Charles N. Flagg
E-mail: Charles.Flagg@stonybrook.edu
School of Marine and Atmospheric Sciences
Stony Brook University
Stony Brook, NY 11794-5000
Thomas R. Rossby
E-mail: trossby@gso.uri.edu
Sandy Fontana
E-mail: sfontana@gso.uri.edu
Graduate School of Occeanography
University of Rhode Island
Narragansett, RI 02882
The MV Norrona is a large, high-speed ferry, based in Torshavn on the Faroes Islands and operated by the Faroese company, Smyril Lines p/f. that makes weekly runs between Denmark and Iceland. This route crosses the northern limb of the Meriodional Overturning Circulation and thus, appropriately instrumented, the ship affords an opportunity to monitor one of the most important components of the world climate system. The Norrona Project is a joint effort by Stony Brook University and the University of Rhode Island, funded by the National Science Foundation, to equip the Norrona with an Acoustic Doppler Current Profiler (ADCP) to begin a long-term monitoring of the northward flow of the warm North Atlantic waters through the Faroes-Shetland Channel and over the Faroes-Iceland ridge into the Greenland and Norwegian Seas. European collaborators have recently joined the effort and have established a “Ferry Box” system on the Norrona to record near-surface temperature and salinity. This web-site describes the program goals, the installation of the ADCP system on the Norrona, the efforts to over-come significant bubble sweep-down effects, and provides access to the data as it becomes available.
Introduction
Background
The ADCP System and Installation
Bubble Sweep-Down
Bubble Fairing
Results and Data
Pictures
Acknowledgements:
This project has been possible only through the extraordinary help and collaboration from Smyril Lines, especially that of Captain Jogvan Davastovu, the captains, engineers and electronics officers of the ship, and shoreside network administrators. The work has been funded under a grant from the U.S. National Science Foundation.
The warm waters flowing from the Atlantic into the Nordic Seas past the Faroes play the primary role in moderating the climate of northern and central Europe. This flow, popularly known as the Gulf Stream, is such a natural part of our lives that we take it for granted. It forms the northernmost link of a global circulation that goes under names such as the global thermohaline circulation (THC), the meridional overturning circulation (MOC), but perhaps the most widely known term is the global conveyor belt, illustrated by the system of dark and light lines in the figure below. These warm and salty waters from the North Atlantic sink in the Norwegian Sea and spill back into the deep Atlantic and from there spread out into the global ocean (the dark blue band). They gradually warm, rise and flow back towards the North Atlantic (light blue line). Significantly, all the waters in this global circulation system flow past the Faroes even if the figure is too simplified to indicate this. Of central importance to this circulation as we know it, is that the waters sink and flow back into the deep North Atlantic. If they did not do so, there would be no demand for waters to replace this loss, and the inflow would decrease or stop (as it did during the last glacial period some 20,000 years ago).
In recent years, as our information of the ocean and its variability has improved, we have learned that the flow varies considerably, and may be quite sensitive to changes in weather and climate. It is already known that the Shetland Current varies in response to winds over the northeast Atlantic. Recent research has indicated that as a consequence of global warming climate at high latitudes including the Nordic countries could actually become cooler. The reason for this is curious. Under present conditions the warm waters flowing north are salty, enabling them to sink to great depths when cooled off in the high latitude winter. These dense waters then flow or spill back out into the deep North Atlantic and the global ocean. But if increased rainfall and ice melt freshens these waters, they may not become dense enough to sink in which case the demand for more warm water will cease – leading to a cooler perhaps even a cold climate at high latitudes. Thus, in these days of global warming there is much interest in monitoring the strength and salinity of this flow past the Faroes. The vessel Norrona provides an excellent platform from which to do this.
