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Advanced Topics in Offshore Wind Turbines Design

F. Bontempi1

1 Professor of Structural Analysis and Design, School of Engineering, University of Rome La Sapienza, Via Eudossiana 18, 00184 Rome, ITALY; phone: +39-06-44585265; email: [email protected] ABSTRACT Offshore Wind Turbine (OWT) is a relatively complex structural and mechanical system located in a highly demanding environment. In this study the fundamental aspects and the major issues related to the design of such structures are inquired. The System Approach is proposed to carry out the design of the structural parts: in accordance with this philosophy, decomposition of the system (environment, structure, actions/loads) and of the structural performance is carried out in order to organize the qualitative and quantitative assessments in various sub-problems. These aspects can be faced by sub-models of different involvedness both for the structural behavior and for the load models. Numerical models are developed accordingly to assess safety, performance and robustness under aerodynamic and hydrodynamic actions. INTRODUCTION Offshore Wind Turbine (OWT) emerges as an evolution of the onshore plants for which the construction is a relatively widespread and consolidated practice providing a renewable power resource (Hau, 2006). In order to make the wind generated power more competitive with regards to conventional exhaustible and high environmental impact sources of energy, the attention has turned toward offshore wind power production (Breton and Moe, 2009).

Besides being characterized by a reduced visual impact, since they are placed far away from the coast, OWT can take advantage from more constant and intense wind forcing, something that can increase the regularity and the amount of the productive capacity and make such a resource more cost-effective if the plant is lifelong and operates with minimum interruption through its lifespan.

From a general point of view, an OWT is formed by both mechanical and structural elements. As a consequence, it is not a “common” Civil Engineering Structure: it behaves differently according to different circumstances related to the specific functional activity (idle, power production, etc), and it is subjected to highly variable loads (wind, wave, sea currents loads, etc.). In the design process, different structural schemes for the supporting structure can be adopted (Figure 1), mainly depending on the water depth which determines the hydrodynamic loads acting on

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the structure and drives the choice of the proper techniques for the installation and maintenance of the support structure. Moreover, since the structural behavior of OWTs is influenced by nonlinearities, uncertainties and interactions, they can be defined as complex structural systems, as other recently designed constructions (Bontempi, 2006).

seabed

transitionplatform

foundation

tower

support structure

sub-structure

blades – rotor - nacelle

foundation

sea floor

water levelTRIPODMONOPILE JACKET

Figure 1. Main parts of OWT for different support structure typology.

The above considerations highlight that a modern approach to study such

structures has to evolve from the idea of “structure” itself, intended as a simple device for channeling loads, to the one of “structural system”, intended as “a set of interrelated components which interact one with the other in an organized fashion toward a common purpose” (NASA, 2007). The consequent System Approach includes a set of activities which lead and control the overall design, implementation and integration of the complex set of interacting components (Simon, 1998; Bontempi et al., 2008): here, the original definition by NASA has been extended in such a way that the “structural system” organization contains also the actions and


loads: the latter derives from, and is strictly related to, the environment (Figure 2). A certain amount of complexity arises from the lack of knowledge and from

the modeling of the environment in which the OWT is located. In this context two main design issues can be individuated: the consideration of the uncertainty deriving from the stochastic nature of the environmental forces (in particular aerodynamic and hydrodynamic) and the proper modeling of the possible presence of non linear dynamic interaction phenomena among the different actions and between the actions and the structure.

Figure 2. Structural system organization.

Generally speaking, the uncertainties can spread during the various analysis

phases that are developed in a cascade. The incorrect modeling of the involved uncertainty can lead to an incorrect characterization of the structural response from a stochastic point of view and, thus, to an improper quantification of the risk for a given structure subjected to a specific hazard.

Having as goal the schematization of the problem and the individuation of the uncertainty propagation mechanisms, reference can be made to the Figure 3, where the process of the environmental actions generation is qualitatively represented also with considerations on the involved uncertainties.

ENVIRONMENT ZONE

Structure

Non environmental solicitations

EXCHANGE ZONE

Wind and wave flow

Structural (non-environmental)

systemSite-specific environment

Wind site basic parameters

Other environmental

agents

Wave site basic parameters

Wind, wave and sea current actions

Aerodynamic and Aeroelastic phenomena

Hydrodynamic phenomena

1. Aleatoric2. Epistemic3. Model

Types of uncertainties



Propagation Propagation

Figure 3. Generic depiction of the uncertainties and the interaction mechanisms

in the design of OWT structure.


