TABLE OF CONTENTS
List of Figures ... 3
List of Tables ... 6
Indices and Acronyms ... 7
Latin letters ... 7 Greek letters ... 8 Acronyms ... 8 Preface ... 10 1. Introduction ... 11 1.1 Background ... 11 1.2 Problem description ... 11 1.3 Thesis objective ... 11 1.4 Report set up ... 13
2. State of the Art ... 14
2.1 Support structures ... 14 2.1.1 Monopiles... 15 2.1.2 Gravity-based structures ... 15 2.1.3 Tripods ... 16 2.1.4 Jackets ... 16 2.1.5 Deep foundations ... 17 2.2 Definitions ... 17
2.3 Wind turbines technology ... 19
2.4 Foundation Piles ... 22
2.4.1 Axial capacity ... 25
2.4.2 Lateral response ... 28
2.4.3 Transportation of Piling ... 29
2.4.4 Installing Piles ... 31
3. Actions on Offshore Wind Turbines ... 38
3.1 Gravity loads ... 38
3.2 Hydrostatic loads ... 38
3.3 Current loads on structures ... 38
3.3.1 Current drag and lift force ... 38
3.4 Wind ... 42
3.5 Wave loads on structures ... 44
3.5.1 Morison equation ... 44
3.6 Waves theory ... 45
3.6.1 Airy wave theory ... 46
3.6.2 Higher order and stretch wave theories... 47
4. CFD (Computational Fluids Dynamics) ... 48
4.1.1 Introduction ... 48
4.1.2 CFD techniques ... 48
4.1.3 Typical problems... 48
4.1.4 What CFD is searching for? ... 50
4.1.5 Solution method ... 54
4.1.6 Grids ... 55
4.1.7 Boundary conditions ... 56
4.2 Fluid-structure interaction (FSI) ... 56
4.2.1 Monolithic approach ... 57
4.2.2 Partitioned approach ... 57
4.2.3 Stability issues for partitioned algorithms ... 57
4.2.4 ALE method (Arbitrary Lagrangian Eulerian) ... 59
4.3 Abaqus ... 62
5. Wind Turbine ... 64
5.1.1 Background ... 64
5.1.2 Geometry and model ... 64
5.1.3 Co-simulation model ... 66
5.1.4 Euler-Lagrange Coupling model ... 69
5.2 Results ... 73
5.2.1 Introduction ... 73
5.2.2 Co simulation ... 73
5.2.3 Euler Lagrange Coupling ... 80
5.3 Case history ... 85
5.3.1 Objective of the study ... 85
5.3.2 Material definitions ... 85
5.3.3 Description of the structure ... 87
5.3.4 Geometry ... 87
5.3.5 Site conditions ... 88
5.3.6 Natural frequency analysis ... 90
5.3.7 Co-simulation model ... 96
5.4 Results ... 104
Conclusions ... 109
Appendix ... 112
LIST OF FIGURES
Fig. 2.1: Different support structures for offshore wind turbines in relation to maturity and water
depth (Kaldellis 2013) ... 14
Fig. 2.2: Monopile foundation(Association 2011) ... 15
Fig. 2.3: Tripods foundations for offshore wind turbines(Association 2011) ... 16
Fig. 2.4: Jacket substructure(Association 2011) ... 17
Fig. 2.5: Different components of a monopile wind turbine ... 18
Fig. 2.6: Construction stages of offshore wind turbine(Wikipedia s.d.) ... 18
Fig. 2.7: Transition piece of a monopile wind turbine, details(Wikipedia s.d.) ... 19
Fig. 2.8: Description of the blade's shape(Wikipedia s.d.) ... 20
Fig. 2.9: (a) Tubular steel tower, (b) Lattice tower(Wikipedia s.d.) ... 21
Fig. 2.10: Internal view of a wind turbine's tower(Wikipedia s.d.) ... 22
Fig. 2.11: Typical piles for offshore construction ((Gerwick 2007), pg.256) ... 23
Fig. 2.12: Offshore terminal dolphin ((Gerwick 2007), pg. 257) ... 24
Fig. 2.13: Typical pile-structure-soil interaction ((Gerwick 2007), pg. 258) ... 25
Fig. 2.14: Local shear stresses during installation of Pile(Lehane 1993) ... 27
Fig. 2.15: Pile subjected to horizontal force(Wikipedia s.d.) ... 28
Fig. 2.16: Comparison with different approaches(Barton 1982) ... 29
Fig. 2.17: Barge shipment of piling ((Gerwick 2007), pg. 261) ... 30
Fig. 2.18: Transportation of piles in self-floating mode ((Gerwick 2007), pg. 258) ... 30
Fig. 2.19: Installation stage of a jacket structure ... 32
Fig. 2.20: Driving of offshore piling. Note suspension of hammer and boot leads for batter piles ((Gerwick 2007), pg.265) ... 32
Fig. 2.21: Installing and driving piles in jacket, Gulf ofMexico. ((Gerwick 2007), pg.266)... 33
Fig. 2.22: Large hydraulic hammer driving 3.15-m diameter steel tubular pile. Note size comparison with menat lower right hand corner. ((Gerwick 2007), pg.267 ... 33
Fig. 2.23: Step 1- Ballasting the launch end of barge ... 34
Fig. 2.24: Step 2- Moving jacket along skid beams, Step 3- Jacket pivots on rocker arms ... 34
Fig. 2.25: Step 4- Floating in water ... 34
Fig. 2.26: Step 5- Upending with derrick barge ... 35
Fig. 2.27: Step 6- In place ... 35
Fig. 2.28: Stage 1- Lifting from barge, Stage 2- Upending: phase 1 ... 35
Fig. 2.29: Stage 3- Upending: phase 2, Stage 4- Setting in final position ... 36
Fig. 2.30: Installation steps of monopile foundation(Association 2011). ... 37
Fig. 3.1: Drag coefficient for a smooth circular cylinder in steady flow (redone after (Handbook of offshore engineering 2005), pg.138) ... 39
Fig. 3.2: Drag coefficient for a rough circular cylinder in steady flow (redone after (Chakrabarti 2005), pg.139) ... 39
Fig. 3.3: Effect of surface roughness on C, at high Reynolds number (redone after (Chakrabarti 2005), pg.139) ... 41
Fig. 3.4: Drag coefficient for flat surface ((Chakrabarti 2005), pg.140) ... 41
Fig. 3.5: Lift coefficient for a smooth circular cylinder in steady flow ((Chakrabarti 2005), pg.141) ... 42
Fig. 3.6: Morison force on a vertical pile ((Chakrabarti 2005), pg.144) ... 44
Fig. 3.7: Definition diagram for an Airy wave(Haritos 2007) ... 46
Fig. 3.8: Applicability of Wave Theories(Haritos 2007) ... 47
Fig. 4.1: Aeronautical application(Wikipedia s.d.) ... 49
Fig. 4.2: Automotive application(Wikipedia s.d.) ... 49
Fig. 4.3: Biomedical application(Wikipedia s.d.) ... 50
Fig. 4.4: Average solution of Navier Stokes equations (Wikipedia s.d.) ... 53
Fig. 4.5: Structured and Unstructured Grids (Wikipedia s.d.) ... 56
Fig. 4.7: Approches and examples of softwares ... 57
Fig. 4.8: One-dimensional example of Lagrangian, Eulerian and ALE mesh and particle motion ((Donea s.d.), chapter 14) ... 60
Fig. 4.9: The motion of the ALE computational mesh is independentof the material motion ((Donea s.d.), chapter 14) ... 60
Fig. 5.1: The first nine eigen modes of the structures ... 65
