inappropriatness of the current offshore guidelines …

INAPPROPRIATNESS OF THE CURRENT OFFSHORE

GUIDELINES FOR THE DESIGN OF LARGE-DIAMETER

MONOPILES IN SANDS – DEMONSTRATION THROUGH

5-MW REFERENCE WIND TURBINE FROM NREL

Djillali Amar BOUZID

Department of Civil Engineering, University of Blida, Route de Soumaa

Blida 9000, Algeria

ABSTRACT

Since the World’s demand on energy is constantly raising, many countries built and are

currently building their offshore wind farms to produce cheap, clean and renewable energy as

a substitute to fossil fuel based energy which is the source of green-house gases leading the

main cause of global heating. Despite the increasing in turbine capacity, the preferred

foundations for these offshore wind turbines (OWTs) are large diameter monopiles due to

their ease of installation in shallow to medium water depth.

For decades the well-known curve method (adopted by API and DNV Offshore Guide

Lines) in which the monopile/soil interaction is considered by non-linear curves

derived from field tests, has been applied to analyze monopiles supporting OWTs. Although

the formulation has been applied successfully in designing piles supporting gas and oil

platforms in offshore industry due to the low failure rate observed in piles, its application to

design large-diameter monopiles supporting offshore wind energy converters has not been

validated for monopiles of diameter up to 6 m as many authors reported inconsistencies in the

use of these design rules. In this paper the weakness points of the Winkler model based on

curves are also given along with the recent propositions to enhance the performance of

the Winkler models which are described. Then a new finite element procedure based on the

combination use of 2D FE analysis and finite differences method suitable for 3D soil/structure

interaction problems is briefly described. By considering a 5-MW reference offshore wind

turbine from NREL embedded in sand, the study focus then, on the examination monopile

head stiffness which is the key element in quantifying the monopile/soil interaction. The

monopile head stiffness is significantly important for the determination of the first OWT

natural frequency which is in turn a crucial parameter for the monopile design.

1. INTRODUCTION

As wind energy is one of the most promising and fastest-growing renewable energy sources

around the world, there has been a rapid development in the installation of wind farms in both

onshore and offshore locations to secure clean electricity as a permanent source of energy.

Because offshore winds tend to have higher speeds and less turbulent than onshore winds,

resulting in an increase in the potential energy produced in offshore locations than in onshore

ones, several offshore wind farms have been already installed all over the planet the last

decade and a vast number is being planned for the near future. Although, there are many

Offshore Wind Turbines (OWTs) support options depending on the water depth (ranging from

gravity foundations for shallow depths of to floating foundations for very deep

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7. Geoteknik Sempozyumu 22-23-24 Kasım 2017, İstanbul

waters of ), the most widely used option for supporting offshore wind-turbines is

the monopile foundations, which consist of welded steel pipe piles driven open-ended into

soil. This type of foundations has proven to be an efficient solution in water depths ranging

between and and consequently it is the most appropriate choice for offshore wind

farms, due to the ease and speed of installation and cost of construction (Achmus et al. 2009;

Adhikari and Bhattacharya (2011, 2012); Carswell et al. 2015; Laszlo et al. (2016, 2017);

Galvin et al. 2016).

The primary objective of this paper is twofold. The first, is to show that a FE analysis based

on a nonlinear soil model is the best choice as a method of analysis for safe design of OWTs.

The second, is to confirm what has been stated about the behavior of laterally loaded

monopiles since the emerging of OWT industry by many researchers who ascertain that the

O’Neill and Murchison’s (1983) formulation which has been adopted first by the API and

then by the DNV, is no longer appropriate for the analysis of large-diameter monopoles under

horizontal loading.

2. P-Y CURVES USED IN THE CURRENT OFFSHORE GUIDELINES

FOR THE DESIGN OF LARGE-DIAMETER MONOPILES

Using this piecewise curve proposed by Reese et al. (1974) and a relatively large

database of laterally loaded pile tests, O’Neill and Murchison (1983) suggested a hyperbolic

formula for curve in order to describe the relationship between soil resistance and

lateral pile deflection in sand:

(1)

In equation (1), is the ultimate soil resistance and represents the initial coefficient of

subgrade reaction depending on the angle of internal friction of the cohesionless soil. The

initial stiffness of the curves , recommended in the design can be obtained by

evaluating the slope of the curve tangent at .

