scholarly journals NREL Assesses National Design Standards for Offshore Wind (Fact Sheet)

2014 ◽  
Author(s):  
Keyword(s):  
Offshore Wind ◽  
Design Standards ◽  
Fact Sheet

Comparison of the Fatigue Performance of Galvanised M72 Bolts With Design Standard Recommendations

2021 ◽  
Author(s):  
Carol Johnston ◽  
Matthew Doré
Keyword(s):  
Fatigue Performance ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Design Codes ◽  
Design Standards ◽  
Threaded Fasteners ◽  
Fatigue Design ◽  
Standard Design ◽  
Wind Turbine Towers ◽  
Thickness Correction

Abstract Now that bolted flanges rather than grouted connections are used to join the transition piece to the monopile in offshore wind turbine towers, many large bolts are being used in applications which subject them to fatigue loads. The bolts in these ring flanges are typically M64 or M72 in size (ie 64mm of 72mm nominal diameter). The fatigue design codes, BS 7608, DNVGL-RP-C203 and Eurocode 3 do provide S-N curves for threaded fasteners, but the reference diameter in those documents is 25mm or 30mm. A thickness correction is provided, to account for larger diameter bolts, but this was originally derived by analysis of the performance of welded joints. It is unclear whether the S-N curves and the recommended thickness correction are appropriate for larger diameter threaded fasteners. The offshore wind industry usually specifies hot dip galvanised bolts, to provide some corrosion protection in the offshore environment. Again, there is uncertainty over whether the S-N curves in fatigue design standards apply to bolts with a galvanised coating. Since the fatigue design codes provide S-N curves for air, free corrosion or seawater with cathodic protection, it is also unclear which of these should be used to predict the fatigue performance of bolts with a galvanised coating. In order to provide data to address these uncertainties, hot-dip galvanised, grade 10.9, M72 bolts from two manufacturers were tested in both air and a seawater environment. In order to represent the conditions experienced by bolts in internal ring flanges, the artificial seawater was sprayed onto the bolts during testing. Tests were conducted with a mean stress corresponding to 70% of the specified minimum 0.2% proof strength of the bolts. Tests were also performed in air, on uncoated M72 bolts, and uncoated M64 bolts for comparison. The results suggest that the current thickness correction in DNVGL RP C203 and BS 7608 is appropriate for M72 bolts. The results in air from the galvanised bolts were below those from uncoated bolts. Although the galvanised results were above the thickness corrected in-air standard design curves (BS7608 Class X -20%, DNVGL Class G and DNVGL ST 0126 FAT 50), they were below the mean curves, suggesting that the performance of galvanised bolts is slightly lower than the existing recommendations.

Large-Scale Resonant Fatigue Testing of Welded Tubular X-Joints for Offshore Jacket Foundations

2019 ◽  
Author(s):  
Jeroen Van Wittenberghe ◽  
Philippe Thibaux ◽  
Maarten Van Poucke
Keyword(s):  
Large Scale ◽  
Fatigue Testing ◽  
Water Pressure ◽  
Limit State ◽  
Fatigue Tests ◽  
Offshore Wind ◽  
Design Standards ◽  
Initiation Phase ◽  
Out Of Plane ◽  
Jacket Structures

Abstract Offshore wind turbines are being installed in deeper water and with more powerful generators resulting in more severe loading conditions on its foundations such as jacket structures. Because the main loading is due to wind and currents, the dominant design limit state is fatigue. The fatigue performance of the tubular joints used in jacket structures has been assessed several decades ago based on test results with limited component dimensions (diameter and wall thickness). In addition, improvements of welding methods and evolution of steel grades are not considered in the current design standards. To provide experimental fatigue-life data on large-scale structures a test program has been carried out on 4 welded tubular X-joints. Each X-joint consists of two horizontal braces with a diameter of 711 mm welded to a central vertical tubular member with 806 mm diameter. The X-joint has a total length of 7.5 m and has two identical welds that are fatigue tested. The fatigue tests are carried out on an innovative resonant bending fatigue test rig that allows to load the specimen in in- and out-of-plane direction at a different amplitude to obtain an even stress distribution over the circumference of the welds. The tests are carried out at a speed close to the resonance frequency of the X-joint. During the test, hotspot strains are measured using strain gauges and a limited amount of water pressure is used to detect through-thickness cracks. The tests are carried out in two phases. During the crack initiation phase, the sample is loaded in both the in- and out-of-plane mode. Once cracks are detected, the test is continued in the crack propagation phase with loading in the plane where cracks had been initiated until through-thickness cracking appeared. During this phase the beach marking technique has been used to mark the shape of the fracture surface at different moments during the fatigue tests. The testing program is part of the RFCS project JABACO that aims to reduce offshore wind cost by incrementing prefabrication of the jacket substructure.

