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).