(2)

It is quite clear from the equation (2), that the initial stiffness is independent on pile properties

(diameter and bending stiffness) and it is linearly dependent on the depth .

Standards for designing laterally loaded piles, e.g. the American Petroleum Institute (API,

2011), Det Norske Veritas (DNV, 2013) are based on the Winkler model using curves

whose expressions are given by equation (1).

2.1 Drawbacks of the formulations of curves

Although, they have been employed for designing offshore piled foundations by the offshore

oil and gas industry for decades, these design recommendations (API, and DNV) are not

appropriate for designing foundations for offshore wind turbine structures, for many reasons:

a) They have been developed on the basis on full-scale load tests on long, slender and

flexible piles with a diameter of 0.61 m, whereas monopiles are relatively shorter and

stiffer piles with diameters up to 6.0 m in the offshore wind turbine industry.

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b) The widely used API model is calibrated against response to a small number of cycles

for offshore fixed platform applications. However, an offshore wind turbine may

undergo 107-10

8 cycles of loading over its lifetime of 20-25 years.

c) Under cyclic loading, the API and DNV models always predict degradation of

foundation stiffness in sandy soil. However, it has been showed that the monopile

stiffness in sandy soil will increase as a result of densification of soil in the vicinity of

the monopile.

For further details about the shortcomings inherent to the model formulation the reader

is referred to Amar Bouzid and Medjitna (2017).

2.2 Most recent modifications to improve the performance of the winkler

model The p-y curve given by the equation (1) is composed by to fundamental elements which are

the initial stiffness and the ultimate soil resistance . Most researchers found that the

linear variation with depth of as indicated in equation (2) is inappropriate for monopoles

from the fact that it heavily overestimates the soil stiffness at depth. However, the

overwhelming majority of authors considered appearing in the formulation by Reese et

al.1974, adequate for design of large-diameter monopiles and kept it unaltered in their new

curve formulations. Consequently, the most recent proposed modifications are listed in

Table 1.

Table 1. Proposed formulae for the curve initial stiffness.

Authors Proposed formulae Assigned parameters

O’Neill and Murchison

1983 (API 2014 and DNV

2011)

appearing in equations (1) and (2)

Wiemann et al. (2004)

for a medium dense sand

for a dense sand

Sorensen et al. (2010)

,

Kallehave et al. (2012)

Sorensen et al. (2012)

3. COMPUTER CODES USED IN THIS PAPER

A pseudo 3D finite element model has been performed to analyze soil/structure interaction

problems in non-linear media. This numerical technique, which has been called Nonlinear

Finite Element Vertical Slices Model (NFEVSM), is based on the discretization of the 3D

soil/structure medium into a series of vertical slices, each one represented by a 2D boundary

value problem (Figure 1). The procedure involves the combination of the finite element (FE)

method and the finite difference (FD) method for analyzing the embedded structure and the

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surrounding soil sub-structured. The soil in this approach, was considered to obey the

hyperbolic model proposed by Duncan and Chang (1970). The numerical procedure has been

implement in a computer code called NAMPULAL (Details of this computer code are found

in Otsmane and Amar Bouzid (2017).

Figure1. (a) Offshore Wind Turbine as SSI problem, (b) The Vertical Slices Model showing

the interacting slices subjected to external and body forces.

For the Winkler model computations another computer code called Winkler-ROWKSS has

been written by the author including in addition, to Reese et al. and O’Neill and Murchison

formulations, all the proposed modifications presented in Table 1. Winkler-ROWKSS uses

the concept of a Beam on Nonlinear Winkler foundation (BNWF) to discretize the monopile

in finite differences method and the curves to model the soil lateral response.

4. ASSESSMENT OF HEAD MOVEMENTS OF THE MONOPILE

SUPPORTING 5-MW REFERENCE WIND TURBINE FROM NREL

IN SAND

The monopile head movements (lateral displacements and rotations) are the crucial

parameters required to design a monopile supporting an offshore wind turbine under

horizontal force or an overturning moment. In deed they are the key elements for the

determination of the monopile head stiffness which can be quantified by three springs, two for

controlling horizontal and rocking movements and one for the cross-coupling interaction. The

Monop

ile

Tower

Nacell

e

(a

)

(b

)

Medium to

be

discretised

V M

H

The fisrt three

slices

The

slice

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relationship between lateral stiffness , rocking stiffness and cross-coupling stiffness

may be expressed in a maytrix form as:

(3)

Where, and are respectively the shear force and the overturning moment applied at the

monopile head and and are respectively the lateral displacement and rotation of the

monopile head.