In Figure 3, following the wind and the hydrodynamic flows impacting on the structure, it is possible to distinguish two zones:

• Environment zone: it is the physical region, sufficiently close to the structure to assume the same environmental site parameters of the structure, yet far enough to neglect the flow field perturbations (in terms of particle’s trajectories, pressure field, etc.) induced by the presence of the structure itself. In the environment zone, the wind and the hydrodynamic flows can interact with each other and with other environmental agents, changing their basic parameters. The physical phenomena and uncertainties in the environment zone propagate themselves in the neighborhood regions.

• Exchange zone: it is the physical region adjoining the structure. In this zone, the structure itself, the wind and the hydrodynamic field experience the mechanical interchange (aerodynamic and hydrodynamic phenomena) from which the actions arise. In the exchange zone, some non-environmental solicitations are present; these solicitations may change the dynamic or aerodynamic characteristics of the original structure; so the actions are generated considering this structural sub-system (original structure combined with non-environmental solicitations) instead of regarding only the original structure itself. By definition, physical phenomena and uncertainties cannot propagate themselves from the exchange zone to the environment zone.

In general the uncertainties can be subdivided in three basic typologies: • epistemic uncertainties (deriving from the insufficient information and the

errors in defining and measuring the previously mentioned parameters); • aleatory uncertainties (arising from the unpredictable nature of the

magnitude, the direction and the variance of the environmental actions); • model uncertainties (deriving from the approximations in the models).

Regarding for example the wind model and considering the turbulent wind velocity field as a Gaussian stochastic process, an epistemic uncertainty related to the hypothesis of gaussianity is introduced. For the sake of simplicity, the epistemic uncertainties are not considered in this study.

The aleatoric uncertainties can be treated by carrying out a semi-probabilistic (looking for the extreme response) or a probabilistic analysis (looking for the response probabilistic distribution) analysis.

Finally, a possible way to reduce the model uncertainties is given by differentiating the modeling levels. This can be carried out not only for the structural models, but also for the action and interaction phenomena models; for this reason different model levels are adopted.

In this way, a suitable strategy to govern the complexity is given from the structural system decomposition, represented by the design activities related with the classification and the identification of the structural system components, and by the hierarchies (and the interactions) between these components. As mentioned before, the decomposition regards not only the structure, but also the environment and the actions and loads, and it is the subject of the first part of this study.

From the design point of view, due to the complexity of these structures one has to adopt a Performance-Based Design philosophy: different aspects and performance under different loading conditions (with reference to all possible system


configurations that can be assumed by the blades and the rotor) have to be investigated for this type of structures. In this framework, additional design issues related to the structural aspects can be accommodated and are mentioned below with some proper references:

− aerodynamic optimization (Snel, 2003); − foundation design and soil-structure interaction (Westgate and DeJong,

2005; Ibsen and Brincker, 2004; Zaaijer, 2006); − fatigue calculations (Veldkamp, 2007; Tempel. 2006); − vessel impact and robustness (Biehl and Lehman, 2006); − life cycle assessment (Martinez et al., 2009; Weinzettel et al., 2009); − marine scour (Sumur and Fredsøe, 2002; Tempel et al., 2004); − possible floating supports (Henderson and Patel, 2003; Jonkman and

Buhl, 2007); − standards certification (API, 1993; BSH, 2007; DNV, 2004; GL, 2005;

IEC, 2009). The most important issue for OWT is anyway the choice of the kind of the support structure, related principally to the water depth, the soil characteristics and economic issues. If the water depth (h) is considered as the major parameter, the following rough classification can be made: monopile, gravity and suction buckets (h<25m); tripod, jacket and lattice tower (20m<h<40÷50m); low-roll floaters and tension leg platform (h>50m). Due to optimization processes, cross structural forms can emerge, as the “strutted” system shown in Figures 4 and 5, with the ability of the structural scheme to adjust different water levels (Polnikov and Manenti, 2009).

Figure 4. Scheme of strutted support structures for OWT positioned in sea with

water level ranging from 20 to 35 m.


Figure 5. Main loading systems for strutted support structure for OWT. STRUCTURAL SYSTEM DECOMPOSITION As previously stated, the decomposition of the structural system is a fundamental strategy for the design of complex structural systems, and it has to be performed together with the decomposition of the performance the structure has to fulfill


(Figure 6). The decomposition is carried out focusing the attention on different levels of detail: starting from a macro-level vision and moving on towards the micro-level details (for more details see Bontempi et al., 2008).

Figure 6. Structural system and performance decomposition of an OWT.

Decomposition of the environment. The first step of the structural system decomposition concerns the environment. This is due to the fact that, in a global approach, the structure is considered as a real physical object placed on an environment where a variety of conditions, strictly related to the acting loads, should be taken into consideration. The environment decomposition is performed in the first column of Figure 6.

