ICSE6 Paris - August 27-31, 2012
- Annelies Bolle, Jonas De Winter, Wendy Goossens, Piet Haerens, Geert Dewaele
ICSE6-65
Scour monitoring around offshore jackets and gravity based
foundations
Annelies BOLLE1, Jonas DE WINTER2, Wendy GOOSSENS2,
Piet HAERENS2, Geert DEWAELE3
1
IMDC nv
Postal address1: Coveliersstraat 15, 2600 Berchem, Belgium - e-mail : [email protected]
2
IMDC nv
Postal address: Coveliersstraat 15, 2600 Berchem, Belgium - e-mail : [email protected],
[email protected], [email protected]
3
C-Power nv
Postal address: Scheldedijk, 30, 2070 Zwijndrecht, Belgium - e-mail : [email protected]
At the C-Power wind farm Thornton bank, two foundations types have been applied: gravity based foundations (GBFs)
and jacket foundations. Since the Thornton Bank is a sandbank, the scour potential has been assessed for these
foundations during the design. Whereas around the GBFs a rock armour scour protection has been installed, the
jackets have been designed to be able to cope with the expected scour holes around the piles.
Since the design and installation of the GBFs and the surrounding scour protection has been extensively described in
previous papers, this paper focuses mainly on the jacket foundations. For the GBFs constructed during the first phase
in 2009 regular surveys of the scour protection are included in the Operation and Maintenance program. The latest
surveys indicate that the scour protection is still in good shape and no damage was observed.
As was the case for the GBFs, also for the jacket foundations only limited design guidance was available and none
which was immediately applicable. This paper describes how was dealt with the development of the scour hole
formation during in view of the design and which assumptions and choices had been made.
Since during construction, regular multi-beam measurements are available from the surrounding seabed, the actual
scour hole formation could be compared to the theoretically predicted values. The comparison showed that the choice
to design for the maximum expected scour expected for a pile group, was certainly not a bad one. Furthermore the
recently observed scour pits are close to the predicted expected scour. Further investigations are still needed to reveal
the causes for some locally observed deep scour pits. The further analysis of these types of measurements in
combination with the site conditions could possibly result in an adapted formula for the scour depths and the scour
development around jacket foundations.
Key words
Scour, jacket foundation, gravity based foundation, offshore wind farm, scour protection
I
INTRODUCTION
The concession area of the C-Power wind farm Thornton Bank is situated 30 km offshore from the Belgian
Coast. The first (demonstration) phase consisted of 6 wind turbines of 5MW which are in operation since
2009. These six turbines have been installed on Gravity Based Foundations (GBFs) and electricity is brought
onshore with a 40km long 150 kV cable. During the second phase in total 30 turbines of 6.15MW each will
be installed on jacket foundations between 2011 and 2012, together with a second high voltage cable (same
specifications) and an Offshore Transformer station (OTS) (see figure 1). The remaining 18 jacket
foundations (with 6.15MW turbines) will be installed between 2012 and 2013 during the third and last phase.
The total capacity of the wind farm will be 325.2MW in 2013.
Since the Thornton Bank is a sandbank, the scour potential has been assessed for these foundations during
the design. Whereas around the GBFs a rock armour scour protection has been installed, the jackets have
been designed to be able to cope with the expected scour holes around the piles.
1
Corresponding author
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ICSE6 Paris - August 27-31, 2012
- Annelies Bolle, Jonas De Winter, Wendy Goossens, Piet Haerens, Geert Dewaele
Throughout the construction and operation phase, the seabed around the foundations is regularly
monitored. In combination with the registered hydrodynamic conditions this results in a valuable dataset for
the assessment of the scour protection performance and the occurrence of edge scour around the GBFs of the
first phase, and the scour hole formation around the jacket foundations of the next two phases.
Within this paper the key findings from this monitoring program are presented and the scour depth
estimates made during the design of the jackets foundations are compared with the values and patterns
measured in the field.
Figure 1: C-Power wind farm Thornton Bank – phase1 (GBFs) & phase 2-3 (jacket oundations).