Background
Scandinavians have always been very interested in the oceans. As far back as the 1500s a Swedish priest, Olaus Magnus, made a famous map of the Nordic countries called the “Carta Marina”, published in 1539. As its name indicates, it also includes a tremendous amount of detail about the ocean, not only of life in the sea, but also of whorls and eddies between Iceland and Norway just north of the Faroes. A copy of Carta Marina will be on display here on the Norrona shortly. In the late 1800s Norwegian oceanographers conducted a number of expeditions to chart the Nordic Seas. They also developed the observational and theoretical skills to interpret their findings. The left figure below shows a sketch of the surface circulation in the Nordic Seas, published in 1909 by Helland-Hansen and Nansen. They clearly show flow from the Atlantic all along the Iceland-Faroe-Scotland ridge: a rapid narrow flow west of Scotland, and a more eddy-like pattern southeast of Iceland, not unlike what Olaus Magnus drew in Carta Marina almost 500 years earlier! Since then, a number of studies have shown that the Iceland-Faroe inflow organizes itself into the eastward flowing Faroe Current just north of the Faroes. The right figure provides a schematic summary of our present understanding of the ocean currents in and between the northeast Atlantic and the Nordic Seas.
Recent Research
A number of research programs have been measuring currents in the Denmark Straits between Greenland and Iceland and along the ridge between Iceland, the Faroes and Scotland. The classic measurement technique is the moored (or anchored) recording current meter that measures the flow of water past the mooring and records the results internally. Such instruments have been deployed across the Faroe Bank Channel, located between the Faroes and Faroe Bank to the south to measure the outflow of cold water from the Nordic Seas back into the Atlantic. (This 800 m deep channel is the deepest connection between the Atlantic and Nordic Seas.) Similar instruments have been deployed across the Shetland Channel to measure the inflow of warm waters into the Norwegian Sea, and north of the Faroes to measure the volume of water transported by Faroe Current. These programs have greatly improved our knowledge of the exchange rates and how they vary with time, from season to season and from year to year. To date, the best estimate of inflow between Iceland the Faroes is 3.8 x 106 m3/sec and just about the same amount in the Shetland Channel. For comparison, the outflows from all the rivers of the world sum to about 1 x 106 m3/sec.
The Norrona Project:
The Norrona measurement program takes a different approach: It will measure currents from the surface to depths as great as ~800 m depth all along along the ship’s track. Thanks to the twice weekly transits between Denmark and Iceland, throughout the year, year after year, the inflow and how it varies in time along the ridge can be determined with unprecedented accuracy. Unlike moored current meters, the Norrona will provide substantially improved spatial coverage. Why does this matter?
We know that the inflow of water through the Shetland Channel varies depending upon winds over the northeast Atlantic. We also know that a decrease in transport cannot persist without causing the inflow to increase elsewhere (or the outflow to decrease) for otherwise the sea level in the Nordic Seas would likely begin to drop. By measuring currents throughout the region, we can begin to understand in detail how changes in one place lead to variations in another. Because the Norrona can measure currents to ~800 m it will also be able to measure the flow from the Nordic Seas back into the Atlantic. At depths below the 800 m (the sill depth of the Faroe Bank Channel) there should be little net flow south through the Shetland Channel. At shallower depths the Norrona will be able to measure any waters going south and eventually spilling into the deep north Atlantic. Thus the Norrona will be able to measure flows in and out and how they vary spatially and in relation to each other.
But the most important question the Norrona program seeks to address is the long-term stability of the flow into the Nordic Seas of warm salty water, the upper branch of the global meridional ocean circulation. Some observations have indicated a possible weakening of the MOC, but the measurement uncertainties are huge. The complete coverage of the inflow provided by the Norrona will allow oceanographers to determine with significantly improved accuracy the inflow and how it varies in time and along the ridge. One might say that the Norrona will provide an early warming system for any change in this inflow and thus change in European climate.
The Norrona System and Installation
The water current measurements from the Norrona make use of a device called an acoustic Doppler current profiler, or ADCP. The ADCP’s acoustic transducer is installed on the bottom of the ship and sends out four narrow acoustic beams down into the water and receives back the Doppler shifted signal reflected from plantonic particles and thermal inhomogenities in the water. The Doppler shift of the returned signal is proportional to the water velocity along the beam relative to the acoustic transducer. This data incombination with the ship’s compass, GPS navigation data and a little bit of trigonometry, can convert these velocities into estimates of the absolute water movement in terms of its east, north and vertical components.
