Preliminary Design of Bottom-Fixed Lattice Offshore Wind Turbine Towers in the Fatigue Limit State by the Frequency Domain Method

2012 ◽  
Vol 134 (3) ◽  
Author(s):  
Haiyan Long ◽  
Geir Moe
Keyword(s):  
Frequency Domain ◽  
Fatigue Limit ◽  
Limit State ◽  
Offshore Wind ◽  
Design Stage ◽  
Frequency Domain Method ◽  
Preliminary Design ◽  
Early Design Stage ◽  
Lattice Towers ◽  
Fatigue Limit State

The fatigue assessment of support structures is one of the most significant challenges in the design of offshore wind turbines (OWT). Fatigue analysis can be conducted in either the time-domain or the frequency-domain. The advantage of frequency-domain analysis is its time efficiency. This paper shows how the frequency domain method can be used to dimension lattice-type OWT towers such that they meet the fatigue criteria in the preliminary design stage. Two types of lattice towers, a three-legged and four-legged version, were redesigned in the fatigue limit state for the NREL 5 MW baseline wind turbine sited at a water depth of 35 m. The wall thickness of the members was chosen as the only variable and varied during the design process until the towers could survive for at least 20 years. In comparison with designs based upon ultimate strength, the mass of both types of towers increased no more than 30% when the fatigue limit state was considered. It is concluded that the lattice type structure requires only half as much material as its tubular counterpart. The three-legged tower is promising because of its simple geometry, even though it displayed a lower torsional stiffness than the four-legged tower. All the analyses in this paper were performed by an in-house FE code, intended for the early design stage of lattice towers. Once the optimum configuration is found in the early design stage, integrated time-domain analyses for the entire OWT system might be required to refine the design, taking all the nonlinear parameters into account.

scholarly journals A Comparative Study between Three-Legged and Tripod Sub-structures in Design of Offshore Wind Turbines in the Transition Water Depth

2018 ◽  
Vol 7 (3.36) ◽  
pp. 23
Author(s):  
Aliakbar Khosravi ◽  
Tuck Wai Yeong ◽  
Mohammed Parvez Anwar ◽  
Jayaprakash Jaganathana ◽  
Teck Leong Lau ◽  
...  
Keyword(s):  
Water Depth ◽  
Limit State ◽  
Cost Effective ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Ultimate Limit State ◽  
Offshore Wind Turbines ◽  
Welding Joints ◽  
Fatigue Limit State ◽  
Ultimate Limit

This research aimed at investigating tripod and three-legged offshore wind turbine substructures. A comparison between the two substructures based on their weight as well as the installation method of piles, i.e. pre-piling and post-piling, was carried out. The in-place (Ultimate Limit State), Dynamic, natural frequency check and fatigue (Fatigue Limit State) analyses were conducted considering aerodynamic and hydrodynamic loads imposed on substructures in 50m water depth. An optimisation process was carried out in order to reduce the mass of substructures. The results revealed that the three-legged substructure is more cost effective with 25% lesser structure mass. However, the construction of the three-legged structure usually takes more time due to increased number of members and subsequently welding joints. The results, furthermore, showed that the pre-piling method reduces the time and cost of offshore installation, and reduces the weight of piles by 50%.  

A Method for the Frequency Domain Seakeeping Analysis of Offshore Structures in the Early Design Stage

2020 ◽  
Author(s):  
Maximilian Liebert
Keyword(s):  
Frequency Domain ◽  
Cross Sections ◽  
Degrees Of Freedom ◽  
Equations Of Motion ◽  
Offshore Structures ◽  
Design Stage ◽  
Design Environment ◽  
Early Design ◽  
Early Design Stage ◽  
Response Amplitude Operators

