5.3 Ship Speed Trials and Analysis
5.3.8 Updated Ship Speed Trials Procedures
The IMO has introduced an Energy Efficiency Design Index (EEDI) in order to monitor and control CO2 emissions, see Section 17.9. As the EEDI is a regulatory requirement and the EEDI formula has ship speed as a basic parameter, this puts a greater importance on speed and its measurement at model and full scale. Amended and updated procedures for ship speed trials have been produced, and details of the new recommended procedures and guidelines are laid out in some detail in [5.17] and [5.18]. Amendments to the analysis procedures are relatively small and the following sections outline changes that have been made to the existing procedures, described earlier in Sections 5.3.1 to 5.3.5.
5.3.8.2 Speed
Verification of speed for use in the EEDI formula takes place at two stages:
Pre-verification: ship’s speed is predicted from model test results, see Section 5.3.8.6.
Final verification: ship’s speed is confirmed by speed trials at sea.
Methods of trial speed measurement and correction for current:
(i) Mean of means: in this method the current is assumed to vary parabolically over time. The method is described in Section 5.3.4.
(ii) Iterative method: in this method it is assumed that the current speedVCvaries with the lunar semi-diurnal period, and a current curve as a function of time is created. In the same process, a regression curve representing the relationship between the corrected power and ship’s speed through the water is determined:
PV s=a+bVSq. (5.29)
The current and regression curves are created in one process. The current speed for each run is calculated by subtracting the updated ship speed through the water VSfrom the measured ship speed over the groundVGat each run as follows:
VC=VG−VS. (5.30)
The process is repeated with new values of VC until the power–speed curve converges.
For equivalent accuracy:
Mean of means method requires: 2+2+2 double runs Iterative method requires: 1+2+1 double runs.
Strasseret al.[5.19] analysed both methods and concluded that the methods are equally adequate.
Figure 5.5. Definition ofLBWL.
5.3.8.3Resistance Increase Due to Wind Recommended methods are as follows:
(i) Wind tunnel tests: wind resistance coefficients can be derived from wind tunnel tests for that particular ship. Examples of such data are given in Section 3.2.2.
(ii) Data sets of wind resistance coefficients: data for tankers, bulk carriers, LNG carriers and container ships are presented in diagrams in [5.17].
(iii) Regression formula: a general regression formula, developed by Fujiwaraet al.
[5.22], is proposed, which is based on model tests in wind tunnels for various ships. Full details of the method are given in [5.17].
5.3.8.4Resistance Increase Due to Waves Recommended methods are as follows:
(i) Direct correction method STAwave–1
The resistance increase in head waves, provided that heave and pitch are small, is given by Equation (5.31). This supersedes Equation (5.25). The application is restricted to waves in the bow sector (within±45° off bow). For wave directions outside this sector, no wave correction is applied.
RAW L= 1
16ρg HW1/32 B
B LBW L
(5.31) whereHW1/3is significant wave height.
LBWL is length of bow on the waterline to 95% maximum beam, as shown in Figure 5.5.
The formula has been validated for head waves, small heave and pitch during trials (vertical acceleration at the bow <0.05g) and significant wave height H≤ 2.25
LBP/100.
(ii) Empirical transfer function method STAwave–2
The empirical method uses parametric transfer functions, for long-crested irreg- ular head waves, based on the ship main parameters. The empirical transfer functions cover both the mean increase due to wave reflections and the motions-induced resis- tance. Full details of the method are described in [5.17].
(iii) Theoretical method
The mean resistance increase is made up of the mean resistance in regular waves, based on Maruo’s theory [5.23], which is induced by ship motion, and the mean
resistance increase due to wave reflection. A full description of the method is given in [5.17].
(iv) Seakeeping model tests
Transfer functions of the resistance increase in waves may be derived from model tank tests in regular waves. The tank tests have to be conducted for the specific ves- sel geometry at the trial’s draught and trim. The test set up and procedure should follow the ITTC recommended procedure [5.21]. Further requirements for the tests are given in [5.17].
5.3.8.5 Water Depth
It is recommended at present to continue to use the Lackenby shallow water cor- rection, see Equation (5.24). An alternative method for correcting for shallow water effects on speed has been developed by Raven [5.24]. The new procedure corrects separately for the effects of shallow water on the different components of resistance.
These have been identified by computational studies. The components investigated were: viscous resistance, wave resistance, the influence of sinkage and propulsive effi- ciency. The outcomes are as follows.
Viscous resistance:
RV
RV deep =0.57 T
h 1.79
(5.32) whereTis draught andhis depth of water.
Wave resistance: investigations concluded that there was no need for a correction to wave resistance up to Frh =0.65, except for sinkage effect, which is considered separately.
Sinkage: correction for increase in displacement δ∇ and resistance R with sinkage:
d(Sinkage)
L = 1.46∇
L3
F rh2
1 −F rh2 − F rhD2 1− F rhD2
(5.33) whereF rh=V/
gh and F rhD=V/
0.3gL.
δ∇ =d(Sinkage)×AWP/∇ (5.34)
where∇is displacement volume andAWPis area of the waterplane.
Resistance increase factor due to sinkage in shallow water:
RSINK=(1+δ∇)2/3. (5.35)
Finally, corrected power
PDdeep= PDshallow
RSINK −RV×Vs ηD
, (5.36)
wherePDshallow is the power measured on trial andηD is the propulsive efficiency coefficient from model test. The power increase can be converted to a speed loss using Equation (3.70) or Equation (3.71).
Propulsive efficiency: the effect of shallow water on propulsive efficiency was investigated. Since the correction would need detailed information on the changes in ηO,wT andt, and the investigated examples showed relatively small changes in
shallow water, this correction was disregarded. An unchanged propulsive efficiency is assumed.
