Elsevier

Applied Ocean Research

Volume 114, September 2021, 102707
Applied Ocean Research

Effect of hydrodynamic coupling of floating offshore wind turbine and offshore support vessel

https://doi.org/10.1016/j.apor.2021.102707Get rights and content

Highlights

  • The hydrodynamic coupling is studied between Semi-sub wind turbine and support vessel.

  • The extreme response and motion characteristics are assessed by different coupling strategies in the time domain.

  • Impact of wave drift force and hydrodynamic interaction is evaluated and discussed.

Abstract

The hydrodynamic coupling of Semi-submersible floating wind turbine and offshore support vessel during walk-to-work operation is studied, which is aimed at investigating the interaction of seakeeping and stationkeeping on both floating structures. The complex multi-body hydrodynamic problem is analyzed by 3-D diffraction and radiation computation in the frequency domain. To identify and evaluate coupling effects, three strategies are proposed in the modeling of the time domain simulation i.e. fully-coupling, no-coupling, and semi-coupling which ignores the interactive wave damping terms between the wind turbine and support vessel. The dynamic and mechanical properties of the wind turbine such as nonlinear mooring tension and damping are validated and reach a good agreement with existing measurements. A series of sea-state that combining different wave conditions and wave headings are taken into account in the study. By analyzing the maximum and spectrum of dynamic response, the surge and sway of support vessel show over-predicted motion due to the overestimate of drift force if hydrodynamic interaction was ignored. Under the following sea, the apparent ’splitting’ phenomenon can be observed that makes both structures drift towards opposite directions. The sway and roll of the wind turbine reduce significantly by up to 40% because of the ’shielding effect’ that ’cut down’ both linear and nonlinear wave forces in the entire frequency range. Sway and heave of support vessel present behavior of ’gap resonance’ where different modes of peak or trough are observed whose occurrence is however opposite to sway and heave.

Introduction

The offshore wind industry is rapidly moving forward, the installed capacity is expectedly scaling up to seven-fold by 2030 from the current 29 GW to 228 GW worldwide (IRENA (2019)). Such significant expansion creates challenges to the operation and maintaining service which costs from 31.3% to 34% of the total cost over the life cycle (Stehly and Beiter (2019)). The main reason is the relatively poor accessibility to the wind farm compared to onshore installation, which makes it hazardous to induce longer downtime and consequently the suspension of energy production.

In the past few years, a relatively new approach walk-to-work (W2W) accessibility received great attention, because it is favorable to deeper water and severe weather compared to the work boat of near-shore usage. W2W requires the offshore support vessel (OSV) of 60 m long or above, the operation is conducted by relying on the dynamic positioning system (DP) and active compensated gangway in most of the time. Fig. 1 illustrates an example of W2W access between an OSV vessel and a wind turbine. Technical staffs and cargo, as a result, can be transferred easier and safer to the wind turbine than slightly rough boat landing. Although OSV compared to the work boat has a certain improvement in general, the operable window highly depends on the seakeeping behavior and weather condition on a case-by-case basis. Even so, the fatal accident during gangway operation is not a rare event, unfortunately. When OSV services a wind turbine, she needs to stay in proximity, though the gangway is employed. In terms of OSV’s seakeeping behavior, it may not be a trouble for the fixed wind turbine, because most of them are monopile or jacket of relatively small size structures. Nevertheless, the floating wind turbine especially the popular Semi-submersible (SemiSub) type, the volume of hull concentrates near the water surface compared to the deep draft Spar type. To OSV, the wave forces should be affected due to the presence of SemiSub that changes the boundary condition compared to that of OSV alone, and vice versa. Meanwhile, considering the comparable displaced volume of two floating structures, the dynamic motion of one structure may impact obviously on the other one due to hydrodynamic coupling. Besides, the relative distance between SemiSub and OSV is limited, which reminds the phenomenon of ’gap resonance’ that happens to FLNG and LNG carrier. Therefore, the hydrodynamic interaction is believed intuitively to influence the performance of seakeeping and stationkeeping. The coupling effect between two moving floating structures i.e. OSV and SemiSub floating wind turbine are therefore worthy of study.

There is little research that has been reported so far regarding the topic proposed above, to the author’s best knowledge. The most experience that we could refer to is contributed by the industry of offshore oil and gas. The multi-body hydrodynamic analysis of side-by-side FLNG and LNG carrier were investigated by quite a few researchers (Buchner, De Boer, De Wilde, 2004, Fournier, Mamoun, Chen, 2006, Huijsmans, Pinkster, De Wilde, 2001, Kashiwagi, Endo, Yamaguchi, 2005, Kim, Ha, Kim, et al., 2003, Koo, Kim, 2005, Pauw, Huijsmans, Voogt, 2007, Pessoa, Fonseca, Soares, 2015). The study of gap resonance mentioned earlier was also reported by scholars such as (Feng, Bai, 2017, Li, 2020, Watai, Dinoi, Ruggeri, Souto-Iglesias, Simos, 2015, Zhao, Pan, Lin, Li, Taylor, Efthymiou, 2018). Focusing on the hydrodynamic interaction, the author previously studied relative motions of vessels in proximity and gangway’s motion during offshore operation (Huang, Li, Chen, Araujo, 2018, Li, Huang, Chen, 2018). The outcome of either simulation or experiment demonstrated a significant impact on seakeeping characteristics due to hydrodynamic interaction. In the aspect of the floating wind turbine, Jiang et al. (2018) conducted simulation of installation of Spar type wind turbine by employing a catamaran installation vessel. The conclusions also confirmed the importance of hydrodynamic interaction during operation.

