Influence of liquefaction on scour around offshore monopile foundations Msc Thesis March 2014 Ferdinand van den Brink Influence of liquefaction on scour around offshore monopile foundations March 2014 Ferdinand van den Brink In partial fulfilment of the requirements for the degree of Master of science in Civil Engineering Structural Engineering Track at Delft University of Technology & Master of science in Civil Engineering Water Management and Engineering at University of Twente Committee: Prof. dr. ir. W.S.J. Uijttewaal Delft University of Technology Prof. dr. S.J.M.H. Hulscher University of Twente Dr. ir. Pieter Roos University of Twente Ir. T.C. Raaijmakers Deltares Dr. ir. W. Broere Delft University of Technology Ir. F. Renting Delft University of Technology 1 2 Summary Scour hole formation around offshore monopile foundations is a threat for the structure’s stability. Large uncertainty leads to conservatism in design codes and hence to high construction costs. Under the same hydrodynamic loading as for scour hole formation the soil can liquefy due to structural vibrations or waves. The aim of this study is to determine the effect of liquefaction from vibrations of offshore monopile foundations on scour by performing scaled flume experiments, where liquefaction is induced by a monotonic excess pore water pressure (EPWP). Liquefaction is known to be caused by EPWP build up under cyclic loading and normally lasts for limited time, because the built up pore water pressure drains off to the bed surface. Therefore, monotonic EPWP is introduced at the bottom of a pile, which is placed in a flume filled with fine sand. During the experiments a current is used to induce scour, while the EPWP is used as independent variable. In the experiments the EPWP gradient is observed to take some time to bring the sediment into liquefaction. First the soil is lifted, but as soon as the vertical resistance of the sand is lost a current breaks through. Subsequently, the flow concentrates in one feeder and sediment is transported as if it is in suspension. When this occurs depends on the magnitude of the EPWP. During the scour experiments this resulted in a sudden collapse of the scour hole. The scour depth decreased and a new balance arises between slope sliding and erosion due to the horseshoe and lee-wake vortices. It is concluded that under liquefaction the equilibrium scour depth decreases for a larger negative excess pore water pressure gradient. Furthermore, the angle of repose is decreased. The equalising effect of liquefaction on the scour hole is also expected in field situations, but the extent is unknown. The potential gain of the decreased scour depth to the structure’s stability is limited, since the liquefied area may not be expected to provide any contribution to the stability of the structure. 3 Samenvatting Ontgronding rondom offshore monopile funderingen vormt een bedreiging voor de stabiliteit van de constructie. Grote onzekerheid leidt tot conservatisme in de ontwerpvoorschriften en deswege een verhoging van de bouwkosten. Bij dezelfde belasting als voor ontgronding kunnen trillingen zorgen voor verweking van de bodem. Het doel van het onderzoek is om het effect the bepalen van de verweking ten gevolge van trillingen in de constructie op het ontgrondingsgedrag door middel van geschaalde experimenten in een stroomgoot, waarbij de verweking is aangebracht met een monotone grondwateroverdruk (EPWP). Verweking kan ontstaan door accumulatie van EPWP onder cyclische belasting en heeft normaal gesproken een beperkte duur, omdat de opgebouwde poriewaterdruk naar boven wegvloeit. Daarom is EPWP aangebracht aan de onderkant van een paal, die geplaatst is in een met fijn zand gevulde stroomgoot. Tijdens het experiment wordt een constante stroming gebruikt om ontgronding te veroorzaken en de EPWP is gebruikt als onafhankelijke variabele. Tijdens de experimenten kost het enige tijd voordat de EPWP gradiënt leidt tot verweking. Eerst worden de bodem iets opgetild, maar nadat de verticale weerstand in het zand is overwonnen, komt een stroming op gang. Wanneer dit gebeurd hangt af van de grootte van de aangebrachte EPWP. Tijdens het experiment resulteert dit erin dat het ontstane gat plotseling voor een deel instort, maar zich vervolgens verder kan ontwikkelen met een andere natuurlijke hellingshoek. De evenwichtsontgrondingsdiepte is afgenomen waarbij zich een nieuwe balans heeft ingesteld tussen afschuivend sediment en erosie ten gevolge van “horseshoe” en “lee-wake” wervelingen. Er kan worden geconcludeerd dat de evenwichtsontgrondingsdiepte afneemt naar mate de toegepaste grondwateroverdruk toeneemt. Daarnaast is de natuurlijke hellingshoek afgenomen. Dit gelijkmakende effect van verweking wordt ook in het veld verwacht, hoewel de mate op basis van de experimenten niet kan worden geschat. De potentiele winst van de verminderde ontgrondingsdiepte fundering is beperkt, want van het verweekte gebied mag nauwelijks een bijdrage worden verwacht aan de stabiliteit van de windmolen. 4 Acknowledgements With this thesis I will complete both my MSc specialisation Water Engineering & Management at the university of Twente and Structural Mechanics at Delft University of Technology. Since this is a cooperation between two universities, this project was not so self-evident and required more dedication of my supervisors. This research would not be succeed without their help and support. Great thanks goes to my daily supervisor at Delft University of Technology Wim Uijttewaal for his great guidance and critical feedback. I experienced our meetings as very interesting and constructive. Besides, I would like to thank my everlastingly enthusiastic daily supervisor Twente University Pieter Roos for his useful feedback and understanding. I would like to thank Suzanne Hulscher for the pleasant and useful meetings and here support throughout the project when I needed it. I am very grateful to the other members of the committee Tim Raaijmakers, Wout Broere and Frank Renting, for their feedback. For the construction of the research setup a lot of people were assisting, including Hans Tas, Jaap van Duin, Frank Kalkman and Tom from DEMO. Special thanks goes out for Sander de Vree, who was always willing to listen and help with the practical problems I faced in a constructive way. Next I thank my fellow students at the MSc room for their companionship, motivating eeuwige roem punten system and numerous kleintjes. Finally I would like to thank my family and friends for their aid and confidence. 5 6 Contents Summary ................................................................................................................................. 3 Samenvatting .......................................................................................................................... 4 Acknowledgements ................................................................................................................. 5 List of symbols ...................................................................................................................... 11 1. Introduction ................................................................................................................... 13 1.1 Current developments on offshore wind energy ........................................................ 13 1.2 Goal of research. ......................................................................................................... 16 1.3 Research questions ..................................................................................................... 16 1.4 Methodology ................................................................................................................ 16 1.5 Outline of report .......................................................................................................... 16 2. Theoretical background ................................................................................................ 17 2.1 Introduction ................................................................................................................. 17 2.2 Fundamental hydrodynamic and morphodynamic processes ................................... 18 2.2.1 Bed shear stress ................................................................................................... 18 2.2.2 Shields parameter ................................................................................................ 19 2.2.3 Important dimensionless quantities ................................................................... 20 2.3 Classifications and definitions ................................................................................... 21 2.3.1 Slender piles and large piles ............................................................................... 21 2.3.2 Local and global scour.......................................................................................... 22 2.3.3 Clear-water and Live-bed .................................................................................... 22 2.3.4 Current-induced and wave-induced .................................................................... 22 2.3.5 Liquefaction .......................................................................................................... 23 2.4 Theory on scour hole development ............................................................................. 23 2.4.1 Mechanism ........................................................................................................... 24 2.4.2