Investigations on Scour at Tripod Foundation Structures in the German Offshore Test Site alpha ventus

A. Stahlmann and T. Schlurmann1

In 2009, the first German offshore test site alpha ventus has been installed in the North Sea in water depths of 30 m, where experience shall be gained and made available for future offshore wind farms. Regarding foundation structures for offshore wind turbines in deep water it is well known that in most cases scouring phenomena occur around the foundations. Due to the complexity of new structures, significant knowledge gaps in scour progression and its probable effects on the stability and durabil-ity are given. Therefore, investigations on scouring phenomena around complex foundations like the tripod are carried, consisting of a unique combination of physical and numerical modeling and local scour monitoring. The physical modeling part was carried by means of 1:40 laboratory tests and 1:12 large-scale physical model tests in wave flumes, whereas numerical simulations on scouring proc-esses and sensitivity analyses are carried out using CFD code. The results so far show that scours at the tripod foundation do not only occur directly around the foundation piles, but also in the near-field and esp. under the structure. The pnenomena are currently further investigated using CFD code, by studies on flow pattern and scour development under combined effects of wave and current loads, as well as by studies on structural variations leading to enhanced or reduced erosion and therefore scour formation.

Introduction and Background

Regarding foundation structures for offshore wind turbines in deep water it is well known that in most cases scouring phenomena occur around the foundations, owing to the presence of the supporting structure and hence affecting changes in the natural flow regime at the sea bed around the foundation, leading to increased sediment mobility. In the first German offshore wind test site alpha ventus in the North Sea, located 45 m off the coast of the Borkum Island, six converters have been founded on tripod structures in water depths of 30 m in 2009. Due to the complexity of this foundation type, significant knowledge gaps in scour initiation and progression in general and especially in detail and its probable effects on the stability of the structure are given, leading to lacking practical design guidelines.

In order to gain experience for future wind farms, the German Federal Ministry for the Environ-ment (BMU) is supporting the research project Gigawind alpha ventus at the Leibniz University Hanover within the research initiative RAVE (Research at Alpha Ventus), with its overall objec-tive of cost reduction and structural optimization of support structures for offshore wind energy deployment in deep water.

Objective and Methodology

Within a sound research initiative, investigations on scour around complex structures like the tripod are carried out. It consists of a unique combination of physical and numerical modeling of the scour processes as well as local scour monitoring in the test site, undertaken by the Ger-man Federal Maritime and Hydrographic Agency (BSH) as a long-term measuring campaign using echo sounder equipment. Therefore, physical modeling was carried out by means of 1:40 laboratory tests and 1:12 large-scale model tests (Fig.1) in the wave flume of the Franzius-Institute and the Large Wave Flume (GWK) in Hanover, whereas numerical simulations on scouring processes and sensitivity analyses are carried out using CFD code. Using this ap-proach, the influences of sea state and tidal-induced flow conditions as well as physical proc-esses in general and in detail can be investigated and structural parameter leading to scour can be identified. Furthermore, scour prediction can be optimized by extending existing approaches, which might not be applicable for complex foundation types.

                                                                                                               1  Franzius-Institute for Hydraulic, Waterways and Coastal Engineering, Leibniz University Hanover,

Nienburger Str. 4, 30167 Hannover, Germany. Phone: +49 511.762-2584, fax +49 511.762-4002, e-mail: stahlmann@fi.uni-hannover.de  

Physical Modeling

From the investigations carried out so far it can generally be concluded that scour does not only occur directly at the foundation piles but also around and below the structure as a combination of local and global scour (Fig. 2), which in this form could not be expected when using common practice design rules and approaches, e.g. given in [1] or [2], but which has to be considered when regarding the stability of the soil and the dynamics of the structure.

Figure 1: Tripod structure installation and measuring instrumentation in the Large Wave

Flume (GWK) for the 1:12 large-scale physical model tests on scouring.

 Figure 2: Scour development for a 1:12 test series in the GWK (regular waves, d=2.5m,

Hm=0.76m, Tm=5.48s). Rel. depths S/D after 3000 waves: 1.13 (1P), 0.82 (3P), 1.11 (MC).

