the wreck. However it was not always possible to avoid mooring wires crossing the wreck site.

As the two 200kg anchors used as stern mooring during the Remotely Operated Vehicle session proved insufficient to hold MV Flatholm on station in strong tides and less favourable weather conditions, two sinkers of 2t and 3t respectively were bought for the diving session.

A 40m length of chain with a buoy and a shackle at the end was attached to each sinker. The use of sinkers required a lengthy mooring procedure to avoid dragging long wires through the water and damaging wreck sites.

On arrival on site, the two buoyed sinkers were dropped on one side of the wreck and their positions marked. The vessel than deployed the bow anchors on the other side of the wreck site and then dropped back. Then the stern winch wires were connected to the sinkers using a small rigid inflatable boat.

The Acoustic Tracking System

An Ultra Short Baseline (USBL) SCOUT acoustic tracking system from Sonardyne was used during all fieldwork sessions to track the Remotely Operated Vehicle and divers.

The SCOUT system consists of three main components: the vessel mounted acoustic transceiver, one or more Remotely Operated Vehicle or diver mounted transponders and the surface command module running the control software.

In a USBL system, the position of subsea targets is calculated by measuring range and bearing from the vessel mounted transceiver to the submerged transponder, which emits acoustic signals.

In a short baseline system, only one transducer (transponder on the seabed) transmits sound, but many transducers on the surface receive signals. In a USBL system such as SCOUT this array of transducers is built into a single transceiver assembly. The baselines between the individual transducers are very (ultra) short, usually in the range of centimetres.

The waterproof and portable surface command module running the SCOUT USBL software was installed in the control room.

The SCOUT USBL Transceiver was mounted on a pole over the side of Flatholm. In order to provide maximum stability for the transceiver the pole had a diameter of 10cm and was attached to a bracket welded onto the rail and supported by another bracket further down the ship’s side. The pole could be swivelled up when Flatholm was in passage.

SCOUT’s very light and rugged Type 7815 HF transponders could easily be attached to either the Remotely Operated Vehicle or diver umbilicals.

The transponder worked on frequencies between 35 and 55kHz. The stated operating range for the system was 500m and the acoustic coverage was +/- 90 degrees below the transceiver.

Even when moored, a vessel yaws, pitches and rolls. As the calculated diver/Remotely Operated Vehicle position is based on the transceiver position, this movement introduces inaccuracies into the positioning and has to be compensated. The SCOUT system allows tracking in two modes: The transceiver includes an internal heading and attitude sensor, which can be used for compensation. Alternatively an external motion reference unit (MRU) and a gyrocompass can be used to compensate for vessel movements.

Using the system on internal sensors is easier, as no external instruments have to be configured, but this decreases the stated accuracy of the system from ± 0.5% of the slant range to ± 2.75% of the slant range.

For both the Remotely Operated Vehicle and diver fieldwork sessions, an external MRU and gyrocompass were installed on the Flatholm.

A TSS HRP-10 MRU was fitted in the survey container. X, Y and Z offset values to the transceiver were entered in the SCOUT software.

An ASG Brown Meridian Surveyor gyrocompass was also installed in the survey container and positioned exactly parallel to the vessel's lubber line.

An onboard GPS provided positioning information. The GPS was a CSI Vector Sensor Differential GPS capable of sub-metre accuracy.

Prior to the fieldwork, all instruments were surveyed on the vessel with the help of a total station by a Wessex Archaeology surveyor to achieve the highest possible positioning accuracy. The offset values to the USBL transceiver were entered into the SCOUT software. All external instruments were connected to the SCOUT surface command module.

Both external and internal sensors had to be calibrated before use. This required the Flatholm to sail predefined patterns around a transponder on the seabed and to steam a full 360 degree circle. A calibration wizard in the SCOUT software automatically applied the necessary corrections to the USBL settings.