In order to assess the finite element results through the use of NAMPULAL against those of

API and those from the recently developed p-y curves implemented in Winkler-ROWKSS, it

is useful to consider the NREL 5-MW baseline wind turbine from the National renewable

energy laboratory (NREL) which is mounted atop a monopile with a flexible foundation in 20

m water depth (Jonkman et al., 2009 and Jung et al., 2015).

The NREL 5-MW mass and structural details are listed respectively in Tables 2 and 3. In this

study a representative soil profile was assumed, which was adopted from the work of Passon

(2006) as shown in Figure 2.

Figure 2. Soil profile and monopole dimensions for the NREL-5MW reference wind turbine.

Table 2. Reference wind Turbine masses

Table 3. Reference wind Turbine geometrical properties

Tower height (m) Tower diameter (m) Tower wall thickness (m)

At the base At the top At the base At the top

87.6 6.00 3.87 0.027 0.019

Rotor mass (tons) Nacelle mass (tons) Tower mass (tons)

110.0 240.0 347.5

MSL 0.0

- 20.0

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The geometrical characteristics are given in Table 4.

Table 4. Monopile properties

Monopile length (m) Monopile diameter (m) Monopile wall thickness (m)

Embedment length overhang

36.0 30.0 6.0 0.06

The evolution of monopole head movements (displacements and rotations) with the

increasing applied loading at the monopole top constitutes the main purpose of the present

investigation. In order to achieve this target both NAMPULAL and Winkler-ROWKSS

have been applied to examine the different tendencies. Figures 3 shows the monopile head

displacement and rotation with increasing lateral load or overturning moment. It seems from

the first sight that the results are separated in the three global categories. From the first,

relevant to finite element results, it is quite clear that the results of present method are nearly

identical to those of the FEM provided by Jung et al. (2015). The second, results by Reese et

al., O’Neill and Murchison and those of LPILE yield almost the same values and

consequently their curves are nearly above each other. However, the third category, relevant

to the Winkler methods based on the latest curves improvements, are occupying quite

different locations. Some near the FE curves, whereas others are even behind those of API

models.

Figure 3. Evolution of monopole head movements with applied loading: (a)

curves, (b) curves and (c) curves.

(a) (b)

(c)

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5. CONCLUSIONS

In this paper the author presented a numerical investigation of a laterally loaded monopoles

supporting 5-MW reference wind turbine in a sandy subsoil. Two types of numerical codes

have been applied to perform the numerical investigations. The first called NAMPULAL has

been used to discretize the medium into vertical slices, each is analyzed using the 2D

conventional finite element method with soil obeying to Duncan and Chang’s hyperbolic

model. The second computer code called Winkler-ROWKSS has been written on the basis of

finite difference scheme to model the monopole as a Beam on Nonlinear Winkler Foundation

(BNWF).

Through the examination of the monopole head movements, the paper investigated the lateral

behavior of a monopile supporting 5-MW reference wind turbine embedded in sands. From

the close examination of the load-deformation curves three major conclusions were reached:

[1] The results by NAMPULAL have been found to be in a close agreement with those

provided Abaqus reported in Jung et al. 2015. This confirms first, that the NAMPULAL

results are reliable since they are in perfect match with those of well-known FE package.

Secondly, the finite analysis reasonably assesses the monopile head stiffness which is a

crucial criterion of its design, ascertaining thus, that this rigourous method is not restricted to

parametric studies but also should be used in daily routine analysis of large-dialeter

monopiles.

[2] The Winkler methods (Reese et al. 1974 and O’Neill and Murchison 1983) on which

API and DNV have drawn their design guideline for large-diameter monopiles overestimate

the soil stiffness which may lead to inaccurate estimation of some dynamic characteristics of

offshoe wind turbine. This confirms that they are inappropriate for designing large-diameter

monopiles under lateral loading.

[3] From the load-deformation curves relevant to the winkler models based on the last

improvements in curves, different tendencies are noticed. Some are close to finite

element analysis, whereas others are even far from those adopted by both API and DNV. It

seems that sorensen and al. 201 have brought some improvement whereas the proposed curve by Kallehve et al. is not suitable at all to deal with large-diameter monopiles.

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