- Hydroacoustic Position Reference System consists of transducer(s) onboard a vessel communicating with transponder(s) placed on the seabed. The transducers are lowered beneath the hull, and when a transponder is deployed on the seabed, the transducers can start interrogating. The transponder is listening for the interrogation and it answers with its own frequency. The transducer receives the answer and is able to determine range and direction to the transponder. This defines the vessel position with reference to the seabed transponder and this information is fed to the dynamic positioning system. Jan. 2010 Kongsberg Maritime AS Page 6.3.1 Rev. 07 Training
Sound in Water Various physical laws influence the signals travelling through water. The strength of the signal, the direction from where it comes, and noise conditions are examples. The speed of sound in sea water is approximately 1500 m/s. The sound waves will decrease in power when they travel through water and the path from the surface to the seabed will depend on salinity and temperature layers. Page 6.3.2 Kongsberg Maritime AS Jan. 2010 Training Rev. 07
Ray bending When the velocity increases from the surface to the bottom (higher salinity and/or temperature), the signal path will be bent up. When the velocity decreases from the surface to the bottom (lower salinity and/or temperature), the signal path will be bent down. Lower velocity Higher velocity Higher velocity Lower Higher velocity velocity Without information about sound velocity in the system, the transponder position will be calculated along the dotted lines. Sound profile Below is an example of a profile. On the picture to the left you can see the profile. The velocity increases quickly from the surface down to 20-30 m. This sudden change is also found on the ray trace picture to the right. SIMRAD 418 1482.19 941028 11:02:14 CALC, Oslofjorden 26 okt MEAN : 1505 1470 SOUND PROFILE VALUE ENTER POSITION 0 SYSTEM SOUND VELOCITY SOUND DATA : PR OFILE DISPLAY DATA COM 2 RX ERROR 32 SIMRAD 418 CALC, Oslofjorden 26 okt 941028 11 :03:54 700 0 RAY DIAGRAM VALUE ENTER POSITION 0 SYSTEM SOUND VELOCITY SOUND DATA PROFILE : DISPLAY DATA 100 DRAW ING PR OFILE TRANSD. DEPTH : 5.0m SOUND V. LOW : 147 0m /s SOUND V. HIGH : 150 5m /s UPPER DEPTH : 0m LOWER DEPTH : 200m RANGE RAY DIAG: 800m RAY START : 3 RAY STOP : 89 RAY STEP : 3 50 DRAW ING PROFILE TRANSD. DEP TH : 5.0m SOUND V. LOW : 1470m /s SOUND V. HIGH : 1500m /s UPPER DEPTH : 0m LOWER DEPTH : 100m RANGE RAY DIAG: 700m RAY START : 60 RAY STOP : 89 RAY STEP : 1 200 100 Off line Nc Nc Nc Off line Nc Nc Nc (CD3175) The ray trace tells us it is impossible to have any direct contact with a transponder at, for example, 700 m range and 40 m depth, since all the rays are bent up to the surface or down to the seabed. If you received a reply from a transponder using the above example, it would be a signal bounce, where the pulse would be going between the seabed and surface one or more times. The mean sound velocity is used to calculate the range, while the transducer sound velocity is used to calculate the angles to the transponder. Jan. 2010 Kongsberg Maritime AS Page 6.3.3 Rev. 07 Training
Signal Loss The signal strength is reduced as a function of distance and frequency. db Transmission loss 120 115 110 105 100 95 90 85 80 75 70 65 60 55 50 45 40 100 400 Transmission loss in db Alpha 1.5 db/km - 13kHz, 5dB - 25kHz, 8dB/km - 32kHz 700 1000 1300 1600 1900 2200 2500 Range 2800 3100 3400 3700 4000 4300 4600 4900 TL 13 khz TL 25 khz TL 32 khz From the table can be seen that low frequencies have less loss than higher, and should work at longer range. The size of the transducer is determined by the frequency. To have low frequency transducers with the same requirements as medium frequency transducers regarding opening angles, requires bigger physical size. Noise The factor that causes most problems is noise. Noise can be generated from the vessel itself (motors, thrusters etc.), from neighbouring installations or vessels, ROVs or from the waves. SPECTRUM LEVEL (DB RE 1UPA) 140 120 THRUSTER NOISE 100 DRILLING NOISE 80 SHIP NOISE WIND FORCE BEAUFORT 7 60 40 4 2 0 20 (CD3942) 1 10 100 1000 10.000 100.000 FREQUENCY HZ ENVIRONMENTAL ACOUSTIC NOISE LEVEL The diagram shows different types of noise with frequency and noise level. Page 6.3.4 Kongsberg Maritime AS Jan. 2010 Training Rev. 07
The curves indicate clearly that thruster noise is far the strongest. Thrusters generate noise, but they can also make air bubbles in the water, and if these come between the transducer and the transponder, the signal can be blocked. Going astern with the vessel using the main propellers normally pulls a lot of air under the hull, this might cause signal blockage similar to that caused by the thrusters. Operational Principles - LBL / SBL / SSBL Jan. 2010 Kongsberg Maritime AS Page 6.3.5 Rev. 07 Training
