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					The Hydrographic Society in Scotland

    Underwater Spoolpiece Metrology

        Thursday 19th May 2005

       Frank Prytz, Blom Maritime
      Underwater Photogrammetry

Blom Maritime as
Photogrammetry explained
Some Photogrammetry work done since 2000
Spoolpiece metrology
  • Onshore test
  • Kristin and Snøhvit test – Photogrammetry results
  • Kristin and Snøhvit test – As seen with the eyes of DeepOcean
  • Lesson learned
  • Acoustics and photogrammetry combined
Blom Maritime AS

47 employees
41 engineers
Located in Stavanger
Working world wide

                       Office in Stavanger
More than 30 years of experience worldwide with industrial metrology
and maritime surveying services
                Photogrammetry under water

Blom Maritime AS started using photogrammetry under water in 2000.

Photogrammetry under water require different combined skills that Blom has in house

 Photogrammetry specialists
 Dimensional control specialists
 Software experience
 Computer science

Photogrammetry may be described as the science of creating 3D models of objects
from images.
Photogrammetry principle
Fieldwork marking example

Millimetre and sub-millimetre accuracy

Reliable results every time

Fast mobilization and field work

Additional dimensions without new field

Increased survey possibilities

Same accuracy in all axes

Can be combined with other methods like
             Production line



Some projects using photogrammetry sub sea
                 Spool-piece test on shore
                             Compared to 52 control points

                                70 meter (97%)                          ±      10 mm
                                20 meter (97%)                          ±       6 mm

                             Statistics from the photogrammetry software

                                 70 meter (97%)                         ±      12 mm
Surveyed route                   20 meter (97%)                         ±       4 mm

                             Result flanges compared to total station survey

                                Tilt and rotation                       ±      0.2deg
                           Spool piece metrology

Preparation on shore if possible
  • Installing and survey of targets
  • Survey of structure and hubs
  • Survey of transponders if used

Mobilization at quayside
  • Adaptation of camera system

Work offshore
  • Preparing metrology scale bars
  • Placing of scale bars
  • Acquire images

Processing and reporting
Spool piece accuracy

Error budget
Description                                         Distance (mm)   Angle (deg)
Survey of template HUBs onshore                           2             0.14
Photogrammetric model (161 images and 708 points)         4             0.01
Photogrammetry results of termination head                2             0.04
Transformation                                            2             0.02
Temperature effect on scale                               4             0.01
Total (67 meters at 1 sigma)                              7             0.15
                       Metrology projects
                        Kristin   Snøhvit – Template D   Snøhvit – Plem

Image quality

3D model

Route length              67              54                  51

Images used              108              350                 328

Hours used               123              95                  184

Images per meters        1,6              6,5                 6,4
Hours per meter
                        1,8 t            1,8 t               3,6 t

Hours used per image   57 min           16 min              34 min
The next few slides is from a presentation held by DeepOcean at
the Geilo conference in Norway this year.

Advantages and disadvantages using acoustics

• Independent of sighting
• Known and accepted method
• Performed by the onboard survey personnel
• Good accuracy if performed well

• Relative time demanding offshore
• A lot of planning and production onshore have to be put into the
  project to achieve optimal results
• Vounerable to acoustic noise
Three tests have been performed where acoustics and photogrammetry were
done in parallell. Blom Maritime was subcontractor to DeepOcean.

1 Spool at Kristin and 2 spools at Snøhvit. Both for Statoil.

Purpose: Find out if photogrammetry may replace acoustics as a spool metrology

Factors to be tested:

•   Time and resource used on shore
•   Time and resource used offshore
•   Processing time
•   Result comparison
Test experience – Preparations onshore:

• Site visit / Planning of method and transponder positions
• Production of brackets
• Test of bracket positions / surveying / calibration of brackets and transponders

• Surveying of photogrammetry targets

Conclution: Photogrammetry approximately 1/3 of the time / cost
compared to acoustics.
Test experience – Time used Offshore

Kristin (1 Spool)
Acoustics:                             34 hours
Photogrammetry:                        17 hours

Snøhvit (2 spools)
Acoustics:                             98 hours
Photogrammetry:                        34 hours

Conclution: Photogrammetry approximately 1/2 of the time/cost
compared to acoustics.
                Results from Kristin N 101

                         Acoustics   Photogrammetry   Difference

Grid distance             44.36m            44.34m       0.02m

Grid bearing              355.33º           355.30º       0.03º

Depth difference         -0.233m           -0.194m      -0.04m

Pitch difference           -6.54º            -6.42º      -0.12º
         Results from Snøhvit at D Template

                     Acoustics   Photogrammetry   Difference

Grid distance        36.866m            36.87m      -0.01m

Grid bearing         280.264º          280.336º      -0.07º

Depth difference     -0.189m           -0.297m       0.11m

Pitch difference      -0.638º           -0.710º      -0.07º
                   Results from Snøhvit at PLEM

                         Acoustics   Photogrammetry   Difference

Grid distance             36.39m            36.36m      -0.03m

Grid bearing             333.071º          333.215º      -0.14º

Depth difference         -0.503m           -0.688m       0.18m

Pitch difference           0.172º            -0.84º       1.01º
Development and improvements

Inclinometer                  QC for pitch, roll and relative elevations.
Depth sensor
                              Inclinometer and depth sensor mounted on a frame. Use two or more
                              systems. Each system calibrated on shore and calibrated offshore
                              relative to each other.
                              Require little or no extra time offshore and in processing.
Reduce the time required to   Coded Targets reduses overall time spent.
final results
                              Used on Snøhvit.
                              Redused time with 65% compared with noncoded targets.

                              If used on Kristin, the prosessing and reporting time would
                              have been reduced from 123 hours to 41 hours.
Photogrammetry and LBL

 Combine the best from the two
 Use results from CRP for LBL
 Use CRP to connect to the
 objects at each end

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