Fig. 5.2: Fluid domain ... 67
Fig. 5.3: Mesh of the model- Abaqus... 68
Fig. 5.4: Interaction surface structure/fluid- Abaqus ... 68
Fig. 5.5: Structure model- Abaqus ... 69
Fig. 5.6: Mesh of the structure- Abaqus... 69
Fig. 5.7: Boundary conditions on the fluid domain- Abaqus model ... 71
Fig. 5.8: Boundary conditions on the fluid domain (1)- Abaqus model ... 72
Fig. 5.9: Assembly of the model- Abaqus model ... 72
Fig. 5.10: Mesh of the model- Abaqus model... 73
Fig. 5.11: Velocity field around the wind turbine, t=0 s ... 73
Fig. 5.12: Velocity field around the wind turbine, t=1 s ... 74
Fig. 5.13: Velocity field around the wind turbine, t=2,6 s ... 74
Fig. 5.14: Velocity resultant field around the wind turbine, t=2,4 s ... 74
Fig. 5.15: vonMises stresses on the structure t = 0,1 s ... 75
Fig. 5.16: Displacements on the structure t = 0,1 s ... 75
Fig. 5.17: Displacement (m) in drag direction (∆t = 3s) ... 76
Fig. 5.18: Displacement (m) in lift direction (∆t = 3s) ... 76
Fig. 5.19: The Strouhal- Reynolds number relationship for circular cylinders (Lienhard, J. (1966))77 Fig. 5.20: Approximatinggraphic (drag displacements) ... 78
Fig. 5.21: Comparison (drag displacements) ... 78
Fig. 5.22: Approximatinggraphic (lift displacements) ... 79
Fig. 5.23: Comparison (lift displacements) ... 79
Fig. 5.24: Acceleration (a/g)-Time(s), Lift direction ... 80
Fig. 5.25: Acceleration (a/g)-Time(s), Drag direction ... 80
Fig. 5.26: Velocity fluid is zero, t=0 sec ... 81
Fig. 5.27: Velocity fluid is 60m/s, t=0,1 ... 81
Fig. 5.28: Velocity field, t=0,5 sec ... 82
Fig. 5.29: Velocity field, t=2 sec ... 82
Fig. 5.30:Von Mises stresses in the blades at 0,3 sec ... 83
Fig. 5.31:Displacements of the blades in Euler Lagrange Coupling ... 83
Fig. 5.32: Displacement condition in the Co-simulation ... 84
Fig. 5.33:Offshore wind farm (Wikipedia s.d.)... 87
Fig. 5.34:Location of the offshore wind farm (Wikipedia s.d.) ... 87
Fig. 5.35:Soil stratification... 89
Fig. 5.36: The first nine eigenmodes of the structures ... 92
Fig. 5.37: The first two eigen modes of the structures in SAP2000 ... 93
Fig. 5.38: The first nine eigen modes of the structures ... 95
Fig. 5.39:Geometry of the model- Abaqus model ... 97
Fig. 5.40:Mesh of the fluid domains- Abaqus model ... 98
Fig. 5.41:Fluid domain ... 98
Fig. 5.42:Interaction surface structure/fluid- Abaqus ... 99
Fig. 5.43:Mohr-Coulomb criterion (Wikipedia s.d.) ... 100
Fig. 5.44:Mohr-Coulomb criterion in principal stress space (Wikipedia s.d.) ... 100
Fig. 5.45:Shear resistance of interface (Wikipedia s.d.) ... 101
Fig. 5.46:Assemby of the model- Abaqus model... 102
Fig. 5.47:Mesh of the structure- Abaqus model... 102
Fig. 5.49:Interaction surfaces with the fluids- Abaqus model ... 103
Fig. 5.50: Velocity field around the wind turbine, t=0 s, Air... 104
Fig. 5.51: Velocity field around the wind turbine, t=0 s, Water ... 105
Fig. 5.52: Velocity field around the wind turbine, t=0,4 s, Air... 105
Fig. 5.53: Velocity field around the wind turbine, t=0,4 s, Water ... 106
Fig. 5.54: Velocity field around the wind turbine, t=1s, Air... 106
Fig. 5.55: Velocity field around the wind turbine, t=1s, Water ... 107
Fig. 5.56: Von Mises stresses in the pile, t=1s ... 107
Fig. 5.57: Displacements of the pile ... 108
Fig. 5.58: Displacements pile in the flow direction, t=0sec ... 108
Fig. 5.59: Displacements pile in the flow direction, t=0,5sec ... 108
LIST OF TABLES
Tab. 2.1: Design Parameters for cohesionless siliceos soil ... 26
Tab. 3.1: Average values of the wind drag coefficients ... 43
Tab. 5.1: Natural frequency values of the wind turbine ... 64
Tab. 5.2: Participation factors ... 66
Tab. 5.3: Strouhal Number for a variety of shapes (Emil Simiu, R. H. (1996)) ... 77
Tab. 5.4: Material parameters ... 86
Tab. 5.5: Soil stratification and parameters ... 89
Tab. 5.6: Natural frequencies of the stucture ... 91
Tab. 5.7: Natural frequencies of the stucture ... 93
Tab. 5.8: Partecipation factors ... 94
Tab. 5.9: Percentage differences between the natural frequencies of the stucture in the different cases ... 96
INDICES AND ACRONYMS Latin letters
A - area of the object (m2);
c - cohesion (KPa);
Cd - drag coefficient;
Ci - coefficient accounting for the shape and loading rate of the structure;
Cm - inertia coefficient;
Cs - shape coefficient;
D - diameter (m);
E - elasticity modulus;
Es - secant elasticity modulus;
F - wind force (kN);
FD - drag force vector per unit length (N/m);
FI - inertia force vector per unit length (N/m);
Fi - impact force (kN);
g - gravitational acceleration (m/s2);
H - wave height (m);
Hb - wave breaking height (m);
Iu - turbulence intensity;
T - wave period (s);
U - wind speed (m/s);
U - displacement (m);
Umo - maximum particle velocity at storm water level (m/s);
w - weight density of water (N/m3); W - weight of an object in air (kN); V - current velocity (m/s);
Greek letters
σ - normal stress (kPa);
τ - tangential stress (kPa);
ν - Poisson’s ratio;
γ - specific weight (kN/m3); ϕ - internal friction angle (0); Φ - velocity potential;
ρ - mass density of air (approximately 1.293 Kg/m3); η - wave surface elevation (m);
Acronyms
API - American Petroleum Institute; FABIG - Fire and Blast Information Group; FEM - Finite Element Method;
GBS - Gravity Based Structure; TLP - Tension Leg Platform;
PREFACE
The following script is the result of the work made to subscribe the thesis for the Master of Science ‘Ingegneria Edile e delle Costruzioni Civili’ curriculum of the Scuola di Ingegneria, Pisa, Italy. It is titled ‘Interaction fluid-structure modelling and Geotechnical design of Offshore wind turbines’, a work carried out by Carmine Donatelli at the Geotechnical Laboratory of the Technical University of Civil Engineering in Bucharest (Romania).
My sincere thanks to Prof. Manole Serbulea and Prof. Diego Carlo Lo Presti for their professional guidance, patience and support during this final thesis.
