NEW SEAFLOOR INSTALLATIONS REQUIRE ULTRA-HIGH RESOLUTION SURVEYS

Transcription

1 NEW SEAFLOOR INSTALLATIONS REQUIRE ULTRA-HIGH RESOLUTION SURVEYS Donald Hussong (Fugro Seafloor Surveys, Inc.) Fugro Seafloor Surveys, Inc., 1100 Dexter Avenue North (Suite 100), Seattle, Washington, 98109, USA Abstract: Submarine cables are increasingly needed in active and rugged areas of the seafloor around the world that have been rigorously avoided in the past. These include connections to deep ocean observatory systems that are being placed in geologically complex and active areas, and connections to seafloor energy generation systems purposely placed in extreme high-energy environments. To meet their objectives, these cable systems require very high-resolution preinstallation surveys, including the use of expensive AUV and ROV systems. 1. NEW CABLE APPLICATIONS Continuous quantitative data about physical oceanographic conditions, geochemistry, ecology, and geological activity in the deep oceans is becoming increasingly important for understanding rapidly evolving changes in the environment. New ocean observatories require cable connections to land in order to provide power and constant broadband data transmission to worldwide real-time users of this essential information. These observatory cables must run to sensors that are installed in rugged seabed, often in areas that are geologically active and even have volcanism and hydrothermal venting. Figure 1 is a view of a subaerial portion of the Mid-Atlantic Ridge in Iceland, showing the kind of terrain that would be expected in this geologic region. Note the size of the people in the photo. The footpath runs along the axis of crustal spreading and is surrounded by cliffs rising nearly vertically for over 50 meters. This is just the kind of region where scientists want to install observatory sensors. Figure 1: Ocean crustal spreading center. Not just are the cables required to go into these rugged regions and have a normal 25- year life expectancy, they also should have no effect on the environment. Conventional surveys for typical telecommunication cables identify a corridor where the cable will be laid along a Copyright 2010 SubOptic Page 1 of 6

2 relatively benign route. Since the cable ship can only position the cable within several tens of meters of the planned route, hazards must be widely avoided. Typical telecommunication cable route surveys cover large areas in order to define generally featureless terrain. In many cases in deep water where the cable will not be buried the route selection depends on broad geologic interpretation of the data to minimize risk. Figure 1 shows typical hull-mounted swath multibeam data acquired from a surface vessel operating in a little over 1500m water depth on the Juan de Fuca spreading center in the north-eastern Pacific Ocean. These data are provided by the University of Washington s OOI Cabled Observatory Program. bathymetry is accurate to about 1 meter in depth, but each depth point is defined by a sonar beam of one degree, so there is only one interpreted depth point for an area that is 30 meters square referred to as the data pixel size. Figure 3 shows the same area as Figure 2, but is based on bathymetry acquired using an Autonomous Underwater Vehicle (AUV) flying at an altitude of about 150 meters above the seafloor. At this altitude each bathymetry point can be defined within a 1.2 meter pixel, and is accurate to about 10 centimeters in depth. So there are about 625 bathymetry points, each accurate to about 20 centimeters, in the AUV data for each bathymetry point in the surface multibeam bathymetry. As a result, the target rift zone, Figure 2: EM300 (30 khz) bathymetry acquired from the surface defined by 30m pixels. Although the location of the crustal spreading rift zone is roughly defined by the regional bathymetry, as well as magnetics and other geophysical data, the actual rift is not discernible in the surface data. The Figure 3: Reson 7125 (400 khz) bathymetry acquired from AUV near the bottom and defined by 1.2m pixels. which at most is only about 15 meters deep and 15 meters wide, shows very clearly in the AUV data. Copyright 2010 SubOptic Page 2 of 6

