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Full Scale Measurements – Sea trials
Experimental Methods in Marine Hydrodynamics Lecture in week 45 Contents:
•Types of tests •How to perform and correct speed trials •Wave monitoring
Dedicated sea trials are conducted under the following circumstances:
• Delivery of newbuildings (Contractual Trials) – Speed-power (compliance with contracted performance) – Bollard Pull test (tugs and offshore vessels – compliance with
contracted performance) – Maneuvering (compliance with IMO criteria) – Sea keeping (only high speed craft)
• If a special problem has arisen, for instance: – Propeller noise and/or erosion – Steering problems – Excessive fuel consumption
• For research purposes (quite rare due to high costs)
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Delivery Sea trials (Contractual trials)
• Ship building contracts contain specific requirements for speed-power performance – Failure to meet requirements means fees to be paid and ultimately
that the ship owner has the right to refuse to accept the ship
• For tugs and offshore vessels, there will be requirements for bollard pull as well
• There might be requirements also for maneuvering trials : – Emergency stop test – Turning circles – Zig-zag tests
• High speed craft – requirements also for seakeeping tests – IMO: 2000 HSC Code (IMO 185E)
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Applicable standards
• ISO 19019:2005 Sea-going vessels and marine technology -- Instructions for planning, carrying out and reporting sea trials
• ISO 15016 Guidelines for the assessment of speed and power performance by analysis of speed trial data
Preparation and Conduct of Speed/Power Trials • IMO: 2000 HSC Code (IMO 185E) – Requirements for
testing of high speed craft
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IMO HSC testing requirements
• Stopping – Normal stop from max speed to zero – Emergency stop – Crash stop
• Cruise performance in two sea states – Normal conditions – Worst intended conditions – Measurements of accelerations, speed, relative wave heading
• Failure tests – Check that the ship, crew and passengers are not at risk if for
instance the steering fails
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Organization of Delivery Trials
• The Shipbuilder is responsible • Trial Leader
– From the shipbuilder – Responsible for the execution of all phases of the trial
• Ship masters – There is one ship master hired by the shipbuilder who is in charge
of handling the ship – There is usually one or more ship masters hired by the shipowner
who is going to take over the ship • Measurements are performed by shipbuilder or by third
party (like Marintek or Maskindynamikk)
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Execution of speed trials
• Always run back and forth at same engine setting • Run back and forth at the same track • Perform runs at different speeds (at least three) • If possible, orient the track with and against the wind
direction
•Steady Approach
> 5 min and 1 mile
•Steady Approach
> 5 min and 1 mile
Wind, current
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Measured mile
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Trial Conditions – max acceptable
• Sea state – Preferably ≤ sea state 3 – Ultimately ≤ sea state 5 (or up to sea state 6 for ships with L>100 m)
• Wind – ≤ Beufort 6 (20 knots) (for ships with L>100 m) – ≤ Beufort 5 (for ships with L ≤ 100 m)
• Water depth h – h>6.0*Am
2 and h>½V2
– Smaller depths require corrections for shallow water • Current
– Current of more than a few knots is unacceptable
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Trial Conditions – Contractual
• Sea state – No waves – In practice: Beufort 1 (Wave height 0.1 m)
• Wind – No wind – In practice: Beufort 2 (Wind speed ≤ 6 knots)
• Water depth h – Deep, – In practice: h>6.0*(Am)½ and h>½V2
• Current – No current – No practical limit for when corrections are made. Use of double runs
means that corrections are always included
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Correction of trial results
• When trial conditions are not fulfilled corrections must be made
• Typical corrections: – Draught – interpolation in model test results on two draughts – Wind – calculation of wind resistance using empirical drag coef. or
results from wind tunnel tests – Shallow water – empirical formulas – Waves – calculation of added wave resistance and speed loss
• Standards for how corrections shall be performed: – ISO 15016 Guidelines for the assessment of speed and power … – ITTC Procedure for the Analysis of Speed/Power Trial Data – STAWAVE by Marin
• Comes with a free software package for performing the analysis
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IMO Energy Efficiency Design Index - EEDI
• Increases the need for standardized trial and correction procedures
• The speed at 75% MCR in calm water must be accurately determined
• Now longer just a matter for yard and ship owner – Shall be approved by classification society
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Recent developments
• The ISO 15016 is about to be discarded – Too complicated to use – Too much freedom to manipulate results – Outdated correction methods
• IMO has tasked ITTC to develop a new standard • ITTC works with Marin, and the new guideline is based on
the STAWAVE methods
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Speed measurement
• “Speed over ground” and “Speed through water” • Timing a measured mile
– the old-fashioned way, only applicable to dedicated speed trials – Gives speed over ground
• GPS – The obvious choice, always used – Gives speed over ground
• Speed log – Device to measure speed through water – Always installed on ships – The accuracy is questionable!