In additionI would like to thank also Dr. Arnaldo delli Carri, who gave me the opportunity to use his powerful workstation for the numerical analysis at the University of Bristol and all the people helped me during this work.
Pisa, October 5th 2015
1. INTRODUCTION 1.1 Background
Offshore wind parks promise to become an important source of energy in the next future as a growing number of parks will be installed in the European Seas.
Onshore wind energy has already grown enormously over the last years, but its further expansion is limited for many reasons (noise nuisance and aesthetic issues), so the problems in the development of wind energy can be resolved by the installation of wind turbines in offshore regions.
The advantages of this solution are the following: • Availability of large continuous areas; • Higher wind speeds;
• Less wind turbulence.
Of course there are also some disadvantages that are the installation and maintenance costs which are much higher than onshore, furthermore the integration of the offshore wind park with the electricity network is much more expensive.
1.2 Problem description
The considered structure is a monopile offshore wind turbine, located in the North Sea with a sea depth of 18m.
The tower of the turbine has a total height of 68m with a variable section, the bottom section has a diameter of 5.6m with a thickness of 32 mm, while the upper section has a diameter of 4m with a thickness of 30mm. The rotor is composed of three blades and has a diameter of 72m, while the foundation pile has a diameter of 5,6m with a thickness of 50mm.
The maximum wind speed on site, as according to INMH, at a height of 10 meters above water level is v10=30 m/s. The current velocities have been established as Vs=1.10 m/s at the surface level and Vd=0.28 m/s at the level of the seafloor.
The soil stratification, below the seafloor is comprised of clays, sands, silts and even gravel.
In the classical approach the current and wind actions are found out with complex equations that are linked to the fluid density, area of the structure, velocity of the flow and empirical coefficients related with the shape of the structure.
The main goal of this work is to use an innovative method to get the environmental actions on the structure.
1.3 Thesis objective
This thesis is aim to verify the sub-structure of an offshore monopile wind turbine, located in the North Sea.
The monopile foundation is a very common solution in shallow waters (0-30m), that is an open ended large diameter steel cylindrical pile driven into the soil; it is the cheaper choice in terms of installation and material costs.
The design problems are several and complex because different engineering fields met: in order to solve the issues Structural, Geotechnical, and Fluid-dynamics competences are necessary.
The selected topic has relevant technical interests and need specific competences that have been developed already in the past for the design of platforms for oil extraction. In this case the particular shape of the structure requires a deeper study of the wind and current sea effects.
In order to get this goal we decided to choose a very innovative analysis for this kind of structure: the Co-simulation, a modelling widely used in other engineering fields (aerospace,mechanical andbiomedical) where the fluid-structure interaction in the design has to be considered with particular attention.
The software used to realize our object-study is Abaqus 6.14 that gives us the possibility to couple a Computation Fluid Dynamics (CFD) analysis with a structural one, using Abaqus/Standard as the structural solver and Abaqus/CFD as the incompressible flow solver.
In the structural model we modelled also the soil-structure interaction, using an elasto-plastic constitutive law, showing how the natural frequencies of the structure change in this case.
The result is an iterative procedure which can evaluate the mutual interaction between fluid dynamic field and the shape of structure and it can also predict the final shape of the body (that we expect as deformed).
To describe the fluid acting on the skew surface of generator wings different pre processing software programs can be used to prepare the meshed geometry of the model or with a trick it is possible to make it directly in Abaqus working on the input file as we will show in the following chapters.
In the first part of the work the on shore wind turbine’s case is analysed. With the results obtained from theCo-simulation, we focused on the study of drag and lift displacements that we used to elaborate a spectral analysis aims to read more clearly the results. The latter has been obtained thanks to MathCad 14.
In the second part a comparison in terms of displacements of the blades in the flow direction is carried out, considering two cases: in the first one the wind speed is 25 m/s (standard case), while in the second one the wind speed changes from 0 m/s to 60 m/s (210km/h) in a very short time. The latter model is made using the Euler Lagrange Coupling (CEL).
At the end we faced the problem of finding the foundation pile’s displacements in the working situation, to see if they are acceptablefor the proper functioning of the turbine.
1.4 Report set up
This thesis consists of five chapters:
1. Formulation of the problem description and the objective of the thesis;
Chapter 1 discusses the research motivation, research questions and scope of the thesis. At the end of the chapter the thesis outline is presented.
2. State of Art;
A review of support structures of offshore wind turbines are presented, focusing our attention on the foundation piles usually used and the installation steps
3. Actions on Offshore Wind Turbine;
We made a description of the all actions with particular consideration of the enviromental ones (wind, waves and currents).
4. CFD and Fluid-structure interaction (FSI) problems formulation;
Chapter 4 illustrates the CFD analysis and about the two main approaches exist for the simulation of FSI-problems: monolithic and partitioned approach.
5. FSI modelling and Geotechnical design of Offshore Wind Turbines;
Chapter 5 defines a fluid-dynamic and structural problem in Abaqus 6.14 in which we analysed both the onshore and offshore wind turbine. Furthermore, in the same chapter, the Euler-Lagrange coupling modelling is presented and compared with the Co-simulation technique.
This section concludes the approch and results of this research in a comprehensive way. This discussion is followed by highlighting the contribution of the research to the state of the art knowledge.
2. STATE OF THE ART 2.1 Support structures
For the installation of offshore wind turbines in different depths and distance from the shore, the support structure used plays a key role, and it constitutes the main infrastructure and the main difference with respect to an onshore wind parks.
Besides the water depth and the shore distance, there are also other criteria that need to be considered when deciding on the most appropriate type of support structure to be used for offshore wind turbines in a specific area. These criteria are the wave conditions, the specific characteristics of the seabed, the potential consequences on the marine environment, the turbine characteristics, the technical and commercial risk factors and the proximity in the electricity grid.
TheFig. 2.1shows the different structures currently available, in relation to the depth in which they can be employed.
Fig. 2.1: Different support structures for offshore wind turbines in relation to maturity and water depth (Kaldellis 2013)
As it can be seen from the figure, several support structures for offshore wind turbines have been developed, which however are currently in different technological status. Generally, the support structures are divided into the ones for shallow water, transitional water and deep water, which are further described below.
Until now, many different support structures have been used, with the most prominent being the monopile in water depths of less than 20 – 25 m, due to the fact that this type of foundation has an easier production and installation phase and lower cost.
Then, gravity-based structures have also been used in many offshore wind projects because they are also appropriate for shallow waters and are quite easy to produce.
Finally, the so-called space-frame structures (jackets, tripods and tripiles) have also been used so far, but in a much smaller quantity than the others.
2.1.1 Monopiles
The monopile support structure (shown in Fig. 2.2) is a relatively simple design by which the tower is supported by the monopile, either directly or through a transition piece. The monopile continues down into the seabed. The structure is made of a cylindrical steel tube with a diameter of 2.5 to 4.5 m.
The pile penetration depth is adjustable to suit the actual environmental and seabed conditions. A limiting condition of this type of support structure is the lateral movement along the monopile and vibration, and are subjected to large cyclic, lateral loads and bending moments (due to the current and wave loads).
Depending on the soil conditions, monopiles are embedded from 3.5D to 8D (where D is the diameter of the monopile) into the bottom of the sea; 3.5D to 4.5D in stiff clay, 6D in average soil and 7D to 8D in soft silt (Lehmann 2007).
Fig. 2.2: Monopile foundation(Association 2011)
Monopiles are currently the most commonly used foundation in the offshore structure wind turbine due to their ease of installation in shallow to medium water depths. This type of structure is well suited for sites with water depth ranging from 0-30m.