Inspired by the above-mentioned works, the effect of hydrodynamic coupling between SemiSub floating wind turbine and OSV in the operational configuration is studied. The popular OC4 floating wind turbine (Robertson et al. (2014)) originally developed for the DeepCwind project is used. The complex multi-body hydrodynamic computation is solved by employing the 3-D diffraction and radiation computation, which is conducted using in-house software HyDra. The time domain approach is chosen by integrating different factors such as hydrodynamic coupling matrix, convolution integral, nonlinear damping, nonlinear mooring tension, nonlinear wave force, etc.

The fully-coupling strategy (FC) takes into account wave loads and the full damping matrix provided by multi-body hydrodynamic computation. Inversely, no-coupling (NC) strategy employs hydrodynamic parameters based on the single body configuration of each floating structure i.e. neither diffraction nor radiation considers the hydrodynamic coupling. For semi-coupling (SC) strategy, the wave load and damping coefficient are adopted from multi-body computation except for coupling damping terms between OC4 and OSV. Compared to FC, SC strategy reduces the computational effort on convolution integral of wave damping at each time step. To better identify and evaluate the hydrodynamic coupling effect, three strategies are proposed in the modeling of time domain simulation. The fully-coupling strategy takes into account wave loads and the full damping matrix provided by multi-body hydrodynamic computation; the no-coupling strategy employs hydrodynamic parameters based on the single body configuration of each floating structure i.e. neither diffraction nor radiation computation considers the hydrodynamic coupling; lastly, the semi-coupling strategy still uses the wave load and damping coefficient from multi-body computation while the coupling damping terms between OC4 and OSV is neglected. The simulations of time domain are carried out by in-house software KraKen. The theoretical background is introduced briefly in Sect. 2. The engineering and physical configuration of OC4 and OSV in operational condition are presented in Sect. 3. The dynamic and mechanical properties of OC4 such as nonlinear mooring line tension and damping are validated by existing measurement. A series of sea-state combining different wave conditions and wave headings are defined in Sect. 4. The extreme response and spectrum under irregular waves are presented in Sects. 4.1 and 4.2, the discussion is made regarding the impact on drift force, slow-drift, and wave-frequency motion. Meanwhile, the shielding effect and relative motions are interpreted too. The hydrodynamic coupling and effect of gap resonance within the wave-frequency region are further investigated in Sect 4.3, where the response amplitude operator (RAO) is presented from white noise simulations. Quite a few interesting findings are discovered, the surge and sway of OSV show overpredicted motion due to overestimating of drift force if hydrodynamic coupling was not taken into account. Under the following sea, the ’splitting’ phenomenon can be observed that makes both floating structures drift towards the opposite direction transversely. The sway and roll of OC4 reduce significantly by up to 40% thanks to the ’shielding effect’. Sway and heave of OSV show obvious behavior of ’gap resonance’.

Section snippets

Theoretical background

In this section, the theoretical foundations and numerical modeling that used in this study is introduced, and only the most necessary formula are given and explained. In general, two different coordinate systems are defined to describe the kinematics of floating structure (floater), including: the Earth-fixed global system OXYZ and local inertial system oxyz attached to each moving floater. The floater is considered as a rigid body of 6 DOFs (degree of freedom). Fig. 2 illustrates both the

Engineering specifications and validation

In this section, the specifications of OSV vessel and OC4 SemiSub wind turbine are given briefly. In addition, the validation compared with the existing experimental result is performed for OC4 such as free decay with nonlinear damping, natural period, mooring line tension, etc.

Results and discussion

In this section, simulation as well as results are conducted and presented based on three different coupling strategies. The fully-coupling strategy (FC) takes into account wave loads and the full damping matrix provided by multi-body hydrodynamic computation. Inversely, no-coupling (NC) strategy employs hydrodynamic parameters based on the single body configuration of each floating structure i.e. neither diffraction nor radiation considers the hydrodynamic coupling. For semi-coupling (SC)

Conclusions

The hydrodynamic coupling of OC4 Semisub wind turbine and OSV vessel is studied during the walk-to-work operation. The hydrodynamic interaction is solved by multi-body analysis based on 3-D diffraction and radiation computation in the frequency domain. The coupling effect is studied by investigating three coupling strategies in the time domain i.e. full-coupling (FC), no-coupling (NC), and semi-coupling (SC) which ignores the interactive damping terms between OC4 and OSV. The simulation of the

CRediT authorship contribution statement

Binbin Li: Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing, Visualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (31)

  • H. Wu et al.

    Wave component in the green function for diffraction radiation of regular water waves

    Appl. Ocean Res.

    (2018)
  • W. Zhao et al.

    Estimation of gap resonance relevant to side-by-side offloading

    Ocean Eng.

    (2018)
  • ABS

    Design guideline for stationkeeping systems of floating offshore wind turbines, final report

    American Bureau of Shipping

    (2013)
  • C. Brun et al.

    The impact of the use of the full qtf on lng moored in shallow water

    Offshore Technology Conference, Houston, Texas, USA

    (2008)
  • B. Buchner et al.

    The interaction effects of mooring in close proximity of other structures

    Proceeding of the 14th International Offshore and Polar Engineering Conference, Toulon, France

    (2004)
  • Cited by (16)

    View all citing articles on Scopus
    View full text