II
FOUNDATION TYPES
Two types of foundations are used within the C-Power wind farm Thornton Bank: the GBFs of phase 1,
and jacket foundations for phase 2 and 3 (see figure 2). The design and installation of the GBFs and the
surrounding scour protection has been extensively described in previous papers (Bolle et al., 2009; Bolle et
al., 2010). Therefore this paper focuses on the jacket foundations.
The jacket foundations are located in water depths ranging from 12 up to 30m. A jacket foundation is a
steel tubular structure designed to support the wind turbines. The height of a jacket is 40 to 50 m. Firstly,
steel piles of 40 to 50m long are driven into the seabed by a hammer installation mounted on a jack up
platform. These "pin-piles" are the anchors to seal the jacket foundations into the seabed. The length of the
piles depends on the soil conditions. To assure the correct locations of the piles a template is lowered onto
the seabed, after it has been dredged to assure horizontality. The pile driving works started in April 2011 and
on Sunday August the 21st all pile driving works for the project were completed. The first jacket foundation
was installed on 12/06/2011 and was grouted to the pin-piles afterwards. On 28/09/2011, so after a period of
3.5 months, all the 24 foundations of area B were successfully installed and grouted.
III
SCOUR AROUND JACKET FOUNDATIONS
Scour phenomena around jacket foundations are highly dependent on the structural design of the
foundation (for example the pile diameter and the distance between the piles). Apart from the structure, also
the soil properties influence the extent of the scour hole.
For the design of the jacket foundations, the lowest expected seabed level is important. Apart from the
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ICSE6 Paris - August 27-31, 2012
- Annelies Bolle, Jonas De Winter, Wendy Goossens, Piet Haerens, Geert Dewaele
Figure 2: Foundation types C-Power wind farm. Left: Gravity Based Foundation, right : jacket foundation.
natural erosion, the seabed will also erode (locally) due to the presence of the structure. Two types of scour
are expected to develop due to the structure’s presence: local scour around each individual pile and global
scour around the entire structure.
The general evolution of the sea bottom (due to general erosion of the seabed or due to the movement of
sand waves) is taken into account in the reference seabed level (RSBL) which corresponds to the lowest
expected seabed level, without the presence of structures, during the considered lifetime of the project.
The expected scour depth and extent describe the scour that will most probably occur. The maximum scour
depth is used to define the Design Seabed Level (DSBL), which is the lowest bottom level over the next 30
years (including the effect of the structures). The Pile Design Tolerance Dredging level (PDTDL) is the
lowest value of the RSBL and the dredged level (in order to allow the placement of the frame for the pinpiles).
To compute the global and local scour depth and extent around a jacket structure the following method has
been applied, based on the combination of predictions for separate elements:
1. The pile diameter for scour calculations Dcalc is determined taking into account marine growth (DNV,
2007). For the C-power jackets Dcalc becomes 2.028m.
2. The global scour depth (based on a 2x2 pile group) is defined by SG = 0.37 x Dcalc (Sumer and Fredsoe,
2002). The global scour extent is defined by the radius rG = SG / tan α, with α = equal to φ/2 [°] and φ =
the friction angle of the soil [°].
However global scour has not be taken into account since the distance between the pile centres
(dpc=18m) is more than 6 x Dcalc (Breusers, 1972 and Hirai and Kurata, 1982).
3. The local scour depth SL is defined by: the expected value SL,e = 1.3 x Dcalc (DNV, 2007 ) and the
maximum value SL,m = 2 x Dcalc (which takes into account the standard deviation of the measurements
and which could allow some contribution of the joints, which are situated at 2.5 to 5.0m above the
seabed.) (Sumer et al., 2002).
4. The local scour extent rL is defined by: the expected radius rL,D = ½ Dcalc + SL,e / tan α, with α = φ/2 and
the maximum radius rL,D = ½ Dcalc + SL,m / tan α, with α = φ/2, taking into account recommendations
about the slope from. The inclined members and secondary structures increase the turbulence, and
possibly the scour extent. Therefore the more conservative approach of Hoffmans and Verheij (2007)
is followed, and the value for the downstream slope (αdownstr =0.5 * αupstr) is applied all around the piles.
