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Collaborating Authors
Angelides, Demos C.
ABSTRACT This paper deals with the derivation of a mathematical model for the dynamic analysis of flexible offshore structures using the finite element method. This model was developed recently by the authors and refers to a three dimensional corotational beam element that can undergo large rotations and translations, but small strains. The applicability of the model has been tested in the past only for pure structural systems and for submerged systems subjected to currents. In the present paper, the mathematical model is extended to include also the influence of the free surface on flexible structures. For the part of the structure close to the free surface, the model evaluates the instantaneous wave profile in the vicinity of the element's nodes. By this way it is deduced whether the element is completely, partially or not at all submerged and then the submerged length is evaluated. Regarding the hydrostatic forces, they are evaluated from the potential energy for the submerged part of the element. Further on, the equivalent hydrodynamic force is evaluated using a modified Morison equation. For completely submerged elements, the applied force is formed using the nodal fluid accelerations and relative velocities normal to the beam axis. In case of a partially submerged element, the evaluation of the hydrodynamic force is based on the fluid accelerations and relative velocities at the submerged node and at the point that the element pierces the free surface. Finally, the consistent stiffness matrix due to the hydrodynamic and hydrostatic forces is meticulously evaluated considering both the cases of a completely submerged element and of a piercing beam. INTRODUCTION Commonly, for the analysis of offshore and space structures, three dimensional beam elements that undertake finite displacements may be applied. Currently, there are several models in the literature for the structural dynamic analysis of flexible beams (e.g. Le et al., 2014; Hsiao et al., 1999; Crisfield et al., 1997; Ibrahimbegovic and Mikdad, 1998; Cardona and Geradin, 1988; Simo and Vu-Quoc, 1986). Especially, the papers of Le et al. (2014), Hsiao et al., (1999) and Crisfield et al., (1997) refer to corotational formulations of spatial beams for pure structural applications. However, it seems that only the investigation of Crisfield et al., (1997) was extended to consider the existence of the fluid. The extended model was presented in Yazdchi and Crisfield (2002) with application to marine pipes and risers. Of course there are other dynamic formulations that use other methods than the corotational one for the analysis of flexible structures considering the fluid environment (e.g. for risers: Felippa and Chung 1981; O'Brien et al. 1988; Kordkheili et al. 2011).
Clustering Techniques for Data Analysis and Data Completion of Monitored Structural Responses of an Offshore Floating Structure
Panapakidis, Ioannis P. (Technological Educational Institute of Thessaly) | Michailides, Constantine (Liverpool John Moores University) | Angelides, Demos C. (Aristotle University of Thessaloniki (A.U.Th.))
ABSTRACT Offshore Floating Structures (OFSs) present a major category of offshore structures that are often subjected to severe environmental conditions and harsh critical loading scenarios. The state of an OFS during its life-cycle must remain in the domain specified in the design, although this can be altered by normal aging due to usage, the action of the environment and accidental events. In recent years, the field of Structural Health Monitoring (SHM) has been growing at a fast rate, especially in different applications within the offshore structures' field (e.g. platforms and systems in oil and gas technology, risers, and offshore wind technology). Based on the monitored data of the SHM a diagnosis and most importantly a prognosis of the health status of the OFS can be assessed. Usually, measured data in long time span of different structural response quantities are used for the aforementioned assessment with, in some cases, unmeasured data. This paper deals with two objectives for the case of monitored structural response data of an OFS:the implementation of clustering techniques for analysis of the structural response data and the completion of missing structural response data based on appropriate clustering techniques. INTRODUCTION A big number of offshore structures are in operation in the different related ocean technologies that exist, namely, oil and gas, renewable energy, coastal, fishery and transportation. It is estimated that more than 10,000 offshore structures of different types, floating or fixed, have been installed and are in operation worldwide (Chakrabarti, 2005). The aforementioned number of offshore structures is expected to increase the coming years. Offshore structures are placed in the ocean environment, where particular conditions exist with harsh induced loadings (Hirdaris et al., 2014). The long term exposure of the offshore structures to these conditions (e.g. severe wave and wind conditions, water pressure actions, vortex shedding, salinity, accidental events, etc.) will affect their design and service life, while anomalies, degradation and damages may incur as a result. Damages in offshore structures may have catastrophic effects in different aspects.