Fatigue of Welded Tubular X-Joints in Offshore Wind Platforms

2019 ◽  
Author(s):  
Theocharis Papatheocharis ◽  
Gregory C. Sarvanis ◽  
Philip C. Perdikaris ◽  
Spyros A. Karamanos
Keyword(s):  
Research Work ◽  
Critical Evaluation ◽  
Offshore Wind ◽  
Automatic Welding ◽  
Design Standards ◽  
Fatigue Design ◽  
Weld Toe ◽  
Reliable Design ◽  
Bending Loading ◽  
Large Wind

Abstract The paper is part of the European research program JABACO (2015–2018), on the optimization of design and construction of offshore jacket platforms for supporting large wind turbines (5–10 MW) in water depths ranging from 30m to 80m. In particular, the paper describes an experimental investigation on the high-cycle fatigue performance of welded tubular connections, subjected to in-plane bending loading. Experimental results from seven (7) X-joint specimens are presented. The specimens were manufactured with 18-inch-diameter tubes and a brace-to-chord-diameter ratio equal to 1. Furthermore, the brace-to-chord-thickness ratio is equal 0.6, and the brace-chord angle is 90-degrees. The specimens are made of regular carbon steel grade 355, and have been fabricated using two different welding techniques: (a) manual (semi-automatic) welding (5 specimens); and (b) robot (automatic) welding (2 specimens). The comparison of the fatigue design life of those welding methods is a major objective of the present study. Prior to testing, numerical simulations have been performed to determine the critical locations around the weld toe, for proper instrumentation of the tubular specimens in terms of strain gage locations. This research work aims at a critical evaluation of available design standards, towards the development of more reliable design tools and reduction of the construction cost of the platform.

scholarly journals Active Load Mitigation to Counter the Fatigue Damage Contributions From Unavailability in Offshore Wind Turbines

2018 ◽  
Author(s):  
Stian H. Sørum ◽  
Emil Smilden ◽  
Jørgen Amdahl ◽  
Asgeir J. Sørensen
Keyword(s):  
Fatigue Damage ◽  
Expected Value ◽  
Offshore Wind ◽  
Design Standards ◽  
Offshore Wind Turbines ◽  
Active Load ◽  
Design Value ◽  
Unused Capacity ◽  
Conservative Values ◽  
Offshore Wind Industry

The offshore wind industry continues to grow, but there is still a need for more economical designs. As unavailability conditions can be critical for the fatigue damage in support structures, design standards use conservative values for availability. This leads to most turbines having an unused fatigue capacity at the end of the lifetime. This paper investigates the potential for reducing this unused capacity in order to reduce the capital expenses. The proposed strategy is to design the turbines for a higher availability, closer to the expected value. For individual turbines that experience lower availability than the design value, active load mitigation is imposed to reduce the fatigue damage. The potential of this methods is explored, together with its limitations. It is found that the effect of faults occurring early in the turbines lifetime can be reduced. This is not the case for faults occurring towards the end of the lifetime.

Effect of Foundation Modelling Methodology on the Dynamic Response of Offshore Wind Turbine Support Structures

2011 ◽  
Author(s):  
Eric Van Buren
Keyword(s):  
Wind Turbine ◽  
Dynamic Properties ◽  
Dynamic Performance ◽  
Optimization Procedure ◽  
Operating Conditions ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Support Structures ◽  
Design Standards ◽  
Modelling Methodology

When preliminarily investigating offshore wind turbine tower concepts it is common to develop optimization software for determining the best possible structural layout. This type of optimization procedure requires a large number of iterations to determine the best possible design and can be quite time consuming, particularly if the dynamic performance of each structure is to be investigated using an aero-hydro-servo-elastic type solver. When performing this type of “dynamic optimization” it is convenient to simply assume fixed boundary conditions at the soil-structure interface and ignore the dynamic properties of the foundation. Using fixed conditions allows for each of the layouts to be compared quickly and makes the computer models simple to create and more efficient in computation than if the foundation is included. Alternatively, the foundations of offshore wind turbine support structures can be represented with several different methods of varying complexity and detail. The most widely used method is the use of a distributed spring model commonly known as the p-y method. This approach is the primary method in most offshore wind turbine design standards for determining the static and cyclic reaction of offshore piles. In this work, two offshore wind support structure layouts are modeled and analyzed in the wind turbine analysis program HAWC2. Dynamic time series analyses under operating conditions are carried out for each tower with fixed conditions and with foundation models based on the p-y method in order to determine the appropriateness of utilizing fixed foundation conditions for optimization procedures.

System Reliability for Offshore Wind Turbines: Fatigue Failure

2013 ◽  
Author(s):  
S. Márquez-Domínguez ◽  
J. D. Sørensen
Keyword(s):  
Wind Turbines ◽  
Limit State ◽  
Wind Farms ◽  
Offshore Wind ◽  
Design Standards ◽  
State Equations ◽  
Offshore Wind Farms ◽  
Offshore Wind Turbines ◽  
Cost Of Energy ◽  
Wake Effects

Deeper waters and harsher environments are the main factors that make the electricity generated by offshore wind turbines (OWTs) expensive due to high costs of the substructure, operation & maintenance and installation. The key goal of development is to decrease the cost of energy (CoE). In consequence, a rational treatment of uncertainties is done in order to assess the reliability of critical details in OWTs. Limit state equations are formulated for fatigue critical details which are not influenced by wake effects generated in offshore wind farms. Furthermore, typical bi-linear S-N curves are considered for reliability verification according to international design standards of OWTs. System effects become important for each substructure with many potential fatigue hot spots. Therefore, in this paper a framework for system effects is presented. This information can be e.g. no detection of cracks in inspections or measurements from condition monitoring systems. Finally, an example is established to illustrate the practical application of this framework for jacket type wind turbine substructure considering system effects.

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