semifloatingspar final
TRANSCRIPT
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Feasibility study of a semi floating spar buoy wind turbine anchored with
a spherical joint to the sea floor
María Sanz Martínez
DTU Wind Energy
DK-4000 Roskilde, Denmark
Anand Natarajan
DTU Wind Energy
DK-4000 Roskilde, Denmark
Lars Christian Henriksen
DTU Wind Energy
DK-4000 Roskilde, Denmark
Abstract:
The feasibility of a semi floating platform
offshore wind turbine system is investigated at
120m water depth. The semi floating system
consists of a 5MW wind turbine on a floater
with mooring lines similar to a spar buoy and
strongly anchored with a spherical joint to the
sea soil.
The stability of the newly designed floater and
mooring assembly are analyzed from static
and dynamic simulations. The design loads on
the universal joint on the sea floor are tuned
with the needs for a ballast chamber. Using
load simulation in the HAWC2 software,
ultimate and equivalent fatigue loads are
obtained and compared with the loads from
the same wind turbine mounted on a spar
buoy system and a land based wind turbine at
the same points. The results show a
decrement in the ultimate and equivalent
fatigue loads for the new system.
Keywords: Design loads, spherical joint, semi
floating platform, mooring system
1. Introduction
Fixed offshore wind turbine structures may be
cost effective till 60 metres water depth.
Monopile fixed sub structures are usually
used in wind turbine installations till 30m
water depths and frame structures till 60m.
Floating wind turbine prototypes are designed
for water depths near 150m-200m. Therefore,
the wind industry may require other designs of
cost effective sub structures in the range of 60
- 120 metres. A floating spar-buoy
substructure which is anchored with a
universal spherical joint to the sea soil is
analyzed at 120m water depth as potential
solution for moderate water depths (60m
to120m). Oil industry has used similar
substructures [1]. However, the wind loads
experienced by the oil rig structures are
negligible compared to the rotor generated
dynamic loads experienced by wind turbines
so a further study is required.
In this study, the wind turbine used is the
NREL 5 MW [2]. The simulations were
performed in the aero-hydro-servo-elastic
code HAWC2 [3]. For a better analogy, the
new system was compared with the NREL
spar buoy [4] and a land based wind turbine.
Hence, the same simulations were run for all
systems.
2. Design of the platform and static
analysis of the system
The new sub structure configuration should
achieve static and dynamic stability. Therefore
the submerged volume needs to be large
enough to equilibrate the system with its
restoring force (the higher the angle of tilt, the
bigger the submerged volume). To match
these requirements, the final geometry was
chosen as two cylinders of different radii
linked by an interface with the shape of a
truncated cone (see Figure 1).
The design of the new platform was obtained
through an iterative optimization process
between the total weight of the system and
the submerged volume under normal
conditions. In each iteration, the weight of the
system was determined and then, the
submerged volume needed to stabilize that
weight was computed. We considered the
weight for the initial iteration as the weight of
the spar buoy system. The final mass of the
platform was seven times lower than the mass
of the spar buoy floater because there is no
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ballast requirement in the semi floating
system.
The final geometry of the floater consists in: a
bottom cylindrical part of 80 m length and 2 m
radius, a transition conical part of 40 m length,
and a top cylindrical part of 10 m length and 3
m radius. The total draft of the platform is 130
m; the floater is designed for a water depth of
120 m.
As the floater was redesigned, the Morison
equation applicability was verified with 8
different significant wave heights. A stability
analysis using two tools was performed: an
approximate Matlab script that considers the
weight, buoyancy and thrust loads and
calculates the pitch angle of equilibrium of the
platform; and detailed HAWC2 simulations
that includes also the mooring line dynamics
and hydrodynamics. The results of both
analyses are listed in Table 1.
