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TECHNICAL REPORT

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JOINT INDUSTRY PROJECTSUMMARY REPORT FROM THE JIP ON THE CAPACITY

OF GROUTED CONNECTIONS IN OFFSHORE WIND

TURBINE STRUCTURES

REPORT NO. 2010-1053REVISIONNO. 05

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Reference to part of this report which may lead to misinterpretation is not permissible.

SummaryReportCapacityGroutedConnections12May2011.doc

Table of Content Page

1 INTRODUCTION ....................................................................................................... 1

1.1 Background 1

1.2 Scope 2

1.3 Grouted connection concepts 2

1.4 Definitions and general safety format 2

1.4.1 Definition of symbols 2

1.4.2 Terms used to document capacity 4

1.4.3 The partial safety factor format 4

2 CYLINDRICAL CONNECTIONS WITHOUT SHEAR KEYS ................................ 6

2.1 Axial capacity 6

2.1.1 Axial capacity due to steel surface irregularity 6

2.1.2 Prestressed flexible supports for the transfer of axial loads 11

2.2 Bending moment capacity 11

2.2.1 Global structural behaviour of grouted connections 11

2.2.2 Structural behaviour at the ends of grouted connections 16

2.2.2.1 Discontinuity in geometry and force flow 16

2.2.2.2 Discontinuity due to axial load in the transition piece 16

2.2.2.3 Discontinuity due to pressure loads acting between the pile and transitionpiece 17

2.2.3 Assessment of grout capacity and design to avoid progressive grout

crushing 18

2.2.4 Assessment of local pressure on ends of grouted sections 20

2.2.5 On improvement of local structural behaviour 21

2.2.5.1 Reduction of contact pressure by increasing the size of the connection 21

2.2.5.2 Local softening of structure 21

2.3 Torsion capacity 21

2.4 Friction coefficient 22

2.5 Abrasive wear of contact surfaces 22

3 CAPACITY OF GROUTED CONNECTIONS WITH SHEAR KEYS ................... 23

3.1 Capacity for static loading 23

3.2 Capacity for dynamic loading 23

4 CAPACITY OF CONICAL SHAPED GROUTED CONNECTIONS..................... 24

4.1 Axial capacity 24

4.1.1 Background 24

4.1.2 Equation for calculating the axial capacity 24

4.2 Moment capacity 274.2.1 Global behaviour 27

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4.2.2 Local capacity at the ends of a grouted connection 27

4.3 Torsion capacity 28

4.4 Abrasive wear of contact surfaces 29

5 FATIGUE CAPACITY OF GROUTED CONNECTIONS IN AIR AND

SEAWATER ENVIRONMENTS ............................................................................. 30

5.1 Design S-N curves for grout material 30

5.2 Watertightness 30

5.3 Grout seals 31

6 FINITE ELEMENT ANALYSES OF GROUTED CONNECTIONS...................... 32

6.1 Requirements as to analysis programs 32

6.2 Finite element modelling 32

6.3 Acceptance criteria 32

7 REQUIREMENT AS TO THE IN-SERVICE INSPECTION AND

MONITORING OF NEW DESIGNS........................................................................ 33

8 MITIGATION OF EXISTING STRUCTURES WITH LOW AXIAL

CAPACITY................................................................................................................ 34

8.1 Inspection for settlement 34

8.2 Inspection to reveal cracking of the grout 34

8.3 Inspection to reveal potential fatigue cracks 34

8.4 Mitigation with respect to axial force 35

8.5 Mitigation with respect to fatigue cracks 35

9 REFERENCES........................................................................................................... 36

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Acknowledgement

This report is developed within the Joint Industry Project on the Capacity of Grouted

Connections in Wind Turbine Structures.

Acknowledgement is made to the JIP Partners for their support and contribution to thiswork:

Ballast Nedam Engineering

BASF Construction Chemicals Denmark A/S

Centrica Renewable Energy Limited

Densit A/S

DNV

DONG Energy

GustoMSC

MT Hjgaard a/s

Per Aarsleff A/S

RWE Innogy GmbH

Statoil ASA /Statkraft AS

Vattenfall Vindkraft A/S

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1 INTRODUCTION1.1 BackgroundDuring the summer and early autumn of 2009, DNV assessed the current industry practice for

calculating the axial load capacity used in the design of grouted connections in offshore

monopile wind turbine structures, ref. Figure 1. It was found that the applied design methods for

capacity analysis did not properly represent the actual physical behaviour of such connections.

Based on this situation, it was decided to initiate a joint industry project on the capacity of large

diameter grouted connections in offshore wind turbine structures. The project was started in

November 2009. The objective of the project is to improve the basis for a reliable designmethodology for grouted connections in large diameter structures typically used in wind turbine

structures.

The new data derived in this project will be used to improve the design basis for large diameter

grouted connections in monopile structures.

A background document for the work is presented in DNV Report no. 2010 - 0650.

A grouted connection is used to connect the transition piece to the monopile as indicated in

Figure 1. The transition piece is installed on top of the monopile resting on temporary supports.

The transition piece is then jacked up to the correct verticality before the grouting is carried out.

After curing, the jacks are removed and there is a gap between the supports and the monopile.

