Optimised screw pile design for offshore jacket foundations in medium–dense sand

Cerfontaine, Benjamin; Brown, Michael; Davidson, Craig; Sharif, Yaseen; Huisman, M.; Ottolini, Marius

doi:10.1680/jgele.21.00105

Cited by 8 publications

(5 citation statements)

References 16 publications

(27 reference statements)

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“…The increased stress above the helix can push the pile downward and consequently reduce or even eliminate the vertical installation force. In addition, the increased stress and potential densification in the soil above the helix can result in better monotonic uplift performance [3,4,5,6,7].…”

Section: 𝐴𝑅 = ∆𝑧 𝑝 ℎmentioning

confidence: 99%

Effects of installation advancement ratio on cyclic uplift response of a single-helix screw pile: experimental and numerical investigation in sand

Wang,

Brown,

Ciantia

et al. 2023

SEG

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A screw pile consists of one or several helices connected on a central straight shaft or core. These kinds of piles are widely used onshore and typically installed by applying torque at the pile head with additional vertical compressive force (also referred to as crowd force) if required. Current standards and industrial guidelines recommend that installation follows the pitch-matched approach i.e. advancement ratio AR = 1.0 [1], which means that the pile vertical penetration for each rotation equals the helix pitch, because it is suggested that this leads to reduced installation disturbance. where is the vertical penetration per rotation and is the helix geometric pitch. Recently this kind of pile has been proposed for upscaling from typical onshore sizes as an alternative silent foundation/anchoring solution for future offshore renewable energy application. This may include use as foundations for jacket structures or anchors for wind turbine in deeper water. However, the increase in pile sizes may require prohibitive vertical force if pitch-matched installation is adopted [2] in sand. One of the solutions to this is over-flighting (AR < 1.0) where the pile is over rotated during installation. When the pile is over-flighted, sand below the helix can move upward through the helix opening resulting in a higher stress and potential densified zone above the helix (Figure 1(a)). The increased stress above the helix can push the pile downward and consequently reduce or even eliminate the vertical installation force. In addition, the increased stress and potential densification in the soil above the helix can result in better monotonic uplift performance [3, 4, 5, 6, 7]. As foundations for offshore renewable energy applications, the screw piles also need to be able to perform under cyclic loading from wind, wave and current for example. Investigations of uplift cyclic performance of pitch-matched screw pile installation have been reported, but the over-flighting effects have not received significant attention. To give confidence in the maintenance of the beneficial effects of over-flighting on cyclic performance, centrifuge tests and discrete element modelling (DEM) were used in this study. The centrifuge tests provide more reliable physical evidence than 1g tests but are less costly and more accessible than field test. Based on the results of centrifuge test, the DEM models are validated and used to allow separate evaluation of the mechanism of the helix and the pile shaft and assessment of soil state evolution during cycling, which are difficult to capture in field or centrifuge tests. The centrifuge tests show that the screw piles installed at lower AR accumulate displacement at lower rates with cycling. which is also captured by the DEM (Figure 1(b)). In addition, DEM suggests that shaft behavior controls cyclic performance because the shaft friction resistance is stiffer than the helix bearing resistance at small displacements. The radial stress on the pile shaft can be enhanced by over-flighting installation. Although it degrades with cycling, the enhanced radial stress due to over-flighting remains higher than that of the pitch-matched pile and therefore the over-flighted piles have improved cyclic performance. On the basis of improved cyclic uplift performance and low vertical installation force, over-flighting installation may be utilized for offshore screw piles performing under cyclic uplift loading where installation forces are a concern or loading is predominantly one-way tensile loading.

