Green Water on Ship-type Offshore Structures Deift University of Technology Ship Hydromechanics Laboratory Library Mekeiweg 2, 2628 CD Deift The Netherlands Phone: +31 15 2786873 - Fax +31 15 2781836 Bas Buchner Green Water on Ship-type Offshore Structures .1 On the cover: 'The Tempest', Bart van der Leck, 1916 120 x 60 cm, oil painting Kroller-Müller Museum ©Bart van der Leck The Tempest, 2002 do Beeldrecht Hoofddorp Printed by: Grafisch Bedrijf Ponsen & Looijen by, Wageningen, The Netherlands Green Water on Ship-type Offshore Structures Proefschrift ter verkrijging van de grand van doctor ann de Technische Universiteit Deift, op gezag van de Rector Magnificus Prof. dr. ir. J.T. Fokkema, voorzitter van het College van Prornoties, in het openbaar te verdedigen op dinsdag 5 november 2002 te 16.00 uur door Bastiaan BUCHNER scheepsbouwkundig ingenieur geboren te Sliedrecht Dit proefschrift is goedgekeurd door de promotor: Prof.dr.ir. J.A. Pinkster Samenstelling promotiecommissie: Rector Magnificus, voorzitter Prof dr.ir. J.A. Pinkster, Technische Universiteit Deift, promotor Prof.ir. J. Meek, Technische Universiteit Deift Prof. A. Incecik, University of Newcastle Prof., J. Juncher Jensen, Technical University of Denmark Prof.dr. A.E.P. Veidman, Rijksuniversiteit Groningen Prof.dr.ir.. A.J. Hermans, Technische Universiteit Deift Dr.mg. C.T. Stansberg, Marintek ISBN: 90-4646-883-X Copyright © Bas Buchner, 2002. All rights reserved. "And behold, there arose a great storm on the sea, so that the boat was beiAg swamped by the waves..." Matthew 8.24 CONTENTS INTRODUCTION I 1.1 The problem 1.1.1The problem of 'green water' 1 1.1.2The problem of green water for ship-type offshore units 2 1.2 Historical overview of green water research 5 1.2.1Early studies 5 1.2.2Experimental studies, focussing on relative wave motions 5 1.2.3Experimental studies, focussing on loads on deck 6 1.2.4Effect of bow shapes 7 1.2.5Stability of fishing vessels 8 1.2.6Prediction methods 8 1.3 Problem definition and objective of this study 9 1.4 Outline of the thesis 10 1.5 Model tests and their interpretation 11 2.THE PHYSICS OF GREEN WATER ON THE BOW 13 2.1 Introduction 13 2.2Model tests 14 2.2.1Test Series A: Pilot tests FPSO with traditional full bow 14 2.2.2Test Series B: Comparative tests traditional full bow and alternative thin bow 16 2.3 Observations 17 2.3.1Visualisation techniques applied 17 2.3.2Summary of observations 20 2.4Motions and relative wave motions 22 2.4.1Ship motions 23 2.4.2Relative wave motions 28 2.5 Water flow onto the deck 33 2.6Effect of above water hull shape 34 2.7 Water behaviour and loading on the deck 35 2.8 Green water impact on structures 39 2.9Summary of the physics of green water on the bow 43 3. SYSTEMATIC MODEL TESTING 45 3.1 Introduction 45 3.2 Structure of the semi-empirical design evaluation method 46 3.3 Systematic model series 47 3.3.1Bow shape variations 48 3.3.2Bow flare variations 49 3.3.3Stem shapes 50 3.3.4Sunimaiy of basic hull models 51 3.4Structures at the bow 51 3.5Protective breakwaters 53 3.6Set-up: soft spring mooring system 53 Contents 3.7Environmental conditions 54 3.8Analysis of combined extremes 55 4.NON-LINEAR RELATIVE WAVE MOTIONS AT THE BOW 57 4.1 Introduction 57 4.2Description of non-linearities 58 4.3Physical background of non-linearities 63 4.3.1The effect of the water on deck on the ship motions 64 4.3.2The effect of the above water hull shape 65 4.3.3The effect of the non-linearity in the waves 68 4.4Review of existing descriptions of non-linearities 69 4.5 Proposed method for the prediction of relative wave motion extremes 72 4.5.1Introduction 72 4.5.2Relative wave motions below the freeboard 73 4.5.3Relative wave motions above the freeboard 77 4.6Validation of the developed expression 84 4.6.1Variations available for validation 84 46.2Validation results 85 4.7Effect of current 86 4.8Summary of the development of the modified Rayleigh distribution 90 5. WATER FLOW ONTO AND ON THE BOW DECK 93 5.1 Introduction 93 5.2Observation of the flow onto the deck for different flare angles 94 5.3 Relation between relative motions and water height on the deck 95 5.4 Flow patterns and velocities over thedeck 102 6.GREEN WATER IMPACT LOADING 109 6.1 Introduction 109 6.2 Relation freeboard exceedance and impact loading 110 6.3 Vertical and horizontal load profiles 122 6.3.1Vertical load profile 122 6.3.2Horizontal load profile 129 6.4Effect of structural shape on impact loading 130 6.4.1Method 130 6.4.2Direct comparison between the different structutal