DRIFT CHARACTERISTICS of 50,000 to 70,000 DWT TANKERS (First Edition – 1982) The OCIMF mission is to be the foremost authority on the safe and environmentally responsible operation of oil tankers and terminals, promoting continuous improvement in standards of design and operation. Oil Companies International Marine Forum WSIL - 191 Drift Characteristics.indd 1WSIL - 191 Drift Characteristics.indd 11/24/2009 3:13:21 PM1/24/2009 3:13:21 PM Issued by the Oil Companies International Marine Forum First Published 1982 ISBN 0 900886 67 6 © Oil Companies International Marine Forum, Bermuda British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. The Oil Companies International Marine Forum (OCIMF) is a voluntary association of oil companies having an interest in the shipment and terminalling of crude oil and oil products. OCIMF is organised to represent its membership before, and consult with, the International Maritime Organization and other governmental bodies on matters relating to the shipment and terminalling of crude oil and oil products, including marine pollution and safety. Notice of Terms of Use While the advice given in this document (“document”) has been developed using the best information currently available, it is intended purely as guidance to be used at the user’s own risk. No responsibility is accepted by the Oil Companies International Marine Forum (OCIMF), the membership of OCIMF, or by any person, firm, corporation or organisation [who or which has been in any way concerned with the furnishing of information or data, the compilation or any translation, publishing, supply or sale of the document] for the accuracy of any information or advice given in the document or any omission from the document or for any consequence whatsoever resulting directly or indirectly from compliance with or adoption of guidance contained in the document even if caused by a failure to exercise reasonable care. Printed and bound in Great Britain by Bell & Bain Ltd. Glasgow Published in 2009 by Witherby Seamanship International Ltd. 4 Dunlop Square, Deans Estate Livingston, EH54 8SB Scotland, UK Tel No: +44 (0)1506 463 227 Fax No: +44 (0)1506 468 999 www.info@emailws.com www.witherbyseamanship.com Notice of Terms of Use While the advice given in this document (“document”) has been developed using the best information currently available, it is intended purely as guidance to be used at the user’s own risk. No responsibility is accepted by the Oil Companies International Marine Forum (OCIMF), the membership of OCIMF, or by any person, firm, corporation or organisation [who or which has been in any way concerned with the furnishing of information or data, the compilation or any translation, publishing, supply or sale of the document] for the accuracy of any information or advice given in the document or any omission from the document or for any consequence whatsoever resulting directly or indirectly from compliance with or adoption of guidance contained in the document even if caused by a failure to exercise reasonable care. WSIL - 191 Drift Characteristics.indd 2WSIL - 191 Drift Characteristics.indd 21/24/2009 3:13:22 PM1/24/2009 3:13:22 PM FOREWORD Following investigations into drift characteristics, it was discovered that results obtained for VLCCs were not necessarily indicative of the behaviour of smaller vessels. Concern was expressed by the American Institute of Merchant Shipping (AIMS) for the need to include within the scope of the studies details for vessels of the 50,000 to 70,000 dwt range, particularly in view of the present numbers of these ships. Computer programs were developed by the National Maritime Institute (NMI) to assess drift behaviour of these ships, which required validation. Some free model experiments were carried out to investigate wave drift forces and also to determine the accuracy of results obtained. The Oil Companies International Marine Forum (OCIMF) and the NMI worked together upon the project to estimate the effects of wind, waves and current upon the drift track, heading and speed of such vessels under a variety of conditions. The Appendix to the study contains some 28 of the 72 runs, being representative of the whole series or effects obtained. The raw data on the experiments may be obtained from: The