2.1.2 Gravity-based structures
These support structures are stable in all environmental conditions, but only appropriate for water depths up to 40 m. They are based on gravity, and that is why they utilize a heavy base with a diameter of 12 to 18 m, which is filled by reinforced concrete, steel, rock or pumped-in sand.
In order for these structures to be installed, seabed preparation is necessary in order for the ground to become uniform and level, which is one of the main drawbacks. Also, it is a heavy equipment to transport. However, this type of support structure cause low noise level during installation and it is quite affordable in comparison with other support structures.
In general, gravity foundations are designed with the objective of avoiding tensile loads between the bottom of the support structure and the seabed. This is achieved by providing sufficient dead loads such that the structure maintains its stability in all environmental conditions solely by means of its
own gravity. Gravity foundations are usually competitive when the environmental loads are relatively modest and the natural dead load is significant.
2.1.3 Tripods
They are three-legged support structures of cylindrical steel tubes (The European Wind Energy Association 2011), which are appropriate for water depths from 20 to 50 m. As with the monopiles, tripods are also embedded into the bottom of the sea, but the penetration depth and the width of the base depend on the environmental and seabed conditions (Chaviaropoulos 2009)They are also quite difficult to be constructed and transported on site(Chaviaropoulos 2009). However, these support structures (see Fig. 2.3) are very durable and appropriate for big and heavy wind turbines (Chaviaropoulos 2009).
Fig. 2.3: Tripods foundations for offshore wind turbines(Association 2011) 2.1.4 Jackets
The jacket, or template, structures are still the most common offshore structures used for drilling and production and from few years also for offshore wind turbines (as shown in Fig. 2.4).
Some structures contain enlarged legs, which are suitable for self-buoyancy during its installation at the site. Fixed jacket structures consist of tubular members interconnected to form a three-dimensional space frame. These structures usually have four to eight legs battered to achieve stability against toppling in waves. Main piles, which are tubular are usually carried with the jackets and driven through the jacket legs into the seafloor.
The term jacket structure has evolved from the concept of providing an enclosure (“jacket”) for the well conductors. These platforms generally support a superstructure having 2 or 3 decks with drilling and production equipment and workover rigs or the wind turbine.
These support structures are appropriate for water depths of 30 m and more for the wind turbine, and they also require to be embedded into the seabed.
Despite the fact that they need to penetrate the seabed they do not cause high noise levels during installation and as with tripods, they are appropriate for big heavy wind turbines.
On the other hand, they are quite expensive structures, require a long time for installation and can be affected by extreme wave conditions.
The transition piece forms the connection between the main jacket and the tower of the wind turbine.
Fig. 2.4: Jacket substructure(Association 2011) 2.1.5 Deep foundations
Currently, offshore wind farms have been using three main types of deep offshore foundations, adapted from the offshore oil and gas industry:
Spar Buoy: a very large cylindrical buoy stabilises the wind turbine using ballast. The centre of gravity is much lower in the water than the centre of buoyancy. Whereas the lower parts of the structure are heavy, the upper parts are usually empty elements near the surface, raising the centre of buoyancy.
Tension Leg Platform: a very buoyant structure is semi submerged. Tensioned mooring lines are attached to it and anchored on the seabed to add buoyancy and stability
Semi-submersible: combining the main principles of the two previous designs, a semi submerged structure is added to reach the necessary stability.
2.2 Definitions
In principle the monopile is an extension of the main tower into the soil under the seabed level. The turbine is supported by a tower construction, which is connected to the monopile via the transient piece.
Fig. 2.5: Different components of a monopile wind turbine
The monopile is an open ended cylindrical steel pipe and the installation can be driving, drilling or a combination, depending on soil properties and water depth.
Fig. 2.6: Construction stages of offshore wind turbine(Wikipedia s.d.)
Between the monopile and the tower there is a transition piece, see theFig. 2.7. The transition piece is joined to the monopile by a radial connection of grout and it has to transfer the vertical and horizontal loads from the tower to the foundation.
The transition piece also makes it possible to raise the tower to a completely vertical position even if the foundation pile is not completely level. The transition piece is completed with pre-installed features such as boat landing arrangement, cathodic protection and cable ducts (J-tube) for submarine cables.
Fig. 2.7: Transition piece of a monopile wind turbine, details(Wikipedia s.d.)
The tower is installed on the transition piece by bolts. The turbine is installed on the tower. The turbine consists of the nacelle and the rotor. The blades are connected with the hub.
As result of the wind, the blades start to turn, making the main shaft rotates. The main shaft is connected to the generator, which transforms the rotation movement of the blades into electricity. This generator electricity is transported by cables in the tower to a grid connection on the bottom of the seabed and then transported to onshore facilities.
2.3 Wind turbines technology
Wind turbines consist of four large main components: a foundation unit, a tower, a nacelle (turbine housing) and a rotor. The following section provides a brief explanation of the various parts of modern horizontal axis wind systems.
The nacelle: it contains the key components of the wind turbine, including the gearbox and the electric generator. Its size is such that the maintenance operators can stand in it and walk from side to side for perfect handling and repair of machinery.
The hub: it is at the junction of the blades with the rotation system. The hub and blade assembly is called wind rotor. There are two types of hub: rigid and swivel, depending on whether the blades behave as a cantilever beam or can swing freely with the rotation’s system. The rotor of the turbine is one of the most visible wind energy systems. Most wind turbines made nowadays, are horizontal axis machines, upwind with two or three blades. The main type of rotor has an axis which is parallel to ground, and therefore, horizontal to wind.
The gearbox: it is responsible for converting the low speed of the blades rotation (about 24 revolutions per minute (rpm)) at high speed, around 1.500 rpm to match the speed of the generator. In some turbines, the gear ratio may exceed 1:100. This is achieved in three separate stages. The first stage is generally a planetary gear, while the others are parallel or helical gears. The gearbox is force-lubricated and the oil is continuously filtered and cooled. With preventive maintenance being standard practice, it is common to monitor the temperature of the gearbox, as well as the vibrations. The electrical generator is actually an alternator which is coupled to the gearbox through the small shaft. It has the charge of producing electricity, which is conveyed through the interior of the tower to the transformer. It is basically the unit that transforms the mechanical energy into electric power. Usually it is an asynchronous or induction generator.
The anemometer: it measures wind speed and sends this information to the controller, which logs it and acts accordingly on the brake. Anemometer electronic signals are used by the electronic controller to connect the wind turbine when the wind reaches about 5 m/s. The computer will stop automatically if wind speed exceeds 25 m/s, in order to protect the turbine and its surroundings. Motor orientation: the yaw motor turns the nacelle so that the rotor faces the wind. The controller indicates the yaw motor when to turn the nacelle.
The slewing ring: it is located at the bottom of the nacelle and is responsible along with the guidance system to position the nacelle in the direction best suited for optimal use of wind, and hence increase the power generated.
The wind vane, which is associated to the guidance system, is which informs the control system of the wind direction at any time.
The braking system: it is used to block the rotor when the maintenance is being carried out or the system must be repaired.
Radiator: the generator is hot when spinning. But if it becomes too hot it will spoil. This is the reason why it should be refrigerated. In some wind turbines, the generator is cooled by water. The electronic controller is a computer that continuously monitors the conditions of the turbine and controls the yaw mechanism. Orients the nacelle into the wind and allows the rotor to start when the wind vane indicates that there is enough wind. In case of any malfunction (e.g. overheating in the gearbox or generator), it automatically stops the turbine and calls the computer operator in charge of the turbine through a telephone link.