Limits to the use of the formulae are:Frh<0.65,T/h<0.5,δ∇0.05.
Investigations into the use of the new procedure indicate that it is more reliable than the Lackenby correction, which tends to overestimate and, after further valida- tion, the new procedure is expected to be included in the ITTC recommended trials procedure.
5.3.8.6Model Tank Tests
These basically follow the ITTC recommended procedures for resistance and self- propulsion tests [5.20], see Section 8.7. The model tests should be carried out for the predicted EEDI speed at 75% MCR. Tests should also be carried out for the ballast condition when the EEDI condition cannot be achieved during sea trials. Load varia- tion tests should be carried out and reported on, for use when correcting the sea-trial results. In the load variation test, the revolutions are varied while keeping the speed constant, the revolutions being chosen to pass through the ship self-propulsion point, see Section 8.7.4.2.
5.3.8.7Power Correction
The measured power is corrected directly with the power increase due to the added resistanceR, due to wind, waves and temperature deviations, in the trial condi- tion. This is termed the direct power method. The results of the load-variation tests are included. The corrected delivered powerPDCis found from the measured shaft power, taking into account the propeller efficiency as:
PDC = PDM − R.VSM ηD0
1 − PDM PDC. ξp
(5.37) wherePDC is the corrected power,PDM the measured delivered power,VSM the measured ship speed,ηD0the propulsive coefficient in ideal conditions from model tests andξP is derived from model load-variation tests. A shallow-water correction and correction for deviation in displacement may then be applied if required; see Section 5.3.8.5.
The correction for shaft revolutions is also based on the results of the model load-variation tests. The corrected shaft revolutionsnCare:
nC = nM
ξnPDM−PDC
PDC +ξvVVSM +1 (5.38)
wherenMis the measured propeller revolutions,VSMis the measured ship speed, and ξnandξvare overload factors derived from the model load-variation tests.
REFERENCES (CHAPTER 5)
5.1 NPL. BTTP 1965 standard procedure for the prediction of ship performance from model experiments,NPL Ship TM82. March 1965.
5.2 NPL. Prediction of the performance of SS ships on measured mile trials,NPL Ship Report 165, March 1972.
5.3 NPL. Performance prediction factors for T.S. ships,NPL Ship Report 172, March 1973.
5.4 Scott, J.R. A method of predicting trial performance of single screw merchant ships.Transactions of the Royal Institution of Naval Architects. Vol. 115, 1973, pp. 149–171.
5.5 Scott, J.R. A method of predicting trial performance of twin screw merchant ships.Transactions of the Royal Institution of Naval Architects, Vol. 116, 1974, pp. 175–186.
5.6 ITTC Recommended Procedure. 1978 Performance Prediction Method, Proce- dure Number 7.5-02-03-01.4, 2002.
5.7 Townsin, R.L. The ITTC line – its genesis and correlation allowance.The Naval Architect. RINA, London, September 1985.
5.8 ITTC Report of Specialist Committee on Powering Performance and Predic- tion,24th International Towing Tank Conference, Edinburgh, 2005.
5.9 ITTC Report of Specialist Committee on Powering Performance Prediction, 25th International Towing Tank Conference, Fukuoka, 2008.
5.10 Lindgren, H. and Dyne, G. Ship performance prediction, SSPA Report No. 85, 1980.
5.11 Holtrop, J. A statistical re-analysis of resistance and propulsion data.Interna- tional Shipbuilding Progress, Vol. 31, 1984, pp. 272–276.
5.12 Bose, N.Marine Powering Predictions and Propulsors. The Society of Naval Architects and Marine Engineers, New York, 2008.
5.13 ITTC Recommended Procedure. Full scale measurements. Speed and power trials. Preparation and conduct of speed/power trials, Procedure Number 7.5- 04-01-01.1, 2005.
5.14 ITTC Recommended Procedure. Full scale measurements. Speed and power tri- als. Analysis of speed/power trial data, Procedure Number 7.5-04-01-01.2, 2005.
5.15 ITTC Report of Specialist Committee on Speed and Powering Trials,23rd Inter- national Towing Tank Conference, Venice, 2002.
5.16 Lackenby, H. Note on the effect of shallow water on ship resistance, BSRA ReportNo. 377, 1963.
5.17 ITTC Recommended Procedure. Preparation, Conduct and Analysis of Speed/Power Trials. Procedure No. 7. 5-04-01-01, 2017. Available from www.ittc .info.
5.18 ISO Guidelines for the Assessment of Speed and Power Performance by Analysis of Speed Trial Data. 15016.2015, 2015.
5.19 Strasser, G., Takagi, K., Werner, S., Hollenbach, U., Tanaka, T., Yamamoto, K. and Hirota, K. A verification of the ITTC/ISO speed/power trials analysis.Journal of Marine Science and Technology, 2015, 20: 2–13.
5.20 ITTC Recommended Procedure. Propulsion Test. Procedure No. 7.5-02-03-01.1 Revision 01, 2002. Available from www.ittc.info.
5.21 ITTC Recommended Procedure. Prediction of Power Increase in Waves from Model Tests. Procedure No. 7.5-02-07-02.2, 2011. Available from www.ittc.info.
5.22 Fujiwara, T., Ueno, M. and Ikeda, Y. A new estimation method of wind forces and moments acting on ships on the basis of physical component models.JAS- NAOE, Vol. 2, 2005.
5.23 Maruo, H. On the increase of the resistance of a ship in rough seas. (2nd report), J.SNAJ, Vol. 108, 1960.
5.24 Raven, H.C. A new correction procedure for shallow-water effects in ship speed trials.Proceedings of International Symposium on Practical Design of Ships and other Floating Units, PRADS’2016, Copenhagen, Denmark, September 2016.
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