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Research Papers: Ocean Renewable Energy

Soil–Structure Interaction Effects in Offshore Wind Support Structures Under Seismic Loading

[+] Author and Article Information
Fani M. Gelagoti

Grid Engineers P.C.,
Pampouki 3, N. Psychiko,
Athens 15451, Greece
e-mail: fanigelagoti@grid-engineers.com

Rallis S. Kourkoulis

Grid Engineers P.C.,
Pampouki 3, N. Psychiko,
Athens 15451, Greece
e-mail: rallisko@grid-engineers.com

Irene A. Georgiou

Department of Geotechnical Engineering, School of Civil Engineering,
National Technical University of Athens,
9, Iroon Polytechniou Street,
Zografou 15780, Greece
e-mail: irelimni@central.ntua.gr

Spyros A. Karamanos

Mem. ASME
Department of Mechanical Engineering,
University of Thessaly,
Pedion Areos, Volos 38334, Greece;
Department of Structural Engineering, School of Engineering,
The University of Edinburgh,
Sanderson Building, Robert Stevenson Road, The King’s Buildings,
Edinburgh EH9 3FB, Scotland, UK
e-mail: skara@mie.uth.gr

Presented in an early form at the 36th International Conference on Ocean Offshore and Arctic Engineering, Trondheim, Norway, June 2017, and in the companion conference paper OMAE2017-61525.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the Journal of Offshore Mechanics and Arctic Engineering. Manuscript received July 9, 2018; final manuscript received April 8, 2019; published online May 9, 2019. Assoc. Editor: Sungmoon Jung.

J. Offshore Mech. Arct. Eng 141(6), 061903 (May 09, 2019) (15 pages) Paper No: OMAE-18-1097; doi: 10.1115/1.4043505 History: Received July 09, 2018; Accepted April 10, 2019

This paper explores the performance of a 10 MW offshore wind turbine (OWT) supported either on a large diameter monopile or a 4-legged jacket emphasizing on the nonlinear response of its belowseabed foundation. The seabed foundation alternatives, a monopile and a multipod foundation, are compared under monotonic, cyclic, and seismic loading. For all nonseismic scenarios considered, the monopile is more flexible than the jacket and transmits higher rotations at the OWT base. The differences between the two alternatives are amplified in the case of nonsymmetric cyclic loading; the monopile not only deforms more than the jacket but tends to accumulate irrecoverable rotation with increasing loading cycles. The seismic performance of the alternative support structures is assessed for a comprehensive set of earthquake motions. It is concluded that both systems are seismically robust especially when subjected to pure earthquake loading, neglecting the simultaneous action of wind and waves. Alarming issues for OWT performance may arise when a nonzero steady wind force is superimposed to the kinematically induced stressing of the seabed foundation due to the seismic wave action. Jacket legs settle unevenly, while monopiles are building up rotations at increasing rates. Assuming a design-level earthquake and a wind thrust of the order 60% of the NC wind loading amplitude, this seismically induced residual rotation for the monopile may often exceed the deformation tolerance criterion. For the same loading combination, the corresponding rotation of the Jacket installation remains safely within the prescribed limits.

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Figures

Grahic Jump Location
Fig. 1

The problem under consideration: a 10 MW wind turbine installed in a site of normally consolidated clay is supported either on a 4-legged steel jacket structure or a monopile

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Fig. 7

(a) Force-controlled cyclic loading scenarios, (b) evolution of foundation rotation for the loading protocols, and (c) foundation rotation accumulation (only for protocol “C”) with increasing number of cycles for three foundation alternatives: monopile (D = 9 m, L = 36 m), jacket on type-1 pile group (D = 2.5 m; Lp = 49 m), and jacket on type-2 pile group (D = 2.5 m; Lp = 39 m).

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Fig. 2

OWT supported by a jacket structure: view of the symmetrical FE mesh. The tower of the OWT (lying above the jacket) has been omitted from section A.

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Fig. 3

OWT supported on a monopile: view of the symmetrical FE mesh

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Fig. 4

(a) Numerical calibration of kinematic hardening model to best fit the experimentally derived shear modulus–shear strain (Gγ) curve provided by Raptakis et al. [39] and (b) example hysteresis shear stress–shear strain (τγ) loops of a clay specimen subjected to a cyclic simple shear quasistatic loading of 10 cycles at two characteristic stain levels: γ = 5 × 10−3 and γ = 1 × 10−2

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Fig. 8

(a) First and second eigenmode of the monopile and jacket installation assuming fixities at the mudline and (b) power density spectrum of the external loads acting on the OWT

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Fig. 5

Contours of plastic deformations around the foundation elements illustrating the resisting mechanisms of (a) a monopile and (b) a jacket at the instant of application of EC loads on the turbine

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Fig. 6

Foundation response in terms of global rotation (θ) under monotonically applied environmental (wind and waves) loads: (a) rotation at the monopile head and (b) rotation of the base of the jacket. The individual markers on the figures represent permanent (irreversible) rotation upon removal of the environmental loads.

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Fig. 10

The elastic response spectra of the five earthquake motions under study. The EN 1998-1 (EC8) design spectrum at the location of the turbine is also portrayed.

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Fig. 9

The seismic excitations: acceleration time histories at the ground surface (black line) and the rock-outcrop (thin dotted line)

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Fig. 11

The 10 MW turbine is excited by SM3: recorded acceleration (a) at the nacelle level and (b) at the base of the tower; evolution of foundation rotation at the seabed for a turbine supported on (c) a monopile and (d) a jacket.

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Fig. 12

Wind load effect on the seismic performance of the 10MW wind turbine: foundation rotation time history for (a) the monopile and (b) the jacket. Excitation Record: SM3. Dashed line for OWTs at parked state (i.e., no wind). The rotation operability limit (which according to the DNV-OS-J101 standard equals 0.25 deg) is also denoted.

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Fig. 13

Time evolution of the dimensionless foundation rotation θ/θο for an OWT supported on (a) a jacket and (b) a monopile. Denoted on the plots are the rotation evolution trendline (dotted line) and two characteristic time instants t1, t2 defining the onset and termination of the strong seismic shaking (on the ground surface), respectively. Excitation record: SM3.

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Fig. 14

Time evolution of dimensionless foundation rotation θ/θο for the five seismic scenarios analyzed: (a) jacket and (b) monopile

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