Posted by C & C Technologies on 06/29/2017

DEEP WATER AUV EXPERIENCES

DEEP WATER AUV EXPERIENCES DEEP WATER AUV EXPERIENCES

Thomas S. Chance, President

Jay Northcutt, Geophysical Operations Manager

C & C Technologies, Inc.

730 E. Kaliste Saloom Road

Lafayette, Louisiana USA 70508


DEEP WATER AUV EXPERIENCES

ABSTRACT


DEEP WATER AUV EXPERIENCESDEEP WATER AUV EXPERIENCES

In August 1999, C & C Technologies placed an order with Kongsberg Simrad for a Hugin 3000 autonomous underwater vehicle (AUV) for operations worldwide. C & C's AUV team worked with Simrad during the development, and the system was completed in November 2000. The 3000-meter rated vehicle was successfully used to perform several deep-water projects in the Gulf of Mexico. This paper describes the Hugin 3000 AUV. It also discusses the operational experiences C & C Technologies has had with the system. Multibeam, sidescan, and subbottom profiler data samples from the AUV are shown. Technical and cost comparisons between AUV and deep-tow systems are also made.

1. INTRODUCTION

Comprehensive high-resolution geophysical data is required by a number of commercial and government sectors worldwide. These groups include the offshore oil and gas industry, the marine telecommunications industry, academic institutions, and various military and civilian government groups worldwide. Other user groups include marine mining, archeological investigation, accident investigation, and exploration.Examples of the data required include multibeam bathymetry and imagery, side scan sonar data, and subbottom profiler data. Multibeam bathymetry data, as shown in figure 1, allows underwater topography to be detailed. In this case, the shelf and slope off ofMonterey, California is shown in green and blue, while the land portion is shown as red and brown in the background. Multibeam imagery, or backscatter, is a function of the reflectivity of the bottom, indicating bottom hardness and roughness.

<span></span>DEEP WATER AUV EXPERIENCES

 Multibeam bathymetry (Gardner & Mayer)

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Fig. 1. Multibeam bathymetry (Gardner & Mayer)

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Side scan sonar data, as shown in figure 2, is used to detect surface features including boulders, debris, pockmarks, drag marks, gas vents, and faults.

 Side scan sonar data

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Fig. 2. Side scan sonar data

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Figure 2 shows a typical deep tow sonar towfish, which often includes a subbottom profiler and a magnetometer. Figure 3 (below) shows typical sub-bottom profiler equipment and a data sample. This equipment is used to detect faults, gas pockets and help determine sediment structure.

Subbottom profiler equipment and data

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Fig 3. Subbottom profiler equipment and data

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The combination of these different data sets significantly helps geophysicists in their understanding of the ocean bottom. However, acquisition of good data, particularly in deep water, has historically been challenging and expensive.

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2. GENERIC AUV DESCRIPTION

An autonomous underwater vehicle (or AUV) is a self propelled, unmanned underwater vehicle that is controlled by an onboard computer. Over the last decade, more than 40 different AUVs have been designed and many have been demonstrated (see figure 4 below). Yet, the technology is not yet fully matured.

Examples of AUVs

Fig. 4. Examples of AUVs

These AUVs have been designed for a variety of functions including mine hunting, water sampling, and surveying. Some AUVs are designed to hover; others are designed to be streamlined cruisers. Some AUVs are long (10m), while others are short (1m). Some AUVs are built around a single sensor, often an echosounder or sidescan, others a rebuilt to support one of a variety of sensors. One of the historic problems with AUVs is that they have limited power due to battery technology. A second problem is that, with the concern over building theAUV itself, little attention has been paid to the launch and recovery system.People often hear about AUVs and UUVs. An AUVis an autonomous underwater vehicle, while a UUVis an untethered underwater vehicle. To be truly autonomous, one could, for example, launch an AUV from the dock, let it go out and perform the required survey without external supervision, and the AUV would return to the dock a week later with all the data. From a technical standpoint, many AUVs are really UUVs.

UUV verses AUV

Fig. 5. UUV verses AUV

However, because “AUV” is the more recognised commercial term, most vehicles are referred to as “AUVs” (as done in this paper). As the technology develops, it also becomes more apparent that some vehicles are more autonomous than others. The result is that a UUV – AUV spectrum is more appropriate (see figure 5). Because this technology is still young, most of today’s vehicles dominate the UUV side of the spectrum.