National Maritime Institute, Feltham, Middlesex, TW14 0LQ, England. WSIL - 191 Drift Characteristics.indd 3WSIL - 191 Drift Characteristics.indd 31/24/2009 3:13:23 PM1/24/2009 3:13:23 PM CONTENTS Page 1 INTRODUCTION 1 2 CONDUCT OF THE STUDY 1 3 MODEL EXPERIMENTS 1 3.1 The ship model 1 3.2 The experiments 2 3.3 Results obtained 3 3.4 Discussion of results 4 4 VALIDATION OF COMPUTER PREDICTION 5 5 PREDICTIONS OF DRIFT 5 5.1 General 5 5.2 Drift estimation curves 5 5.3 Effect of latitude 6 5.4 Effect of ship size 6 6 GENERAL DISCUSSION 7 7 CONCLUSIONS 7 8 REFERENCES 8 9 NOMENCLATURE 8 APPENDIX 17 WSIL - 191 Drift Characteristics.indd 4WSIL - 191 Drift Characteristics.indd 41/24/2009 3:13:23 PM1/24/2009 3:13:23 PM 1 1 INTRODUCTION The drift of disabled large tankers has been studied at NMI using a combination of model and theoretical investigations (Refs. 1 and 2). An attempt was made during these studies to generalise the results so that drift characteristics of smaller tankers could be deduced from data collected from models of their larger counterparts. This work was incomplete and required validation, so that when the National Maritime Institute was approached by the Oil Companies International Marine Forum (OCIMF) to predict the drift of tankers in the 50-70,000 dwt range, it was decided to carry out in addition to the predictions a short series of experiments to aid in validating the calculations. The project ultimately became a joint venture between NMI and OCIMF, with the model experiments being funded by the Ship and Marine Technology Requirements Board (now the Mechanical and Electrical Engineering Requirements Board) of the Department of Industry. The computational studies and report analysis were funded by OCIMF. Results of both the experimental and computational studies are included in this report. 2 CONDUCT OF THE STUDY When a ship drifts in the open sea, its equilibrium track, speed and heading depend on a balance between the external forces and moments acting on the ship (from wind, current, waves and Coriolis acceleration (Ref. 1)) and the hydrodynamic forces and moments due to its resultant drift through the water. Wind effects may be estimated from the results of wind-tunnel experiments on models of the ship in question (Ref. 2) while a uniform current may be assumed to cause a translation in the direction of the current. Coriolis or geostrophic acceleration effects may be calculated directly (Ref. 1) while the hydrodynamic forces resisting motion may be deduced from model experiments (Ref. 2). This leaves forces and moments due to waves to be estimated. A method by which this may be accomplished is outlined in Ref. 1, but, as it was based on data for VLCC models, it was desirable to carry out some model experiments to confirm its applicability to tankers in the deadweight range of interest. Accordingly a series of experiments was carried out in which a representative model tanker was allowed to drift in waves alone in the absence of wind and current. Although the main aim of these experiments was to provide validation data for the NMI drift prediction computer program, the results were of interest in their own right, especially since the opportunity was taken to explore the effect of heel, trim, rudder angle and draught on drift in waves. 3 MODEL EXPERIMENTS 3.1 THE SHIP MODEL A stock NMI model, number 5528, was used for the experiments. Originally a model of a 40,000 dwt bulk carrier, it was chosen as that closest in form to the tankers under investigation. Its principal particulars are shown in Table 1 where they are compared with those of the 50,000 dwt tanker Ondina. (Model dimensions are scaled to give the same length between perpendiculars as Ondina.) The body plan, bow and stern profiles of the model are shown in Figs. 1 and 2. It is clear that model 5528 could represent a ship of about 50,000 dwt albeit with slightly less beam than would be expected for a tanker of similar size. This, however, did not affect the validation which was carried out for the model as tested, the results then being generalised to a more typical tanker form. WSIL - 191 Drift Characteristics.indd 1WSIL - 191 Drift Characteristics.indd 11/24/2009 3:13:23 PM1/24/2009 3:13:23 PM 2 TABLE 1 SymbolUnitModelShipOndina Length between perpendiculars