The Hydrographic Society in Scotland

WorkshopUnderwater Spoolpiece Metrology

Thursday 19th May 2005

Frank Prytz, Blom MaritimeUnderwater Photogrammetry

Agenda

Blom Maritime asPhotogrammetry explainedSome Photogrammetry work done since 2000Spoolpiece 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 employees41 engineersLocated in StavangerWorking world wide

Office in Stavanger

More than 30 years of experience worldwide with industrial metrology and maritime surveying services

Blom Maritime AS started using photogrammetry under water in 2000.

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

Photogrammetry specialistsDimensional control specialistsSoftware experienceComputer scienceElectronics

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

Photogrammetry under water

Photogrammetry principle

Fieldwork marking example

Advantages

Millimetre and sub-millimetre accuracy

Reliable results every time

Fast mobilization and field work

Additional dimensions without new field survey

Increased survey possibilities

Same accuracy in all axes

Can be combined with other methods like acoustics

Results

Processing

Computer

Camera

Production line

Some projects using photogrammetry sub sea

Spool-piece test on shore

Surveyed route

Compared to 52 control points

70 meter (97%) 20 meter (97%)

±±

10 mm6 mm

Statistics from the photogrammetry software

70 meter (97%) 20 meter (97%)

±±

12 mm4 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

Work offshore

• Preparing metrology scale bars

• Placing of scale bars

• Acquire images

Mobilization at quayside

• Adaptation of camera system

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

Hours per meter survey

1,8 t 1,8 t 3,6 t

Hours used per image 57 min 16 min 34 min

Images per meters 1,6 6,5 6,4

The next few slides is from a presentation held by DeepOcean at the Geilo conference in Norway this year.

Advantages and disadvantages using acoustics

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

Disadvantages• 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 tool

Factors to be tested:

• Time and resource used on shore• Time and resource used offshore• Processing time • Result comparison

Test experience – Preparations onshore:

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

Photogrammetry• 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 hoursPhotogrammetry: 17 hours17/34=0.5

Snøhvit (2 spools)Acoustics: 98 hoursPhotogrammetry: 34 hours34/98=0.35

Conclution: Photogrammetry approximately 1/2 of the time/cost compared to acoustics.

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 Kristin N 101

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 D Template

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

Results from Snøhvit at PLEM

Development and improvements

InclinometerDepth sensor

QC for pitch, roll and relative elevations.

Inclinometer and depth sensor mounted on a frame. Use two or more systems. Each system calibrated on shore and calibrated offshorerelative to each other. Require little or no extra time offshore and in processing.

Reduce the time required to final results

Coded Targets reduses overall time spent.

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 methodsUse results from CRP for LBL QCUse CRP to connect to the objects at each end