3. C & C’s SELECTION OF HUGIN AUV

C & C Technologies began working on the U.S.Naval Research Laboratory’s (NRL’s) survey UUV program in 1994. Over the next five years, C & C integrated several survey sensors into the navy’s twoUUVs. These sensors included multibeam bathymetry, a subbottom classification system, an acoustic Doppler current profiler (ADCP), a forward-looking sonar, a conductivity-temperature-depth(CTD) sensor, video, DGPS, and a high-speed radio link. Integration of these sensors required an extensive amount of survey software to be developed by C & C Technologies. This exercise, with the associated sea trials, gave C & C extensive AUV experience.

In 1998, C & C developed a small UUV for nearshore beach surveys. Finally, after more than two years of research into potential AUV manufacturers,C & C Technologies chose Kongsberg Simrad, headquartered in Norway, as its AUV vendor.

Simrad’s Hugin 3000 was chosen for a number of reasons. First, Simrad had a proven track record with more than 100 successful surveys performed by earlier generation Hugin AUV systems. Second, C &C felt that Simrad, being a commercial electronics manufacturer, was in a better position to deliver and support an AUV as compared to other less commercially oriented groups. Simrad had also already mastered the multibeam, inertial navigation system, and fuel cell technologies. Fourth, Simrad had demonstrated their launch and retrieval system.

Reasons for C & C’s selection of Hugin

Fig. 6. Reasons for C & C’s selection of Hugin

Simrad’s only weakness was that they lacked the sensor integration software. C & C on the other hand, had developed a significant amount of sensor integration software and had a tremendous team of AUV software and hardware engineers in place. The result was a perfect complement.

4. HUGIN 3000 AUV DESCRIPTION

The Hugin 3000 is a survey AUV. Unlike someAUVs, it is not designed to hover, but to cruise. This reduces its flexibility somewhat, but makes it very efficient at what it is designed to do – run survey lines. The Hugin can operate in two modes, one more autonomous, the other less autonomous.

Hugin 3000 features

Fig. 7. Hugin 3000 features

The Hugin 3000 is rated for 3000 meters (10,000ft). It’s survey sensors include the following:

  • 200 kHz multibeam bathymetry & imagery

  • Dual frequency sidescan (120 & 410 kHz)

  • Subbottom profiler (2-10 kHz)

  • CTD


Other ancillary sensors include:

  • HiPAP USBL

  • Inertial Navigation system

  • Doppler velocity log

  • Fiber optic gyro

  • High & Low Speed Acoustic links

  • DGPS & UHF radio (Surface)


The Hugin is powered by an aluminium oxygen fuel cell with ni-cad back up batteries. With the fuel cell power source, the AUV can travel at 4 knots for at least 40 hours with all sensors running. Hugin is5.3m long (17ft) and is1.0m in diameter (3.3ft).The Hugin 3000 has numerous safety devices to allow it to surface when a significant problem arises.Various triggers are linked to two drop weights and an air bladder.Survey lines and sensor settings can be pre-programmed into the AUV. Lines can be aborted and the survey program can be altered via the acoustic telemetry link while the survey is underway. 

Most of the settings for the survey sensors can also be remotely controlled via the link. Multibeam, sidescan, subbottom, and positioning data is recorded in the AUV. Decimated data from these sensors is transmitted from the AUV to the ship. This allows real time quality assurance of the data, and real time feedback on the bottom conditions. Positioning of the data is done using HiPAP USBL, the inertial navigation system, and the Doppler velocity log integrated in a Kalman filter.In the more autonomous mode, the QA data is not transmitted and positioning is accomplished by using the inertial navigation system and the Doppler velocity log with the Kalman filter.

5. ADVANTAGES OF THE HUGIN 3000 AUV

Historically, deep towed systems, incorporating aside scan sonar and subbottom profiler, have been used to collect high-resolution geophysical data in deep water. More recently, some of these systems include swath bathymetry systems and magneto-meters. Generally, deep towed systems include a topside control / recorder (on the vessel), a winch, tow cable, and the towfish.