The ADCP installed on the Norr�na is a 75 kHz RD Instruments Ocean Surveyor (OS75) mounted in a void space between frames 105 and 106, about 60 meters from the bow (see figure below). The figure to the right (click on thumbnail image) shows a schematic of the entire ADCP system from the transducer inside its “seachest” to the deck unit and data acquisition system in one of the ship’s electronics rooms on deck 8. The “seachest” forms a water tight connection to the hull while the cofferdam provides a secondary containment. A multi-conductor power and signal cable connects the transducer to the data acquisition and archiving system. The primary heading data is supplied from a compass repeater tied to the ship’s gyro compass. Navigation information is supplied from a Thales ADU5 GPS receiver which provides very accurate heading information as well as the ship’s position. The data collection is controlled by the AutoADCP software which, in turn, controls and monitors RD Instruments’ VMDAS data acquisition system. When the vessel docks in Torshavn a two-way wireless internet connection to shore provides a means to control the system and down-load the data.
The installation of the ADCP system on the Norrona presented some unexpected difficulties. The initial installation took place in January 2006 at the Blohm and Voss shipyard in Hamburg, Germany. We have installed and run ADCP systems on commercial and research vessels in the past but the Norrona presented a number of new challenges. The first was that the ship is actually a full-scale floating hotel with all the amenities and finished accommodations that one would expect. Leading several cables (see figure below) from the top of the ship to the bottom and from fore to aft meant that the panelling along seeming miles of corridors had to be carefully removed and then reinstalled. And because we were dealing with a finished vessel, over a dozen watertight through-bulkhead cableways needed to be opened, the ADCP and other cables passed through, and then the pass-throughs re-cemented.
Despite some surprises with the ADCP’s initial installation, the ADCP was successfully installed during the one-week dry dock period. It was during the shake-down cruise during the week following the relaunching of the ship that we encountered the real difficulties with the system although we were not sure what was happening for some time. Various tests right after launch as well as later tests by an RD Instruments technician indicated that the instrument itself was working properly. However the data return was extremely spotty and poor. There were somewhat better results when the Norrona steamed through the fjords approaching Bergen, one of the ship’s usual ports of call. But as soon as the ship passed out of the fjords into the open sea the backscatter amplitude became irratic, the correlation level dropped to low levels (the OS75 is a broad-band instrument) and the percent good went to zero. Occasionally, as in once every few minutes, we would get a possible profile but when the seas got rougher even those occasional profiles ceased.
Candidate sources of our problems with the data collection included internal machinery-generated vibration, propeller noise, electronic interference due to the long length of cable that necessarily ran along-side some of the ship’s power cables, and the outside possibility that we were experiencing bubble sweep-down. We talked to people experienced in ship vibration which suggested that there would be very little if any vibration energy in the ADCP’s 75kHz frequency band even from the extremely noisy, to our ears, centrifugal oil separators located above and aft of the ADCP transducer. Various tests with and without the ADCP connected and with various ship systems turned on and off finally showed that we were not getting any electronic interference. That left bubble sweep-down as the only reasonable explanation for the poor rate of data return.
Bubble sweep-down, during which bubbles generated by the bow wave are swept under the hull, is something that the US Navy has dealt with but there does not seem to be much if any literature on how to deal with it. The usual approach if bubble sweep-down is encountered is to mount acoustic devices on streamline pods below the ship. On the relatively slow vessels of the research fleet, bubble sweep-down has sometimes been encountered in rough weather but is not usually a problem. However in the case of the RV Iselin operated by the University of Miami, bubble sweep-down was shown with cameras mounted below the hull to be a significant problem. This led to the installation of a simple pod for its acoustic sensors which generally corrected the problem. The RV Thomas Thompson operated by the University of Washington has a permanent 19″ deep pod in which all its acoustic sensors are mounted. This has resulted in extremely good data from its ADCPs even in very rough weather.
In the past, bubble sweep-down has not appeared to be a significant issue in the voluntary observing ship programs that make use of commercial container vessels and cruise ships. Our own experience with the MV Oleander, a relatively small (~400′) container vessel operated by Bermuda Container Lines that makes weekly runs between New York and Bermuda traveling at about 17kts, has indicated that when the ship is loaded during the Bermuda-bound legs that the data return is usually quite acceptable. On the return New York bound leg of the trip when the ship is lightly loaded, draws ~1m less and the bow rides higher, it takes reasonably good weather for good data return. The hull of container vessels, and this includes the Oleander, are generally fairly blunt and generate a significant bow wave which one would think would lead to major bubble sweep-down problems. But that does not appear to be the case. The Norrona runs only somewhat faster than the Oleander at 21kts. The hull, on the other hand, looks similar to that of high speed cruise ships with a sharp entry which one would normally expect to generate a minimal bow wave and thus, minimal bubble sweep-down. In addition, the Norr�na is almost always ballasted to maintain a constant draft, whether it is empty or full. One potential source of the bubbles is that the Norrona has two enormous bow-thrusters located just below the waterline (see the photo below) which could draw in bubbles when the ship pitches. Another possibility brought to our attention after consultations with naval architects familiar with high speed ferry design and their bulbous bows is that the sharp intersection between the bulbous bbow and the hull creates a potential pathway for bubbles to get under the hull.