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Section: 𝐴𝑅 = ∆𝑧 𝑝 ℎmentioning

confidence: 99%

Effects of installation advancement ratio on cyclic uplift response of a single-helix screw pile: experimental and numerical investigation in sand

Wang,

Brown,

Ciantia

et al. 2023

SEG

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“…In recent years, research on the installation effect and required load of helical anchors has been increasing, including studies on the disturbed zone, density change, soil parameters after disturbance [8][9][10], lateral earth pressure and uplift or compressive performance after disturbance [11][12][13][14], installation requirements, particle motion during screwing and the torque correlation factor [15][16][17][18]. Some studies have investigated the impact of the installation advancement ratio (AR) on the load-bearing performance of single-helix anchors, where the AR is defined as the ratio of vertical displacement per one rotation ∆z to helix pitch p h [16,19,20]. Cerfontine et al [16,20] found that a lower vertical displacement per rotation can significantly reduce the necessary crowd force during installation or even generate some pull-in, and the uplift stiffness and capacity of the pile were enhanced by this installation process, but the torque remained relatively unchanged.…”

Section: Introductionmentioning

confidence: 99%

“…Some studies have investigated the impact of the installation advancement ratio (AR) on the load-bearing performance of single-helix anchors, where the AR is defined as the ratio of vertical displacement per one rotation ∆z to helix pitch p h [16,19,20]. Cerfontine et al [16,20] found that a lower vertical displacement per rotation can significantly reduce the necessary crowd force during installation or even generate some pull-in, and the uplift stiffness and capacity of the pile were enhanced by this installation process, but the torque remained relatively unchanged. In addition, the pile capacity in tension generally increases as the AR is reduced and reaches a maximum for AR = 0.5, while the compressive capacity reduces.…”

Section: Introductionmentioning

confidence: 99%

Installation Disturbance of Helical Anchor in Dense Sand and the Effect on Uplift Capacity Based on Discrete Element Method

Chen,

Liu,

Hao

et al. 2024

JMSE

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Helical anchors have been extensively employed as foundation systems for carrying tension loads due to their installation efficiency and large uplift capacity. However, the installation influences of helical anchors are still not well understood, especially for multi-helical anchors. The matrix discrete element method was used to model the process of helical anchor penetration and pull-out in dense sand to investigate the effects of the anchor geometry and advancement ratio (AR, the relative vertical movement per rotation) on soil disturbance, the particle flow mechanism, and the uplift capacity. For shallow helical anchors, the overall disturbance zone is the shape of an inverted cone after installation, while for deep helical anchors, it is funnel-shaped. The advancement ratio has significant effects on the soil particle movement and uplift capacity of helical anchors. The soil particle flow mechanism around helical plates has been identified for single-helix anchors at various advancement ratios, and for double-helix anchors, the influence of the top plate on particle movement during installation was investigated. The uplift capacities of both single- and double-helix anchors increase with the decrease in the AR (AR = 0.5~1), and the influence decreases with the anchor embedment ratio. The efficiency of double-helix anchors induced by installation is close to 1 at pitch-matched installation (AR = 1), indicating that the impact of the top plate during installation is minimal in this case.

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“…Secondly, stricter regulations on underwater noise make mitigation methods for pile driving more expensive and silent piling techniques could be used as an alternative [2]. Figure 1(a) describes a new screw pile foundation that meets those two challenges [3].…”

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confidence: 99%

“…The main challenge during the pile installation is the very low reaction force that may be available offshore at the pile head, which consists only of the pile self-weight and the installation tool. The aim of this work is to demonstrate that screw piles can be installed for offshore applications by rotary jacking at low reaction force, via (geotechnical centrifuge, [2]) and numerical (DEM, [5]) modelling. Figure 1(c) shows the vertical force associated with each pile component during the DEM simulation (AR = 0.5).…”

mentioning

confidence: 99%

Silent piling for offshore jacket foundations in sand: DEM and centrifuge modelling