5.1 POSITIONING

Positioning the conventional towfish remains a challenge in deep water surveys. In depths less than about 800 meters, the towfish can be acoustically positioned from the tow vessel. However, in greater depths, this method is no longer viable. Alternatives, such as deploying a long baseline (LBL) array or using a second vessel, sometimes called a chase boat, to position the towfish are employed. Of these two methods, the chase boat technique is more cost effective than using LBL.

Figure 8 (below) depicts a tow vessel with a deep towed sonar followed by a chase boat to position the towfish. In this example, the water is about 2000m deep.

Deep towed sonar deployment

Fig. 8. Deep towed sonar deployment

Because the cable-out to towfish-depth scope of about three to one is required to keep the towfish down, about 6000m of tow cable is used. The ship in the figure that is over the towfish “chases” the tow vessel and it acoustically positions the towfish. The towfish position is radio telemetered from the chase boat to the towboat so the towboat can try to steer the towfish along the prescribed survey line.

If an AUV were used instead of a deep towed sonar, the program can immediately move from a two vessel operation to a one vessel operation. That one vessel, the AUVs mother ship, can transit over the AUV justas the chase boat tracked over the towfish. From a cost and logistics standpoint, this makes the project much better.

5.2 REDUCED SURVEY TIME

The survey time with the Hugin 3000 AUV is dramatically reduced over conventional towed systems in two ways.First, the survey speed of the AUV is much higher than a deep towed sonar. A deep towed fish is limited to about 2.5 kts. At faster speeds, the towfish will tend to rise towards the surface, making it too high from the bottom to get good data. Alternatively, the Hugin AUV routinely travels at 4.0 knots, or about 60 percent faster than a deep tow (figure 9).

DEEP WATER AUV EXPERIENCES

AUV advantages in speed & quick line turns

Fig. 9. AUV advantages in speed & quick line turns

Second, line turns take far less time for an AUV than for a deep towed sonar. For example, deep tow systems can take anywhere from two to six hours to make a 180 degree turn. These long wide turns are required to prevent the towfish from colliding with the ocean bottom. Historically, up to 50 percent of the time spent on a deep tow project is used for line turns. On the other hand, the Hugin can make a line turn in just a few minutes. In fact, the Hugin can usually end a survey line, turn, and be on the next survey line before the survey ship itself.

The effect of the faster survey speed and the quick line turns can reduce the required survey time by about 60 percent as compared to using a deep tow.

5.3 BETTER SURVEY LINE

One of the difficulties of using a deep towed sonar is getting onto the survey line and staying on the survey line. In fact, rarely is the first line of a deep tow survey in the right place. Currents can push the towfish off line by hundreds of meters depending on the tow vessel’s speed and direction, as well as the varying current conditions. If a target is missed, it usually means a long slow turn and hope that the current / boat speed effect is understood well enough from the opposite direction to come relatively close to the target. This is illustrated on the left side of figure 10. As shown, the AUV may crab just a bit to overcome the currents, however, it will stay within a few meters of the programmed line.

 AUV advantages in maintaining line

DEEP WATER AUV EXPERIENCES

Fig. 10. AUV advantages in maintaining line

The survey is also improved because the AUV can maintain a constant height off the ocean bottom.This is particularly difficult when using a deep tow in rough terrain. If the deep tow is too high, data quality will be poor. If the deep tow is too low, cross track coverage is limited and the possibility of colliding with the bottom becomes much higher.Additionally, if the deep tow has a multibeam sonar, varying towfish height will result in data holidays between lines that are very time consuming to fill.Alternatively, the Hugin AUV can be preprogrammed with three dimensional survey line information, which can be obtained from reconnaissance information. More importantly, the Hugin can track the bottom just ahead of itself and adjust its depth to maintain a constant height off bottom.

The numerous advantages of the Hugin AUV over deep towed systems are illustrated in figure 11 below.

Summary of AUV advantages over deep tow

Fig. 11. Summary of AUV advantages over deep tow

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5.4 OTHER HUGIN 3000 AUV ADVANTAGES

Another advantage of the AUV is that it can work close to obstructions where tethered systems could become tangled. For example, deep towed systems cannot operate close to structures with anchor cables such as anchor rigs, anchor barges and moored boats, as well as platforms (see figure 12).

