WORLD CLASS through people, technology and dedication WORLD CLASS through people, technology and dedication

Transcription

1 WORLD CLASS through people, technology and dedication WORLD CLASS through people, technology and dedication 1

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3 Acoustic Positioning Systems Hydrofest Aberdeen University Wednesday 31 st March Ian Florence Subsea Construction Manager Kongsberg Maritime Ltd. 3

4 Acoustics are not a Black Art Most people do not understand acoustics so think they need people with special skills and of course chicken bones to do acoustic jobs Most acoustic jobs go wrong because they are specified with the wrong equipment for the task in hand. 4

5 CRITICAL TECHNOLOGY IN ALL ASPECTS OF OFFSHORE 5

6 POSITIONING SYSTEM OVERVIEW Long Base Line (LBL) Range measurements to subsea transponders Short Base Line (SBL) Range measurements from several transducers to a subsea transponder Super Short or Ultra Short (SSBL or USBL) Range and angle measurements to a subsea transponder Integrated Inertial Positioning Kongsberg Hydroacoustic Aided Inertial Navigation (Hain) Fugro FASTRACK 6

7 Accuracy & Precision Accuracy is probably one of the most misused words in the offshore poisonings industry We must look at the difference between accuracy and precision Also what inherent weaknesses and strengths each system has 7

8 "If you have only one watch, you always know exactly what time it is. If you have two watches, you are never quite sure..." 8

9 Precision Vs Accuracy 9

10 Error Budgets Each position system is made up of various components and processes Each will have an inherent error Each will have a systematic and random error Sometimes called Coloured and white noise respectively 10

11 Typical Error Budget 11

12 Courtesy of Subsea7 Acoustic Positioning 12

13 ACCURACY Each method has its own accuracy We need to look at this word and define it more Absolute accuracy Relative accuracy Repeatable accuracy 13

14 ABSOLUTE ACCURACY This is when we need a subsea position with real world coordinates (e.g. latitude/longitude) For example if the subsea position has to be related to an UKOA block boundary or other subsea objects Usually consists of tying in a DGPS (Differential GPS) and subsea positioning system You Are Here! Lat: N Long: W 14

15 RELATIVE ACCURACY Usually used when you are not interested in where in the world you are If you want to position a structure say from an existing wellhead for example 15

16 REPEATABLE ACCURACY This is when you need to place something and come back to it Say you wet store something and you have to return to it on a regular basis 16

17 CONSIDER THE ACCURACY REQUIREMENTS Each position method will have an achievable accuracy Each method will equate to an amount of equipment and more importantly an amount of vessel time It is important that the accuracy requirements are not over-specified Generally if you double the accuracy the cost to achieve that accuracy will increase four fold Even more important will the increased accuracy cost vessel time? 17

18 USBL/SSBL TECHNIQUES The signal is sent to the transponder and turned around so the range is a simple V=m/s equation. The phase is detected by the wave front hitting element 1 first and at a T later hitting the reference element. This is done on the fore/aft and port/starboard axis. T x C Φ Baseline Range =( T x C)/2 The baseline is the distance between the elements (super short) 18

19 USBL/SSBL TECHNIQUES Many companies manufacture USBL systems but they can not change the laws of physics All USBL system work on this basic range / phase measurement principle 19

20 USBL/SSBL TECHNIQUES Advantages Easy to calibrate Most vessel are equipped with systems Require only one transponder but can track multiple transponders Disadvantages As the depth (or distance to target) increases the accuracy decreases Can be prone to systematic errors Requires Vertical reference unit and gyro compass Deep water high accuracy = high$ 20

21 LONG BASELINE TECHNIQUES Long baseline acoustic positioning requires an array of seabed transponders 21

22 LONG BASELINE TECHNIQUES Advantages: Used for higher accuracy and redundancy Decouples surface navigation Mitigate the effects of gyro and VRU error Survey relative to known points on the seabed Independent of the depth of water Mitigates the problem with speed of sound 22

23 LONG BASELINE TECHNIQUES Disadvantages Requires the deployment of four or more transponders (costly in deepwater) Require lengthy calibration Usually arrays are job specific and require recovery at the end of the project Usually only one user for each array 23

24 INTEGRATED INERTIAL SYSTEMS Inertial systems are used to smooth the inherent white noise inaccuracies of USBL or LBL systems 24

25 INTEGRATED INERTIAL SYSTEMS Advantages Used to augment USBL can avoid deploying an LBL array Can be post processed to increase accuracy (precision) Easy to use once set up 25

26 INTEGRATED INERTIAL SYSTEMS Disadvantages: Inertial units are expensive ($120,000+) Restricted exportability Needs high-accuracy heading sensor for the vessel Set up can be complicated Pre-operation vessel calibration required 26

27 INS is on the increase! INS systems have now become accepted Kongsberg are still the leaders but a sign of success is companies manufacturing components and packages to fill this increasing market demand 27

28 MULTI-USER LBL 28

29 Nasnet A digitally encoded system allowing multiple users Long range requiring minimum assets to be deployed Single seabed reference frame Highly robust acoustic signalling to mitigate noisy vessels and ROVs Long duration deployment A simple to use system Expandable system architecture providing multiple capabilities 29

30 Legacy Tone-Based Technology Tone Signal single frequency, no modulation Each channel has its own individual frequency With separation guard bands between adjacent channels Hence limited number of channels active at any one time 30

31 Correlation Processing - Tone Burst Incoming Signal Stored Replica Reasonable Timing Detection Threshold Correlation Response 31

32 Wideband Signal Unique digital code phase modulated onto the frequency carrier Many unique codes possible on a single frequency carrier Also known as Phase Shift Keying (PSK) 32

33 Wideband Range Correlation Processing Incoming Signal Stored Replica Perfect Match other codes rejected Large Peak Very Accurate Timing Validation Threshold Correlation Response 33

34 More Wideband Wideband and DSP (Digital Signal Processing) is making real advances in navigation telemetry and data logging This will only increase the accuracy that can be achieved in even deeper water. Problem is the client will always want more preferably cheaper 34

35 WIDEBAND (SPREAD SPECTRUM) Allows more channels Faster calibration time Allows operations in a noisier environment Give greater range accuracies Allows high-accuracy measurements (e.g. jumper measurements) from standard transponder array 35

36 Too Many Choices? Read the procedure Some poor project surveyor and bidding department have spent months working out all this for you. They spent a long time getting the procedure accepted by the client take the time to read it and more importantly understand it. If you don t Know ASK 36

37 WORLD CLASS through people, technology and dedication WORLD CLASS through people, technology and dedication 37