The Recording System

Since 2002, Wessex Archaeology has developed a Microsoft Access based diver recording system known as ‘DIVA’ which can be used for real time recording of three-dimensional positions and information in conjunction with the mapping programme ESRI ArcGIS 9.0.

DIVA consists of a Microsoft Access database which stores the information, and a GIS interface for graphic display. This system was used in Round 1 of the project and has since been further improved.

The position of the tracked vehicle or diver is output from the Sonardyne SCOUT surface command module in real world co-ordinates and can consequently be displayed in real time on top of geophysical survey data in ArcGIS 9. The vehicle or diver track can also be displayed and saved separately

To allow integration with the previously acquired geophysical survey data, Universal Transverse Mercator (UTM) coordinates, based on the WGS84 datum and the WGS84 ellipsoid were used for display and recording. The targeted wrecks were spread over UTM zones 30N and 31N.

A laptop running DIVA and ArcGIS was set up in the survey container, so that the recorder could see the screens showing the underwater footage being collected by the ROV and divers.

Observations made by divers or recorded from the Remotely Operated Vehicle video feed can be entered into the database by the recorder. These observations, stored in the DIVA database, include the three-dimensional position, comments typed in by the recorder and mapping labels for the display in the GIS system.

Observations can be taken as quick, but less accurate, spot fixes, or as average fixes. Average fixes obtain a number of positions over a short period of time and calculate an average position for the diver or tracked vehicle using a software application developed by Wessex Archaeology and known as Accu-fix. The more accurate average fixes were generally used for recording datum points and important features on the wrecks.

Diver observations can be displayed as different layers in ArcGIS, grouped for example by mapping labels, observation type, etc.

A summary form allows the archaeological supervisor to summarise individual dives, whole events and monuments. The diver can also enter archaeological, environmental and operational summaries. Paper records such as drawings can be referenced to individual diver observations.

A general log allows the archaeological supervisor to keep track of daily events, working hours, weather forecast and tidal predictions.

Records of earlier events, such as the Round 1 diving or the collection of geophysical data were readily available in the DIVA database. This was a useful source of reference and comparison, especially where earlier diver observations were concerned.

ROV Operations

The Remotely Operated Vehicle (ROV) survey of shallow wreck sites took place between the 11 and 26 June 2005. The survey was carried out from the MV Flatholm, Coastline Survey with two Wessex Archaeology staff acting as archaeological supervisor / tender and recording system operator to carry out the recording, and an external ROV operator.

The ROV and pilot were supplied by SUBCO Underwater Equipment. A Seaeye 600 high power inspection and observation ROV was used throughout the project.

The 1m long and 76cm wide ROV weighed 75kg. It was powered by 4 thrusters which provided a maximum forward speed of 2.5 knots. The ROV was supplied with 150m of umbilical. The maximum payload was 10kg.

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The ROV had two integrated cameras: a PAL colour camera; and a low light black and white (B/W) camera. Two 150 watt lights were dimmable and could be tilted together with the cameras to ± 80 degrees.

The ROV was equipped with forward scanning sonar for navigation, a built in fluxgate compass and a digital depth gauge. Information from these instruments could be displayed over the video output.

The surface controls consisted of an isolation transformer, the main ROV surface unit for power distribution, a hand controller with joystick, two screens and a Panasonic digital video capture unit. Video was recorded on miniDV tapes.

The ROV surface unit was installed in the survey container on deck. The umbilical was stowed on deck. Even though the ROV could be safely handled manually it was usually launched and recovered with the crane on deck.

Diving Operations

The diving survey was split in two units of 15 days with a six day break in between and took place between 4 July and 8 August 2005. Again MV Flatholm of Coastline Surveys was used as support vessel. The project staff consisted of five archaeologists and three crew aboard MV Flatholm. The archaeological team consisted of four divers and an archaeological recorder. Diving roles were rotated within the team. Two divers were appointed as diving supervisors.

All dives were conducted using surface supplied diving equipment with Kirby Morgan 28 band masks.