The Hugin can work around obstructions

Fig. 12. The Hugin can work around obstructions

Finally, the Hugin AUV is advantageous because it can get very high resolution multibeam data as compared to hull mounted multibeam systems. As shown in figure 13, in 2000 meters of water, the bathymetry can be improved from 7.0m of vertical noise to about 0.2m, an improvement of over 30times. Similarly, the calibrated backscatter from the multibeam sonar would improve from a 40m pixel size to a pixel size of less than 1.0m.

AUV advantages over hull mounted sonar

Fig. 13. AUV advantages over hull mounted sonar

6. REAL-TIME AUV DATA

C & C’s Hugin 3000 AUV may be set up in a more autonomous configuration (AUV mode), or a less autonomous configuration (UUV mode). Both configurations have their advantages; however, theUUV mode allows beneficial data to be acoustically transmitted to the surface and monitored in real time.

AUV control screen

Fig. 14. AUV control screen

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Figure 14 shows one of several real time displays.This display snapshot was recorded during sea trials in October 2000. It indicates the status of various systems internal to the AUV and the current value of various parameters. This example shows the current depth, altitude, AUV speed, motor rotational speed, and values for pitch and roll. It is interesting to note that during sea trials the pitch of +2.3 degrees reversed during reciprocal lines as one would expect from the slight grade of the ocean bottom in the test area.

Figure 15 shows one of the sea trial test areas. A rose pattern of test lines was designed around a subsea well in 1200m (4000ft) of water in the Gulf ofMexico. The real time display shows the AUV (in orange) and surface ship geographically located with the survey lines in the background.

15 AUV test site

DEEP WATER AUV EXPERIENCES

Fig. 15 AUV test site

The real time position of the AUV displayed on the screen (figure 15) confirmed the quick speed and reduced line turn time that was expected.

Figure 16 shows the real time decimated multibeam bathymetry waterfall and coverage map screens as the test survey was in progress. Multibeam imagery was not sent up in real time because of the limited data rate of the acoustic link. Problems with the DVL caused the AUV to porpoise at times, which accounts for the data gaps, and interference between sensors caused the checking in the data. Both problems were subsequently resolved.

Real time multibeam bathymetry display

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Fig. 16. Real time multibeam bathymetry display

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Real time side scan data is show in figure 17. The decimation and compression techniques developed by the C & C software team worked extremely well in this case as it had done with the multibeam and subbottom data. In fact, the difference between the decimated data and the actual data was relatively small, and most of the time, imperceptible.

Real time side scan display

DEEP WATER AUV EXPERIENCES

Fig. 17. Real time side scan display

In this data sample, the AUV was to survey along aline that goes over the subsea well. Because the well was nadir of the sidescan, only well cuttings were detected as seen in the figure.

The next screen snapshot (figure 18) shows the realtime subbottom data. Again, the data is intelligently thinned and compressed on the AUV before it is acoustically transmitted to the vessel.

Real time subbottom display

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Fig. 18. Real time subbottom display

As see in the figure, the acoustic layers are readily recognized while the well is shown on the right.

Because the acoustic link is limited in its bandwidth,C & C engineers developed a technique by which the user can adjust how much of the link is dedicated to each of the three primary survey sensors. For example, if there is more interest in an expected subbottom feature, more of the bandwidth can be dedicated to the subbottom, thus providing a higher rate of subbottom updates at the expense of the other decimated data.

One of the advantages of the real time transmission of sub sampled data is to confirm the correct operation of the sensors. Based on this feedback, realtime adjustments can be made such as changing the swath width, AUV height, or pulse length. Finally, if the ocean bottom terrain is not conducive to development, you can abort the current line and reprogram the AUV in real time to survey a different area.

7. POST PROCESSED AUV DATA

The AUV uses two 50 gigabyte hard drives to record the sensor and positioning data. Once the AUV is back on board the survey ship, about once every two days, the raw data is downloaded over a 100base T Ethernet. The full data sets are then used to produce finished survey charts and reports while onboard the survey ship. A high speed Inmarsat terminal is used to transmit these finished products to C & C’s office.It is then placed on a secure website for client’s immediate access.

