The practical differences between the acoustic environment in the North Sea, the Atlantic Ocean and the Caspian Sea. Dave Philip BP Exploration

Transcription

1 The practical differences between the acoustic environment in the North Sea, the Atlantic Ocean and the Caspian Sea Dave Philip BP Exploration

2 Introduction Offshore Surveyors Aims Make all my errors small and random Needs Sufficient redundancy Good geometry, method, and quality of measurements Awareness of possible errors Knowledge of achievable accuracies and precisions KNOWLEDGE OF THE ACOUSTIC ENVIRONMENT

3 ACOUSTIC ARRAY CALIBRATION FUNDAMENTALS Sound Velocity of seawater Empirical Formulae using CTD information or Direct reading velocimeter or both? Position and orientation Box-in s (3 minimim) Surface positioning accuracy transferred to seabed Baseline measurements Acoustic Ranges of high precision Array adjustment Combining observations of varying precisions without distortion

4 Acoustic Sound velocity Significance of the variables Temperature - 1 Degree C = 3 m/s Salinity - 1 ppt = 1 m/s Pressure - 10 m = 0.17 m/s Empirical nature of Formulae Evaluation of available formulae Selection of most appropriate Examples of seasonal and localised variations North Sea West of Shetland Caspian Sea or Lake?

5 Oceanographic Questions Do Offshore Surveyors have sufficient oceanographic awareness? Do we use and research the readily available Oceanographic Databases? Do we brief Field Personnel on what conditions to to expect? Are we ever surprised?

6 SALINITY The original definition of salinity was based on the chlorinity of seawater and required careful chemical analysis. There is an almost constant ratio of the salt constituents with chlorine in ocean seawater. A practical salinity scale was agreed in 1978 which is based on the electrical conductivity of seawater compared to a standard solution of KCl at a standard temperature (15 Centigrade) Effectively we have a Standard Ocean which we measure with standardised instrumentation. The empirical velocity formulae use this standard. 6

7 The STANDARD OCEAN Globally very well mixed 7

8 COMPOSITION of STANDARD OCEAN 8

9 Ion composition of Standard Seawater Ions g/kg % Chlorine Cl % Sodium Na % Sulphates SO % Magnesium Mg % Calcium Ca % Potassium K % Bicarbonates HCO % Bromine Br % Boric Acid H3BO % Strontium Sr % Fluorine F % 9

10 Surface Salinity Averages Acknowledgement World Ocean Atlas 2001 Figures 10

11 Surface Temperature Averages Acknowledgement World Ocean Atlas 2001 Figures 11

12 West of Shetland Part of Atlantic Ocean Acknowledgement BP Metoc Briefing Note No

13 Main currents entering the Faroe-Shetland Channel (acknowledgement to Orvik and Niiler) 13

14 North Sea - Seasonal Sea Environment SALINITY TEMPERATURE VELOCITY 0.0 Parts per thousand Degrees Centigrade Metres per second January February March April D E P T H m e t r e s D E P T H - m e t r e s D e p t h May June July August Sept ember October November December 14

15 West of Shetland - Ocean Environment SALINITY TEMPERATURE Foinaven and Schiehallion operating depths

16 ENVIRONMENTAL ASSESSMENT of New Area CASPIAN SEA NON standard seawater Unique salt ion composition CTD Standard Ocean in error Density Computations Standard Ocean in error Seasonal variability Areal and vertical mixing References The density of Caspian Sea waters by Frank J Millero and Peter V Chetirkin. Deep-Sea Research Vol 27A pp 265 to Analysis of deep-water exchange in the Caspian Sea based on environmental tracers by F.Peeters et al. Deep-Sea Research Vol 1 47 pp

17 Comparison of Average Ion Composition in the World Oceans versus the Caspian Sea Ions World Ocean (Lyman, Fleming, 1940) Caspian Sea (Blinov, 1962) g/kg % equ. g/kg % equ. Na K Ca Mg Cl Br SO CO H 3 BO Total

18 1 Day Variation in Caspian Sea surface Temperatures Acknowledgement Sputnik.infospace.ru Images April 2006 SHAH DENIZ SHAH DENIZ 18

19 TEMPERATURE Comparison CTD v SVP TEMPERATURE PROFILE COMPARISON 18th February 2006 Temperature Pressure SVP Down SVP Up SVP MEAN CTD 1 MEAN CTD1 Down CTD1 Up CTD2 Down CTD2 Up CTD 2 MEAN