shapes 135 6.5 Effect of distance from the forward perpendicular 136 6.6Summary of load prediction procedure for structures on the deck 137 6.7Efficiency of protecting breakwaters 138 6.7.1Design considerations 138 6.7.2Observations 138 6.7.3Quantification of breakwater efficiency 139 6.7.4Loading on the breakwater 141 GREEN WATER FROM THE SIDE AND STERN 143 7.1 Introduction 143 7.2 Model tests 144 7.3 Observations of relative wave motions along the side 145 7.4 Relative wave motions along the side of a hull 145 7.4.1Calculated linear relative wave motions 145 7.4.2Measured non-linear relative motions along the side 146 7.5Physical background of observed behaviour 147 7.6Description of non-linear relative wave motions 149 7.7Green water flow and loading from the side 153 7.7.1Transverse water flow onto the deck 153 7.7.2Hydraulic models for green water flow from the side 155 7.7.3Green water loading from the side on slender structures onthedeck 158 7.8 Green water in stem area 162 7.8.1Traditional tanker stems with a relatively low freeboard 164 7.8.2 New design full and flat stems with a high freeboard 164 REVIEW AND APPLICATION OF THE METHOD 167 8.1 Introduction 167 8.2 Range of applicability of the method 169 8.3 Coupling with specific metocean data 172 8.3.1Use of the contour ofjoint probabilities of wave height and period 172 83.2Determination of the extreme heading angle with respect to the waves 174 8.4Dynamic structural response 174 NUMERICAL PREDICTION OF GREEN WATER 179 9.1 Introduction 179 9.2 Requirements for numerical methods 180 9.3 Evaluation of existing numerical methods 181 9.3.1General 181 9.3.2. Method using height functions Glimm's method 181 9.3.3Method using line segments: non-linear boundary integral method 184 9.3.4Method using fluid regions: VOF 188 9.3.5Evaluation of a modified-VOF method 189 9.4Mathematical and numerical model of the modified-VOF method 190 9.4.1Mathematical model 190 9.4.2Numerical model: geometry and free surface description 190 9.4.3Discretisation of the Navier-Stokes equations 192 9.4.4Other aspects in the numerical model 194 Contents 9.5 Case study 1: Flow onto a deck with impact on different structural shapes 195 9.5.1General 195 9.5.2Computational domain and initial conditions 195 9.5.3Results 197 9.6 Casestudy 2: Water entry of a 2D wedge 202 9.6.1General 202 9.6.2Model tests 203 9.6.3Modified-VOF simulation model 205 9.6.4Results 205 9.7Evaluation of Modified-VOF method related to the green water problem 209 9.8Possibilities for the present application of the Modified-VOF method210 10. CONCLUSIONS AND RECOMMENDATIONS 213 10.1 Summary ofobjective 213 10.2 Conclusions 213 10.3 Recommendations for further research 217 APPENDIX I: SCALING OF GREEN WATER PHYSICS 219 APPENDIX II: EFFECT OF STRUCTURAL ELASTICITY 223 APPENDIX A: Pilot tests traditional full bow (Test Series A) 235 APPENDIX B: Comparative tests traditional full bow and alternative thin bw (Test Series B) 239 APPENDIX C: Systematic testseries with different hull shapes and flare angles (Test Series C) 243 APPENDIX D: Pilot tests green water from the side (Test Series D) 257 REFERENCES 259 NOMENCALTURE 267 SUMMARY 273 SAMENVATTING 275 ACKNOWLEDGEMENTS 277 CURRICULUM VITAE 279 1 1. INTRODUCTION 1.1 The problem 1.1.1The problem of 'green water' In heavy storms, the waves and ship motions can become so large that water flows onto the deck of a ship. This problem is known as 'shipping of water', 'deck wetness' or 'green water loading'. The term 'green water' is used to distinguish between the spray (small amounts of water and foam) flying around and the real solid seawater on the deck. Because seawater is rather green than blue, the term 'green water' is widely used. For a long time already, green water has been considered to be an important problem for the safety and operability of naval and merchant vessels. Tan (1969) noted that shipping of water was the most important reason for changing course and speed for Dutch merchant ships in order to avoid serious damage to the ships or their deck loads. Also the widely known accident with M.V. Derbyshire has shown the destructive force of green water loading and its fatal consequences, see for instance Faulkner (2001). The same applies to naval vessels. Andrew and Lloyd (1980) quote Captain D. Maclntyre from his book 'The Battle of the Atlantic': "Their hulls whipped and shuddered in the huge Atlantic seas.., solid green water swept destructively along their decks.. .For hour after hour this process repeated itself.