Abstract
This paper investigates the cyclic deformation behaviour of S355 G10+M steel which is predominantly used in offshore wind applications. The thick weldments were identified as regions prone to fatigue crack initiation due to stress concentration at weld toe as well as weld residual stress fields. The monopile structure was modelled using a global-local finite element (FE) method and the weld geometry was adopted from circumferential weld joints used in offshore wind turbine monopile foundations. Realistic service loads collected using SCADA and wave buoy techniques were used in the FE model. A non-linear isotropic-kinematic hardening model was calibrated using the strain controlled cyclic deformation results obtained from base metal as well as cross-weld specimen tests. The tests revealed that the S355 G10+M base metal and weld metal undergo continuous cyclic stress relaxation. Fatigue damage over a period of 20 years of operation was predicted using the total elastic-plastic strain energy accumulated at the root of the weldments as the life limiting criterion. This study helps in quantifying the level of conservatism in the current monopile design approaches and has implications towards making wind energy more economic.
Keywords: Service loads; Offshore wind turbine; S355 welds; Finite element modelling; Fatigue life prediction.
Introduction
Over the last 15 years, constant efforts have been directed towards promoting renewable energy technologies and the deployment of new offshore wind farms has rapidly accelerated around the world, particularly in Europe. A recent example of such efforts is the new offshore wind farm being constructed by Seimens Gamesa Renewable off the coast of Yorkshire-UK, (which is the largest project among the current wind farms) and is approaching completion. It is estimated that by 2024, this wind farm could result in meeting the energy demand of 1.2 million households in the UK [1]. Nevertheless, being a relatively newer technology as compared to fossil fuels, considerable efforts have been put towards bringing down the levelised cost of energy (LCOE) for offshore wind power, which would render the technology commercially competitive [2,3]. It has been established that scaling up the offshore wind turbine (OWT) size, which includes taller mast (thereby giving access to stronger winds) and larger rotor blades (which increases the swept area), enhances the efficiency, but requires advanced designs to withstand greater structural loads [4].
Steady efforts have been focused towards development of optimized features for rotor blades [5,6], tower [7,8], foundation [9,10] and structural health monitoring [11,12]. In addition to exploring newer methods, many studies have also been carried out to enhance the structural integrity of existing designs. Jacob et al. [13] investigated the residual stress profile in a typical circumferential butt weld of OWT monopile made of S355 G10+M and found compressive residual stresses in the heat affect zone (HAZ). Since, this would lead to a reduction in the value of stress intensity factor (crack driving force), such weld residual stresses would be beneficial in life extension of the OWT. Regardless, residual stress redistribution phenomenon and interaction of environmental factors with fatigue crack can significantly alter the material behaviour. Mehmanparast et al. [14] studied the effect of environment (i.e. air and seawater) and microstructure (basemetal and heat affected zone) on the fatigue crack growth in S355 G8+M and reported that in free corrosion condition, the crack growth rate was increased by a factor of 2 as compared to tests conducted in air. Moreover, an independent study was conducted by Mehmanparast et al. to characterise the mechanical and fracture properties of monopile weldments to improve the structural integrity assessment of monopiles [15].