Blades: they are the components which interact with the wind. Their shape (see Fig. 2.8) is designed in order to obtain good aerodynamic efficiency. In fact, rotor blade designers often use classical aircraft wing profiles as cross sections in the outermost part of the blade. The next figure shows a typical wind turbine blade outline, together with several cross-sections at different locations along the length.
Fig. 2.8: Description of the blade's shape(Wikipedia s.d.)
Near the hub, the blade has a circular section. As the distance from the hub axis (radius) increases, the thickness of the wing decreases, as well as the chord. The aerodynamic forces vary with the square of the local relative air velocity and increase rapidly with the radius. It is thus important to design the part of the blade near the tip with good lift and low drag coefficients. The blades are flexible and therefore can be deflected by the wind. In order to avoid the blades hitting the tower, the rotor axis is frequently tilted at a small angle. The cross-section of a wind turbine blade is quite thick in order to obtain the high rigidity necessary to resist the different mechanical loads acting during operation.
The wind exerts a force which is not constant; fluctuations result from turbulence, but also from the fact that wind velocity increases with altitude. A blade in a high position is subjected to a stronger wind than in a low position and thecorresponding load fluctuation is also repeated at each revolution.All these alternating loads are responsible for fatigue, which is the biggest technical challenge in blade design. A specific analysis is required to eliminate the risk of resonance between the different mechanical oscillators (blades, tower, drive train, etc.). Nowadays, the turbine manufacturers make computer models of their machines before building them, to ensure that the vibrations of different components do not interact to amplify noise.
Blade’s materials:
The blades are made of lightweight materials, like fibre -reinforced plastics, which exhibit good fatigue properties. The fibres are mainly glass woven fabrics but for the largest blades, carbon fibres are used in the blade parts where loads are most critical. Some blades are entirely made of carbon fibres whereas wood laminates are also used by some manufacturers. The fibres are incorporated in a matrix of polyester, vinyl ester or epoxy resin and the blades are made up of two shells which are bonded together. Internal webbing reinforces the structure. The external blade surface is covered with a smooth coat of coloured gel intended to prevent ultraviolet ageing of the composite material. Tower: it carries the nacelle and the rotor. Towers for large wind turbines may be tubular steel towers, lattice towers, or concrete towers.
Tubular steel towers:
Most large wind turbines are delivered with tubular steel towers (see Fig. 2.9), which are manufactured in sections of 20-30 metres with flanges at either end, and bolted together on the site. The towers are conical (with their diameter increasing towards the base) in order to increase their strength and to save materials at the same time.
Lattice towers:
Lattice towers (shown in Fig. 2.9) are made using welded steel profiles. Its basic advantage is cost, since a lattice tower requires only half as much material as a freely standing tubular tower with a similar stiffness. The basic disadvantage is their visual appearance. Due to aesthetic reasons lattice towers have almost disappeared from use for large modern wind turbines.
Fig. 2.9: (a) Tubular steel tower, (b) Lattice tower(Wikipedia s.d.)
In theFig. 2.10, it can be seen the inside of a tower, a description and an explanation of each of its parts is provided.
Fig. 2.10: Internal view of a wind turbine's tower(Wikipedia s.d.)
2.4 Foundation Piles
Over 95% of offshore oil and gas platforms are steel structures founded on drivenopen-ended steel pipe piles. To economize on structural costs and on installation costs the number of piles is reduced as much as possible, requiring piles with very high load bearing capacities.
Recently these piles have found increasing use as monopile foundations for offshore wind power generators.
These steel pipe piles have diameters from 0,76 m up to 2,5 (in exceptional cases piles of 5,1m diameter has been driven successfully for offshore wind turbine ) and from 40m to 300m or more in length.
The wall thickness of the pile will vary along the length, with thicker walls used near the pile head where bending moments are maximum. The ratios (d/t) are usually equal to 40, giving a net steel area of 10% of the overall pile cross-section.
Drilled and grouted piles are generally more expensive to install than driven piles, owing to the long construction period required, which may amount to several weeks. They are sometimes preferred when a drilling barge with necessary pile-handling capability is already on location or in calcareous sediments (or in other crushable material, where the shaft friction obtained with driven piles can be extremely low).
The Fig. 2.11shows the typical piles of an offshore structure.
Fig. 2.11: Typical piles for offshore construction ((Gerwick 2007), pg.256) As alternative to a primary pile, mud is used to stabilise a drilled hole.
For resisting axial compression, the pile transfers its load by skin friction along its outside perimeter and by end bearing on its tip , if the tip is either closed or plugged in such a way as not to yield in relation to the pile.
Large diameter tubular piles may not plug and the end bearing is lost. However, the interior surface will develop skin friction.
End bearing and skin friction do not develop their resistances simultaneously and so are not usually directly additive at serviceability levels load, but we can consider partially them at ultimate load. For this reason deep piles are designed primarily as friction piles. Internal skin friction generally develops to its maximum within a one-diameter length of the tip.
For resisting lateral loads, most offshore structures in deep water (30-40m) depend on the bending resistance of the pile interacting with the passive resistance of the soil in the near-surface zone. Since the soil resistance is a function of its deformation, the analysis is based on the interaction of the lateral load P with the displacement y at each incremental level below the seafloor. This is called P/y effect.
The pile must have sufficient strength to resist the resultant moments and shears at these levels and to prevent biaxial buckling.
The capacity to resist lateral loads can be improved by increasing the stiffness and moment capacity of the pile in the critical zone near and just below the mud line by increasing the wall thickness of the steel pile in this zone, or by filling the pile with concrete in this region.
An alternative method to of resisting lateral loads, used in harbour structures and some offshore terminals, is the use of batter piles, sufficiently inclined to develop a substantial horizontal component of their axial capacity (as shown in Fig. 2.12). Batter piles must have a reaction in order to be effective; this is usually provided by a mating pile battered in the opposite direction. (Gerwick 2007)
Fig. 2.12: Offshore terminal dolphin ((Gerwick 2007), pg. 257)
Under lateral load, these connection must develop high shear and moment capacity. Their performance in earthquakes and ship collision has often been unsatisfactory due to the local buckling near the pile head.
Special methods and equipment have had to be developed to install the large piles required for offshore structures.
Driving with very large hammers is still the preferred method for most cases because it is fast. However, where soil conditions do not permit driven installations and in other special cases, drilling may be employed, with the pile being grouted into the drilled hole.
The effects of all installation operations on the supporting soil must be considered. In some cases, they have good effects, but in other cases not, so special precautions must be taken.
American Petroleum Institute (API) warns that piles drilled and grouted may have resisting values very different from those of driven piles. Large-diameter piles (over 1.5 m) may not develop their full internal skin friction. For piles driven in undersized drilled or jetted holes in clays, the skin friction will depend on the amount of soil disturbance, including the relief of stress, which is occasioned by the installation.
In overconsolidated clays, drilled and grouted piles may develop increased skin friction. If excess drilling slurry is present in clays or in soft rock, the coefficient of friction may be significantly reduced.
In calcareous sands and some silts, driven piles may have very low values of friction compared to those attainable by drilling and grouting.
API further warns that the lateral resistance of the soil near the surface is significant to pile design (Gerwick 2007).