Abstract The motion analysis of floating offshore structures is a major design aspect which has to be considered in the early design stage. The existing design environment E4 is an open software framework, which is being developed by the Institute of Ship Design and Ship Safety, comprising various methods for design and analysis of mainly ship-type structures. In context of the development to enhance the design environment E4 for offshore applications this paper presents a method to calculate the response motions of semi-submersibles in regular waves. The linearised equations of motion are set up in frequency domain in six degrees of freedom and the seakeeping behaviour is calculated in terms of the amplitudes of the harmonic responses. The hydrodynamic forces onto the slender elements of the semi-submersible are accounted for by a Morison approach. As the drag and damping forces depend quadratically on the amplitudes, these forces are linearised by an energy-equivalence principle. The resulting response amplitude operators of the semi-submersible are validated by comparison with model tests. The method represents a fast computational tool for the analysis of the seakeeping behaviour of floating offshore structures consisting of slender elements with circular cross sections in the early design stage.

Computation of Drift Forces for Dynamic Positioning Within the Very Early Design Stage of Offshore Wind Farm Installation Vessels

2014 ◽  
Author(s):  
Philip H. Augener ◽  
Stefan Krüger
Keyword(s):  
Wind Farm ◽  
Wind Farms ◽  
Offshore Wind ◽  
Irregular Waves ◽  
Design Stage ◽  
Dynamic Positioning ◽  
Offshore Wind Farm ◽  
Early Design ◽  
Early Design Stage ◽  
Drift Forces

The German government has decided upon the changeover from fossil and nuclear based electrical power generation to renewable energies. Following from this offshore wind farms are erected in the exclusive economic zones of Germany. For the transportation and installation as well as the maintenance of the wind turbine generators very specialized vessels are needed. The capability of dynamic positioning even in very harsh weather conditions is one of the major design tasks for these vessels. For this reason it is important to know the external loads on the ships during station keeping already in the very early design stage. This paper focuses on the computation of wave drift forces in regular and irregular waves as well as in natural seaway. For validation the results of the introduced calculation procedure are compared to measured drift force data from sea-keeping tests of an Offshore Wind Farm Transport and Installation Vessel.

Calculation of the Dynamic Positioning Capability of an Offshore Wind Farm Vessel During the Jack-Up Process in the Early Design Stage

2019 ◽  
Author(s):  
Maximilian Liebert
Keyword(s):  
Wind Farm ◽  
Offshore Wind ◽  
Design Stage ◽  
Force Model ◽  
Dynamic Positioning ◽  
The European Union ◽  
Early Design ◽  
Early Design Stage ◽  
Offshore Industry ◽  
Jack Up

Abstract As a consequence of the planned exit from fossil-based energy in the European Union the exploitation of renewable energies has become a major aspect of the Offshore Industry. Especially the construction and operation of offshore wind energy turbines pose a challenge which is met by the use of jack-up vessels with extendible legs. In order to dimension the vessel’s manoeuvring devices in the early design stage and to ensure a safe jack-up process for given environmental loads the dynamic positioning capability during the jacking including the influence of the legs has to be calculated. As part of the development of a holistic dynamic analysis this paper presents the implementation of the legs’ influence in an existing manoeuvring method. The manoeuvring method solves the equations of motion in three degrees of freedom (surge, sway, yaw). It is based on a force model which comprises various modular components. Therefore another component for the leg-forces is added. A Morison approach is chosen to calculate the hydrodynamic forces on the cylindrical legs. The legs’ hydrodynamic added masses are accounted for and added to the hull’s inertial terms. The benefit of the presented method is the possibility to calculate the dynamic positioning capability with extended legs without being dependent on the results of either time-consuming or non-specific model tests. Therefore the method represents a fast computing tool to design the vessel for the specific environmental conditions of the site of operation.