5. The total scour depth is defined by: the expected total scour depth ST,e = SG + SL,e =2.6m and the
maximum total scour depth ST,m = SG + SL,m=4.1m.
The total scour extent is defined by a radius, starting from the centre of each pile: the expected radius
rT,e = ½ Dcalc + ST,e / tan α = 9.4m and the maximum radius rT,m = ½ Dcalc + ST,m/ tan α =13.9m.
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ICSE6 Paris - August 27-31, 2012
- Annelies Bolle, Jonas De Winter, Wendy Goossens, Piet Haerens, Geert Dewaele
In this case the total scour depth and extent equals the local values, since no global scour has been
found.
It should be remarked however that the jacket structure is more complex than a pile group. Apart from the
piles, also the inclined tubular elementsmembers, the joints between the different members and the
secondary structures such as the cables and J-tubes, contribute to the scour. These parts cause external
turbulence, and scour will be increased. It can be expected that scour depths at the piles with cable are
somewhat deeper than at the other piles (but this effect is not taken into account in the above determined
scour depths). The contribution of the members and joints to the global scour will depend on the distance of
these elements above the seabed. For a greater distance the contribution to the scour will decrease.
An estimate of the total scour for this structure, including the contribution of all these effects, cannot be
made based on existing formulas, but requires specified (physical) models. The available theory only allows
giving an estimate for a simplified configuration (the pile group).
The local scour depth and extent as defined above is independent from the water depth, so for all wind
turbine locations the same local scour depth and extent has been applied.
IV
FIELD MEASUREMENTS
IV.1
Gravity based foundations
The Operation and Maintenance program for the GBFs has been discussed previously in [IMDC, 2010]
and [Whitehouse et al., 2011] and consists of the execution of multi-beam bathymetric surveys of the scour
protection and seabed in an area of 200m diameter around the six GBFs. This survey is executed twice a
year, at the start and end of the good weather period, with roughly 6 months in-between each other. The
latest available survey dates from May 2011. The analysis showed that for none of the 6 foundations action
was required, since the levels were all situated above the alarm line (Figure3). Since then, no major storms
occurred, so damage of the scour protection is not expected. However, monitoring continues.
Figure 3: Scour protection GBF D6, May 2011. Left: observed levels, right: difference with the alarm level.
IV.2
Jacket foundations
The seabed around the jacket foundations has been regularly measured during the last year. Multi-beam
measurements have been performed before dredging (August 2010 – March 2011), after dredging (MarchApril 2011) , after pre-piling (June – September 2011), throughout the cable installation process (October –
December 2011) and during the first winter season (December 2011 - February 2012). In total 12 datasets are
available for each location. An example of these measurements is shown on the contour plots for jacket
foundation G2 in Figure 3. Hydrodynamic measurements are available from the Flemish banks monitoring
network. Only two storms occurred during this period: 8-9 December 2011 ( Hs up to 4m), and 3-5 January
2012 (Hs 3.5- 4.5m). However, since for the scour hole calculations, empirical formulas have been used
which are independent from these parameters, no further hydrodynamic analysis has been made up till now.
With regards to scour development, two clear phases can be distinguished: the first after pre-piling, but
before jacket installation (the ‘monitoring1’ survey), the second when the entire foundation is in place (all
later surveys). After pre-piling distinct circular scour holes developed around the piles. The average erosion
depth is 1.3m (0.65D), compared to the expected value of about 2.65m (1.3D) for indefinitely high piles. The
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ICSE6 Paris - August 27-31, 2012
- Annelies Bolle, Jonas De Winter, Wendy Goossens, Piet Haerens, Geert Dewaele
maximum observed scour depth is 2.4m (1.2D) below PDTDL, which is still lower than the expected value.
The time between the survey and the pile installation is on average 2 to 3 months, which should be enough to
speak about fully developed scour, according to the DNV formula (DNV, 2007) which is similar to the
relationship suggested by Sumer and Fredsoe (2002) for the depth of the scour hole as a function of time. For
typical conditions of U= 0.6m/s, Hs=0.5m, Tp=4s and water depth = 25m, the expected scour depth is
reached after nearly one month. Since the waves are very small, the non-dimensional time scale for a steady
current has been applied.