Breadth, moulded Load draught Trim Scale Displacement volume Block coefficient Prismatic coefficient Maximum section coefficient Wetted surface coefficient S/ Rudder area/(Lpp · TL) Maximum rudder angle Screw particulars Model screw number Number of blades Diameter Boss diameter (max.) Design face pitch (mean) Blade area ratio Pitch/diameter ratio LppB TL ? ? CBCpCx m m m m3 degs m m m 2.841 0.363 0.149 level 76.178 0.1270 0.816 0.823 0.992 2.631 0.0155 ±35 W228 4 0.0921 0.0203 0.0677 0.453 0.735 216.42 27.65 11.35 level — 56,143 0.816 0.823 0.992 2.631 0.0155 ±35 — — 7.02 1.55 5.16 0.453 0.735 216.42 31.27 12.19 level — — — — — — — — — — — — — — Model 5528 was constructed of rigid polyurethane foam with a rudimentary superstructure added as in Fig. 3 which shows the model under tests in waves. Fully-proportional radio-control of screw revolutions and rudder angle was provided to allow initial positioning and final retrieving of the model during experiments. The model was also fitted with an onboard heading gyro and telemetry system together with the transmitter of the NMI ultrasonic tracking system. The flux-gate and cover associated with the gyro, as well as the ultrasonic probe amidships can all be seen in Fig. 3. Before each series of experiments the model was dynamically balanced and inclined to find both the yaw gyradius in air and the metacentric height. 3.2 THE EXPERIMENTS The experiments were carried out in the number 4A manoeuvring tank at the National Maritime Institute. This tank is some 30 m square and 2.44 m deep. It is equipped with a wedge-type wave-maker along one side with a beach along the side opposite. Regular or irregular uni-directional long-crested waves can be generated in this tank with the irregular waves conforming to a fixed spectrum. The spectrum does not correspond to a standard form (such as the Oceanic International Towing Tank Conference spectrum) and is dominated by frequencies at or near the modal frequency. It was possible, however, to pre-set both the required significant wave-height, HS , and modal period and the resulting HS , zero up-crossing period and spectrum shape were monitored throughout the experiments using a conductive wave probe, a spectrum analyser and micro-computer. All experiments started with the model at-rest close to the wavemaker either heading directly into, just less than or just more than beam-on to the waves. The wavemaker was then started and the track and heading of the model recorded using the NMI ultrasonic tracking system to record the position of the midships of the model and the gyro to record heading. The results were combined on shore and presented in real-time on a visual display unit. Results were also punched on to paper tape for further analysis. The conditions of the model for the various experiments are given in Table 2 which also lists measured roll, heave and pitch periods together with measured metacentric heights. Displacement in load or ballast conditions remained constant while heel, trim, etc. were varied; all results are scaled to full-size at ? = 76.178. WSIL - 191 Drift Characteristics.indd 2WSIL - 191 Drift Characteristics.indd 21/24/2009 3:13:24 PM1/24/2009 3:13:24 PM 3 The wave conditions tested are referred to below in terms of Beaufort Numbers. These follow the standard ITTC Beaufort Number/significant wave height/windspeed relationships given in Table 3. TABLE 3 Beaufort Number HS (m) Modal frequency (Hz) Wind speed (m/s) 5 6 7 8 9 10 2.71 3.90 5.24 7.16 9.24 11.83 0.121 0.101 0.087 0.075 0.066 0.058 9.3 12.4 15.4 19.0 22.6 26.8 Note: HS = sigilificant wave-height. 3.3 RESULTS OBTAINED In all some 72 runs were carried out of which three were ignored due to equipment malfunction. In the majority of experiments the model was allowed to drift freely in the tank under the action of waves, but for some (numbers 27, 33, 47 and 48(1)) attempts were made to manoeuvre the model using limited power. Examples of the results obtained are given in the Appendix. elapsed time in minutes down-wave (x) and across wave (y) positions in nautical miles ship’s heading in degrees track velocity (or drift speed) in knots track direction in degrees In all cases the axis system and definitions of Fig. 4 have been used. The results are summarised in Table 4 in