AQUALINK - The TelstraSaturn Cable - Survey and Cable Lay
by Bob Waugh, BTW Hydrographic Ltd. NZ
MV Searanger off Lyall Bay
BTW Hydrographic over the past couple of years have been involved with the installation of the Aqualink fibre optic submarine cable festoon system around the coast of New Zealand. The cable owner is the communications company Telstra Saturn Ltd The cable was built and installed by the prime contractor Ericsson Communications Ltd who sub contracted the marine lay and beach installation work to Seaworks NZ Ltd.a prime contractor carrying out the cable lay operation. The cable has a diameter of a 50 cent coin with a capacity of one Terabit or the equivalent of 15 million voice calls.
The initial plan was to lay this cable offshore from Auckland, along the west coast of the North Island landing at Raglan, New Plymouth, South Taranaki, Wanganui, Waikanae, overland to Paraparamu, Titahi Bay, again overland to Lyall Bay, north and south of Kaikoura and Christchurch.The maximum length of leg allowed between landings for technical reasons was about 160 kilometres.
BTW was contracted by Seaworks Ltd, the cable layer to assist with the cable route design by preparing a desktop study, to survey the proposed route and then position the cable lay vessel.
The purpose of the desktop study was to identify from published material a recommended route, which would be subject to a detailed marine survey. For this BTW retained the National Institute of Water and Atmosphere to assist. The key factors taken into account by this study included - Fishing activity, Cables, pipelines or other man made obstacles, Seabed conditions, type of material, hard, soft, mobile etc, Bathymetry, Currents and wave conditions, Resource consent issues, Regulatory requirements.
The main criteria used to select the cable route were that it could be buried by plough during the lay operation. Other criteria were to keep the cable at water depths greater than 30 metres to avoid the effect of wave action, avoid hard areas where it may be difficult to bury the cable and avoid areas of sand waves where the seabed may move due to the currents.The primary references used for this study were the NIWA Coastal Sedimentary Charts, scientific papers published by NIWA and GNS and BTWs experience carrying out surveys in the seas around New Zealand over the past 30 years.The data available although mostly of a regional nature could be confirmed in several areas where there were very detailed surveys.Our confidence in this data was largely confirmed by the subsequent surveys.
The study indicated that a cable route was feasible with most of the route along being a flat featureless seabed.The areas of particular interest were the Cook Strait crossing and the Manukau Harbour entrance, sandwaves off Wanganui and the landings on the north and south Taranaki coasts.The ocean environment in these areas is very dynamic with for example strong currents in the Cook Strait and Manukau Harbour and strong wave action. The area off Kaikoura was avoided due to the very steeply shelving unstable seabed down into the Kaikoura Canyon and the Conway Trough. The desktop study also recommended that only one survey line along most of the route should be necessary due to the nature of the seabed.
The next task was to appear as an expert witness for the resource consent hearings. This process was most interesting to observe and confirmed my opinion that the consent process could be considerably simplified with a clearer definition of matters that need to be considered to approve a consent.
In January 2000 the route survey was commenced on the survey vessel Sea Surveyor with a survey crew of seven. The survey crew included a project manager, a geophysicist seconded from the Institute of Geological and Nuclear Sciences, two surveyors, two electronic engineers and a draughtsman/data processor. The survey task was to determine along the proposed route that there was sufficient depth of sediment to bury the cable, no obstructions or steep slopes that would affect the cable lay operation, establish where the seabed may be mobile, and determine the water depths and length of cable required. The survey required some very sophisticated equipment to obtain and position the data required.
The survey vessel was positioned with differential GPS and the ships gyro. Offsets to the towed sensors were determined with an underwater short baseline acoustic system. This measured acoustically the bearing and distance to a towed fish with sensors installed. The NGL navigation and positioning software system that was developed by BTW was interfaced to the GPS, ships gyro, acoustic system, motion sensor measuring heave pitch and roll and all the various sensors collecting data. NGL calculates and logs the position of each sensor at one-second intervals, marks sensor record every 20 seconds to reference the data and provides an onscreen display to assist the helmsman steer down the line. NGL also converted GPS geographic coordinates in the WGS-84 datum to Universal Transverse Mercator projection coordinates used for plotting of charts.
Geoff Marshall at the navigators workstation.
Three sensors were used to acoustically determine the nature of the seabed. A single beam echosounder was used to determine water depths along the route.Corrections for speed of sound in water and predicted tides were applied in real time to the depths logged by NGL. I t is of interest to note that the latest swathe bathymetry echosounder systems were not considered as a relevant tool for several reasons. Most of the proposed route is across very flat seabed, the cost to hire a system is high, and they are difficult to set up and calibrate on a vessel of opportunity, and require very sophisticated processing preferably onboard. In hindsight the decision was correct although a swathe system may have been useful surveying across Cook Strait where there are steeper slopes.
A side scan sonar was used to map the seabed topography either side of the route, assist in identifying the type of seabed material and determine the position of any hazards close to the route. A side scan sonar system has acoustic transducers mounted in a fish towed close to the seabed. Sound beams are directed obliquely towards the seabed either side of the fish. The reflections from the seabed material are recorded to produce a picture similar to an aerial photo mosaic of the seabed.
A sub bottom profiler was mounted in the same fish as the side scan sonar to determine the properties to at least a depth of two metres below the seabed.The profiler transducers directed vertically downwards produce an acoustic signal at a lower frequency to the echosounder to penetrate the seabed. The signal reflections are recorded via a hydrophone into a processor to produce a profile showing layers of different material. The resolution of depth of the layers is to better than 20 centimetres.
From the profiler and side scan sonar record s a geophysicist can determine, with the assistance of collected samples of the seabed, the properties of the seabed.
The survey was successfully completed after 51 days at sea with only seven days of bad weather downtime. Approximately 3000 line kilometres were surveyed for 953-kilometre route length.As each section of the cable route was surveyed the geophysicist and senior surveyor would change out to complete the processing and interpretation of data and prepare charts.
The survey data was presented on 122 charts each with three display panels. The top panel, is a seabed feature map of the route showing the route, seabed contours, seabed material and sample positions. The second panel shows a geological profile or longsection along the route showing the different seabed layers. The bottom panel is for recording data obtained relating to the cable as it is laid.
The final route chosen is very close to the route proposed by the desktop study. The survey however identified that it would be difficult to bury a cable ashore along the south Taranaki coastline.The cable owner decided then that the cable route between Wanganui and New Plymouth would be routed overland.
The cable lay operation has also been a most interesting operation. The cable is buried through a large 12 tonne plough towed astern of the Seaworks cable lay vessel the Searanger. Similar positioning systems to those used for the route survey were to position the vessel and the plough on the seabed. The NGL positioning system is required to process the positioning information used to keep the plough on route but also record installation data such as burial depth, tow force and cable tension for presentation on the as laid charts.
The Aqualink project for BTW Hydrographic has been an interesting and exciting project to have worked on particularly as we were involved from the beginning in all aspects of the process to install the cable. Our thanks go to Seaworks, Ericsson, and TelstraSaturn for their confidence in our company to assist them with this very significant project for the future of New Zealand.