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Thesis Van De Leur TU Ver ... 101213.Pdf Post-trenching with a Trailing Suction Hopper Dredger Master thesis Hydraulic Engineering Kevin Van de Leur - 1170279 Van Kevin This thesis may not be released before 13-12-2012 Post-trenching with a Trailing Suction Hopper Dredger Master Thesis Kevin Van de Leur 1170279 December 13, 2010 Dredging Development Department Royal Boskalis Westminster NV Papendrecht, The Netherlands Faculty Civil Engineering & Geosciences Delft University of Technology Delft, The Netherlands Preface This master thesis forms the culmination of the time I spent studying at the university in Delft. Taking the trip down memory lane I can't be anything less than grateful for the opportunities I had. One of these opportunities was my graduation at Boskalis. The combination of performing a literature study, execute DIY model tests and develop my own numerical model made my graduation a very diverse and interesting project. Hereby I would like to express my gratitude towards the members of my committee, their support and knowledge was very valuable. Special grati- tude goes out to Mark Biesheuvel for his patience and guidance during the past nine months. I am convinced that the broad education and extracurricular activities in Delft formed me to be more than an engineer. Therefore I can only hope that the university will provide the same opportunities for future students. Kevin Van de Leur Rotterdam, the Netherlands December 13, 2010 Graduation committee: Prof.dr.ir. C. van Rhee Delft University of Technology ir. G.L.M. van der Schrieck Delft University of Technology ing. M. Biesheuvel Royal Boskalis Westminster N.V. Dr.ir. R.J. Labeur Delft University of Technology iii Abstract It is common practice to protect subsea pipelines by embedding them into the soil. Trenches can be made before or after the pipelines have been laid. In the latter case, the excavation process is called post-trenching. The essence of post-trenching, as handled in this thesis, is erosion of sand by a waterjet. The literature study focused on the processes of jets and erosion. A lot of research has been done in the field of water jets and use- full information is widely available. Nevertheless the available information on the subject of impinging jets is rather limited and the validity remains questionable. Water jets used for post-trenching create high flow velocities for which the traditional erosion equations are not valid. Therefore use is made of a special set of equations for high speed erosion. With the information provided by the literature study a description of jet- ting in sand was made. The known processes were arranged resulting in a set of equations. Following the rules for scaling the set of equations was converted into a properly scaled model. Preliminary model tests were conducted to observe the jet-process and nar- row down the possible jet angles. These preliminary tests were followed by scale model tests to determine the erosion depth for different nozzle angles. A numerical model was developed to simulate the jet erosion. Since the known erosion equations could not model the erosion behaviour of a jet, a turbulence term was introduced. The results of the simulations were com- pared with the model tests. Though the numerical erosion model showed promising results, it could not be validated due to a lack of data. The most important conclusions are that soil can be eroded to the desired depth, a data-set has been created and much insight is gained with respect to the post-trenching process. Last but not least, a numerical model was made that can prove to be useful after better validation. v Contents 1 Introduction 1 1.1 Background . 1 1.2 Post-trenching with a TSHD . 