The scour which occurs is caused by the 4 piles only, and no additional effects have to be taken into
account. Also the pile-stick-up is limited (about 1.5m), so not the entire water column is disturbed. The effect
Figure 3: Scour formation around jacket foundation G2: August 2011 till February 2012.
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ICSE6 Paris - August 27-31, 2012
- Annelies Bolle, Jonas De Winter, Wendy Goossens, Piet Haerens, Geert Dewaele
of the pile height can be taken into account be a relation developed by DHI & Snamprogretti (1992). For a
pile stick-up of 1.5m, the expected scour depth would be reduced to 0.9m (0.45D), the maximum depth to
1.4m (0.7D), which is lower than the observed values. For a pile stick-up of 3m, the formula gives estimates
Sexp = 1.5m and Smax=2.3m (1.1D), which are closer to the observed values.
After jacket installation however, the scour increases. During the period October 2011 – February 2012 the
averaged scour depth ranges between 1.4 and 1.9m (0.7 – 0.95D). The average value throughout the wind
farm of the largest scour depth at each location (= maximum of the four piles) ranges between 1.7 and 2.7m
(0.85 – 1.35D). Also some very large individual values where noticed (up to 10m), for which the reason is
still not clear. Since these values originate from local spots, and don’t occur all around the piles, they might
be due to construction (e.g. during cable placement and or trenching) or faulty measurements (due to
influence of the piles).
Another observation is the enlargement of the width of the scour hole during time. After the pre-piling,
only small scour holes were observed. After jacket installation they scour holes become instantly deeper, but
only wider during time. For the lower four plots from figure 3 for example, average scour depths are between
2.3 and 2.6m (without big changes over time), while it is clearly visible that the scour extent increases over
time. This evolution from the holes around the pre-piles only, towards the observed scour holes in January
2012 is also illustrated on the cross-sections (E-W and S-N, through the center of the foundation) in figure 4.
Figure 4: Comparison measurements ‘monitoring1’ (August 2011) and ‘ post storm January 2012’ at jacket
foundation G2: top 90°, bottom 180°. RSBL = PDTDL in this case.
On figure 4 also the dredging levels and the monitoring and design lines are indicated. Similar to the
monitoring program for the scour protection of the GBFs (Bolle et al., 2009), three monitoring levels have
been defined: the alarm line which corresponds with the expected scour depths, the danger lines
corresponding to the maximum expected scour and an intervention level in between. To each of these levels,
required actions have been coupled (e.g. more intense monitoring or repairs).
However, since the scour phenomenon and the geotechnical stability cannot be simplified to a 2D problem,
additional figures have been produced to investigate if certain levels are exceeded, for which an example is
shown in figure 5. These type of figures shown the difference between the measured seabed level and the
monitoring levels, and represent thus the sediment thickness above such a level. The upper two and lower
left plot in figure 5 show the layer thickness of the sediment above the alarm level (i.e. the expected scour):
red means sediment is present above this level, blue means that the level has been exceeded. A tolerance of
+/- 0.2m has been blanked to account for measurement accuracy.
The first plot (monitoring 1) shows that in some areas around the foundation, dredging occurred till below
PDTDL. The scour holes however are less deep than the alarm level. After the ‘in-survey’ for the cable,
some very local scour pits have been observed, whereas for the survey in January 2012 no local scour was
observed, but a slight lowering of the area in between the expected scour holes occurred. The lower right plot
shows however that this more widespread scour still does not exceed the danger level.
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- Annelies Bolle, Jonas De Winter, Wendy Goossens, Piet Haerens, Geert Dewaele
Figure 5: Difference of the measured bathymetry at foundation jacket foundation G2with the alarm levels (top
and bottom left) and the danger level (bottom right)
V
DISCUSSION
A global view on the average observed scour depths is presented in figure 6. The average scour depths
(average of the 4 scour holes at each location) are plotted against the bottom level for all the surveys. The
gross part of the scour depths stay below the alarm depth, which is the theoretically expected scour depth of
1.3D. The in general lower values for the ‘monitoring1’ survey (the green stripes) were already discussed
above and can be explained be the limited lengths of the pre-piles.