The diving panel, communication box and video recording equipment were set up in the survey container next to the recording computers and the SCOUT system. The air supply consisted of five 300bar J-bottles secured to the outside of the survey container. After each dive the cylinders were recharged using an onboard diesel HP compressor.

On the starboard side of the vessel a gate was cut into the rail to provide safe access to the water via a diving ladder. The umbilicals were stowed on the open working deck.

The dive team generally consisted of diving supervisor and recording system operator in the survey container, standby diver and tender on deck and one diver in the water. On some of the shallower sites the standby diver was used in-water alongside the main diver to aid with recording or to obtain digital video and stills footage of the main diver.

Every dive was recorded on miniDV tape using a colourwatch digital video system with hat mounted camera. A Canon Powershot G2 in Ikelite housing with Ikelite DS 125 substrobe was used to take digital still photographs. A .56 Sea and Sea wide-angle lens allowed the photography of larger objects from close distances.

On night dives and deeper sites with low visibility a 100 Watt Lumb Bros FIII underwater torch was employed.

In addition to the diver recording system based on acoustic tracking, conventional recording methods, such as offset drawing, were used on the dived wreck sites.

Depending on the desired level of recording, underwater photography, videography, sketches and drawings were used. Trilateration and offset measurements were acquired with folding rules or tape measures.

You can see how the methods worked by looking at the Bottle Wreck. This wreck was recorded using traditional marine archaeologcial methods. A 32m by 10m grid was established around the visible remains of shipwreck and cargo. The pipe cargo mound and individual artefacts were then drawn in relation to the grid using offset and trilateration methods. To link the site grid to real world co-ordinates accurate fix points were taken for each of the grid corner points with the USBL system. To speed up the drawing and recording process, measurements were obtained by the diver, but the drawing was carried out by the supervisor on the surface. This was done using the live video link provided by the colourwatch system.

Remotely Operated Vehicle Fieldwork Summary

The work support vessel MV Flatholm was mobilised on 11 June 2005 at Lowestoft. The acoustic tracking system was installed and secondary instruments such as GPS and gyrocompass measured in. After loading and setting up the Remotely Operated Vehicle equipment, a test dive was conducted in Lowestoft harbour.

The transit to site took place on the 12th June 2005, and the first wreck was surveyed on the 13th June.

Altogether 20 Remotely Operated Vehicle dives were conducted on seven different sites in 10 days. Two days were down days due to strong winds (above force 6). The total bottom time for the 20 conducted dives is 1431 minutes, or 23.85 hours. The table below provides further details of the sites dived by Remotely Operated Vehicle:

WA ID	Wreck  Name	Dive Days	Down Days	No of Dives	Bottom Time/min
5004	SS Concha	2	-	3	340
5005	SS Umba	2	-	3	241
5006	SS Devon Coast	2	-	3	54
5009	Unknown	1	-	3	280
5011	Unknown	2	1	4	182
5013	Unknown	1	1	2	200
5014	SV Thomas Lawrence	1	-	1	94

Diving Fieldwork Summary

At the start of the first diving session, unfavourable weather conditions forced MV Flatholm to stay in Lowestoft, so that the diving equipment had to be mobilised in Lowestoft instead of Shoreham as planned. Five days fieldwork time were lost due to weather at the start of the session.

The remaining session was spent on site 5013, the so called ‘Bottle Wreck’. On the second session, the equipment was mobilised and demobilised in Shoreham. Two wrecks, site 5009, the ‘1906 Wreck’ and site 5004, the ‘Concha’ were surveyed.

Altogether 55 dives were conducted on three different sites in 19 days. Nine days were down days due to bad weather or technical defects. The total bottom time for the 55 dives is 2295 minutes or 38.25 hours. The table below provides further details on the sites that were dived:

WA ID	Wreck  Name	Dive Days	Down Days	No of Dives	Bottom Time/min
5004	SS Concha	5	2	13	675
5009	Unknown	6	2	13	455
5013	Unknown	8	5	29	1165