Evolution of the Bubble Fairing
With the painful conclusion that bubbles were all but completely blanking out the ADCP we were left with two choices. Either remove the ADCP and give up on the scientifically ideal route between Europe and Iceland, or try to come up with a viable solution. We chose to try to save the installation and the route by investigating the feasibility of installing a fairing. And rather to our surprise, Smyril Line was willing to go along with this approach. For those who are unfamiliar with commercial ship design, virtually all commercial vessels are flat-bottomed over most of the hull with no keel projecting down. Thus, any fairing we might install would be the deepest and most exposed portion of the hull.
The first item of business was to find out whether a fairing would do any good at all. Luckily for us, the Norrona was scheduled for a dry dock period in May 2006 which afforded us an opportunity to attach a temporary fairing. By temporary we mean that it was expected that at some point the fairing would fall off either through finally yielding to the flow past the device or because the unit was bumped while maneuvering in the shallow waters of the ports of call. Given the lack of any references on fairing design the sizing of the fairing was a somewhat arbitrary exercise. We made the assumption that the bubbles would be contained in the turbulent boundary layer running along the bottom. The boundary layer thickness was estimated to be about 15 cm thick at the location of the transducer so a fairing about 20 cm tall with a suitable lip should penetrate the boundary layer and push the bubbles aside. The free-flooding temporary fairing was foil shaped, 2.75 m long x 0.81 m wide x 0.20 m tall, made from 1/8″ stainless steel, filled with expandable foam and covered with a 3/8″ thick Lexan sheet. At ~20 cm from the transducer the Lexan sheet was acoustically transparent to the ADCP. The drawing and photos below show the fairing. In the first photo the bottom of the fairing is facing up and the conical cutout for the acoustic beams is visible. The fairing was glued to the ship using Dow Corning 5200 marine cement.
The temporary fairing stayed attached to the ship for about 10 days and during that time we were able to collect data as the ship made a trip from Torshavn to Europe and almost all the way back. The percent good from that run is shown below indicating that we were getting nearly 100% good profiles down to more than 400m in the Faroes-Shetland Channel. The fairing seems to have fallen off just before the ship reached Lerwick in the Shetland Islands. The contrast with and without the fairing is obvious and the value of a bubble fairing was demonstrated. But gluing a fairing to the hull was not going to be a permanent solution.
As an aside, when we finally got some data to look at it was clear that the ADCP itself may have been defective. Shortly after the temporary fairing was installed in May 2006, we discovered that the ADCP instrument had developed a malfunction, probably shortly after the first two weeks of operation. With the temporary fairing the ADCP appeared to work but the velocities calculated from the Doppler shifts did not seem plausible. After extensive attempts at troubleshooting with RDIntruments technicians, we first received a replacement deck unit as a loan from RDI, but this did not help. The problem appeared that the problem was with the transducer. So when the vessel was drydocked in the 3rd week of January 2007 for work on the ship’s stabilizers, the transducer was removed and returned to RDInstruments for analysis and repair. The repaired transducer was reinstalled when the vessel was drydocked at the end of October 2007.
With the evidence that a fairing might do the job, Smyril Lines was amenable to the idea of a permanent, non-frangible fairing provided that it passed inspection by Det Norska Veritas (DNV), the Norwegian ship certification agency. Bay Marine Inc of Barrington, RI undertook the task of designing the fairing and submitting the drawings to DNV. The new fairing differed from the temporary fairing in that it was slightly narrower, the lip around the bottom was thicker, and by the presences on three rugged bumpers installed ahead of the fairing. The bumpers were intended to ward off debris that might hit the fairing. The drawings were OK’d in time for another dry dock period in late October, 2007.
