Breaking and Non-Breaking Wave Load on Pile Group-Supported Marine Structures - Knowledge Generation and Model Development (WaPiGS)
Final Report Abstract
For closely-spaced piles, which are common in both offshore and coastal environments the interference effects between piles may significantly change the flow around the piles, and thus the wave load as compared to that on a single isolated pile. In this case, wave loads on piles within pile groups cannot be calculated by the commonly applied formulae for a single isolated slender pile which are generally based on the Morison equation. Therefore, the main objectives of this research study were: 1) The generation of a knowledge base for a better understanding of the physical processes involved in the interaction of waves and pile groups considering the effects of the most relevant influencing parameters which include different hydrodynamic and structural conditions, and 2) The development of more physically-based and more generic formulae for the prediction of wave loads on a slender pile within a pile group as a function of the most influencing hydrodynamic and structural parameters. To achieve these objectives, the following work has successfully been conducted: 1) A comprehensive review and analysis of the current knowledge on breaking and non-breaking wave loads on pile groups was conducted. As a result, the missing knowledge and data required to achieve the objectives of the research project were identified. 2) The available large-scale tests carried out in the Large Wave Flume (GWK) were re-analysed. The shortcomings of the large-scale experiments were identified which underlined the necessity of performing new scale experiments. 3) New small-scale laboratory experiments were designed and performed in two distinct phases in the 2 m-wide wave flume of the LWI. In Phase 1, similar wave and structural conditions tested in GWK were successfully reproduced in the LWI flume. Based on the results obtained in Phase 1 of the small-scale laboratory tests, Phase 2 was planned and performed in the LWI wave flume to (i) specify more precisely and systematically the most relevant influencing wave and structural parameters on the wave loading of a pile in a pile group with different arrangements and, consequently, to improve the understanding of the processes associated with the wave-pile group interaction, and (ii) generate a comprehensive data set which covers a wide range of wave and structural conditions. 4) An artificial intelligence (AI) - based computational tool, a combination of M5 tree (M5MT) and genetic programming (GP) named "hybrid M5MT-GP model", was developed. This tool was applied for the systematic analysis of the generated data, which resulted in the development of new generic wave formulae for the prediction of wave loads on a slender pile within a pile group in different arrangements as a function of the most significant wave and structure parameters. 5) A "hybrid 2D-3D Computational Fluid Dynamics model" was developed for studying wave-pile group interaction. In the hybrid model approach, a one-way coupling was established between the 2D model for the far-field and the 2D model for studying the fluid-structure interaction in the near field. The far-field model is a 2D incompressible Navier-Stokes multiphase solver for the proper reproduction of phase-focused (freak) waves generated in the laboratory experiments. The near-field model is a multiphase 3D CFD model that utilizes compressible Navier-Stokes equations to enhance the simulation of entrapped air compressibility effects associated with breaking wave impact on structures. The developed hybrid 2D-3D CFD model was successfully validated using the GWK tests. A procedure was developed to control the convergence in the case of highly nonlinear focused waves entering the 3D domain. 6) The validated numerical model was applied for a more systematic parameter study in order to investigate the effect of parameters which could not be tested in the laboratory. As a part of this numerical parameter study, the influence of the wave direction on the resulting wave loads on a pile within a pile group was studied in this phase by modelling pile group arrangements with angles ranging from 0° to 90° between the line connecting the pile centres and the wave direction. In addition, the numerical model is being applied for scale effect assessment. The numerical models were validated for two different scales using the LWI (1:32.5) and GWK (1:5) tests. Two other numerical models were conducted for other scales including prototype (1:1) and a smaller scale (1:10) which lays between the GWK and LWI models. 7) In addition to the pile group effect on wave loading of a slender pile, which was the main part of this research project, wave run-up on a single pile as well as on a pile within pile groups was investigated by means of small-scale experiments in the LWI flume. Based on the most relevant influencing parameters determined in this study, new formulae were developed to predict regular non-breaking wave run-up on single piles and on a pile within other neighbouring piles. The analysis has resulted in several findings for wave-pile groups’ interaction. Keulegan Carpenter number KC, as well as the pile group arrangement and relative spacing S G/D were found as the most influencing parameters on grouping effect K G. The highest amplification of the resulting wave load was found for side by side arrangement where K G increases up to 2.4 for closely-spaced piles with small relative spacing values (S G/D=0.5). For tandem arrangement, K G decreases up to 0.42 for closely-spaced piles due to sheltering effect. The developed M5MT-GP model and formulae are simple, compact and transparent and by which pile group effect K G can be systematically estimated. The prediction formulae are proved to be generic and physically justifiable. They are expected to be widely applied in calculating wave load on closely-spaced pile groups. New wave run-up formulae are shown to be more accurate than the available ones in predicting regular wave run-up on piles.