Fig. 2.13: Typical pile-structure-soil interaction ((Gerwick 2007), pg. 258)
and some considerations must be given to the effects of soil disturbance during pile installation. 2.4.1 Axial capacity
The design of offshore piles is heavily based on empirical correlations.
A big challenge of offshore pile design is to extrapolate design parameters from a big database, that is based on experiences of piles with 1m in diameter.
The cone resistance is used as the primary measure of the soil strength from which pile design parameters can be deduced.
For sands, design parameters can be expressed in terms of the cone resistance (standard penetration test), while for fine-grained sediments parameters are based either on the undrained shear strength or the in situ vertical effective stress together with an overconsolidation ratio.
The method guidance for engineers designing offshore piling is available in the recommendations of the American Petroleum Institute.
The (2.1)is used to calculate the shaft friction in sand (API 2000)
f = k0σvo’ tgδ’ (2.1)
where:
k0-coefficient of lateral earth pressure (ratio of horizontal to vertical normal effective stress) σvo’ - effective overburden pressure kPa at the point in question
The coefficient K0 is recommended as 0.8 for open ended piles loaded in compression and 0,5 to 0,7
for piles loaded in tension(API 2000).
For long piles f may not indefinitely increase linearly with the overburden pressure as implied by the equation before. In such cases it may be appropriate to limit f to the values given in Tab. 2.1: Design Parameters for cohesionless siliceos soil, where we can find some indications for selection of δ if other data are not available.
Tab. 2.1: Design Parameters for cohesionless siliceos soil
Density Soil Description Soil-Pile Friction Angle,δ Degrees Limiting Skin Friction Values (KPa) Nq Limiting Unit End Bearing Values (MPa) Very Loose Loose Medium Sand Sand-Silt Silt 15 47.8 8 1.9 Loose Medium Dense Sand Sand-Silt Silt 20 67 12 2.9 Medium Dense Sand-Silt Silt 25 81.3 20 4.8 Dense Very Dense Sand-Silt Silt 30 95.7 40 9.6 Dense Very Dense Gravel Sand 35 114.8 50 12
For piles end bearing in cohesionless soils the unit end bearing q in kPa may be computed by the (2.2)(API 2000):
q = σvo’ Nq (2.2)
where:
Nq- dimensionless bearing capacity factor.
In the preceding table we can find some indications to choose Nq. The latter is taken to vary from 12
to 50 according to the grain size and relative density of the material.
Recentstudies, about the distribution of limiting shaft friction with the depth, show that the adoption of a constant value K with depth together with a limiting value for f is not consistent with data from fields tests. The following figure shows profiles of shaft friction recorded by the 3 instruments clusters in a 6m by 100mm diameter pile jacket into sand and the cone resistance profile for comparison.
Fig. 2.14: Local shear stresses during installation of Pile(Lehane 1993)
The leading instrument cluster (4 diameters from the pile tip) shows a shaft friction profile that follows the cone profile closely; the two subsequent instrument clusters show progressively reduced values of friction for any given depth.
The implication is that a maximum value of K occurs close to the pile tip, where the shaft friction is 0.5 to 1% of the cone resistance. The value of K then reduces with distance, h, from the pilei tip. For piles with
L/D > 20 (2.3)
We remind that the shaft friction is usually expressed in empirical forms as qc function (as
suggested by(R.Lancellotta 1987)).
The shaft friction, f, acts on both the inside and outside of the piles.
For piles considered to be plugged the bearing pressure may be assumed to act over the entire cross section of the pile. For unplugged piles the bearing pressure acts on the pile annulus only.
For pipe piles in cohesive soils, the shaft friction, f, in kPa at any point along the pile may be calculated by the equation(2.4):
f = α c (2.4)
where:
c -the undrained shear strength of the soil at the point in question α - a dimensionless factor
The value of α was found on empirically to be around unity for soft soil with shear strength less than 25 KPa, reducing to 0,5 or less once the strength exceed 75 kpa (Tomlinson 1957). Pile load test conducted for offshore developments showed that it was not the shear strength of the clay that mattered in determining the value of α so much as the degree of overconsolidation.
The factor, α, can be computed by the (2.5):
where
Ψ - for the point in question
σvo’ - effective overburden pressure at the point in question kPa
A variation of above relationship was suggested by Kolk and van der Velde(1996) to take account of the length of the pile, expressed by
α =0.55 ( ’ )0.3 ( / )
0.2
(2.6) For piles end bearing in cohesive soils, the unit end bearing q, in kPa, may be computed by the equation(2.7)(Kolk 1996):
q= 9 c (2.7)
2.4.2 Lateral response
The most popular method of analysis for laterally loaded piles, and the method adopted in the offshore design codes, is based on the Winkler model and is commonly termed the p-y approach. Despite its relative simplicity, this method of analysis has successfully been used in the offshore oil and gas industries for many decades. However, the current p-y approach is not well suited to predicting the response of piles with the geometry and loading seen in the offshore wind industry. To study the lateral response of piles in offshore design the interaction pile-soil is modelled by non linear P-y curves, in which P is the lateral load and y is the lateral deflection.
We have to focus on the maximum bending moment in the pile and on the maximum deflection. As shown inFig. 2.15: Pile subjected to horizontal force
Fig. 2.15: Pile subjected to horizontal force(Wikipedia s.d.)
The bending moments and lateral deflection are limited to the upper part of the pile, and for this reason it is ignored any axial shaft capacity in the upper few diameters of the pile in order to allow for lateral movements.
There are precise forms for non linear P-y curves for standard ‘soft clay’ and sand categories of seabed sediment, evolved from the experimentally derived curves proposed by (Matlock 1970). Limiting lateral resistance in clay is taken to vary as shown in (2.8):
with the upper limit of 9cud not being reached until a critical depth, zr , several diameters below the
seabed.
The limiting value at depth is conservative at light of lower bound plasticity solution, which ranges from 9.14 cud for smooth pile and 11.92 cud for a fully rough pile (Randolph 1984).
By contrast , the proposed limiting resistance in sand, which follows a rather complex quadratic variation with depth (API 2000)appears unduly optimistic by comparison with other approaches. Others studies have indicated that adopting a limiting resistance of Pu kp 2 γ’zd(Barton 1982)
provides a good fit to experimental data (see Fig. 2.16)
Fig. 2.16: Comparison with different approaches(Barton 1982) The shape of P-y curves in clay may be expressed by the (2.9):
= 0.5 ( )1/3 ≤ 1 (2.9)
where:
yr is defined as:
yr = 2.5 ε50 d (2.10)
and:
ε50- the nominally the strain at 50% peak stress in an unconsolidated undrained test, and
generally taken in the range 0.005 to 0.01
The shape of P-y curves in sand under static load is(O’Neill M 1983):
= A tanh( y) (2.11)
A is defined as
3-0.8 >0.9 (2.12)
Pr is a reference soil resistance (API 2000).
2.4.3 Transportation of Piling
Large-diameter tubular pile sections of steel or pre stressed concrete are usually lifted onto a barge, which is then towed to the site. The pile sections must be well chained to prevent any tendency to shift and roll in a seaway.
Usually the piles have thick-enough walls so that the stack will not locally deform the piles; however, this should be checked and suitable blocking or supports provided as necessary to prevent damage (see the Fig. 2.17(Gerwick 2007))
Fig. 2.17: Barge shipment of piling ((Gerwick 2007), pg. 261)
Sometimes, it becomes practicable and efficient to transport the piles in a self-floating mode, either singly or in a well-secured (chained) raft (see Fig. 2.18).