Fatigue Limit-State Design

2009 ◽  
pp. 217-256
Author(s):  
Jeom Kee Paik ◽  
Anil Kumar Thayamballi
Keyword(s):  
Fatigue Limit ◽  
Limit State ◽  
Limit State Design ◽  
Fatigue Limit State

Fatigue limit state design of fast boats

2017 ◽  
Vol 55 ◽  
pp. 17-36 ◽  
Author(s):  
Or Neuberg ◽  
Nitai Drimer
Keyword(s):  
Fatigue Limit ◽  
Limit State ◽  
Limit State Design ◽  
Fatigue Limit State

Lattice Towers for Bottom-Fixed Offshore Wind Turbines in the Ultimate Limit State: Variation of Some Geometric Parameters

2011 ◽  
Vol 134 (2) ◽  
Author(s):  
Haiyan Long ◽  
Geir Moe ◽  
Tim Fischer
Keyword(s):  
Wind Turbines ◽  
Limit State ◽  
Steel Structures ◽  
Offshore Wind ◽  
Ultimate Limit State ◽  
Offshore Wind Turbines ◽  
Lattice Towers ◽  
Visual Impact ◽  
Physical Impact ◽  
Ultimate Limit

Optimal solutions for offshore wind turbines (OWTs) are expected to vary from those of their onshore counterparts because of the harsh offshore climate, and differences in loadings, transportation, access, etc. This definitely includes the support structures required for service in the sea. Lattice towers might be a competitive solution for OWTs due to less physical impact from waves and less concern for visual impact. This paper addresses the design methodology of lattice towers for OWTs in the ultimate limit state and presents a FEM code that has been developed to implement this methodology. The structural topologies are specified in terms of tower cross-section geometry, the inclination of bracings, and the number of segments along the tower height. For each topology a series of towers is designed in which the bottom distance between the legs has been varied; the resulting tower mass is evaluated as a major parameter for the cost assessment. The study was conducted using the NREL 5-MW baseline wind turbine for an offshore site at a water depth of 35 m. The optimal design is searched for according to tower mass and fabrication complexity. The most economical tower geometry appears to have a constant inclination of bracing owing to its simplicity of fabrication and strong antitorsion capacity. Three-legged and four-legged alternatives have different advantages, the former having simpler geometry and the latter offering better torsion resistance. As a design driver for offshore steel structures, the fatigue life of the towers designed in the ultimate limit state should be assessed and the structures are consequently modified, if necessary. However, fatigue assessment is out of the scope of this paper and will be done in a later work.

Design of Dynamic High Voltage Cables for Floating Substation

2020 ◽  
Author(s):  
Lucie Guignier ◽  
Riccardo Mariani ◽  
Arthur Cottet-Emard ◽  
Stéphane Toumit ◽  
Thomas Choisnet
Keyword(s):  
Fatigue Limit ◽  
Limit State ◽  
Time Domain Analysis ◽  
Fatigue Properties ◽  
Design Verification ◽  
Coupled Models ◽  
Limit State Design ◽  
Pacific Coast Of Japan ◽  
And Performance ◽  
Fatigue Limit State

Abstract This paper presents the design and performance assessment of 220kV dynamic export cables for a floating substation characterized by a ring-shaped floater known as Damping Pool. The main originality of the design presented is that the cables considered have dry conductors. They are shielded from the water by a longitudinally welded corrugated copper sheath. Similar cables have been operating at lower voltage levels and thus with thinner insulation thicknesses. The export cable configuration has been designed considering environmental conditions representative of both the Central North Sea, Pacific Coast of Japan or the US, in 100m water depth. Ultimate and fatigue limit-state design verification of the configuration are made through nonlinear time-domain analysis using coupled models comprising the floating substation hull, the mooring system and dynamic export cables. Fatigue limit-state design verification is based on the fatigue properties of the cable section, combined with appropriate S-N curves of the armour layers and metallic screen-sheath. Design verifications show that the dynamic export cable configuration proposed could satisfactorily meet the performance requirements for a service life over 25 years, considering proven cable equipment such as a bend stiffener remaining within today’s manufacturer molding capacities.

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