2 1.3 Problem definition . 3 2 Jets 5 2.1 Introduction . 5 2.2 Circular turbulent jet . 5 2.2.1 Flow development region . 6 2.2.2 Developed flow region . 7 2.3 Impinging circular jet . 8 2.4 Oblique impinging circular jet . 9 2.5 Conclusion . 10 3 Erosion 11 3.1 Introduction . 11 3.2 Basic principles . 11 3.2.1 Sedimentation . 12 3.2.2 Erosion . 14 3.3 Van Rijn formula . 14 3.4 Van Rhee stability parameter . 15 3.4.1 Approximation of the van Rhee formula . 16 3.4.2 Conclusion . 20 3.5 Hofland stability parameter . 20 4 Description of jetting in sand 23 4.1 Introduction . 23 4.2 Jet regimes . 23 4.2.1 Penetrating jet regime for translating jet . 24 4.2.2 Deflective jet regime for translating jet . 25 4.2.3 Transitional jet regime for translating jet . 25 4.3 Processes . 26 4.3.1 Jet . 26 vii CONTENTS 4.3.2 Turbulence . 26 4.3.3 Erosion depth . 26 4.3.4 Centrifugal force . 28 4.3.5 Gravity current . 30 4.3.6 Settling . 31 4.3.7 Breaching . 31 5 Scaling of jet processes 33 5.1 Introduction . 33 5.2 Scale factors . 33 5.3 Similarity . 34 5.4 Dimensionless indicators . 34 5.4.1 Froude . 34 5.4.2 Reynolds . 34 5.5 Scaling . 35 5.5.1 Jet pressure and flow velocity . 35 5.5.2 Turbulence . 35 5.5.3 Erosion velocity . 35 5.5.4 Erosion depth . 36 5.5.5 Centrifugal force . 36 5.5.6 Gravity current . 36 5.5.7 Settling . 37 5.5.8 Breaching . 38 5.5.9 Grain diameter . 38 5.6 Scale scenarios, indicators and effects . 39 5.6.1 Froude scenario . 39 5.6.2 Reynolds scenario . 42 5.6.3 Velocity scale nu = 1 scenario . 43 5.7 Conclusion . 44 6 Small scale model tests 47 7 Large scale testing 49 8 Erosion model 51 8.1 Introduction . 51 8.2 Model overview . 51 8.3 Fluid dynamics . 54 8.3.1 Turbulence model . 54 8.3.2 Boundary conditions . 54 8.4 Erosion calculations . 57 8.4.1 Data gathering . 57 8.4.2 Flow driven erosion . 57 8.4.3 Turbulence driven erosion . 59 viii 8.4.4 Computation of the bed level for the next time step . 60 9 Results erosion model 63 9.1 Introduction . 63 9.2 Stability . 63 9.3 Stationary jet . 65 9.4 Trailing jet . 67 9.4.1 Low flow velocity . 67 9.4.2 High flow velocity . 69 9.5 Conclusion . 71 10 Conclusions and recommendations 73 10.1 Conclusions based on model tests . 73 10.2 Conclusions based on numeric erosion model . 73 10.3 Recommendations . 73 A Small scale tests 79 B Conversion of CPT to density 81 C Mass balance 83 D Test results 85 E Boundary conditions 87 F Stagnation point 91 F.1 Introduction . 91 F.2 Pressure gradient . 91 F.3 Turbulence . 92 F.3.1 Friction velocity . 93 F.3.2 LES . 93 F.3.3 Hofland parameter . 95 ix List of Figures 1.1 TSHD Prins der Nederlanden . 2 1.2 Pipe configurations with draghead and nozzle . 3 2.1 Circular jet . 5 2.2 Flow development . 6 2.3 Impinging circular jet . 8 2.4 Oblique impinging jet . 10 3.1 Equations approximating the Shields curve . 19 3.2 Shields curve . 19 4.1 Erosion forms . 23 4.2 Scour hole . 24 4.3 Translating penetrating jet . 25 4.4 Translating deflecting jet . 25 4.5 Translating jet in transitional regime . 26 4.6 Erosion front . 27 5.1 Settling in transitional regime . 37 8.1 Domain for erosion model . 52 8.2 Flow chart of erosion model . 53 8.3 Patches . 55 8.4 Samplepoints . 57 8.5 Calculation of slope angle . 58 8.6 Wall function . 58 8.7 Erosion in the EM . 61 8.8 Translation in the EM . 61 8.9 Bed level for a translating jet . 62 9.1 Area of transition between cilindrical jet inlet and rectangular domain . 64 9.2 Cross section jet inlet . 65 9.3 Velocity distribution stationary jet . 65 9.4 Pressure distribution stationary jet . 66 xi LIST OF FIGURES 9.5 Turbulence distribution stationary jet . 66 9.6 Simulation results . 67 9.7 Cross section of domain after simulation . ..
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