Fig. 2.18: Transportation of piles in self-floating mode ((Gerwick 2007), pg. 258)
This becomes especially attractive when the piles can be subsequently lifted and placed in long sections—for example, skirt piles—that can be entered well below the surface of the sea.
The ends of the piles can be closed with steel closure plates or rubber diaphragms. These need to be strong enough to take wave slap during tow to the site.
Upon arrival, one end is usually lifted clear of the water by a derrick line to permit cutting out the closure, and then that end is allowed to rotate down to the vertical.
In some cases, the trapped air in the other end (sometimes augmented by compressed air) is used to limit the draft when the pile reaches vertical. In such a case that closure must be adequate to resist the internal air pressure. In any event, closures should be provided with a valve so that air or water
can be vented and/or allowed to flood in, permitting ballasting and removal of the end plate in a controllable manner.
Removal of a closure plate below water can be a dangerous operation. In one case off Australia, the diver who was cutting out the closure plate was sucked into the pile by the rush of water and drowned. Provision of a valve and prior equalization of pressure on both sides of the closure plate would have prevented this accident.
In shallow waters, such as those at an offshore terminal, piles have been temporarily stored on the sea floor with recovery pendants and buoys attached.
When a long pile is upended in the water, very large bending moments may be introduced at certain stages of rotation.
For many recently constructed offshore platforms, some piles have been transported with the jacket, either set in the main legs or in the skirt pile sleeves and guides, where they provide additional buoyancy (if closed) as well as additional weight. The purpose of the pre installation is to enable several piles to be driven down immediately after seating of the jacket on location so that the jacket will be stable against the action of waves and current.
Typically, as soon as the jacket is seated and levelled the piles are cut loose so that they can penetrate the soils under their own weight. They are then extended as necessary, and four (or more) are driven down a short distance, where they may be temporarily welded or clamped to the jacket top. Final levelling of the jacket may then take place. One by one, these initial piles may be raised as necessary to eliminate any bending stresses, relowered, and driven down to required penetration. As noted above, the piles, when transported in the jacket, must be adequately secured against the forces of launching and upending.
During the upending of the Magnus platform jacket in the North Sea, a number of piles broke loose and ran down, hit the sea floor, and were badly buckled. For a while it was feared that the entire project would be set back one year. As it was, a mammoth effort, plus favorable weather, permitted replacement of the piles and installation late that same season.
2.4.4 Installing Piles 2.4.4.1 Jacket
The piles for the typical offshore jacket are transported on barges, with the first section of each pile being as long as can be handled and placed by the derrick barge.
Fig. 2.19: Installation stage of a jacket structure
Driving of offshore piling. Note suspension of hammer and boot leads for batter piles(Gerwick 2007) in Fig. 2.20:
Fig. 2.20: Driving of offshore piling. Note suspension of hammer and boot leads for batter piles ((Gerwick 2007), pg.265)
Skirt piles are encased in sleeves bracketed out from the lower end of the jacket. Skirt piles must be driven either with a follower or an underwater hammer. The piles are typically clustered around the corner legs of the jacket and are aligned parallel to them.
Guides are attached at intervals along the jacket legs to aid in setting the piles through the sleeves. Many jackets incorporate both pin piles and skirt piles. (Gerwick 2007)
Fig. 2.21: Installing and driving piles in jacket, Gulf ofMexico. ((Gerwick 2007), pg.266) In these operations different types of hammers are used such as hydraulic hammers or diesel hammers.
Fig. 2.22: Large hydraulic hammer driving 3.15-m diameter steel tubular pile. Note size comparison with menat lower right hand corner. ((Gerwick 2007), pg.267
It is very impressive the size comparison with men at lower right hand corner in theFig. 2.22. 2.4.4.2 Installation stages (Jacket)
The first step is to put onto the barge the jacket in the harbour and get to the installation site. This can be looked like a very simple action, but the experience has shown that the first phase of transportation is the most dangerous.
The stages of jacket installation starting with the removal of the jacket from the barge to its positioning on the sea bed and its stabilization.
The removal of the jacket from the barge can be done in two different ways: launch or lift.
The launch site is normally at or near the installation location. In the case of heavy jackets in shallow water it may be necessary to launch the jacket in deep water and than bring the jacket to the site.
Before the launch, the seafastening securing the jacket to the barge is cut. The jacket is then pulled along the barge by winches. As the jacket moves towards the stern of the barge, the barge start to tilt and a point is reached when the jacket is self sliding.
Fig. 2.23: Step 1- Ballasting the launch end of barge
The jacket is pushed until its center of gravity is off the barge. Further movement causes the rocker arm and jacket starts to rotate. The jacket will then slide under its own self weight into the water, as shown in Fig. 2.24.
Fig. 2.24: Step 2- Moving jacket along skid beams, Step 3- Jacket pivots on rocker arms Once in the water (see Fig. 2.25) the self floating jacket is brought under control with lines from tugs and/or the installation vessel.
Fig. 2.25: Step 4- Floating in water
The jacket must be designed and fabricated to withstand the stresses caused by the launch.
Once launched the jacket must float with a reserve of buoyancy in order to stop the downward momentum of the jacket. This requires the jacket to be water tight. It is common practice to gain additional buoyancy by sealing jacket legs and pile sleeves with removable rubber diaphragms. However, there is frequently a need for even more buoyancy. This is achieved by adding buoyancytanks. These need to be removable and are located where they give most benefit. Buoyancy tanks from previous launches are often used.
The launch of a jacket is clearly a critical phase in the life of the jacket. Considerable design effort is required in order to ensure that the launch sequence is feasible.
A jacket launch naval analysis is required in order to:
a. Ensure that an adequate sliding velocity is maintained during the rocker arm rotation; b.Verify that the trajectory followed has a safe seabed clearance;
c. Determine the jacket behaviour during launch;
d.Define operational requirements during launch, including ballast configuration; e.Check the stability of the jacket during launch and when free floating.
Fig. 2.26: Step 5- Upending with derrick barge
and after this stage the jacket is put in vertical position using temporary supports (see Fig. 2.27):
Fig. 2.27: Step 6- In place
An increasing number of jackets are being installed by direct lift (the jacket is lifted off the barge completely in air).This trend has been encouraged by the availability of large crane vessels and by more lightness achieved in jacket design.
Shallow water jackets may be lifted in the vertical position. In this case no up-ending is required and installation is straight forward. Deep water jackets will in general be lifted on their side. Two cranes will normally be used.
When considering a tandem lift it should be noted that it is unlikely that both hooks will carry the same load, and that the maximum permissible jacket weight will be less than the sum of the two crane capacities. Finally, the weight of lifting slings need to be considered, these contributing as much as 7% of the lift weight.
When the jacket is to be removed from the transportation barge by lifting, it is normal for the installation vessel to be correctly positioned so that up-ending and set-down may proceed as one integral lift operation.
The first stages in lifting a jacket from the transportation barge involve positioning the barge and connecting the slings to the hook. The barge will normally be controlled by tugs. Once everything is ready for lift to proceed the seafastenings will be cut. The next stage is to transfer the weight of the jacket from the barge to the crane. The general requirement here is to lift as rapidly as possible.
.
However, careful control and phasing with barge and crane vessel motions is required in order to ensure that once the jacket is lifted clear of the barge it does not hit the barge as a subsequent wave passes through. The same lift procedure is adopted in both a direct and buoyancy assisted lift. Once the jacket is lifted clear of the barge, the barge is removed by tugs. Up-ending of the jacket will then normally proceed directly.