20 VELOCITY Comparison CTD v SVP Caspian Salinity correction +1.4 ppt VELOCITY PROFILE 18th February 2006 Velocity SVP Velocity Observed CTD 414 Velocity Computed CTD 315 Velocity Computed CTD Adjusted Caspian Millero Pressure

21 ARRAY CALIBRATION PRINCIPLES Box-in Method, strengths and weaknesses Baseline and Depth measurements Acoustic Ranges and Pressure Array adjustment Mixed observable precisions Subsea positioning Extended arrays, Old and new, Variable Velocities 21

22 Transponder Box-in Effects of errors Even/Symmetrical data distribution 3-D Solution Scale Velocity/ Depth - scaling Systematic Offsets/Filtering/bias Random Vessel dynamics Heading, Pitch, and Roll Measurement noise

23 TRANSPONDER BOX-IN Circling vessel Position transfer from surface to seabed Box-in Symmetrical and Balanced data Robust and Reliable Average results +/ metre Standard DGPS WOS 1996 Average results +/ metres High Performance DGPS Caspian

24 WOS CONTROL ARRAY for NEW DRILL CENTRE Straight forward Adjusment 3 Box- ins provide three orientations for adjustment. Azimuth residuals 0.02, 0.04, and 0.11 Degrees Acoustic baselines provide scale and shape of network. Acoustic Baseline residuals < 0.02 metres Existing structure provides the origin of network Standard DGPS Box-in co-ordinates agree within 0.75 metres 24

25 WEST OF SHETLAND DRILL CENTRES 25

26 SCHIEHALLION WEST DRILL CENTRE 26

27 RE-ESTABLISHING CONTROL ARRAY at Schiehallion West Drill Centre 27

28 Positioning new wells at existing Drill Centres At least 3 Existing Wells provide positional control and orientations for new adjustments. In this example 5 wells have been used. Delta Ord X Delta Ord Y a b a b Delta Ord X and Delta Ord Y are the northing and easting residuals from established well coordinates Acoustic baselines still provide scale and shape of network. Acoustic Baseline residuals < 0.02 metres Absolute Accuracy of +/ metres Local Accuracy of +/- 0.3 metres 28

29 WEST OF SHETLAND Baseline residuals SCHIEHALLION WEST CONTROL ARRAY Well Setting WW11, WW12, and WP Residual - metres Baselines in sequence 29

30 WOS DRILL CENTRE Extended 3 New Wells 30

31 Array Adjustment The Acoustic Network Problem Combining Measurements with different precisions and attributes Very high or Ultra high precision relative acoustic measurements for scale Lower or Higher order DGPS control for absolute array position and orientation By monitoring of temperatures we have maintained scale control on the acoustic baseline measurements high precision with high accuracy By always having redundancy in our control whether box-in or existing well we have maintained overall accuracy and not distorted the array by holding control fixed. Adjustment in sympathy with observations

32 TPG500 CASPIAN CONTROL ARRAY DESIGN 32

33 CASPIAN CONTROL ARRAY Absolute Absolute Position and Heading 3 Box-ins Box-ins +/ m Standard error +/ m 78 Baselines Baseline +/ to +/ Standard error +/ m Array adjustment Error Ellipse +/-0.05m Marginally Detectable Error Box-ins 0.48m Baselines 0.13m 33

34 Static Cardinal Point Box-in + Heading reversal Two Template Transponders 34

35 Box-in residuals Template Transponder Box-in residuals 50 Observations per cardinal point Residuals (metres) Tx 111 Residuals Tx 112 Residuals Fixes in sequence Tx111 Residual Sd +/ metres Tx112 Residual Sd +/ metres 35

36 Baseline and Depth measurements Acoustic ranging New Wideband technology Resistant to Acoustic multipath - bounce, reflections very high precision Transponder Temperature Sensor (standard fit) or Velocimeter (optional) or both Temperature Accuracy +/- 0.1 Degrees C Velocimeter Accuracy +/ m/s Temperature Sensor Fitted in every transponder Monitor Temperature and Velocity Database Transponder Depth Sensor Strain gauge - crude and requires calibration Digi-quartz - precise but not often used

37 BASELINE PRECISIONS BASELINE MEASUREMENT PRECISION Precision (One Sigma) Ascending sort 158 Baselines 37

38 VELOCITY Variation during BASELINE Measurements Baseline Calibration SVS Data :00 05:32 11:30 15:10 16:47 19:00 20:10 21:30 22:00 22:30 23:00 23:30 00:00 00:30 01:00 01:30 02:00 02:30 03:00 03:30 04:15 04:30 04:45 Time (hrs) Interp 106 Interp 108 Interp 101 Interp 111 Interp Sound Sp eed ( SVS)