Table 1 – Platform equilibrium angle of both systems depending on wind speed
Mean Wind speed [m/s]
Pitch angle (Matlab)
[deg]
Pitch angle (HAWC2)
[deg]
5 +21.3 +1.2
8 +23.5 +2.8
11 +25.9 +5.2
13 +24.6 +3.9
16 +23.7 +2.9
20 +23.0 +2.3
24 +22.6 +2.0
The largest pitch angle is obtained for mean
wind speeds close to the rated wind speed
because the thrust force is the main input of
the system. The mooring line forces cause the
different equilibrium angles of both analyses.
This shows the system is statically stable
without mooring system, but the resultant
equilibrium angle is too high to be operative.
Mooring lines should be considered, and
designed for improved performance.
A modal analysis was then performed to
obtain the lowest modes of the static system.
The results are listed in Table 2:
Table 2 – Natural frequencies of the semi
floating system
Frequency [Hz]
Description
0.02977 Platform roll
0.03004 Platform pitch
0.03518 Platform yaw
These results are within the same range of
the natural frequencies calculated for the spar
buoy system in other studies as [5].
The floater is depicted in Figure 1.
Figure 1 – Sketch of the floater with a universal joint at the soil
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3. Specification of new mooring
system
We defined a new mooring system based on
[6]. Each mooring line is divided into a number
of elements or Timoshenko bodies that are
analyzed individually together with the rest of
the system in HAWC2. The equation of
motion for each of the mooring bodies is:
( ) ( ) ( )
( )
The main parameters of the mooring system
are presented in Table 3. The mooring system
uses catenary lines because it has simple and
cheap anchors, it is easy to install and it is
suitable for shallower waters. Its sketch can
be seen in Figure 2.
Table 3 – Summary of properties of the mooring system
Number of mooring lines 3
Depth to fairleads, anchors 30, 120 m
Radius to fairleads, anchors 9.4, 850 m
Un stretched line length 853 m
Line diameter 0.1 m
Line mass density (in air) 113.09 kg/m
Line mass density (in water) 86.19 kg/m
Line extensional stiffness 381700 kN
Figure 2 – Mooring system scheme.
4. Wind Turbine System and
Environment
Environment definition
The wave conditions for the aeroelastic
simulations were defined following the
JONSWAP (Joint North Sea Wave Project)
Spectrum [7] with an irregular airy pattern.
The peak enhancement factor, γ was
assumed to have a constant value of 3.3. The
significant height and the period of the waves
are defined in Figure 3.
Figure 3 – Wave parameters depending on the mean wind speed.
We define ‘semi floating system’ as the 5 MW
NREL wind turbine defined in [2], mounted on
the platform described in section 2 and the
mooring system defined in section 3.
To confirm the good operation of the semi
floating system the mean power and thrust
are compared with the corresponding figures
for the floating spar buoy mounted wind
turbine in Figure 4 to Figure 7.
Figure 4 – Power curve comparison.
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Figure 5 – Thrust curve comparison.
Figure 6 – Pitch curve comparison.
Figure 7 – Rotational speed curve comparison.
The differences are minimal.
Estimated damping for the pitch of the
platform
We consider the semi floating system as a
second order system to estimate its damping.
In the steady state, the response of the
system to a step force was simulated. The
same test was completed for the spar buoy
wind turbine and for a land based wind
turbine. The response at the tower top for a
single degree of freedom system is
( ) {
( )
√ ( ) ( ( )
)
The tower top displacement was fitted to the
previous equation and the system parameters
were calculated:
√
The mass used in each case was the whole
system mass (tower, nacelle, rotor and
floater). The obtained results were:
Table 4 – Comparison of damping values
Semi
floating system
Spar buoy
system
Land based system
ξ 0.214 0.147 0.578
c [kNs/m] 7.955∙102 1.756∙10
3 1.063∙10
3
k [kN/m] 3.265∙103 4.748∙10
3 1.210∙10
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The design load cases 1.1 (NTM, normal
turbulence model) and 1.3 (ETM, extreme
turbulence model) from the standard IEC
64100:3 [8] are simulated. Both design load
cases use 6 different wind turbulence seeds
for each mean wind speed.
