Elsevier

Applied Ocean Research

Volume 82, January 2019, Pages 337-345
Applied Ocean Research

Driving energy losses for constant diameter and tapered submerged monopiles

https://doi.org/10.1016/j.apor.2018.11.006Get rights and content

Abstract

Large monopiles are used as foundations for offshore wind turbines and are generally designed with a tapered section or conical shape. Some loss of driving energy is expected to occur during installation of these structures due to the submerged section of the tapered monopile. The current literature on this subject is limited and indicates rather large losses compared to field observations.

A numerical model of the monopile–water–soil system was set up in the general-purpose finite element package Abaqus. By simulating the hammer impact and the resulting stress wave propagation through the monopile and water, the energy losses to be expected can be calculated accurately. The model was verified against independent finite element analyses and experimental data.

A parametric study was performed and the effect of hammer characteristics, submerged monopile length and monopile geometry on the driving energy losses were quantified. The results enable a simple relationship between the energy losses and the monopile geometry to be proposed which increases linearly with pile diameter, taper angle, and submerged length. The losses are typically on the order of 0.15–0.3% per metre submerged length for large tapered monopiles.

Introduction

Large diameter monopiles are widely used as the foundation for offshore wind turbines and the current trend indicates that large diameter monopiles will continue to be selected for larger water depths and for heavier turbines [1]. The increase in monopile length and turbine weight requires an increase in monopile diameter to retain an optimal stiffness response. Monopiles up to 10 m diameter are already being planned at some wind farms. Design optimization of a monopile generally leads to a tapered section over part or all of the submerged pile.

Reliable pile driving predictions prior to installation are required for optimal hammer selection. The hammer energy needs to cover for the required driving energy necessary to overcome the soil resistance to driving and for the energy losses which occur in the process. The energy losses are not normally accounted for during pile driving predictions for small diameter piles (i.e. less than 2–3 m diameter). Recent studies which focus on the noise generated by pile driving of 0.7–0.9 m diameter piles suggest that 0.5–1% of the hammer energy is lost in the water column [2] [3]. However, as the size of the monopiles is increasing and as the geometry is changing from constant diameter piles to tapered monopiles, the driving energy losses now make up a larger part of the required hammer capacity compared to the case for small diameter piles.

Potential energy losses during driving due to the tapered section of large diameter monopiles have been addressed by Barauskis and Jacobsen [4] and Gjersøe et al. [5] by determining the reduced work due to the confined water forcing on the inside of the tapered monopile. In both papers, the use of finite element modelling (FEM) is presented:

  • In Barauskis and Jacobsen [4] FEM was used to obtain the axial stresses during driving, which was then used to calculate the axial strain in the monopile and this the relative change in vertical velocity at the taper.

  • In Gjersøe et al. [5] FEM was used to directly determinate the relative change in vertical velocity at the taper.

The reduced work due to the confined water was then calculated from the pressure in the water calculated from Joukowsky's law, Thorley (2004) [6]:Elosses=ρ·c·Δv·ΔAwhere ρ and c are the water density and speed of sound, and Δv is the relative change in velocity, and ΔA is the difference in horizontal cross-section area at top and bottom of the tapered section.

The energy losses due to submergence reported in both papers for a tapered monopile with top diameter 4.6 m and a bottom diameter 6 m are large and do not reflect the author's installation experience. As such, more reliable driveability predictions are required for optimal driving hammer selection to avoid oversizing or undersizing the hammer. Potential driving energy losses need to be accurately quantified and this is the main objective of the present work.

A numerical model for both constant diameter and tapered monopiles covering a wide range of geometries and hammer energies has been developed to identify the driving energy losses in water. The water is explicitly modelled as an acoustic medium to capture the interaction between the pile and the water. This paper describes the model and the results of the study.

Section snippets

Energy transmission and losses

A typical tapered monopile driven partially submerged is shown in Fig. 1. Energy is transferred from the hammer to the pile and transmitted down the pile primarily in the form of an axial stress wave. Any energy which is transformed into other forms which do not contribute to driving the pile into the ground may be considered “lost” from a driving perspective. This terminology is adopted for this study. Apart from the energy losses in the hammer-anvil-pile system which are common to all impact

General

The simulations performed considered an axisymmetric finite element (FE) model of the full monopile, the water and the soil. The hammer blow was simulated by applying a force–time signal at the pile top.

Non-linear dynamic analyses were performed using the general purpose finite element package Abaqus Standard [10] with acoustic elements for modeling pressure waves in water coupled to the structure, as in Simulia [11].

The simulations covered a time period of 40 ms, and the time interval for each

Finite element model verification

Reinhall and Dahl [7] present a pile driving experiment at a construction site on Vashon Island in Puget Sound. Piles of 0.762 m diameter and 2.54 cm wall thickness were driven with a diesel powered DELMAG D62-22 hammer into the seabed with a 12.5 m water column. The underwater noise during impact pile driving was recorded by a vertical line array (VLA). The acoustic radiation was further investigated using a FE model of a pile being driven into the sediments in shallow water using an implicit

Parametric study

An extensive parametric study was performed with a total of 26 simulations covering the variations shown in Table 2.

In order to develop the input signals, practical combinations of hammer, hammer energy, and anvil were selected aiming for a peak axial stress in the range 90MPa - 140MPa and pile set at least 5mm. A series of GRLWEAP analyses were performed to cover both a smaller hammer operating at maximum energy and larger hammers at much lower energy. The hammer and anvil selection and

Results

The results of all runs are summarised in Table 6. Note that the peak stress (presented at mid-taper height) is always lower than the target peak stress output of GRLWEAP. Similarly, the transferred energy calculated at top control point (Et,top) is smaller than the hammer ENTHRU (with the exception of Case 8a). The differences between the Abaqus results (Et,top) and the GRLWEAP analysis output (ENTHRU) are due to the differences in the modelled pile, hammer and anvil impedances (hammer and

Driving energy losses and stress wave propagation in water

Further insight into the stress wave propagation is gained from the vertical–radial displacement path of the pile. Fig. 13 shows the displacement of a point in the uniform top section and at the centre of the tapered section. Note that the axis scales are not the same for clarity of presentation. The figure shows that even within the taper section, the initial outward movement of the pile is present for the first 5 ms, and only after this time does the pile ‘close in’ on the water inside. The

Conclusions

A 2D axisymmetric finite element model simulating impact driving and energy losses in submerged tapered monopiles has been presented. The hammer blow is introduced in the model as force–time input signal derived by GRLWEAP analyses for different hammers. The FE model and the energy loss estimation were verified by reproducing the impact pile driving results presented in Reinhall and Dahl [7]. Calculated energy losses due to submergence were in agreement with the energy transmitted to the water

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