SSBL Principle (Super Short Base Line) When using the SSBL principle, the distances between the elements inside the transducer (base lines) are used to calculate the transponder position. The position calculation is based on distance and direction measurements to one transponder. An interrogation pulse is transmitted from the onboard transducer, which will interrogate the subsea transponder, which again will answer with a reply pulse. If the transponder on the seabed is slightly out of vertical line, there will be a small time difference from one element to the other when the pulse hits the surface of the transducer. This time difference is extremely small and hard to measure by a clock. Instead the system measures the phase difference when the signal hits for instance the Ref. and Y element in the drawing below. When we know the wavelength, and the phase difference, the time difference can be easily calculated. With this information we can calculate the angle of the reply pulse and determine from which direction the pulse is coming. When we know the speed of sound in water we can find the distance. Angle measurement Computation of position in the forward/aft direction Computation of position in two planes Page 6.3.6 Kongsberg Maritime AS Jan. 2010 Training Rev. 07
LBL Principle (Long Base Line) The LBL system consists of one transducer and an array of transponders, where the exact distance between each transponder is known. The base lines are no longer the elements inside the transducer, but the distance between the transponders. For SSBL the base lines are less than 10 cm, while for LBL the base lines can be more than 1000 m. The distances between the transponders are calibrated, and LBL is therefore more timeconsuming to set up than SSBL. The calibration is done using a built-in mode in the transponders. All the transponders will be interrogated simultaneously, and they will respond with their individual replies. The LBL system will calculate the ranges from the individual transponders, and by using the base lengths of the calibrated transponder array together with these ranges in software algorithms, the vessel can be positioned. The advantage of LBL systems over SSBL systems is that accuracy is maintained down to decimetre level, even if the ranges are several hundred meters. The transducer might be an SSBL type, or it can be a special one with only one element, since angular measurements are not used. LBL requires intelligent transponders that can be commanded to execute different operations using telemetry. Jan. 2010 Kongsberg Maritime AS Page 6.3.7 Rev. 07 Training
Transducers and transducer elements A transducer is an acoustic transmitter/receiver normally placed onboard the vessel, approximately 3 m below the keel. Mounted on a pole which is remotely operated from the bridge, it can be lowered or recovered whenever necessary. A transducer consists of several transducer elements. Electric connection Steel block Electric connection 2 blocks of ceramic crystals Magnesium block Rubber cover Transducer face The elements will start vibrating when voltage is applied, transmitting sound waves with correct frequency. When the pulses from the transponder are received, the elements start generating voltage. The internal distances between these elements are known. Page 6.3.8 Kongsberg Maritime AS Jan. 2010 Training Rev. 07
Typical System Overview - HiPAP transducer There are different systems and transducers on the market. Kongsberg Maritime is well known for the efficient HiPAP transducer, (High Precision Acoustic Positioning). The HiPAP hull unit uses the same relay unit to lower and hoist as the two other hull units. The transceiver is installed close to the hull unit, since the cable from the hull unit to the transceiver is only 5 m. Position data is sent from the operator unit to other equipment. The digital interface can be RS232, RS422, current All interfaces to the transceiver (gyro and VRU) are in serial format (RS422) loop or ethernet. Jan. 2010 Kongsberg Maritime AS Page 6.3.9 Rev. 07 Training
Transponders A transponder is an acoustic receiver/transmitter placed on the seabed or onboard an ROV or any other structure to be positioned. It is triggered from the vessel using acoustic signals, and will in normal operation only answer if it is interrogated from the vessel. The power source is normally a battery, the lifetime of which depends on how often the transponder is interrogated and what kind of battery type is used. The size and weight of the transponders are determined by the depth specification and battery lifetime. Page 6.3.10 Kongsberg Maritime AS Jan. 2010 Training Rev. 07