Cerfontaine,

Brown,

Ciantia

et al. 2023

SEG

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The race to decarbonise the economy has led to an exponential growth of offshore wind farm developments across the globe. While monopiles are the dominating installed foundations, jacket structures are expected to be more and more common, as wind farms extend to deeper waters [1] and innovative technologies are required to alleviate some important challenges. Firstly, straight shafted piles are not particularly efficient to sustain large tensile demand induced by the push-pull axial loading of the foundations. Secondly, stricter regulations on underwater noise make mitigation methods for pile driving more expensive and silent piling techniques could be used as an alternative [2]. Figure 1(a) describes a new screw pile foundation that meets those two challenges [3]. The foundation is installed by rotary jacking, which avoids any impact related noise. The pile is composed of a large upper shaft, that is designed to resist the lateral load applied by the jacket structure, and a smaller lower shaft which reduces the torque demand during installation. A helix is attached close to the pile tip, to provide an enhanced axial resistance and facilitates the installation of the pile [4]. The main challenge during the pile installation is the very low reaction force that may be available offshore at the pile head, which consists only of the pile self-weight and the installation tool. The aim of this work is to demonstrate that screw piles can be installed for offshore applications by rotary jacking at low reaction force, via (geotechnical centrifuge, [2]) and numerical (DEM, [5]) modelling. Figure 1(b) compares the vertical installation force measured in the centrifuge with a DEM simulation, as a function of the advancement ratio (AR), defined as the vertical displacement of the pile during one rotation normalised by the helix pitch. Figure 1(b) shows that at constant installation rate (fixed AR), the lower ARs lead to a tensile force measured at the pile head, i.e. the pile pulls on the installation actuator. The DEM simulation of is in good agreement with the centrifuge result at the same AR and shows that the vertical force trend reverts to a compressive value once the upper part of the shaft enters the ground. Figure 1(c) shows the vertical force associated with each pile component during the DEM simulation (AR = 0.5). The greatest penetration resistance (lowest negative value) is the transition piece, reaching -4MN, when the pile tip reaches a depth of 25m. The shaft penetration resistance is small and probably reduced by the rotary movement that changes the inclination of the shear stress along the shaft, as explained in previous publications [6]. The base and helix contributions are largely positive (in tension), as described in Figure 1(c). Consequently, the tension (“thrust”) created by the helix compensates the penetration resistance of the rest of the pile and enables its penetration with a very low reaction force applied at the pile head. The effect of the AR on the helix behaviour was previously explained as follows [7, 8]. Helix overflighting (AR< 1) forces a displacement of the sand particles upwards. This forced displacement is opposed by the existing soil, which acts as a non-linear spring, which is progressively compressed. The soil above the helix is then compressed, as depicted in Figure 1(d), which represents the vertical stress in the sand bed and around the pile. Another effect of the pile overflighting is the considerable reduction of the vertical stress under the helix, which facilitates the pile penetration. On the contrary, a large compressive stress under the transition piece is at the origin of its large penetration resistance and its geometry may need to be further optimised. In the field, where the pile head condition is a constant reaction force, the AR will simply vary such that the thurst provided by the helix will compensate the pile penetration resistance. Results shown in Figure 1(b) prove that there exists a set of AR between 0.10 and 1.00 such that the necessary force is equal to zero at any depth, thus ensuring the fasibility of its installation at low reaction force, providing a torque of 23MNm can be applied. The physical and numerical modelling of a new type of pile have shown that it is a viable option to support jacket structures, as it can be installed without driving and with a very limited reaction force. The DEM simulations have revealed the underlying mechanisms explaining the centrifuge results, while centrifuge results were used to validate the DEM simulations. This project showed that the combination of the two techniques is very powerful to speed up technology development.

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Optimised screw pile design for offshore jacket foundations in medium–dense sand

Cited by 8 publications

References 16 publications

Effects of installation advancement ratio on cyclic uplift response of a single-helix screw pile: experimental and numerical investigation in sand

Effects of installation advancement ratio on cyclic uplift response of a single-helix screw pile: experimental and numerical investigation in sand

Installation Disturbance of Helical Anchor in Dense Sand and the Effect on Uplift Capacity Based on Discrete Element Method

Silent piling for offshore jacket foundations in sand: DEM and centrifuge modelling

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