3 The AUV used to acquire the data in Figure 4 was a Sentry operated by the National Deep Submergence Facility at the Woods Hole, and also carried dual magnetometers as well as a CT sensor and optical sensors. Most importantly, it carries a precise depth sensor and complex inertial navigation system necessary to provide high resolution 3-dimensional navigation that is essential to utilizing the accuracy of the bathymetry data. Note in Figure 3 there are slight (at most about a meter) mismatches between bathymetry on adjacent tracks, which is likely a function of the pressure measurement which defines the depth of the AUV in the water. of substantial bottom scouring due to relatively low currents. Figure 4 shows coral boulders in about 500 meters water depth on the submarine slope of Oahu in the Hawaiian Islands. These data were acquired for engineering a large diameter, lightweight, pipe that may be installed to carry cold water from about 800 meters water depth to land, where the water will be used as a coolant for a large air conditioning system. Surface multibeam data did not even show the boulders, and completely missed evidence of scouring at these depths. This level of sediment transport on the seabed would be very significant for the pipeline. For ocean observatory installations there will typically be trunk line cables that run to multiple branching units, generally called nodes, where individual cables then run to multiple sensors spaced around the nodes. All these cables must be installed safely across the complex terrain. Another new application for power cables will be to connect ocean turbines and other energy generation devices to land. In these cases we do not expect unusually rugged terrain, but will need to cope with very highenergy environments. Bottom currents of at least 5 knots are typically targeted for energy generation. A mass of water moving at 6 knots will carry the same energy as air moving at hurricane velocities. Thus these seafloor installations, and the cables connecting these, will be in an environment that is the equivalent of being in a constant hurricane on land and again will be expected to operate for many years. Recent surveys for other applications have shown that even where there is little evidence of bottom currents prior to the survey, the precision of an AUV or ROV survey near the bottom will show evidence 100m Figure 4: Bathymetry acquired by an AUV at an altitude of about 120 meters over coral boulders surrounded by scoured seafloor There are also plans for installing submarine power cables between the Hawaiian Islands. These cables are sensitive to the depth of their burial because of the need for predictable heat loss to the ocean. Similarly, buried cables are being considered in Hawaii for connecting to offshore electrical generators driven by surface waves, which Copyright 2010 SubOptic Page 3 of 6

4 necessarily will need to be routed through mobile seafloor environments. Figure 5 shows one meter contours over the same area as Figure 4. It should be noted that there are at least three tracklines of data in this image, and the bathymetry offset between the tracklines is at most less than 25 centimeters. sonar and other navigation control systems do not react quickly enough. An alternative is to use an ROV equipped with multibeam echo sounder and other sensors. The altitude of the ROV can be precisely adjusted in real time from the surface, and the system can be flown along the survey track at a slow speed also controlled by an operator on the surface. Figure 6 shows the amount of resolution available when mapping at a low altitude, in this case with an AUV mapping from 15 meters above the seabed. 100m Figure 5: One meter contours over the scoured area Even more resolution is available if the AUV is flown at a lower altitude over the seabed, or if a Remotely Operated Vehicle (ROV) is used close to the seafloor. An AUV is constrained by the amount of relief expected on the seafloor in the area being mapped. Typically the AUV will have forward-looking sonar which will sense the bottom suddenly rising and start the AUV ascending to avoid running into the bottom. In reality, however, most survey AUVs do not rise under power at an angle of much more than 30 degrees, so if there is significant relief expected on the seafloor the AUV will be set to fly at an altitude where it will not impact the bottom if the Figure 6: 10 centimeter contours over a scoured outflow pipe using an AUV mapping at 15 meters altitude The object is a storm water outflow pipe in Puget Sound near Seattle. There are bottom currents in the area estimated at about one knot, but the sediments near the outflow head and the pipe leading to it have been subject to significant scouring due to lowlevel turbulence around the structure. If this was a seafloor energy generating installation the current velocities in the area would be many times greater, and scouring would be even more severe. The installation of power cables used to transmit the generated Copyright 2010 SubOptic Page 4 of 6