39 WIDEBAND ACOUSTIC BASELINES Average Precision(+/ m) v Accuracy (+/ m) BASELINE RESIDUALS ADJUSTED BASELINE RESIDUAL SD +/ Residuals - Metres Baselines in sequence 39

40 Final Box-In Control Tp03, Tp09, Tx111, Tx112 (High Performance GPS) Tp06, Tp09 (Standard DGPS) 40

41 CASPIAN ARRAY ADJUSTMENT Box-in Fit Ord Y Ord X TP TP Tx Tx POSITION and AZIMUTH COMPARISON Global High Performance GPS versus Local Standard DGPS with single Reference Station Position mn me Azimuth Difference Degrees 41

42 CASPIAN CONTROL ARRAY Relative Relative Position and Heading 2 Template Transponders Slots 5 & 11 Forced centering (Engineered funnel fit) +/ at best +/ at worst m Baseline Check agreement to +/-0.016m Derived Heading accuracy Better than +/ degrees Gross error check against Template As-installed results Delta East Delta North

43 TEMPLATE Transponder Locations 43

44 RESULTS OF CASPIAN CONTROL ARRAY CALIBRATION As Found Differences Position Delta N m Delta E m Azimuth Degrees Redefines Intended Target co-ordinates for TPG500 Redefines Intended Target heading This ensures that all systems are consistent in both an absolute and relative sense. Accuracy potential is tested and confirmed 44

45 FINAL SPUD CAN ACOUSTIC POSITIONING 45

46 ACOUSTIC RESULTS FINAL Position of SHAH DENIZ A jacked to 13.5 m Air gap The seabed Acoustic position for Shah Deniz Slot 8 is 0.14 metres on a bearing of Grid from the as-found seabed Template Slot 8 position. Can metres G from intended Can metres G from intended Can metres G from intended The accuracy for these position is +/ metre Leg Can Heading is Grid ( True) +/ Degrees as determined from acoustics. This is 0.21 higher than intended and within the tolerance (+/-1 ). Average pitch inclination of the cans was 0.39 bows up +/ and the average roll inclination / starboard side down. 46

47 GPS Positioning 47

48 POST PROCESSED GPS(4 Antennae) BAKU Reference Station differences from mean 0.007, 0.019, 0.035, metres 48

49 POST PROCESSED GPS The inclination of the cans has directly affected the surface position of the hull. The observed surface and seabed positions agree well with the observed average inclinations of the cans. The final mean surface position for Slot 8 Shah Deniz A is 0.50 metres on a bearing of Grid from the as-found seabed Template Slot 8 position. (Tolerance +/ metres) The accuracy for this position is +/ metre The mean Platform Heading is Grid ( True) +/ Degrees as determined from the four GPS Antennae. This is 0.32 higher than intended and within the tolerance (+/-1 ). 49

50 50

51 SPUD CANS but not as you know them! 51

52 SAILAWAY Baku to Shah Deniz 52

53 53

54 CAN SUBMERSION Going going.. 54

55 SHAH DENIZ Template + 4 Conductors 55

56 TEMPLATE + 4 CONDUCTORS 56

57 TEMPLATE and SPUD CAN Intended Positions 57

58 Grouting the spud cans 58

59 TPG500 CONCLUSIONS Innovative engineering solutions combined with precise engineering construction created a unique large tonne structure. This required a unique installation methodology that allowed three 1500 tonne spud cans 30 metres in diameter and 12 metres high to be attached to the legs. The combined structure was carefully installed over a preinstalled Template with 4 pre-drilled wells. Each can then penetrated the seabed by approximately 8 metres and was then grouted. The final transport and installation of the TPG500 from Zykh to Shah Deniz was completed in 17 days. To support this operation very accurate positioning using new wideband acoustic technology was provided and ensured the required positioning tolerances were achieved. 59

60 North Sea acoustic conditions have provided a comparatively easy start Offshore positioning has progressed more in the last 30 years than it had previously in the last 300 years. Our ability to measure time very accurately has created the observable. Now we need to support this very high precision measurement with environmental observations of the propagation speed of sound to provide high accuracy acoustics ranges. With two thirds of the world covered in water the future for acoustics is significant but the environments will be ever more challenging and no doubt there are a few more surprises in store!! 60

61 Acknowledgements West of Shetland Well Positioning Fugro Survey Ltd Transport and Installation TPG500 at Shah Deniz Technip France Technip Maritime Overseas Ltd Geo Century Ltd 61