- Europe (1.00)
- North America > United States (0.68)
ABSTRACT The present paper aims mainly at developing a finite beam element that could be applied for the analysis of flexible offshore structures, such as deep water ones of frame type, and the mooring lines of floating structures. The first goal of the paper, is to form the dynamic matrices, i.e. inertia force vector and mass matrix for a corotational beam element. This element can undergo large rotations and translations, but small strains. The second goal of this work, is to extend the proposed structural model in order to consider a fluid surrounding the system studied. Specifically, an equation of the applied hydrodynamic force is used applicable to a beam element arbitrarily oriented. Furthermore, for the beam element, the added mass of the fluid is considered within the element's mass matrix, which is formed through the usage of the element's shape functions. A numerical example is presented that illustrates the efficiency of the pure structural model within the field of large displacements. Finally, a second problem concerning fluid-structure effects is also studied numerically. INTRODUCTION Three dimensional beam elements that undertake displacements unrestricted in size may be applied for the modeling of members of offshore and space structures. Currently, several models exist in the literature regarding the structural dynamic analysis of flexible beams (e.g. Crisfield et al., 1997; Ibrahimbegovic and Mikdad, 1998; Cardona and Geradin, 1988). However, those works, have not been extended to consider the existence of the fluid. On the other hand, there are also dynamic formulations for the analysis of flexible structures considering the fluid environment (e.g. for risers: Felippa and Chung 1981; O'Brien et al. 1988; Kordkheili et al. 2011). The present paper extends a pure structural model of a static beam element to the dynamic regime. Then, the structural model is extended to consider the motion of the beam within a moving fluid. The model for the pure structural analysis is based on the corotational theory. The term corotational is consistent with the idea that a local frame is applied which is able to translate and corotate with the element in order to track its deformation; this allows the decomposition of the total motion of the element into a rigid body motion and a deformational one. This is a great advantage, as the geometric nonlinearity, imposed by the large motion of the element, takes place only during the rigid body motion. The static terms of the model are evaluated based mainly on the work of Nour-Omid and Rankin (1991). The first goal of this paper is to derive the dynamic terms where special care was taken to end up to concise mathematical equations. Only the dynamic terms of the formulation have integrals and they were appropriately handled to confine the pure integrable quantities and then to evaluate them analytically. The previous actions enhance the computational efficiently of the proposed model. The set of equations are highly nonlinear and for this reason they are solved using the Newton-Raphson iterative procedure, while through the linear approximation of the set of equations the effective stiffness matrix for the corotational beam element was evaluated. In the analysis of offshore structures it is important to use formulations which account for torsional-bending coupling (e.g. Felippa and Chung 1981). The present structural model accounts for this coupling and this is validated through a relevant numerical example. It should be noted that the proposed structural model is applicable to beam elements with geometric nonlinearity but without surrounding fluid. The second goal of the paper is the extension of the structural model to consider fluid motion around the beam and also the motion of the beam within the fluid.
Missing Wind Speed Data: Clustering Techniques for Completion and Computational Intelligence Models for Forecasting
Panapakidis, Ioannis P. (Technological Education Institute of Thessaly Larisa) | Michailides, Constantine (Aristotle University of Thessaloniki (A.U.Th.)) | Angelides, Demos C. (Norwegian University of Science and Technology)
Abstract This paper addresses two objectives for the case of missing wind speed data:the implementation of clustering techniques for completion of missing wind speed data and the development of offshore wind speed forecasting models. Various clustering algorithms are compared in terms of better portioning of the wind speed data. With the aid of a robust clustering tool, a more in depth analysis can be held on wind speed data drawing useful conclusions about the data structure. In addition, three novel techniques are developed for the completion of the missing data. Furthermore, two commonly used forecasting models are used, namely a Feed-Forward Neural Network (FFNN) and the Adaptive Neuro-Fuzzy Inference System (ANFIS). The purpose is to train and test these models under the limitation imposed by the incomplete data set. The present paper serves as a necessary step in the problem of handling incomplete wind speed data towards the assessment of offshore wind energy potential. Introduction Fixed bottom Offshore Wind Turbines (OWTs) are considered as an efficient solution to cope with the increasing demand in power grids globally. OWTs have been passed successfully into the phase of commercialization and industrialization and there is an increasing trend for using this very promising renewable energy technology and its products widely in the coming years. During the last years an increasing number of research efforts and pilot programs brought forth the benefits provided by OWTs both for utilities, grid operators, self-producers and societies. OWT is a contemporary topic that is becoming the focus of interest of the research community. In June 2015 3,072 fully grid connected offshore wind turbines existed in 82 wind farms across 11 European Union (EU) countries, with a capacity of 10,393.6 MW. The average water depth of these OWTs is equal to 22.4 m. Around 90% of the already installed OWTs are supported in monopile fixed bottom substructure foundation type. The coming years is expected that OWTs will dominate in renewable energy sector globally. Nowadays, the installed offshore wind capacity is growing with a rate of 40% per year.
Abstract Corrosion represents one of the major causes of structural deterioration in offshore structures. Therefore, the investigation of the effect of corrosion in their life-time structural reliability is considered of great importance for the optimization of the maintenance schemes. In this paper an approach is proposed for the estimation of the long term process of corrosion in offshore structures based on a contemporary long term corrosion model. A specific structure is used for this investigation, where different corrosion zones are considered. The reliability of the structure, expressed through the probability of failure, is evaluated for the main structural components as a function of the exposure period. The effect of corrosion is, also, demonstrated through the displacements of the joints of the structure.