RPT type For 1000 m depth rated transponders an aluminium housing is chosen. The RPT type is a combined transponder/responder, which can use an external DC power source in addition to internal battery. For 3000 m depth rated transponders a stainless steel housing is chosen Transponder Deployment When deploying the transponder it is important to prevent the air produced by main propellers, thrusters, diving bell, etc. from obstructing the path of communication between the transponder and the transducer. When the system is part of a dynamic positioning system the current and wind direction must be considered before deploying the transponder. The transponder must be deployed in a position where the current carries the air from a diving bell or other air producing equipment away from the operating area. Jan. 2010 Kongsberg Maritime AS Page 6.3.11 Rev. 07 Training
The transponders might be deployed with a rope or a wire going to a buoy or the vessel on the surface, or they might be "thrown" over the side of the vessel if they have an acoustic release mechanism. The length of the rope between the transponder base and the weight can be 2-5 m. The recommended weight of the sinker is different for 1000 m and 3000 m transponders. For 1000m transponders we recommend a weight of approx. 60 kg. For 3000m transponders we recommend 100 kg. Keep in mind the current when transponders are deployed. The weight might be increased if the current is strong, it is most important to get the transponder in the exact predetermined position. REMEMBER: Make sure the weight of the transponder and the sinker is brought up in the sinker and NOT in the protective cage on the transponder whenever the transponder is handled. The cage is for protection of the transducer, and is certified for lifting the transponder with flotation collar only. Page 6.3.12 Kongsberg Maritime AS Jan. 2010 Training Rev. 07
Transponder models We have three main groups of transponders: - MPT Multifunction Positioning Transponder - SPT SSBL Positioning Transponder - RPT ROV Positioning Transponder MPT transponders are used in: SPT transponders are used in: RPT transponders are used in: LBL positioning Array calibration SSBL positioning General telemetry commands *) SSBL positioning General telemetry commands *) ROV positioning Tow-fish positioning (The RPT does not have the telemetry option) The transponder model name gives the user information about operating frequency, depth rating, transducer beam width and any option. The transponder name is put together like this: Model name: Transponder name = model name + model number + options - MPT = Multifunction Positioning Transponder - SPT = SSBL Positioning Transponder - RPT = ROV Positioning Transponder Model number: 1. digit 2. digit 3. digit 1=15 khz (low frequency) 1=1000 metre depth 1= ±15 beam width 3=30 khz (medium frequency) 2=2000 metre depth 3= ±30 beam width 3=3000 metre depth 4= ±45 beam width 6= ±60 beam width 9= ±90 beam width Some of the options available: - D = Depth sensor - H = Heading magnetic compass - E = External power - I = Inclinometer - II = Internal and external inclinometers, (diff.inclo. TP) - N = Rechargeable NiCAD or seal lead battery pack - R = Release mechanism - S = Split, separate transducer and housing - T = Temperature sensor - Rsp = Responder - DuB = Dual Beam Jan. 2010 Kongsberg Maritime AS Page 6.3.13 Rev. 07 Training
400 Mode 400 transponders are used by the Kongsberg 400 series, as well as the HiPAP sytem. TP Channel 1 st Interrogation frequency 2 nd Interrogation frequency Reply frequency B12 21000 21500 29250 B13 21000 22000 29750 B14 21000 22500 30250 B15 21000 23000 30750 B16 21000 23500 27250 B17 21000 24000 27750 B18 21000 24500 28250 B19 B21 21500 21000 28500 B22 B23 21500 22000 29500 B24 21500 22500 30000 B25 21500 23000 30500 B26 21500 23500 27000 B27 21500 24000 27500 B28 21500 24500 28000 B29 B31 22000 21000 28750 B32 22000 21500 29250 B33 B34 22000 22500 30250 B35 22000 23000 30750 B36 22000 23500 27250 B37 22000 24000 27750 B38 22000 24500 28250 B39 B41 22500 21000 28500 B42 22500 21500 29000 B43 22500 22000 29500 B44 B45 22500 23000 30500 B46 22500 23500 27000 B47 22500 24000 27500 B48 22500 24500 28000 B49 TP Channel 1 st Interrogation frequency 2 nd Interrogation frequency Reply frequency B51 23000 21000 28750 B52 23000 21500 29250 B53 23000 22000 29750 B54 23000 22500 30250 B55 B56 23000 23500 27250 B57 23000 24000 27750 B58 23000 24500 28250 B59 B61 23500 21000 28500 B62 23500 21500 29000 B63 23500 22000 29500 B64 23500 22500 30000 B65 23500 23000 30500 B66 B67 23500 24000 27500 B68 23500 24500 28000 B69 B71 24000 21000 28750 B72 24000 21500 29250 B73 24000 22000 29750 B74 24000 22500 30250 B75 24000 23000 30750 B76 24000 23500 27250 B77 B78 24000 24500 28250 B79 B81 24500 21000 28500 B82 24500 21500 29000 B83 24500 22000 29500 B84 24500 22500 30000 B85 24500 23000 30500 B86 24500 23500 27000 B87 24500 24000 27500 Page 6.3.14 Kongsberg Maritime AS Jan. 2010 Training Rev. 07