July/August 2022 - 84

background to the use of SBJs as a
foundation solution for WTGs and
examines data from recently installed
full scale suction buckets to provide
insights into the risks which may be
encountered during the suction bucket
installation process.
Background
The Borkum Riffgrund 2 offshore wind
farm (BKR02) is located approximately
40 km (25 mi) north of the island of
Borkum off Germany's North Sea coast.
The BKR02 wind farm comprises 56
8.0-MW wind turbine generators, of
which 20 are supported by SBJ
foundations and 36 are supported by
monopile foundations. Figure 2 shows
the location of the BKR02 wind farm
relative to the German coastline.
At BKR02, the suction buckets and
jacket structures were identical for all
20 locations. The suction buckets had a
Table 1: Typical properties of BKR02 units found in the upper soil profile
Unit
Description
Sand (SS) Clean to slightly silty, dense to
very dense, fine to medium
grained
Sand (VS) Silty to very silty, medium to
dense, medium to fine grained
Silt
Medium dense, fine grained
The ground conditions at BKR02 are
typical of German Bight North Sea
conditions, with predominantly
granular materials dominating the
upper 10-20 m (33-65.6 ft) of the soil
profile. As per the soil profile identified
at Borkum Riffgrund 1 (BKR01) offshore
wind farm (Shonberg et al, 2017), some
silt layers were also identified across
the site. Table 1 shows the typical soil
Relative
density (%)
90-100
Fines
content (%)
0-5
Number of
locations
20
60-80
50-70
10-20
> 20
9
15
assess the likely presence of boulders
and the presence of silt layers which
might affect the flow conditions around
the bucket during installation.
Installation Theory
Figure 2: Location of the BKR02 offshore wind farm
diameter of 10 m (33 ft) and a skirt
length of 7 m (23 ft), with a minimum
required embedded length of 5.5 m
(18 ft). The bucket dimensions were
governed by in place criteria, as the
installation process was not expected to
be critical. However, additional skirt
length, commonly referred to as " stick
up, " was required to allow for the plug
heave which was expected to occur
during the installation.
84 * DEEP FOUNDATIONS * JULY/AUG 2022
units found at the suction bucket
locations. The soil unit Sand (SS) was
observed at all 20 SBJ locations and was
the most commonly encountered unit
relevant to the SBJ installations. Silt
layers were also commonly encountered
across the site, with the Silt soil
unit encountered at 15 WTG locations.
A 3D ultrahigh resolution survey
was carried out at each SBJ location to
assess the potential hazards that could
affect the suction bucket installation.
The 3D survey was undertaken to
The installation of suction buckets,
particularly in coarse grained materials,
has been studied extensively for more
than 30 years. Research into the
installation process was undertaken for
oil and gas application in the North Sea
(Tjelta et al, 1986; Bye et al, 1995; Erbrich
and Tjelta, 1999) and since then,
significant research efforts have focused
on laboratory scale experiments (Tran,
2005; Bienen et al, 2018), large-scale
experiments and field trials (Houlsby et
al, 2006; Ibsen, 2008) and finite element
modelling (Klinkvort et al, 2019). This
has led to both cone penetration testbased
prediction methodologies (DNV,
1992; Senders & Randolph, 2009) and
bearing capacity-based prediction
methodologies (Houlsby & Byrne, 2005;
Andersen et al, 2008). The available
methods for predicting the required
suction pressures for suction bucket
installations in sand and the risks
associated with these installations are
well summarised by Offshore Wind
Accelerator (2019) and Sturm (2017).
The bearing capacity methods, as
well as Senders & Randolph (2009),
generally take into account the effect of
flow around the skirt tip caused by the
differential pressure between the
inside and outside of the bucket, which
leads to a reduction in vertical effective
stresses at the skirt tip and inside the
July/August 2022

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