The response of suction caissons to long-term lateral cyclic loading in single-layer and layered seabeds

1. THE RESPONSE OF SUCTION CAISSONS TO
LONG-TERM LATERAL CYCLIC LOADING IN
SINGLE-LAYER AND LAYERED SEABEDS
Michael Adane

2. SUCTION CAISSONS
• are being increasingly considered as an alternative foundation type to monopiles for offshore wind
turbines.
• Are subjected to lateral cyclic loading from wind and waves acting on the structure.
Response of suction caissons
• Recent studies = few thousand cycles
• Actual scenario = millions of loading cycles
• Lateral cyclic loading was seen to increase caisson capacity by up to 30% – with a bias towards clay-
dominated seabed profiles – and stiffness by up to 50%.
• Previous studies have only focused on single layered seabeds.
• But actual scenario seabeds in many sites considered for offshore wind development are layered.
For example, in Dogger Bank, North Sea, the thickness of the superficial sand overlying stiff clay
varies from several to tens of meters, such that the caisson is likely to be embedded in a ‘sand over
clay’ seabed profile.

3. OFFSHORE WIND TURBINES
• Offshore wind turbines are typically supported by monopile foundations. Eg. Gode offshore wind farm.

4. HOW DOES IT WORK?
• An alternative and potentially less expensive foundation is a suction caisson.
• is a large upturned bucket that is installed in the seabed by pumping water from the caisson
interior.
• First used as a foundation for a 3 MW turbine at Frederikshavn, Denmark in 2002

5. INSTALLATION OF SUCTION CAISSON WITH LARGE UPTURNED BRACKET

6. MAIN SOURCES OF LOADING
• Wind turbine weight
• Lateral cyclic loads due to waves and wind
 Results large overturning moment M
 is expected to experience between 107 and 108 relatively
low-level loading cycles over its 25 year lifetime.
• Resulting in non-recoverable rotation of the turbine.
 This can be an issue for the serviceability design of a
turbine.
 the permanent accumulated rotation of the turbine during
operation may need to be kept within very tight limits
(0.250)

7. RESONANCE
• Occurs when
• natural frequency of the system ≈ forcing frequency.
• potential for cyclic loading to change the foundation stiffness, which influences the stiffness and thus natural
frequency of the wind turbine as a whole.
• We need to reliably predict foundation and system stiffness over the course of the design life of the wind
turbine.

8. WIND INDUCED HARMONIC RESONANCE

9. SOIL PROPERTIES AND SAMPLE PREPARATION
• Conducting reduced scale single-gravity experiments requires careful consideration of scaling to ensure that
the results are meaningful at field scale.
• This study mainly focuses on accumulated caisson rotation.
• So consideration needs to be given to the soil stiffness.
• A layered seabed profile, with dense sand (Dr,f = 80%) overlying stiff clay (su,f = 80 kPa)
• To replicate typical North Sea seabeds.
• model caisson was based on a field-scale caisson with a diameter, D = 16 m and a skirt length, L = 8 m,
modelled at a reduced scale of 1:100 such that the effective stress ratio, σ’v,m /σ’v,f = 1/100.
• Mean effective stress at failure P’ considered at a representative depth equal to half the caisson skirt length
• Field P’ = 120KPa
• Model P’ = 1.2KPa

10. EXPERIMENTAL ARRANGEMENT AT DIFFERENT STAGES

11. EXPERIMENTAL ARRANGEMENT
• Caisson penetration resistance
during installation was
measured by a load cell
• The hinge ensured no bending
moment developed at the load
application point
• Lateral loads were measured by
a second load cell
• Caisson rotation and
displacement were determined
from measurements made by
four laser sensors

12. LOADING

13. PROCEDURE
a) Installation: was deemed complete when the caisson penetration resistance
increased markedly, signifying contact of the caisson lid with the surface of the soil
sample. The caisson vent was then sealed and the installation actuator was removed
to prepare for the lateral load test.
b)Monotonic loading: the monotonic tests were conducted in displacement control,
with a displacement
rate at the hinge of 0.05 mm/s. Each monotonic test was maintained until the caisson
had rotated through
about 2°, which was more than sufficient for failure.
c) Cyclic loading: the cyclic tests were conducted in (lateral) load control, such that the
moment, M, at the load reference point (M = He) was also controlled.
d)Post-cyclic monotonic loading: after cyclic loading the lateral load on the caisson
was reduced to zero before loading the caisson monotonically to quantify post-cyclic
stiffness and capacity.

14. RESULTS: ULTIMATE MOMENT CAPACITY
• single-layer sand sample, ultimate moment capacity, Mult = 2
Nm.
• Layered sample, ultimate moment capacity, Mult = 4 Nm.
• single-layer clay sample, ultimate moment capacity, Mult = 6
Nm.

15. RESULTS: ACCUMULATED ROTATION

16. RESULTS = UNLOADING STIFFNESS
• variation in normalized unloading stiffness, kN/k1, with cycle number, N, where k1 and kN are the unloading
stiffnesses in the first cycle and cycle number N, respectively.
• Unloading stiffness is seen to increase due to very small changes in rotation in each cycle.
• shows that the increases are more immediate in the soil profiles dominated by sand, which reflects densification,
with later
increases in the clay-dominated profiles attributed to consolidation.

17. RESULTS = POST-CYCLIC BEHAVIOR
• A monotonic test was performed after each cyclic
test to assess the effect of cycling on capacity and
stiffness.
• up to approximately 30% in the clay-dominated
samples
• approximately 10% in the sand-dominated samples
• In single-layer clay Mult is taken as this peak
moment capacity, the
• increase in moment capacity is 50%.
• is attributed to cyclic-loading induced
consolidation.
• Consolidation plays a beneficial role in stabilizing
caisson rotation and the unloading stiffness, but
may also allow for design efficiencies with respect
to moment capacity.

18. CONCLUSION
• The study considered the response of a suction caisson to the type of lateral cyclic loading experienced
by
an offshore wind turbine in the layered seabeds prevalent in many of the existing and proposed wind
farm developments.
• The main focus in the experiments reported above is
1. Caisson rotation (as serviceability limit state in design restricts max rotation of 0.5 degrees)
2. Foundation stiffness (as stiffness changes need to be considered in the fatigue limit state) (Resonance)
3. Lateral Capacity (Ultimate limit state design)
• Cyclic loading has the effect of increasing the capacity by up to 30% and foundation stiffness by up to
50%, attributed to densification of the sand and consolidation-induced strength increases in the clay.
• NOTE :- conclusions reached in this study relate to unidirectional cyclic loading and the validity of these
conclusions need to be checked when the load direction varies according to the changing direction of
the wind and waves.

19. REFERENCES
• Zhu FY, O'Loughlin CD, Bienen B, Cassidy MJ, Morgan N. The response of
suction caissons to long-term lateral cyclic loading in single-layer and
layered seabeds. Géotechnique. 2017 Nov 10:1-3.