5. Fatigue analysis
For the site we assumed the mean wind
speed has a Rayleigh distribution with a scale
parameter σ = 6.80 (data from [9]). A yaw
misalignment angle of ± 10 degrees is
included 50% of the simulated time (25%
each) in the normal turbulence model
simulations.
In this study, the fatigue analysis was used as
a methodology to compare the dynamic loads
on both systems (spar buoy and semi floating)
in the bottom of the tower. For a better
comparison, a third system, a land based
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wind turbine, was simulated and compared
together with the semi floating and the spar
buoy system.
In the analysis of the semi floating system, the
joint was also studied as a point of interest,
whose lifetime loads should be used as
guidelines in the design of the joint.
Figure 8 shows the fatigue damage equivalent
loads for the fore-aft bending moment (Mx) at
the bottom of the tower (lowest node of the
tower). The semi floating system obtained
results within the same range of the land
based wind turbine, while the spar buoy
reached higher equivalent loads. These
results were due to higher platform pitch
oscillations for the spar buoy system
compared with the oscillations of the semi
floating system for the same angle. The
reason for the high differences is the semi
floating system has only positive bending fore-
aft moments and this has a lower contribution
on the fatigue loads (see Figure 9). At high
wind speeds, the semi floating system
reached lower negative moments in
comparison to the land turbine.
Figure 10 shows the fatigue loads for the side
to side bending moment (My) in the bottom of
the tower. The semi floating system obtained
lower loads for high wind speeds than both
the land based and the spar buoy systems
due to the lower variations in the amplitude of
the loads than for the other systems (in Figure
11 it is noticeable the lower values of the
standard deviation for the semi floating and
land based systems). For low wind speeds the
spar buoy system reaches high equivalent
fatigue loads while the semi floating and the
land based systems have loads in the same
range.
For the torsional moment at the bottom of the
tower (Mz), the semi floating system achieved
lower equivalent loads compared with the
other two systems, which was again due to
the lower standard deviation of loads on the
semi floating system (Figure 12 and Figure
13).
The equivalent lifetime loads for the three
systems are listed in Table 5 (the lifetime of
the turbine is assumed as 20 years of
operation ≈ 107 cycles).
Table 5 – Equivalent fatigue loads in the tower bottom
[kNm] Semi Float
Spar Buoy
Land Based
Mx 57360 137720 46110
My 22950 50970 21920
Mz 6210 11210 11330
The ratios between the results obtained for
the sea turbines and the land turbine have
been compared with previous studies in this
field ( [10] and [11]) obtaining similar results to
previous publications.
Figure 8 – Equivalent for aft fatigue loads Mx at the tower bottom (continuous line: no yaw
misalignment; dotted line: -10⁰ yaw
misalignment; dashed line: +10⁰ yaw
misalignment).
Figure 9 – Statistical data of the NTM simulations for the Mx loads at the tower
bottom (Max – maximum value; Min - minimum value; Mean - averaged value; SF –
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semi floating system, SB – spar buoy system, Land- land based system).
Figure 10 – Equivalent side to side fatigue loads My at the tower bottom (continuous line:
no yaw misalignment; dotted line: -10⁰ yaw
misalignment; dashed line: +10⁰ yaw
misalignment).
Figure 11 - Statistical data of the NTM simulations for the My loads at the tower
bottom (Max – maximum value; Min - minimum value; Mean - averaged value; SF – semi floating system, SB – spar buoy system,
Land- land based system).
Figure 12 – Equivalent torsional fatigue loads Mz at the tower bottom (continuous line: no
yaw misalignment; dotted line: -10⁰ yaw
misalignment; dashed line: +10⁰ yaw
misalignment).
Figure 13 – Statistical data of the NTM simulations for the Mz loads at the tower
bottom (Max – maximum value; Min - minimum value; Mean - averaged value; SF – semi floating system, SB – spar buoy system,
Land- land based system).
