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Research Papers: Ocean Renewable Energy

Response Analysis of a Nonstationary Lowering Operation for an Offshore Wind Turbine Monopile Substructure

[+] Author and Article Information
Lin Li

Centre for Ships and Ocean Structures (CeSOS);
Centre for Autonomous Marine Operations and Systems (AMOS);
Department of Marine Technology,
Norwegian University of Science and Technology (NTNU),
Trondheim NO-7491, Norway
e-mail: lin.li@ntnu.no

Zhen Gao, Torgeir Moan

Centre for Ships and Ocean Structures (CeSOS);
Centre for Autonomous Marine Operations and Systems (AMOS);
Department of Marine Technology,
Norwegian University of Science and Technology (NTNU),
Trondheim NO-7491, Norway

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received April 7, 2015; final manuscript received June 16, 2015; published online July 27, 2015. Assoc. Editor: Yi-Hsiang Yu.

J. Offshore Mech. Arct. Eng 137(5), 051902 (Jul 27, 2015) (15 pages) Paper No: OMAE-15-1030; doi: 10.1115/1.4030871 History: Received April 07, 2015

This study addresses numerical modeling and time-domain simulations of the lowering operation for installation of an offshore wind turbine monopile (MP) with a diameter of 5.7 m and examines the nonstationary dynamic responses of the lifting system in irregular waves. Due to the time-varying properties of the system and the resulting nonstationary dynamic responses, numerical simulation of the entire lowering process is challenging to model. For slender structures, strip theory is usually applied to calculate the excitation forces based on Morison's formula with changing draft. However, this method neglects the potential damping of the structure and may overestimate the responses even in relatively long waves. Correct damping is particularly important for the resonance motions of the lifting system. On the other hand, although the traditional panel method takes care of the diffraction and radiation, it is based on steady-state condition and is not valid in the nonstationary situation, as in this case in which the monopile is lowered continuously. Therefore, this paper has two objectives. The first objective is to examine the importance of the diffraction and radiation of the monopile in the current lifting model. The second objective is to develop a new approach to address this behavior more accurately. Based on the strip theory and Morison's formula, the proposed method accounts for the radiation damping of the structure during the lowering process in the time-domain. Comparative studies between different methods are presented, and the differences in response using two types of installation vessel in the numerical model are also investigated.

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References

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Figures

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Fig. 1

Lifting arrangement of the monopile using a floating installation vessel

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Fig. 2

Natural periods of the monopile rigid-body motions with varying positions (mode 1: heave motion dominant; mode 2 and mode 3: rotational motion dominant; and mode 4 and mode 5: pendulum motion dominant)

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Fig. 3

Added mass of the MP at different drafts (refer to the point of MP at the mean free surface)

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Fig. 4

Excitation force of the MP at different drafts (refer to the point of MP at the mean free surface)

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Fig. 5

Potential damping coefficients of the MP at different drafts (refer to the point of MP at the mean free surface)

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Fig. 6

Response spectra of MP in irregular waves with Hs = 2.0 m, Tp = 5 s, and Dir = 0 deg (fixed crane tip, FFT, up to z = 0)

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Fig. 7

Response spectra of MP in irregular waves with Hs = 2.0 m, Tp = 8 s, and Dir = 0 deg (fixed crane tip, FFT, up to z = 0)

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Fig. 8

Response spectra of MP in irregular waves with Hs = 2.0 m, Tp = 5 s, and Dir = 0 deg (floating crane vessel, FFT, up to z = 0)

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Fig. 9

Response spectra of MP in irregular waves with Hs = 2.0 m, Tp = 8 s, and Dir = 0 deg (floating crane vessel, FFT, up to z = 0)

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Fig. 10

Response spectra of MP in irregular waves with Hs = 2.0 m, Tp = 5 s, and Dir = 0 deg (fixed crane)

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Fig. 11

Response spectra of MP in irregular waves with Hs = 2.0 m, Tp = 5 s, and Dir = 0 deg (fixed crane, cosine method, up to z = ζ)

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Fig. 12

Retardation functions at different drafts

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Fig. 13

Comparison of the retardation functions at a draft of 7 m

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Fig. 14

Response time series of the entire lowering process (fixed vessel, Hs = 2.5 m, Tp = 5 s, and Dir = 0 deg)

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Fig. 15

Response time series of the entire lowering process (fixed vessel, Hs = 2.5 m, Tp = 12 s, and Dir = 0 deg)

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Fig. 16

Response spectra of the lowering phase (fixed vessel, Hs = 2.5 m)

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Fig. 17

Response spectra of the steady-state phase (fixed vessel, Hs = 2.5 m)

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Fig. 18

STD of the responses in the steady-state phase using a fixed vessel (Hs = 2.5 m, Dir = 0 deg): (a) rotational resonance motion and (b) pendulum resonance motion

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Fig. 19

Ratios of energies from different contributions using a fixed vessel (Hs = 2.5 m, Dir = 0 deg): (a) ME-only method and (b) ME + RT method

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Fig. 20

Response spectra of the steady-state phase (floating vessel, Hs = 2.5 m)

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Fig. 21

STD of the responses in the steady-state phase using the floating vessel ((Hs = 2.5 m, Dir = 0 deg): (a) rotational resonance motion and (b) pendulum resonance motion

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Fig. 22

Ratios of energies from different contributions using the floating vessel (Hs = 2.5 m, Dir = 0 deg): (a) ME-only method and (b) ME + RT method

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