Results of the laboratory and large-scale physical model tests in waves flumes can be found in [3], where differences in scour depths depending on the model scale and load conditions on the one hand, but also a qualitatively good agreement of scour pattern between large-scale tests and in-situ measurements from the measuring campaign could be shown on the other hand (Fig. 3). Differences in scour depths due to general scaling effects and different load conditions are further investigated in detail, also in combination with numerical tests, which is work in pro-gress.

 Figure 3: Qualitative comparison of scour depths. Left: In-situ data from multi beam survey,

April 2010. Source: Lambers-Huesmann & Zeiler, BSH 2010. Right: 1:12 model test (JONSWAP spectrum, d=2.5m, Hs=0.72m, Tp=5.52s)

Numerical Modeling

Due to limitations in the laboratory facilities that have so far been used for the physical model studies, only unidirectional wave loads without the presence of a tide-induced current could be modeled, presumably leading especially to observed differences in scour depths between large-scale test results and in-situ measurements. To circumvent this constraint, numerical simula-tions are additionally carried out using CFD methods.

For the numerical tests, the open-source CFD package OpenFOAM® (released by OpenCFD®) has been used. A fully 3D multiphase solver has been chosen, using RANS equations and a k-Omega-SST turbulence model. The numerical model has been calibrated and validated with regard to flow pattern and shear stresses, generally using empirical formulations and model data from literature, and especially using measurements from the previously mentioned model tests in wave flumes. In addition to investigations on flow pattern and shear stresses on a fixed bed, a moving mesh approach simulating the scour development on a moving bed has been implemented. The scour model including bed load and suspended load is solving the Exner equation for sediment, whereas the sediment transport model of [4] has so far been chosen.

Using this setup, simulations on flow pattern and scour development around a tripod foundation on different model scales and with combined effects of wave and current loads (Fig. 4) with varying directions are performed. Furthermore, parameter studies on structural variations lead-ing to enhanced or reduced erosion and therefore scour formation are carried out. Out of this, new insights on the physical processes of scouring around the foundations are gained, which can lead to optimized design guide lines for such complex foundation types.

Figure 4: Left: Bed shear stress amplification factors for an exemplary 1:40 scenario with (top) wave load, (bottom) wave + current load under the wave crest. Right: Water surface and near-bottom velocity magni-

tudes in a 1:40 CFD simulation under regular wave conditions in OpenFOAM.

Conclusions

Investigations on scouring around complex foundation structures like the tripod have been and are further carried out using a unique combination of physical modeling, in-situ measurements and numerical CFD simulations. Using this approach, the influences of sea state and tidal-induced flow conditions on the scouring phenomena as well as physical processes in general and in detail can be investigated and structural parameter leading to scour may be identified in order to fill gaps in knowledge of scour progression and its probable effects on the stability of complex converter foundation structures. Out of this, scour prediction can be optimized by ex-tending existing approaches, which might not be applicable for complex foundation types.

Acknowledgements

The authors gratefully acknowledge the support of the German Federal Ministry for the Envi-ronment (BMU) within the funded project "Ganzheitliches Dimensionierungskonzept für OWEA-Tragstrukturen hinsichtlich Lasten, Langlebigkeit, Gründung und Gesamtstruktur" (BMU code 0325032).

Project details and cooperation partners can be collected from www.gigawind.de.

References

1. Sumer, B.M.; Fredsøe, J. (2002): The Mechanics of Scour in the Marine Environment. World Scient. Publishing Co. Pte. Ltd.

2. Zanke, U.C.E.; Hsu, T.-W.; Roland, A.; Link, O.; Diab, R. (2011): Equilibrium scour depths around piles in noncohesive sediments under currents and waves. Coastal Engineering, Vol. 58, 986-991

3. Stahlmann, A., Schlurmann, T. (2010). Physical Modeling of Scour around Tripod Founda-tion Structures for Offshore Wind Energy Converters. Proceedings of the International Con-ference on Coastal Engineering, No. 32 (2010), Shanghai

4. Engelund, F. and Fredsøe, J. (1976). Sediment Transport Model for Straight Alluvial Chan-nels. Nordic Hydrology, 7, 293-306