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Centrifuge modelling of the impact of local and global scour erosion on the monotonic lateral response of a monopile in sand

Centrifuge modelling of the impact of local and global scour erosion on the monotonic lateral response of a monopile in sand
Centrifuge modelling of the impact of local and global scour erosion on the monotonic lateral response of a monopile in sand
The majority of offshore wind turbines are founded on large-diameter, open-ended steel monopiles. Monopiles must resist lateral loads and overturning moments because of environmental (wind and wave) actions, whereas vertical loads tend to be comparatively small. Recent developments in turbine sizes and increases in hub heights have resulted in pile diameters increasing rapidly, whereas the embedment length to diameter ratio (L/D) is reducing. Soil erosion around piles, termed scour, changes the soil strength and stiffness properties and affects the system’s load resistance characteristics. In practice, design scour depths of up to 1.3D are routinely assumed during the turbine lifetime; however, the impact on monopiles with low L/D is not yet fully understood. In this article, centrifuge tests are performed to assess the effect of scour on the performance of piles with low L/D. In particular, the effect of combined loads, scour type (global, local), and depth are considered. A loading system is developed that enables application of realistic load eccentricity and combined vertical, horizontal, and moment loading at the seabed level. An instrumented 1.8-m-diameter pile with L/D = 5 is used. A friction-reducing ball-type connection is designed to transfer lateral loads to the pile without inducing any rotational pile-head constraint, which is associated with loading rigs in tests of this nature. Results suggest that vertical and lateral load interaction is minimal. Scour has a significant impact on the lateral load-bearing capacity and stiffness of the pile, leads to increases in bending moment magnitude along the pile shaft, and lowers the location of peak pile bending moment. The response varies with scour type, with global scour resulting in larger moments than local scour. The size of the local scour hole is found to have a significant impact on the pile response, suggesting that scour hole width should be explicitly considered in design.
0149-6115
Li, Qiang
fbe48bd4-925e-40a1-b7cb-36d902955bda
Askarinejad, Amin
4622bacc-a03f-4a7d-9b53-693639279fed
Prendergast, Luke
a6d404b3-30ba-4ed4-bddf-f0577e0ad6fb
Gavin, Kenneth
d46e6c97-9b37-468d-8eab-3f741c153e18
Chortis, Giorgos
6f42ec97-818a-4d4a-bef7-f19458c3997b
Li, Qiang
fbe48bd4-925e-40a1-b7cb-36d902955bda
Askarinejad, Amin
4622bacc-a03f-4a7d-9b53-693639279fed
Prendergast, Luke
a6d404b3-30ba-4ed4-bddf-f0577e0ad6fb
Gavin, Kenneth
d46e6c97-9b37-468d-8eab-3f741c153e18
Chortis, Giorgos
6f42ec97-818a-4d4a-bef7-f19458c3997b

Li, Qiang, Askarinejad, Amin, Prendergast, Luke, Gavin, Kenneth and Chortis, Giorgos (2020) Centrifuge modelling of the impact of local and global scour erosion on the monotonic lateral response of a monopile in sand. Geotechnical Testing Journal, 43 (5). (doi:10.1520/GTJ20180322).

Record type: Article

Abstract

The majority of offshore wind turbines are founded on large-diameter, open-ended steel monopiles. Monopiles must resist lateral loads and overturning moments because of environmental (wind and wave) actions, whereas vertical loads tend to be comparatively small. Recent developments in turbine sizes and increases in hub heights have resulted in pile diameters increasing rapidly, whereas the embedment length to diameter ratio (L/D) is reducing. Soil erosion around piles, termed scour, changes the soil strength and stiffness properties and affects the system’s load resistance characteristics. In practice, design scour depths of up to 1.3D are routinely assumed during the turbine lifetime; however, the impact on monopiles with low L/D is not yet fully understood. In this article, centrifuge tests are performed to assess the effect of scour on the performance of piles with low L/D. In particular, the effect of combined loads, scour type (global, local), and depth are considered. A loading system is developed that enables application of realistic load eccentricity and combined vertical, horizontal, and moment loading at the seabed level. An instrumented 1.8-m-diameter pile with L/D = 5 is used. A friction-reducing ball-type connection is designed to transfer lateral loads to the pile without inducing any rotational pile-head constraint, which is associated with loading rigs in tests of this nature. Results suggest that vertical and lateral load interaction is minimal. Scour has a significant impact on the lateral load-bearing capacity and stiffness of the pile, leads to increases in bending moment magnitude along the pile shaft, and lowers the location of peak pile bending moment. The response varies with scour type, with global scour resulting in larger moments than local scour. The size of the local scour hole is found to have a significant impact on the pile response, suggesting that scour hole width should be explicitly considered in design.

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Published date: 10 January 2020

Identifiers

Local EPrints ID: 500736
URI: http://eprints.soton.ac.uk/id/eprint/500736
ISSN: 0149-6115
PURE UUID: 2dd0dc9f-924a-4ccd-9948-ff1b53902d43
ORCID for Qiang Li: ORCID iD orcid.org/0009-0001-7588-3819

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Date deposited: 12 May 2025 16:40
Last modified: 13 May 2025 02:12

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Contributors

Author: Qiang Li ORCID iD
Author: Amin Askarinejad
Author: Luke Prendergast
Author: Kenneth Gavin
Author: Giorgos Chortis

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