Publications
- (2014): A Hybrid 2D-3D CFD Model System for Offshore Pile Groups Subject to Wave Loading. 33rd International Conference on Ocean, Offshore and Arctic Engineering (OMAE), San Francisco, USA
El Safti, H., Bonakdar, L., Oumeraci, H.
(See online at https://dx.doi.org/10.1115/OMAE2014-23636) - (2014): Pile group effect on the wave loading of a slender pile. PhD thesis, Technische Universität Braunschweig, Germany (ISBN 978-3-86948-383-2)
Bonakdar, L.
- (2014): Small and Large Scale Experimental Investigations of Wave loads on a Slender Pile within Closely Spaced Neighbouring Piles. 33rd International Conference on Ocean, Offshore and Arctic Engineering (OMAE), San Francisco, USA
Bonakdar, L., Oumeraci, H.
(See online at https://dx.doi.org/10.1115/OMAE2014-23110) - (2015): Pile group effect on the wave loading of a slender pile: A smallscale model study. Ocean Engineering, 108, 449-461
Bonakdar, L., Oumeraci, H.
(See online at https://doi.org/10.1016/j.oceaneng.2015.08.021) - (2015): Pile group effect on the wave loading of a slender pile: A summary of laboratory investigations. Pfahl-Symposiums, Braunschweig, Germany (Dr. Bonakdar has won the Edgard-Frankignoul award 2015 for this research study)
Bonakdar, L., Oumeraci, H.
- (2015): Wave load formulae for prediction of waveinduced forces on a slender pile within a group of piles. Coastal Engineering, 102, 49-68
Bonakdar, L., Oumeraci, H., Etemad-Shahidi, A.
(See online at https://doi.org/10.1016/j.coastaleng.2015.05.003) - (2016): Interaction of waves and vertical array of slender piles. 6th International Conference on The Application of Physical Modelling to Port and Coastal Protection (COASTLAB16), Ottawa, Canada
Bonakdar, L., Oumeraci, H.
- (2016): Regular wave run-up on a single pile and on a pile within pile groups. 3rd International Conference on Coastal Zone Engineering and Management in Middle East, Dubai, UAE
Bonakdar, L., Oumeraci, H.
- (2016): Run-up on vertical piles due to regular waves: Small-scale model tests and prediction formulae. Coastal Engineering, 118, 1-11
Bonakdar, L., Oumeraci, H., Etemad-Shahidi, A.
(See online at https://doi.org/10.1016/j.coastaleng.2016.08.008) - (2016): Use of OpenFOAM in Offshore Engineering for Wave-Structure and Wave-Structure-Soil Interactions. 12th International Conference on Coasts, Ports and Marine Structures (ICOPMAS), Tehran, Iran
El Safti, H., Bonakdar, L., Oumeraci, H.
- (2018). Wave forces on slender piles within pile groups: laboratory tests and prediction formulae for design practice, in: PIANC Yearbook 2017. pp. 33-48
Bonakdar, L.