which standard deviations are shown for cases where runs were repeated, the number of runs in the sample being given as n. It is apparent from this Table and from the results in the Appendix that three runs (58, 59 and 60) were undertaken with a ballast draught trim by the stern of 1.5 m (5 ft) rather than the 3.66 m (12 ft) stern trim of Table 2. This gave some indication of the effect of trim in ballast draught, but it should be remembered that the ‘normal’ ballast draught condition is that of Table 2 with a stern trim of 3.66 m. TABLE 2 Draughts Trim Heel (deg) Rudder (deg) GM (m) Periods (s) FP(m)AP (m)RollPitchHeave 11.35 13.41 10.90 4.64 11.35 8.47 12.05 8.30 LEVEL BOW STERN STERN 0, 4S 0 0 0 0, ±35, FF 0 0 0, ±35 3.19 –– –– 1.81 10.1 11.5 11.1 13.4 7.9 9.6 8.7 8.7 10.5 7.9 7.9 –– Note: FF = ‘flying free’. WSIL - 191 Drift Characteristics.indd 3WSIL - 191 Drift Characteristics.indd 31/24/2009 3:13:24 PM1/24/2009 3:13:24 PM 4 3.4 DISCUSSION OF RESULTS The results of Table 4 are of some interest and the following observations, relating to the drift of the model in waves, have been made. Where appropriate, tests of significance have been made using the Student’s ‘t’ test. (i) In load or ballast draught and in normal trim, the angle of the rudder had no significant effect on drift. (ii) Changing from load to ballast draught at Beaufort 9 waves caused a significant change in drift track, but changes in speed and heading were not significant. The effect of ballast draught (with a 3.66 m trim by the stern) was to cause the model to make more headway across the waves, changing its drift direction from 166° to 142°. (iii) At load draught, a 4° starboard heel made no significant difference to track, speed or heading at a significant wave-height of 5.5 m. (iv) At a significant wave-height of 5.5 m stern trim in load draught had no significant effect, while at a significant wave-height of 10.8 m, stern trim in load draught had a significant effect on drift direction, causing the model to make more headway. (v) No effect of changing trim from 1.5 m to 3.66 m by the stern was observed in ballast draught. (vi) At a significant wave-height of 10.8 m, a trim by the bow had a significant effect on drift track only, causing the model to make more headway. TABLE 4 Beaufort Number HS (m) TZ (s) Track heading (deg) Speed (knots) Heading (deg)d°Trimn Load 5 6 6 6 7 7 7 7 7 7 8 8 9 9 9 9 9 10 10 2.607 (0.138) 4.142 (0.257) 3.891 4.053 5.408 (0.291) 5.678 5.719 5.666 (0.110) 5.838 (0.083) 5.853 (0.064) 7.924 (0.282) 8.091 10.946 (0.252) 10.741 (0.393) 10.843 (0.095) 10.776 11.326 15.506 15.046 (0.246) 9.117 (0.942) 10.170 (0.594) 9.860 9.750 12.095 (0.618) 14.790 17.452 11.406 (0.105) 11.172 (0.021) 11.999 (1.046) 13.360 (0.267) 13.322 14.505 (0.253) 14.890 (0.351) 14.729 (0.264) 14.916 14.305 15.582 15.829 (0.071) 154 (3.950) 158 (1.826) 155 155 157 (2.915) 159 160 158 (0) 157 (2.828) 154.5 (0.707) 161.3 (2.160) 160 166 (1.732) 163.75 (2.986) 147.5 (0.707) 143.5 148 171 163.625 (7.181) 1.533 (0.147) 1.375 (0.096) 1.5 1.5 1.27 (0.045) 1.3 1.0 1.175 (0.035) 1.35 (0.071) 1.2(0.141) 1.025 (0.061) 1.0 0.833 (0.236) 0.963 (0.149) 1.2 (0.141) 1.3 1.25 1.1 1.05 (0.071) 94.1 (1.293) 89.5 (1.291) 89.5 90 93.6 (4.736) 92 92 93 (2.828) 97.75 (0.353) 96.5 (0) 93 (4.486) 107 (?) 93.33 (4.509) 94.67 (1.756) 92.25 (0.353) 115 114 101 95.375 (0.946) 0 0 35S 35P 0 35S 35P 0 0 0 0 35S 0 35S 0 0 FF 0 35S N N N N N N N 4° heel B S N N N N S B N N N 6 4 1 1 5 1 1 2 2 2 6 1 3 4 2 1 1 1 4 Ballast 5 6 7 7 7 8 9 10 6 5 2.544 3.929 5.720 5.550 (0.204) 5.496 7.491 11.347 14.564 3.962 (0.263) 2.467 22.374? 12.290 12.036 11.901 (0.168) 12.119 14.074 14.789 16.310 10.388 (0) 8.639 162 144 143.5 141 (1) 141 146 142 145 158.5 (4.95) 150 1.0 1.1 0.85 1.267 (0.076) 1.3 0.9 0.95 0.9 0.55 (0.071) 1.0 92.5 100.5 101.5 121.83 (5.008) 125 112.5 105 110 93.75 (3.181) 91.5 0 0 0 0 35P 0 0 0 0 0 1.5 mS ” ” 3.66 mS „ „ ” ” ” ” 1 1 1 3 1 1 1 1 2 1 Notes: Hs = Significant wave-height; Tz = Zero up-rossing period; d° = Rudder angle; n = Number of runs; S = Trim by stern; FF = Rudder flying free; N = Normal trim (level); B = Trim by head. Figures in brackets are standard deviations. WSIL - 191 Drift Characteristics.indd 4WSIL - 191 Drift Characteristics.indd 41/24/2009 3:13:24 PM1/24/2009 3:13:24 PM