At the joint (Figure 14 to Figure 16), the
maximum equivalent loads were obtained in
the wind and waves direction (y). In the other
axis, the changes in the forces were minor, so
the equivalent fatigue loads were smaller. The
equivalent fatigue loads at the joint for the
semi floating system are listed in Table 6:
Table 6 – Equivalent fatigue lifetime loads in
the joint (semi floating system)
Load Direction
Equivalent fatigue lifetime load (kN)
Fx 664
Fy 1128
Fz 665
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Figure 14 – Equivalent lateral fatigue loads at the joint (Fx).
Figure 15 – Equivalent longitudinal fatigue loads at the joint (Fy).
Figure 16 – Equivalent vertical fatigue loads at the joint (Fz).
6. Ultimate load analysis
The ultimate loads analysis is done according
to two different methods described in the IEC
standards [8] and [12]. The first method used
was simply extracting the maximum values
from the ETM simulations (DLC 1.3). For the
second method utilized, the NTM simulations
were used. The threshold was defined as
Where µ and σ are the mean and the
standard deviation of the NTM simulations. All
the peaks above the threshold were
considered. Their average value was
calculated and multiplied by 1.35 to obtain the
ultimate load expected for each wind speed.
Again, for a better analysis, the three systems
-semi floating, spar buoy and land based-
were compared.
Figure 17 shows the ultimate loads at the
tower bottom fore aft bending moment (Mx) for
the semi floating system. They reached their
maximum around rated wind speed (due to
the maximum thrust). The spar buoy system
obtained higher ultimate loads than the other
systems because of isolated extremes. Even
though the mean values obtained for the three
systems were quite similar, their standard
deviations differed for high wind speeds (see
Figure 18).
Figure 19 shows the ultimate loads of the
three systems for the case of the side to side
tower bottom side to side moment (My). The
semi floating system obtained lower loads
than the other two systems.
For the case of the torsional bending moment
at the bottom of the tower (Mz), the semi
floating system obtained the lowest ultimate
loads (see Figure 20).
Figure 17 – Ultimate loads for fore aft bending moment (Mx) at the tower bottom (SF – semi
floating system, SB – spar buoy system, Land- land based system).
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Figure 18 – Standard deviation of the fore aft bending moment (Mx) at the tower bottom (SF
– semi floating system, SB – spar buoy system, Land- land based system).
Figure 19 – Ultimate loads for side to side bending moment (My) at the tower bottom (SF
– semi floating system, SB – spar buoy system, Land- land based system).
Figure 20 – Ultimate loads for torsional bending moment (Mz) at the tower bottom (SF
– semi floating system, SB – spar buoy system, Land- land based system).
At the joint of the semi floating system (Figure
21 to Figure 23) the maximum loads were
reached around rated wind speed. This is due
to the high thrust force obtained at rated wind
speed. To decrease the values achieved in
Figure 23 (vertical force at the joint), ballast
could be added to the floater.
Figure 21 – Ultimate lateral loads at the joint (Fx).
Figure 22 – Ultimate longitudinal loads at the joint (Fy).
Figure 23 – Ultimate vertical loads at the joint
(Fz).
7. Conclusions
The semi floating system is a promising
solution for moderate water depths. The
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oscillations of the semi floating system under
turbulent wind excitation are much lower than
the spar buoy. The pitch angle of the platform
is steadier and has lower oscillations against
possible excitations and that reduces the
fatigue loads considerably.
According to the results of the fatigue analysis
(Table 5) the semi floating system expects
significantly lower damage equivalent loads
than the spar buoy system. This means that
the structural requirements are less
demanding and, more likely, cheaper than for
the spar buoy system. The design of the joint
should be carefully evaluated and consider
the impact of the fatigue loads depicted in
Table 6 and the ultimate loads displayed in
Figure 21 to Figure 23.
The ultimate loads analysis shows that at the
tower bottom, the maximum loads obtained
for the semi floating system are lower at
almost all wind speeds than for the spar buoy
wind turbine.
Further analysis should be done to verify the
feasibility of this new semi floating system.
Acknowledgement
The work presented in this paper is part of the
Collaborative Project "INNWIND.EU"
supported by the EU Seventh Framework
Program (FP7), grant no. 308974. The
financial support is greatly appreciated.
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