While the idea of a bubble fairing was being pursued, a second method that would acoustically detect the presence or absence of bubbles under the hull was being investigated. On a rather short time frame for such a device, a prototype 250kHz transducer and data logging system was developed in time to run some tests during the week before the Norrona was scheduled for the October dry dock period. These test were rather uncertain but seemed to indicate that there might be a lower concentration of bubbles out board of the transducer’s initial location. With some trepidation, the decision was made to move the transducer and install the permanent fairing about 3.2m outboard of the original position. The photo below shows the fairing from the front during installation and shows the rather substantial bumpers. Note also that the leading edge of the lip around the fairing was angular and not rounded.
Although we had great expectations with the new fairing, it did not live up to our hopes and data return was still rather poor. Better than before, but still poor. This led to further assessment of the problem and more consultation with experts in the fluid dynamics around ship hulls. There were a number of possibilities under consideration, 1) that we had moved the transducer into a region with a greater bubble concentration and/or a thicker bubble layer, 2) that the bumpers had altered the flow such that vortices rolled the bubble layer underneath the fairing, or 3) that the sharp leading edge was forming cavitation bubbles that then flowed passed the transducer. In November 2007 the ship encountered some unusually bad weather which caused one of the stabilizer fins to break off and punch a small hole in the side of the ship. This led to another dry dock period in February 2008 and we took that opportunity to make some relatively minor modifications to the fairing. During this dry dock period we removed all three of the bumpers, added two inches to the lip of the fairing and carefully rounded the leading edge. The modified bubble fairing is shown in the photo below showing a much smoother and less obstructed structure.
Results with this version of the fairing were somewhat better and we have gathered a reasonable amount of data between the Faroes and the mainland. Some of this improvement might simply be that the first few months with the modified fairing have been during the late spring and summer when the winds and waves are less. Comparing the returns with the weather between the Faroes and Norway certainly suggests that the returns are better when the winds and waves are low, and less good under more adverse conditions. Also, we’re still getting poor returns over the sections the run between the Faroes and Iceland which clearly traverses an important limb of the North Atlantic current into the Norwegian and Greenland Seas.
At this stage, nearly three years into the project we had yet to completely solve the bubble problem. The ship was going in to the dry dock one more time in early January 2009 giving us one more opportunity to make alterations. In anticipation of this opportunity to make alterations our efforts were directed at determining exactly how the bubble layer flows around and over the fairing, what alterations to the design might correct the problem, and whether there might be a more advantageous location with fewer bubbles within the restricted area in the ship that can support the ADCP system. There was a three-pronged approach to the assessment of the bubble layer. The first was an improved version of the thru-hull sonar with better data logging capabilities. The second method was develop an underwater camera on a magnetic mount that could be installed by divers when the ship was in Torshavn. The camera we used is the latest version of the CritterCam developed by Greg Marshall’s group at the National Geographic Society. In November and early December of 2008 we deployed the camera at various locations to determine how the bubbles were flowing over the fairing and where to best location for the fairing might be. And the third approach was a serious study of the flow field around the fairing using a computational fluid dynamics program from Flow Works, Inc. The results of these approaches are discussed below.
The updated version of the thru-hull bubble sonar operates at 200 kHz as before but with an improved digitization rate of 24 kHz, decimated to a sample rate of 240 Hz, yielding a vertical resolution under the hull of 3 mm. The operating frequency is determined by the acoustic impedance of the 12.5 mm thick steel hull such that the hull is nearly acoustically transparent and there is maximum signal penetrating into the water. The transducer was moved to 18 places in the void space where the ADCP could be located to see if there was an optimal location. At each location 100 profiles were obtained over a period of one minute returning 120 backscatter estimates between 104 and 600 μsec after the ping. The results that were obtained by the sonar (see below) indicated high backscatter close to the hull that decayed to background at between 10 and 20 cm. The background values corresponded closely with those obtained when the ship was still at the dock. Occasionally, we got high backscatter from a few pings farther from the hull with the farthest high backscatter return from about 40 cm. The weather during the sonar runs was fairly benign by North Atlantic standards, with 10 to 18 kts of wind and average wave heights of 8 to 11 ft as reported by the British meteorological buoy 64046. So the results do not reflect the severe bubble conditions that typify winter seas but they do suggest that there may be a �usual� bubble cloud close to the hull of some 10 to 20 cm thick and that deeper bubbles are also possible but the sonar data are not sufficient to characterize those more extensive bubble clouds.