Fig. 2.29: Stage 3- Upending: phase 2, Stage 4- Setting in final position
When the jacket is in vertical position we have the last operation, in which the piles are driven into the jacket’s legs to the required depth.
When the jacket is stabilized the processwillcontinuewiththeinstallation in different stages of the tower, nacelle and the rotor of the wind turbine.
2.4.4.3 Monopile
The use of this support is very common in the oil platforms offshore, and has thus achieved agreat success for the 160 MW wind farm at Horns Rev. Monopile foundations have been usedfor offshore turbines in Denmark, the Netherlands and Sweden. The “monopile” concept is based on single piles that are driven into the seabed. Pile driving is a fast process, and piles are relatively inexpensive to produce. The current design philosophy for wind farms in water depths up to 20 m is based on the monopile with the installation methodology – driving, drilling or combination – depending on soil properties, water depth and contractors experience.
Monopiles are a relatively compliant structure and hence are more difficult to design; uncertainties in ground conditions for example can result in a structure with a quite different structural frequency than designed for, with all the potential resulting problems of resonance induced dynamic oscillations. The monopile foundation is a fairly simple structure, consisting of several segments of steel and tight stacked one above the other to make a single stack. This configuration provides high density and appears to be little influenced by the effect of slag or the scour effect. Another great advantage is the fact that the installation of this structure does not require the preparation of the seabed. Monopile support gets to be put in the internal seabed, several metres deep, where it is fitted. This condition limits the horizontal movements (thanks to the pressure exerted by the surrounding soil throughout its length) and verticals (due to the friction that is developed along the surface). There are two methods used for the installation of the foundations. The first, driving, consists of putting the support at the selected point and keeping it upright. From the top, with a kind of hydraulic hammer, it is exercised a force pushing down the structure, digging in the area of the chosen measure. This method is quite simple, but obviously can only be taken if in the presence of
sandy bottoms, otherwise it would be very difficult to penetrate the support on the seabed (think of a rocky bottom) and there may be risk of damaging the structure. The second method involves the drilling of the seabed, in order to make a hole where the support will be put. First, it is placed in the bottom of a hollow steel cylinder which serves to drive the auger and to contain the waste coming from the excavation (300-400 m3 of material recovery), after this it comes the phase of the perforation.
When the excavation reaches the desired depth, the auger is removed and the addedsupport, filling the space between it and the excavation of cement. In the case of drilling, the bottom is rocky and the support is stuck at a lower depth than in the driving method, in which the seabed is sandy. Once the foundation is positioned, we proceed with the installation of the scour protection by a crane placed rocks around, controlling this process by sonar. The rocks can be transported from the coast with the use of large barges, and in some cases they stones of the drilling can be reused. Once it is placed the support and the scour protection, the installation of the wind turbine is next. The connection between the support and the tower of the turbine is produced by locking through a transition component which at the top presents a suitable platform to facilitate access to the structure. Besides connecting the base with the tower, the transition element allows to adjust the inclination of the wind turbine in the case that the support is not perfectly placed. Once the tower is set, it is proceeded to install the nacelle, lift with a crane.In the Fig. 2.30we can see an example of monopile foundation and the different stages to install the support and the transition element.
Fig. 2.30: Installation steps of monopile foundation(Association 2011).
One of the main problems of the monopile foundation is related to its flexibility, a problem that becomes even more important with increasing depth water and increased wind speed. If it is wanted to keep high rigidity of the structure when increasing the depth of water, the mass would increase with a cubic law (increasing length, diameter and thickness) and thus a first approximation would be that the costs would follow the same law. At the same time, the installation would require barges for the transportation and much larger hydraulic hammers that are not always available, and in any case would involve major increase in costs. For these reasons, these types of supports are used for shallow waters, varying between 5 and 30 metres. At the end of the useful life of the plant it is expected to cut the column of 2-3 metres below the seabed, leaving it where it is. It was decided to take this decision, taken because the mining operations of the foundation would be too complex and as a result of increased work load there will be a much greater environmental impact.
3. ACTIONS ON OFFSHORE WIND TURBINES 3.1 Gravity loads
Gravity loads include dead loads, operating and equipment weights, live loads and buoyancy loads. The dead loads include the permanent loads of the structure and equipment and other fixtures that are not likely to vary during the service life of the structure.
3.2 Hydrostatic loads
A floating structure when at rest in still water will experience hydrostatic pressures on its submerged part, which act normal to the surface of the structure. The forces generated from these pressures have a vertical component, which is equal to the gravitational force acting on the mass of the structure. In other words, for a freely floating structure, this force is equal to the displacement weight of the structure. In other directions, the net force is zero.
3.3 Current loads on structures
In the design of offshore structures, current is generally considered time-invariant represented by its mean value. The current strength, however, may have a variation with water depth.
The current introduces a varying pressure distribution around a member of the offshore structure generating a steady drag force on the structure in the direction of flow. Since the pressure distribution is not symmetric about the direction of flow, a transverse force is also generated on the structural member.
3.3.1 Current drag and lift force
If a two-dimensional structure is placed in a uniform flow, then the force experienced by the structure will depend on the fluid density, the flow velocity and the frontal area of the structure encountering the flow. The force is found to vary with the square of the flow velocity:
2 F F 0.5C A U F = ρ (3.1) where ρ- fluid density,
A - structure projected area normal to the flow, U - uniform flow velocity and
CF- a constant known as the drag coefficient.
The drag coefficient CF, has been shown to be a function of the Reynolds number, Re based on
mean current velocity and member diameter. For a circular cylinder across the flow, D is the diameter of the cylinder. The drag coefficient for a smooth stationary circular cylinder in a steady flow has been obtained through laboratory testing (most of these with air as the flowing fluid) and is shown in theFig. 3.1.
Fig. 3.1: Drag coefficient for a smooth circular cylinder in steady flow (redone after (Handbook of offshore engineering 2005), pg.138)
For a Reynolds number less than 2 x l05, the flow is considered to be in the subcritical range. For an Re less than 50, the flow is strictly laminar and steady. The drag coefficient in this range decreases linearly as a function of the Reynolds number. The laminar flow is maintained up to an Re of about 200, beyond which the flow starts to become turbulent. The flow becomes fully turbulent for Re > 5000. The range of Re between 2 x l05 and 5 x l05 is termed critical where flow is in the transition mode. This area actually causes a low-pressure region behind the member (called the wake) to grow narrower with a corresponding decrease in the pressure gradient, causing a sharp drop in the actual value of the drag coefficient.
This is known as drag crisis. As the Reynolds number increases, the supercritical range is reached where the flow is strictly turbulent. In this range the drag coefficient slowly increases again. Beyond an Re of 3 x l06 the turbulent flow is called post-supercritical. Here the drag coefficient approaches a steady value because less dramatic changes occur in the boundary layer at still higher velocities. This figure clearly demonstrates the difficulty associated with small-scale testing of structures in which the drag force is important.
Drag force for a rough circular cylinder in steady flow (see Fig. 3.2).
02 . 0 D K =
Fig. 3.2: Drag coefficient for a rough circular cylinder in steady flow (redone after (Chakrabarti 2005), pg.139)
In practice, the surfaces of many structures in operational mode are not smooth. The roughness of the structure surface may be contributed from several sources. The appendages attached to a