Figure 6: Overview of the average scour depth in function of the bottom level for all measurements at all
locations (period March 2011 – February 2012).
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ICSE6 Paris - August 27-31, 2012
- Annelies Bolle, Jonas De Winter, Wendy Goossens, Piet Haerens, Geert Dewaele
Once the jackets have been installed, now and then higher scour depths are observed at a number of
locations, most often in the shallower part of the area. The reasons for this very local scour are yet unclear
but can possibly be found in construction methods are faulty measurements. Slightly higher values than
expected are believed to be really scour induced, and support the choice for the maximum scour value to be
used for the design studies of the jacket foundations.
Furthermore two big groups seem to occur in the graph: the seabed levels between -25 and -20, and those
between -20 and -14. The scour depths for the deeper locations seem to be slightly smaller than for the other
group. Although this trend is not very strong, it has been found for each of the surveys. This suggests that
further investigations into the differences in between these two areas might be useful: apart from different
water depths, different currents or soil properties could influence the scour development. This could provide
more insight into the process and could serve for a calibrated or more specific scour formula for these cases.
Apart from the individual scour holes, also the entire scour pattern and the development in time around the
foundation is worth to be further investigated. The latest surveys showed a limited lowering of the seabed in
between the individual scour holes, which is possible an effect of the jacket foundation. The inclined parts of
the jacket foundation could not be taken into account directly with existing formulas. Additional
measurements would provide an excellent dataset to develop a formula for the scour depths and the scour
development around jacket foundations.
VI
CONCLUSIONS
The main purpose of this analysis was to verify whether the theoretical scour pit development estimates for
the jacket foundations corresponded to the scour holes measured in situ. The comparison showed that the
choice to design for the maximum expected scour expected for a pile group, was certainly not a bad one.
Furthermore the recently observed scour pits are close to the predicted expected scour (or alarm level).
Further investigations are still needed to reveal the causes for some locally observed deep scour pits. The
further analysis of these types of measurements in combination with the site conditions could possibly result
in an adapted formula for the scour depths and the scour development around jacket foundations.
For the GBF foundations, the monitoring went on the last years, and no damage of the scour protection has
been observed.
VII ACKNOWLEGMENTS AND THANKS
We would like to thank C-Power for making the datasets available for this analysis, and for sharing these
findings with the offshore community (see also www.c-power.be).
VIII REFERENCES AND CITATIONS
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solution for offshore gravity based foundations’. Proceedings of ICSE5, San Francisco, 7-10 November
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Bolle A., Haerens P., Trouw K., Smits J. and Dewaele G. (2009) - Scour around gravity-based wind
turbine foundations - prototype measurements, Proceedings of the Coasts, Marine structures and
Breakwaters conference, 16-18 September 2009, Edinburgh.
Breusers H.N.C. (1972) - Local scour near offshore structures, Delft Hydraulics Publication 105, Delft.
DNV-OS-J101 (2007) - DNV Offshore Standard: Design of Offshore Wind Turbine Structures.
DHI & Snamprogretti (1992) - SISS project. Sea bottom instability around small structures. Erodible bed
Laboratory tests (Phase 1). Final report, text and drawings. Contract INGE91/SP/03060, June 1992.
Hirai S. and Kuruta K. (1982) - Scour around multiple- and submerged circular cylinders, Memoirs
Faculty of Engineering, Osaka City University, 23, 183-190.
Hoffmans H.N.C. & Verheij H.J. (1997) - Scour Manual.
Sumer B.M. & Fredsoe J. (2002) - The mechanics of scour in the marine environment, World Scientific.
Whitehouse R., Sutherland J. and Harris J. (2011) - Evaluating scour at marine gravity foundations.
Proceedings of the Institution of Civil Engineers, Maritime Engineering 164 December 2011 Issue MA4,
Pages 143–157 http://dx.doi.org/10.1680/maen.2011.164.4.143
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