Post processed multibeam bathymetry

Fig. 19. Post processed multibeam bathymetry

The post processed multibeam imagery shown in figure 19 is color-coded at one meter contour intervals. It is also sun shaded so that any relief (real or anomalous) is more easily detectable. The artefacts seen in the real time data are still apparent.The depth resolution achieved by the AUV was on the order of 20cm (8 in) or 0.02% of the water depth.

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Post processed side scan data

Fig. 20. Post processed side scan data

Post processed side scan data is shown in figure 20.The data in this example shows slight faulting. The side scan data can be mosaicked if required. It can also be compared to the multibeam imagery that, because of the processing technique, offers better positioned data than the side scan.

Processed subbottom profiler data

Fig. 21. Processed subbottom profiler data

The subbottom profiler data shown in figure 21 is processed and saved in SEG Y and XTF. In the upper portion of the figure, a pair of significant faults is shown. The lower part of the figure shows some unusual uplifted lenses.

In summary, the post processed data from the seat rials looked very good with the exception of the slight interference causing artefacts in the multibeam data. As stated, this error has been resolved since this test was conducted.

8. AUV LAUNCH & RETRIEVAL

One of the attractions of the Hugin 3000 AUV is the proven launch and retrieval system. The launch and recovery system is built into a standard 20 foot shipping container that can be secured to the stern of the survey ship. The launch sequence is depicted in figure 22.

Hugin AUV being launched

Fig. 22. Hugin AUV being launched

To retrieve the system, the AUV is brought to the surface and commanded to release its positively buoyant nose cone. A ten-meter floating rope between the AUV and the nose cone then becomes the target of a grapnel tied to the retrieval winch.

AUV retrieval harpoon

DEEP WATER AUV EXPERIENCES

Fig. 23. AUV retrieval harpoon

A harpoon gun (figure 23) is used to fire the grapnel over the floating AUV rope and retrieve the AUV.This procedure works well in up to 5 meter seas and prevents any personnel from leaving the ship.

Hugin AUV 3000 retrieval

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Fig. 24. Hugin AUV 3000 retrieval

The retrieval sequence is shown in figure 24. This technique was used on Hugin I and Hugin II in theNorth Sea.

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9. AUV COST SAVINGS

While an AUV is far more expensive from a capital standpoint than a deep towed sonar system, its speed and accuracy allow it to be more cost effective. For rectangular survey areas, an AUV survey will be about 10 percent less expensive and will require 50percent of the time needed for a deep tow survey.

For linear surveys, such as pipelines and cable routes, the cost will be about 85 percent of a comparable deep tow survey.Generally, in depths greater than about 800m of water, the AUV becomes more cost effective than a deep towed system. In fact, the deeper the water, the greater the cost and time savings tend to be. Also, the greater the number of survey line turns, the more cost and time effective the AUV becomes.

Many suggest that the capital costs of AUVs will drop significantly. However, the cost of the on board sensors and system advances will probably be high for several more years. For example, a high end PC is just as expensive now than it was ten years ago; however they are more capable. AUVs will likely follow that same path.

10. CONCLUSIONS

C & C Technologies’ Hugin 3000 AUV is the world’s first commercially operated AUV with capability to 3000m. It has a state-of-the-art multibeam bathymetry and imagery system, a dual frequency chirp side scan sonar, and a chirp sub-bottom profiler. AUV survey services are already being provided by C & C with world wide availability.

The Hugin AUV has several advantages including faster survey speeds, faster line turns, better ability to stay on line to avoid data holidays, and the ability to work in congested areas. These advantages have been demonstrated at sea by C & C.

AUV position determination from the integration ofUSBL, inertial navigation, and DVL has shown to be robust and very accurate. This capability has also been demonstrated at sea by C & C.

Simultaneous real time monitoring of AUV multibeam, side scan sonar, and subbottom profiler data has been proven and is very beneficial from a quality assurance standpoint. The raw data, downloaded from the AUV, has shown to be excellent.

The launch and retrieval system has been well proven thanks to plenty of debugging during the AUV development! It is safe and robust.

Finally, while the capital cost of an AUV is greater than that of a deep tow, the efficiency of the AUV allows it to be far more cost and time effective. The deeper the water and the greater the number of survey lines, the more cost effective the AUV will be.



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