Subsea Metrology and Positioning

Metrology is defined by the International Bureau of Weights and Measures (IBWM) as “the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology.” This article describes the subsea positioning systems, which are integrated with the main survey computer in order to provide accurate and reliable absolute positioning of the surface and subsurface equipment.

Transducers

A transducer is a device for transforming one type of wave, motion, signal, excitation, or oscillation into another. Transducers are installed on board the vessels accordingly. A plan is created for the locations of all acoustic transducers and their coordinates that refers to a fixed reference point. A high quality motion sensor (motion reference unit) is used to compensate for transducer movement.

Calibration

Calibration is the process of comparing a measuring instrument with a measurement standard to establish the relationship between the values indicated by the instrument and those of the standard. Calibration of the positioning systems, including all spare equipment, is carried out to ensure that each piece of individual equipment is working properly. Field calibration is performed during both prefield and postfield work. Depending on the length of the field work, additional field calibrations may be required during the course of the work. The following general procedures and requirements are adopted during the calibration process:

• No uncalibrated equipment, including cables and printed circuit boards, is used during any part of the position fixing.
• Each calibration setup should last for at least 20 min and the resulting data is logged for processing and reporting.
• The results of the calibrations, including relevant information on equipment settings, are presented for review and acceptance. The report

includes measured minimum, maximum, mean, and standard deviations for each measurement and recommendations for operating figures. Any odd figures, anomalies, or apparent erroneous measurements are highlighted and explained in the report. If, after further examination of results, doubt still exists as to the integrity of any equipment, the faulty equipment is replaced with similarly calibrated equipment prior to the start of the survey.

• In the event that positioning equipment must be repaired or circuit boards changed, and if such actions alter the position information,

recalibration is performed.

Water Column Parameter

A water column is a conceptual column of water from surface to bottom sediments. The application of the correct speed of sound through seawater is critical to the accuracy of the acoustic positioning. Sound velocity is a function of temperature, salinity, and density. All three properties change randomly and periodically; therefore, regular measurements for velocity changes are required. A salinity, temperature, and depth profiler is used to determine the propagation velocity of sound through seawater. The computed sound velocity value or profile is then entered into the appropriate acoustic system. All procedures need to be properly followed and the results applied correctly.

Field Procedure

A velocity value or profile is obtained at the beginning of the survey and thereafter. Observations are made at suitable depth or intervals during descent and ascent through the water column. The velocity profile is determined with the value at common depth agreed to within 3 m/sec; otherwise, the observation has to be repeated. The sound velocity at the sea bottom level is determined within 1.5 m/sec. Having observed and recorded these values, a computation of the speed of sound is made.

Calibration

The temperature/salinity/depth probe has a calibration certificate verifying that it has been checked against an industrial standard thermometer in addition to testing against a calibrated saline solution. A strain gauge pressure sensor certificate is supplied.

Acoustic Long Baseline

Acoustic long baseline (LBL), also called range/range acoustic, navigation provides accurate position fixing over a wide area by ranging from a vessel, towed sensor, or mobile target to three or more transponders deployed at known positions on the seabed or on a structure. The line joining a pair of transponders is called a baseline. Baseline length varies with the water depth, seabed topography, and acoustic frequency band being used, from more than 5000 m to less than 100 m. The LBL method provides accurate local control and high position repeatability, independent of water depth. With the range redundancy that results from three or more range measurements, it is also possible to make an estimation of the accuracy of each position fix. These factors are the principal reasons for a major increase in the utilization of this method, particularly for installation position monitoring. LBL calibration and performance can be improved significantly by using “intelligent” transponders. These devices calibrate arrays by making direct measurements of the baselines and acoustically telemetering the data to the surface equipment for computation and display. They also reduce errors inherent in the conventional LBL due to ray-bending effects, as measurements are made close to the seabed where propagation changes are generally slight. In addition, they can be supplied with environmental sensors to monitor the propagation conditions.

Field Procedure

The system is operated by personnel who have documented experience with LBL operations, to the highest professional standards and manufacturers’ recommendations. Local seabed acoustic arrays consist of networks with at least six LBL transponders. Ultra-high-frequency (UHF) arrays are used for the installations with the highest requirements for accurate installations. The system includes:

• A programmable acoustic navigator (PAN) unit for interrogation of LBL;
• Transponders;
• A transducer for vessel installation;
• All necessary cables and spare parts.

All equipment is interfaced to the online computer. The online computer system is able to handle the LBL readings without degrading other computation tasks. The software routines allow for the efficient and accurate use of all LBL observations and in particular deal with the problems encountered in surveying with a LBL system. The vessel’s LBL transducer is rigidly mounted. The operating frequency for the required mediumfrequency (MF) system is typically 19 to 36 kHz. The operating frequency for the required UHF system is typically 50 to 110 kHz. A minimum of five lines of position for each fix are available at all times.

MF/UHF LBL Transponder

The latest generation LBL transponder has the following minimum requirements (data presented as MF/UHF):

• Transducer beam shape: hemispherical/hemispherical;
• Frequency range: 19 to 36 kHz/50 to 110 kHz;
• Acoustic sensitivity: 90 dB re 1 mPa/90 to 125 dB;
• Acoustic output: 192 dB re 1 mPa at 1 m/190 dB;
• Pulse Length: 4 ms/1 ms;
• Timing resolution: 1.6 msec/8.14 msec;
• Depth rating: according to project requirements.