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Reservoir Description and Dynamics (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems (1.00)
Abstract The identification of dominant failure modes of large structures is a subject of great importance in modern engineering, because it offers the opportunity to monitor the evolution of failure mechanisms in time and space, thus allowing the designer to interfere with system reliability by reinforcing or altering the structural system, either during the design or the operational phase. The purpose of this investigation is to identify the dominant failure modes of an existing offshore platform of jacket type under dynamic loading. For the stochastic analysis describing the above mentioned system, the random variables being selected are the yield stresses of the n elements of the structure. A genetic algorithm is implemented for the selective searching of n-dimensional space of random variables in order to obtain dominant failure modes in decreasing order of their probability of occurrence. Then, the importance of a failure mode is approximated by matching each failure mode to the respective safety index, ฮฒ.
Effect of Tendonsโ Material on the Dynamic Behavior of a Tension Leg Platform Analyzed in Frequency and Time Domain
Theodoridis, Lazaros (Aristotle University of Thessaloniki (AUTh)) | Loukogeorgaki, Eva (Aristotle University of Thessaloniki (AUTh)) | Angelides, Demos C. (Aristotle University of Thessaloniki (AUTh))
Abstract In the present paper, the effect of tendons' material on the dynamic behavior of a Tension Leg Platform (TLP) is investigated in both frequency and time domain. Specifically, uncoupled frequency domain and uncoupled and coupled time domain analyses are preformed. Three different types of material with different modulus of elasticity are taken into account and the effect of these materials on the TLP's natural frequencies and on its response is presented and discussed. Finally, comparison of results obtained using the coupled and the uncoupled time domain analysis methods, is performed and the observed differences are highlighted and explained.
- North America > United States (0.28)
- Europe (0.28)
Abstract In this paper, the dynamic behavior of a single free span pipeline is analyzed and the effect of various factors/parameters (different design conditions, wave and current characteristics, soil characteristics, length of the free span and boundary conditions at the ends of the pipeline) on its dynamic behavior and its structural integrity is investigated. Nonlinear pipe-soil interaction of the part of the pipeline lying on the seabed is considered. A global buckling check is also implemented. The effect of each examined factor/parameter on the dynamic behavior and the structural integrity of the free span pipeline is discussed and demonstrated.
ABSTRACT In the present paper a recently installed Sensor Network for Monitoring the Response (SNMR) of a Floating Structure (FS) is presented. SNMR is deployed on a pontoon-type FS operating as a floating breakwater, located 300m from the coast in the port of Neos Marmaras in Greece. The developed SNMR consists of:sensors for real time measurement of FS's critical response quantities related with the structural integrity and safety of the FS as well as of environmental parameters and data acquisition and data transfer and storage system. Characteristic examples of time series of the measure quantities obtained during the operation of the SNMR are presented and preliminary assessed.
Dynamic Analysis of Fixed Bottom Offshore Wind Turbines
Throumoulopoulosv, Amfilochios G. (Department of Civil Engineering, Aristotle University of Thessaloniki (AUTh)) | Loukogeorgakiv, Eva (Department of Civil Engineering, Aristotle University of Thessaloniki (AUTh)) | Dimitriou, Aristarchos C. (Department of Civil Engineering, Aristotle University of Thessaloniki (AUTh)) | Angelides, Demos C. (Department of Civil Engineering, Aristotle University of Thessaloniki (AUTh))
ABSTRACT In this paper, a numerical tool (MicroSAS-OWT) for the integrated analysis of Offshore Wind Turbines (OWTs) with fixed-bottom support structure of arbitrary shape is presented. MicroSAS-OWT is developed through the coupling of FAST with MicroSAS. FAST is used for modeling the rotor nacelle assembly, while the tower and the support structure are modeled in MicroSAS. The interface of the two codes is ensured at the tower top. The tool is applied for the case of the NREL 5MW OWT that consists of a monopile support structure with rigid foundation, and is preliminary assessed through comparison of results with the corresponding ones obtained using FAST. Stress analysis of the tower and the support structure is also performed. INTRODUCTION Offshore wind energy represents a very promising kind of renewable energy source. Offshore Wind Turbines (OWTs) are considered nowadays as an attractive alternative solution to the onshore ones offering multiple benefits and addressing effectively the well-known obstacles and problems associated with the latter ones (Henderson et al., 2003; Breton and Moe, 2009; Esteban et al., 2011). The efficient exploitation of offshore wind energy necessitates the successful handling of several challenges related to developmental, economical and technological issues (Musial et al., 2006). Among these challenges, one of the most crucial is the development, investigation, assessment and adoption of new design concepts, especially for the support structure. These new design concepts will allow the placement of OWTs in deeper water and therefore, the operation of larger capacity OWTs. Considering the high complexity characterizing any OWT system, resulting from its inherent characteristics (e.g. variability and intense interaction of components) and from its operation in a complex environment, where different loading sources exist, the development/application of suitable numerical tools is critical for addressing efficiently the previously mentioned challenge.
- North America > United States (1.00)
- Europe (1.00)