Decomposition of the structure. The second step is related to the OWT structure. This is organized hierarchically, considering all the structural parts categorized in three levels (second column of Figure 6):


• Macroscopic (MACRO-LEVEL), related to geometric dimensions comparable with the whole construction or parts with a principal role in the structural behavior; the parts so considered are called macro components which can be divided into: − the main structure, that has the objective to carry the main loads; − the secondary structure, connected with the structural part directly loaded

by the energy production system; − the auxiliary structure, related to specific operations that the turbine may

normally or exceptionally face during its design life: serviceability, maintainability and emergency.

Focusing the attention on the main structure, it consists in all the elements that form the offshore wind turbine. In general, the following segments can be identified: − support structure (the main subject of this study); − rotor-nacelle assembly.

• Mesoscopic (MESO-LEVEL), related to geometric dimensions still relevant if compared to the whole construction but connected with specialized role in the macro components; the parts so considered are called meso-components. In particular the support structure can be decomposed in the following parts: − foundation: the part which transfers the loads acting on the structure into

the seabed; − substructure: the part which extends upwards from the seabed and

connects the foundation to the tower; − tower: the part which connects the substructure to the rotor-nacelle

assembly. • Microscopic (MICRO-LEVEL), related to smaller geometric dimensions and

specialized structural role: these are simply components or elements.

Decomposition of the actions and loads. The next step is the one of the actions derived from the environmental conditions. These can be decomposed as shown in the third column of Figure 6, from which the amount of the acting loads can be comprehended. It is important to underline that, since the environmental conditions in general are of stochastic nature, the magnitude of the actions involved is usually characterized, from a statistical point of view, by a return period TR: lower values of TR are associated with the so called “normal conditions”, while higher values of TR are associated with “extreme conditions”.

Performance decomposition. As a final step, the performance requirements are identified and decomposed as follows (lower part of Figure 6):

• Assurance of the serviceability (operability) of the turbine, as well as of the structure in general. As a consequence, the structural characteristics (stiffness, inertia, etc.) have to be equally distributed and balanced along the structure.

• Safety assurance with respect to collapse, in probable extreme conditions; this is applicable also to the transient phases in which the structure or parts of it may reside (transportation and assembly), and that have to be verified as well.


• Assurance of an elevated level of reliability for the entire life-span of the turbine. As a consequence, a check of the degradation due to fatigue and corrosion phenomenon is required.

• Assurance of sufficient robustness for the structural system, that is to assure the proportionality between an eventual damage and the resistance capacity, independently from the triggering cause, assuring at the same time an eventual endurance of the structure in the hypothetical extreme conditions.

The following performance criterions can be identified for the structural system, leading eventually to the selection of appropriate Limit States:

• Dynamic characterization of the turbine as dictated by the functionality requirements: − natural vibration frequencies of the whole turbine (compressive of the

rotor-nacelle assembly), the support structure and the foundations; − compatibility of the intrinsic vibration characteristics of the structural

system with those of the acting forces and loads; − compatibility assessment for the movement and the accelerations of the

support system for the functionality of the turbine. • Structural behavior regarding the serviceability (SLS - Serviceability Limit

State): − limitation of deformations; − connections decompression.

• Preservation of the structural integrity in time: − durability for what regards the corrosion phenomenon; − structural behavior with respect to fatigue (FLS - Fatigue Limit State).

• Structural behavior for near collapse conditions (ULS - Ultimate Limit State): − assessment of the solicitations, both individual and as a complex, to the

whole structural system, to its parts, its elements and connections; − assessment of the global resistance of the structural system; − assessment of the resistance for global and local instability phenomena.

• Structural behavior in presence of accidental scenarios (ALS - Accidental Limit State) − structural robustness: decrease in the load bearing capacity proportional to

the damage (see for example Starossek, 2009, and Bontempi et al., 2007); − survivability of the structural system in presence of extreme and/or

unforeseen, situations; these include the possibility of a ship impacting the structural system (support system or blades), with consequences accounted for in risk scenarios.

CONCLUSION In this paper, the System Approach has been proposed as a conceptual method for the design of OWT structures. In this sense, structural system decomposition has been performed, with a specific view on the structural analysis and performance. The presented considerations aim to the organization of the framework for the basis of design of offshore wind turbines, as a support to the decision making, with specific


reference to the structural safety, serviceability and reliability for the entire lifespan. ACKNOWLEDGEMENTS The present work has been developed within the research project “SICUREZZA ED AFFIDABILITA' DEI SISTEMI DELL'INGEGNERIA CIVILE: IL CASO DELLE TURBINE EOLICHE OFFSHORE", C26A08EFYR, financed by University of Rome La Sapienza. REFERENCES Hau, E., Wind Turbines: Fundamentals, Technologies, Application, Economics, 2nd

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