5 electrical power would need to account for sediment mobility in the area. By mapping at 15 meters altitude we are able to produce very reliable 10 centimeter contours since the bathymetry points are so dense they can be handled at a 10 centimeter pixel scale. The AUV navigation, which is based on an inertial navigation system with USBL position updates from the surface vessel, is adequate to validate the tiny pixel size. Note that in these data there are multiple tracks of the AUV and the data are so accurate that the trackline boundaries are not discernible. 2. ADDITIONAL COST FOR HIGH RESOLUTION MAPPING Acquisition of increasingly high-resolution seafloor mapping data comes at a cost which is proportional to the depth of the water and the resolution of the data. It is essential to the success of many projects that the required data resolution be very carefully evaluated in order to make this fundamental early installation activity as cost-effective as possible. This means acquiring data with adequate resolution to ensure that the cable and route design are optimized and that the installation will proceed with a minimum of unexpected problems. These considerations will often justify a more expensive survey. A typical offshore survey vessel equipped with a deepwater multibeam sonar will cost around $45,000 per day, with wide cost variation depending on location, the size of a project, and what ancillary equipment might be on the vessel. In 1,000 meters WD such a vessel would likely survey at about 9 knots, acquire data in 3 kilometer swath (allowing for overlap of adjacent tracklines), and given normal time for turns and cross-lines would end up acquiring data that cost about $50 per km². This same offshore vessel working in 1,500 meters WD would work with proportionally wider swaths of data, and would thus acquire data at a cost of about $35 per square kilometer. These data are comparable to what is presented in Figure 2, and provide generalized, but valuable, information for routine installation planning and for looking for routes that avoid geologic hazards. Submarine telecommunication cable surveys often use deep-towed sonar systems, commonly with side-scan sonar but often also equipped with swath bathymetry sonars. These deep-towed systems will provide data resolution that is very greatly improved. If such a system is towed at an altitude of 150 meters above the bottom it will provide data that has essentially the same resolution (depth accuracy and pixel size) as surfaceacquired data in 150 meters WD. The only problem with these data are that the position of the deep-towed sonar is generally positioned from the survey vessel towing it with an accuracy of about 1% of the distance of the vessel to the towed array. In 1,000 meters WD a towed system will likely have about 2,500 meters of cable deployed, so the accuracy of the sonar position will be about +/- 25 meters. This is acceptable for most telecommunication cables where the deployment of the cable will likely have at least that much error in these water depths. It may not be adequate for new applications such as cabled observatories and power cables. Using a deep-towed sonar would only add something like $5,000 to the daily cost of a survey vessel, but the acquisition is expensive. By comparison, in 1,000 meters WD a deep-tow survey at 150 meters altitude would acquire data in a 500 meter swath at about 3.5 knots, and would expend considerable time during turns, so the Copyright 2010 SubOptic Page 5 of 6

6 resultant data would cost something more like $500 per km² -- an order of magnitude more than the cost of less precise data acquired from a hull-mounted system. Extending this comparison, a vessel equipped with an AUV will probably cost 50% more per day, and the AUV will likely acquire something more on the order of about 150 meter-wide corridors of data at 3.5 knots for an average of hours per day. This would bring the cost of high resolution AUV data to something on the order of $5,000 per km². These costs will, of course, vary widely depending on the application. Extending this to ROV surveys we find that costs are even more variable. These mapping systems can be used in a very wide variety of applications and can map very close to the seafloor, but they are inherently much slower because of the connecting cable to the surface vessel. So ROVs will be more expensive in proportion to their slower survey speed even if used at altitudes that are similar to AUV altitudes. Clearly these are very approximate cost estimates, but they are useful to illustrate the point that there is considerable extra expense to acquire increasingly high-resolution surveys data. In very rough terms in deep water it is 100 times as expensive per km² to acquire the data shown in Figure 3 compared to the data shown in Figure 2. Regardless of the precise costs, however, the point is that this additional expenditure for many future installations may be essential for the success of these projects. Copyright 2010 SubOptic Page 6 of 6