Foundation structure acts as a life-limiting component for an OWT as it is subjected to a spectrum of structural loads, such as weight of the rotor and nacelle assembly, bending load from wind, wave currents and vibrations due to rotor blades being some of the significant loads. Most of the installed OWTs consist of monopile foundations which are built by stacking 3-7 m diameter cylindrical sections of 30-125 mm thickness and can cost up to 35% of the total set up cost of an OWT [16–18]. Besides, the circumferential weldments joining the thick monopile sections, lead to material property variations at the weld metal-HAZ-base metal interface which turns into a favourable site for fatigue crack initiation. This is due to difference in microstructure and chemical segregation as a result of rapid heating and cooling associated with the submerged arc welding process. Kolios et al. [19] performed linear elastic finite element (FE) analysis on monopile circumferential weldments and observed that depending on the weld quality, the stress concentration factor at weld toe lies between 1.1 and 1.65. Subsequent fatigue testing [19] on the large scale dog-bone samples extracted from 90 mm thick weldments displayed crack initiation at regions of maximum stress concentration. In [20], the authors studied the stress-strain response at the different sections (base metal, heat affected zone and weld metal) of a cross-weld specimen using digital image correlation technique. It was found that the three regions exhibited comparable strength values, however the elongation to failure of the weld metal and heat affected zone was reduced by a factor of 10 with respect to the base metal. As the OWT weldments are not subjected to any post-weld treatment, a combination of mechanical properties mismatch (between heat affected zone and the surrounding base metal), residual stress and stress concentration factor at weld toe make it a potential site for fatigue crack initiation. Another study [21] used down-sized geometries of monopile section to investigate the effect of bending moment. The stresses were found to be greatest at regions nearest to the fixed bottom of the monopile, which in an OWT would be the section just around the sea-bed. However, the loading conditions in an OWT is governed by multiple factors such as wind (speed and direction), wave (height and frequency) and rotor speed. Therefore, some researchers [17,22,23] used Supervisory Control And Data Acquisition (SCADA) technique to measure the true service loads acting on an OWT.
Another form of relevant damage mechanism in OWT structures is due to corrosion. Corrosion-fatigue process is initiated by the formation of localised corrosion pits at certain parts of the wind turbine structure which are formed as a result of the breakdown of the thin oxide-layer on the surface of metals, which then develop into a critical size large enough to initiate a crack. However, the quantification of corrosion-fatigue mechanism poses an important challenge as the pit-to-crack stage of this process is largely unknown, hence in most instances, the pit itself is often taken as a crack in the fatigue analysis [24–26]. Corrosion damage can be accounted for in the fatigue design process by considering the S-N curves for structural steels in seawater with and without cathodic protection [14]. Nevertheless, such a prediction would have inherent uncertainties due to the variations in corrosion rate with geometric location as well as the water depth. This difference is a result of the variations in the chemical composition of the seawater at different locations. Seawater is generally considered to be composed of 3.5 wt. % of sodium chloride (NaCl) and its pH ranges from 7.8 to 8.3 [16]. From material loss due to corrosion perspective, the splash zone (at the free surface of sea) is considered to undergo maximum uniform corrosion, that could amount to a yearly thickness reduction of 0.2-0.4 mm in structural material [24]. The submerged sections are subjected to a thickness reduction of 0.1-0.2 mm per year [24].
The present study aims to analyse the effect of service loading conditions on offshore wind monopile foundation structure using a full scaled FE model to predict the fatigue life of the structure. A global-local model was used to enable optimized computation of local stress and strain value at the weld toe of circumferential butt weld joints located nearest to the sea-bed. The model was calibrated using strain controlled cyclic test data to improve the prediction accuracy. Development of reliable fatigue life prediction tools will encourage design optimizations on OWT structures and therefore make wind energy harvesting more economic.
Material and test methods
The material considered in this study is S355 G10+M structural steel since it is commonly employed in fabrication of OWT foundation structures. In S355 G10+M notation, the letter S indicates that the material employed in this study is a structural steel with a minimum yield stress of 355 MPa while G10 indicates the steel grade within the material groups specified in EN-10225 standard and +M indicates thermo mechanical rolling process. A 90 mm thick hot rolled plate was welded using submerged arc welding process and three circular round bar specimens were extracted from the weld region as shown in Fig. 1(a), referred to as cross-weld specimens. Details of the welding procedure can be found in a previous study by Jacob et al. [20]. The specimen extraction location was selected such that the 2-3 mm thick heat affected zone was positioned at the centre of the specimen gauge section. This configuration allows the interface between weld metal, HAZ and base metal to be tested under the applied loading condition. Therefore the test results obtained from the cross-weld specimens would represent the material properties of the monopile circumferential welded joint. Further, three more specimens were extracted from the hot rolled plate in a region far from the weld section to represent the base metal material properties. All the specimens were designed according to ASTM E606 design standard [27] as shown in Fig. 1(b).