At least two of the transponders in each array include depth, temperature, and conductivity options. The MF transponder includes anchor weights (minimum of 80 kg) attached to the release mechanism on the base of the unit by a strop (preferably nylon to avoid corrosion) 1.5 to 2 m in length. A synthetic foam collar is used for buoyancy. The UHF array setup includes frames with transponders rigidly installed in, for example, baskets 2.0 to 2.5 m above the seabed. The deployment is performed following a special procedure in accordance with the sea bottom depth. The setup is visually inspected by an ROV after installation. Concrete reference blocks or other transponder stands for MF arrays may be required.

Acoustic Short Baseline

A short baseline (SBL) acoustic positioning system [7] is one of the three broad classes of underwater acoustic positioning systems that are used to track underwater vehicles and divers. The other two classes are USBL and LBL systems. Like USBL systems, SBL systems do not require any seafloormounted transponders or equipment and are thus suitable for tracking underwater targets from boats or ships that are either anchored or under way. However, unlike USBL systems, which offer a fixed accuracy, SBL positioning accuracy improves with transducer spacing [8]. Thus, where space permits, such as when operating from larger vessels or a dock, the SBL system can achieve a precision and position robustness that is similar to that of seafloor-mounted LBL systems, making the system suitable for highaccuracy survey work. When operating from a smaller vessel where transducer spacing is limited (i.e., when the baseline is short), the SBL system will exhibit reduced precision.

Ultra-Short Baseline

A complete USBL system consists of a transceiver, which is mounted on a pole under a ship, and a transponder/responder on the seafloor or on a tow-fish or ROV. A computer, or “topside unit,” is used to calculate a position from the ranges and bearings measured by the transceiver. An acoustic pulse is transmitted by the transceiver and detected by the subsea transponder, which replies with its own acoustic pulse. This return pulse is detected by the shipboard transceiver. The time from the transmission of the initial acoustic pulse until the reply is detected is measured by the USBL system and converted into a range. To calculate a subsea position, the USBL calculates both a range and an angle from the transceiver to the subsea beacon. Angles are measured by the transceiver, which contains an array of transducers. The transceiver head normally contains three or more transducers separated by a baseline of 10 cm or less. A method called phase differencing within this transducer array is used to calculate the angle to the subsea transponder.

Description

SBL systems conventionally replace the large baselines formed between transponders deployed on the seabed with baselines formed between reference points on the hull of a surface vessel. The three or four reference points are marked by hydrophones, which are typically separated by distances of 10 to 50 m and connected to a central control unit. Seabed locations or mobile targets are marked by acoustic beacons whose transmissions are received by the SBL hydrophones. It is more convenient than the LBL method because multiple transponder arrays and their calibration are not required; however, position accuracy is lower than the LBL method and decreases in deeper water or as the horizontal offset to a beacon increases. Additional factors such as vessel heading errors and roll and pitch errors are significant in the accuracy measurements. In the USBL, the multiple separate SBL hull hydrophones are replaced by a single complex hydrophone that uses phase comparison techniques to measure the angle of arrival of an acoustic signal in both the horizontal and vertical planes. Thus, a single beacon may be fixed by measuring its range and bearing relative to the vessel. Although more convenient to install, the USBL transducer requires careful adjustment and calibration.

Field Procedure

A USBL system, with tracking and the latest generation fixed narrow transducer, can be used. A high-precision acoustic positioning (HIPAP) or similar system can also be used. This subsurface positioning system is integrated with the online computer system to provide an accurate and reliable absolute position for the transponders and responders. All necessary equipment is supplied so that a fully operational USBL system can be interfaced to an online computer for integration with the surface positioning systems. It must also meet the operational requirements set forth in this section. The installation of equipment should comply with manufacturers’ requirements, and special attention should be given to the following requirements:

• A system check is performed within the last 12 months prior to fieldwork. Documentation must be submitted for review.
• The installation and calibration of the acoustic positioning system should provide accuracy better than 1% of the slant range.
• The hull-mounted USBL transducer should be located so as to minimize disturbances from thrusters and machinery noise and/or air bubbles in

the transmission channel or other acoustic transmitters.

• The USBL equipment is supplied with its own computer and display unit, capable of operating as a stand-alone system.
• The vertical reference unit (VRU) is fabricated based on recommendations by the USBL manufacturer and installed as recommended.
• The system is capable of positioning at least nine transponders and/or responders.

Calibration of the USBL System

Calibration and testing of the USBL and VRU should be performed according to the latest revision of the manufacturers’ procedures. If any main component in the USBL system has to be replaced, a complete installation survey/calibration of the system must be performed. In the USBL, the multiple separate SBL hull hydrophones are replaced by a single complex hydrophone that uses phase comparison techniques to measure the angle of arrival of an acoustic signal in both the horizontal and vertical planes. Thus, a single beacon may be fixed by measuring its range and bearing relative to the vessel.

Although more convenient to install, the USBL transducer requires careful adjustment and calibration. A compass reference is required and the bearing measurements must be compensated for the roll and pitch of the vessel. Unlike the LBL method, there is no redundant information from which to estimate position accuracy.

References

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[7] P.H. Milne, Underwater Acoustic Positioning Systems, Gulf Publishing, Houston, 1983.

[8] R.D. Christ, R.L. Wernli, The ROV Manual, Advantages and Disadvantages of Positioning Systems, Butterworth-Heinemann 2007.

[9] Fugro Engineers B.V., Specification of Piezo-Cone Penetrometer, <http://www.fugro-singapore.com.sg>.

[10] M. Faulk, FMC ManTIS (Manifolds & Tie-in Systems), SUT Subsea Awareness Course, Houston, 2008.

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