Results from the bubble sonar showing 100 backscatter profiles at a number of locations as a function of distance below the hull.
The underwater camera was an attempt to get unequivocal evidence about the character of the bubbles under the hull. The difficulty with this approach was to find a camera that could operate autonomously, record the pictures internally and be diver deployable. It turns out that we were not the first with requirements for an autonomous underwater video camera. Greg Marshall’s group at the National Geographic Society have been developing their Crittercam for several years so that they could attach it to various sea animals and record their behavior. The latest version of the Crittercam is remarkably compact and can record up to eight hours of video (and audio) data on internal solid state memory. Marshall graciously allowed us to use one of his cameras and Eric Bergenpas set it up so that it could record one minute of video data every four hours for up to five days. The next step was to develop a method to attach the camera to the hull in such a way that a diver could deploy it at a number of locations while remaining attached at the Norrona’s usual cruising speed of 20 kts. That was accomplished using a series of permanent magnets that together developed a grip of about 1600 lbs together with a lever and cam arrangement that allowed the unit to be detached from the hull. Then the whole camera and clamp were housed in streamlined fairing to minimize drag. The magnetic clamp and fairing were tested on the high speed Port Jefferson to Bridgeport ferry with the assistance of Mark Wiggins and graduate student divers Brooke Rogers and Amber Stubler.
The camera was deployed at five different locations with the help of Ebba Mortensen of the Faroese Fisheries Institute and Edvard Kjeld a professional diver operating in the Faores, to study the character of the bubble clouds and to get some idea of their spatial distribution. That there are bubbles under the ship is indisputable from the camera results and they are clearly the cause of the poor data return from the ADCP. The most informative results came from the videos taken during daylight hours when light from the surface illuminated bubble clouds from the side. In the figure below showing the bubble fairing from the side, one can see the bubble cloud approaching from the right. The fairing is 21 cm high indicating that this particular bubble cloud is roughly 30 cm deep. This video was obtained when the winds and sea state were relatively mild. When the conditions are much rougher, the camera’s vision is often obscured by the bubbles next to the lens so our ideas about what is happening may be biased. The second figure below was taken looking forward at a location closer to the centerline of the ship. The grey clouds visible below the hull are the bubble clouds approaching the camera. A few frames later the camera is obscured due to the bubbles hitting the lens. While the bubble clouds are undoubtedly produced in the turbulent bow wave as the ship pitches up and down, the shape of the clouds seem quite steady for the two or three seconds that they show up in the videos. It is also clear that the larger bubble clouds are produced by the pitching of the ship as they come at the camera at fairing regular intervals.
Clicking on the following links will bring you the most illustrative videos for the first four deployments: #1, #2, #3, and #4. The positions of the camera for each of the videos is shown in the figure above right. In the first video the camera is upside down so up is down and left is right. Overall, the combined camera and sonar result together with the results from the first temporary bubble fairing suggested that there will be fewer bubble problems if we moved the ADCP closer to the centerline of the ship.
The last part of the investigation involved computational fluid dynamics simulations performed by Bob Fratantonio, a graduate student in ocean engineering at URI, using Floworks software. The questions we wanted to resolve were: 1) was there a better overall shape to the fairing than the initial one, in particular would a more pointed fairing reduce the stagnation pressure at the leading edge of the fairing, 2) would extending the lip forward to 8 cm (from the current 4 cm) help reduce the boundary layer flow under the fairing, and 3) would vertical fins or chines placed ahead of the fairing cause sufficient upwelling under the hull to bring bubble free water up against the fairing and transducer. Each of these questions was addressed in succession and then finally in combination to see how the overall system would work.
The results of altering the shape of the fairing to be more pointed does reduce the stagnation pressure at the nose of the fairing slightly and produces less of a downward deflection of particles flowing along the centerline as shown in the figure below. The figure shows path of particles released ~0.5m ahead of the fairing and 0.2m below the hull. The reduction of the stagnation pressure on either side of the centerline is more substantial with a greater effect on the flow field.