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Measurement of shaft power • Strain gauges glued directly to the shaft
– Calibration factor must be calculated, so shaft dimensions and material properties must be known exactly
– Tachometer to measure shaft speed
• Commercial power meters – Made for permanent installation – The best, but most expensive alternative
• Poor, but cheap alternatives are – fuel rack measurements (measurement of fuel consumption,
combined with supplier data for fuel quality) – measurement of cylinder pressure (used on large, slow speed
engines)
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Shaft measurements
Torque measurement Thrust measurem.
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Optical torque sensor
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Optical thrust and torque measurement
Required accuracy for thrust measurement is 25 naonometers! Challenging, but possible, according to supplier VAF Instruments
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Bollard Pull Tests
Good location Poor location
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Bollard pull test
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Bollard pull test •2x460 kW
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Maneuvering trials
• Trial types and execution same as in model scale • Measurements:
– (D)GPS position measurement – Gyro compass course – Rate of turn (if possible) – Rudder angle – Propeller revs
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Types of Ship Maneuvers
• IMO standard maneuvers: – Zig-zag tests
• 10º/ 10º to both sides • 20º/ 20º to both sides
– Turning circle test • 35º rudder angle
– Full astern stopping test
• Additional maneuvers: – Spiral test – Reverse spiral test – Pull-out maneuver
• normally added at the end of a turning test
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Zig-zag test
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Test 2011: 20-20 zig zag
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Turning circle
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Testing of position-keeping ability and thruster performance at zero speed
• Important for vessels that have requirements to Dynamic Positioning performance
• No standard tests or commonly recognised procedures – There is a need for development of standardized tests and analysis
procedures for this purpose
• A way to characterise thruster performance at zero speed: – Run the thrusters in different combinations (one by one, and in
specific combination) for a short time – Measure the acceleration of the ship in the horisontal plane – Compute the impulse required to create the acceleration – Compare the effective impulse with the impulse provided by the
thruster(s) to arrive at a kind of efficiency
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Measurements – environmental conditions • Water depth
– Echo sounder (ship instrument) or nautical charts • Water quality
– Temperature: Cooling water intake temperature can be used – Density: From nautical charts or density measurements
• Wind – Velocity and direction from anemometer – A separate, calibrated instrument is preferable – Watch out for influence of superstructure on the measurement
• Current – Nautical charts and tables – the difference in speed between double runs – a 360º turning test at low speed – The difference between log speed and GPS speed
• often, one doesn’t trust the speed log sufficiently for this purpose
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Wave measurements
• Visual observation and estimation – Estimates by yard representative, ship-owner representative, and
possibly a neutral third party are compared and averaged
• Mobile wave buoy – Accurate (but only at a single point) – Recovery of the buoy is difficult (risk of loosing it)
• Fixed weather station – Good solution if one is nearby
• Wave radar (Wavex) • Bow-mounted altimeter • Wave information without measurement: Hindcast data
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Wave buoys • Fugro Oceanor Wavescan
– Directional wave spectrum – Wind – Current – Water temperature and salinity – Must be moored; large, heavy, costly
• Smaller, spherical buoys – Drifting or moored – Measures acceleration to determine wave
elevation, including period, but not direction – Usually measures position – for a drifting
Measurement of motions • Accelerations: Conventional accelerometers • Angles: Gyros, compass, accelerometers • Rate gyro to measure rate of change of angles • Inertial Measurement Units (IMU)
– Consists of a number of accelerometers built into one compact unit – Gives out accelerations, velocities and motions at any point – Konsberg Seatex MRU is a good example of a commercial IMU
• Kongsberg Seapath – Combination of DGPS and IMU – for accurate position
measurement
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Kongsberg Seatex MRU 5+
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Kongsberg Seapath 330
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Measurement of forces: Hull Monitoring
• Strain gauges most common sensor
• Short and long gauges • Cabling exposed to
damage, gauges work loose
• Sensors based on fiber-optics - polarimetric and bragg-grating suggested as alternative
Hull Monitoring System:
Strain gauge in protective casing:
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Rolls-Royce Health and Monitoring System - HEMOS
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Example: Monitoring of loads on an azimuthing thruster of a seismic vessel
• Measurement of ship motions and position with Seapath
• Measurements on the port azimuthing thruster
• Automatic triggering of data storage • Data acquisition system remotely
monitored from land
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Performance monitoring
• Typical merchant ship application: To monitor the development of speed and fuel consumption over time, in order to detect need for maintenance
• Challenges: – Monitoring and correcting for environmental conditions
• Waves, wind, water temperature – Accurate measurement of shaft power and speed through water – Correcting for loading condition – Data processing – Setting-up and running automatic data transmission
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Propeller Cavitation Observations
Seen from below Seen from the side
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Cavitation observation techniques
1. generation borescope
2. generation borescope
Source: marin.nl
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Sample picture from full scale propeller cavitation observation
Summary: •Types of tests •How to perform and correct speed trials •Wave monitoring