Brackets for temporary support

of tower before grouting

Tower

Monopile

Transition piece

Grout

Gap after

installation of tower

and grouting

between pile and

transition piece

Grout seal

Transition piece

Grouted connection

Monopile

MWL

Mud level

Brackets for temporary support

of tower before grouting

Tower

Monopile

Transition piece

Grout

Gap after

installation of tower

and grouting

between pile and

transition piece

Grout seal

Transition piece

Grouted connection

Monopile

MWL

Mud level

Figure 1 Sketch of grouted connection in monopile structure

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1.2 ScopeThe scope of this work is to present a technical basis for achieving the objective of the joint

industry project as outlined in section 1.1.

This work includes guidance on:

The structural capacity of cylindrical shaped grouted connections without shear keyssubjected to axial load.

The structural capacity of cylindrical shaped grouted connections without shear keyssubjected to bending moment.

How to design the top and bottom ends of the grouted connections to prevent thecracking/crushing of the grout.

The axial resistance as a function of the amount of sliding between the grout and steelduring dynamic loading, the wear of sliding surfaces and the friction coefficient of

sliding surfaces.

The design of pre-stressed flexible supports for the transfer of axial loads. The fatigue capacity of grouted connections in air and seawater environments. The capacity of conical shaped grouted connections. An assessment of the safety factors to be applied to large diameter grouted connections.

These items are presented in more detail in the following sections of this report.

The capacity of grouted connections with shear keys has only been briefly assessed in this study.

It is proposed that this should be further investigated in Phase II of the Joint Industry Project on

the capacity of grouted connections.

1.3 Grouted connection conceptsThe following concepts involving grouted connections between the monopile and transition

piece are considered in this report:

Cylindrical shaped steel shells with plain surfaces (without shear keys). Cylindrical shaped steel shells with plain surfaces and additional elastomeric supports for the

transfer of axial loads. Conical shaped steel shells.As indicated above in 1.2, grouted connections with shear keys will be further investigated in a

proposed JIP phase II.

1.4 Definitions and general safety format1.4.1 Definition of symbolskf = characteristic interface shear strength due to surface irregularities and friction

= grout to steel interface coefficient of friction

= height of surface irregularities

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p = pressure in radial direction between grout and steel

Rp = pile outer radius

RTP = outer radius of transition piece (TP)

(The outer tubular is denoted as sleeve in the grouted connection used in jacket

structures; and therefore the index s is sometimes used for this parameter in literature)

tTP = wall thickness of transition piece (TP)

tp = wall thickness of pile

tg = thickness of grout

E = modulus of elasticity for steel

Eg = modulus of elasticity for grout

Lg = length of grouted section

lep = elastic length of the monopile:4 2

pp

ep

)1(3

tRl

=

leTP = elastic length of the transition piece:4 2

TPTP

eTP

)1(3

tRl

=

Pile Grout

L

tp

tTP

RTP

Rp

CL

tg

Transition piece (TP)

Lg

tp

tRp

CL

tg

Pile Grout

L

tp

tTP

RTP

Rp

CL

tg

Transition piece (TP)

Lg

tp

tRp

CL

tg

Figure 2 Definition of symbols used for cylindrical shaped grouted connections

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1.4.2 Terms used to document capacityReference is made to definitions in DNV-OS-J101 (2007) of terms used to document capacity. In

addition, the following definitions are given:

A definition of characteristic values is an important part of a safety format. In the 2007 version

of DNV-OS-J101, the characteristic ultimate capacity (or resistance) is defined as the estimated

mean value of the capacities derived from the tests. This estimate should be obtained with a

confidence of 75%. In a limit state design of structures, it is common practice to define

characteristic capacity (Rk) as the 5% quantile value of the test data. It is proposed to use the

same definition in a revision of the design standard too. As there is a lot of test data on the

compressive strength of grout material, the standard deviation of the compressive strength can be

assumed to be known, and it is therefore considered sufficient to estimate the 5% quantile value

with 75% confidence.

The design capacity is defined as the characteristic capacity divided by a material factor.

The value of the material factor for axial capacity used until 2009 is linked to the format of the

corresponding design equations that were used. New test data shows that the design equations in

design standards from before 2009 did not properly represent the physical behaviour with respect

to axial load capacity in large diameter connections. It is thus likely that a design format that did

not properly represent the physical behaviour can explain some of the scatter in the test data that

has earlier been used to calibrate safety factors.

Also, a lack of knowledge about surface irregularity or fabrication tolerances in the test

specimens is considered to introduce uncertainty which may explain significant scatter in tested

capacities. The large scatter in test data relating to grouted connections has traditionally led torequirements that a large safety factor be used for the design of grouted connections.

The initial axial load capacity in large diameter connections may be greater than that which can

be documented reliably due to structural tolerances from circumferential welds and fit-up

tolerances between cans (pile sections) that are not normally included in a capacity assessment.

However, this additional capacity may be considered lost during alternating moment loading

which leads to wear of the sliding surfaces and hence to a reduction in the axial load capacity.

By formulating a capacity that is considered to represent the physical behaviour more accurately

and with improved control of minimum tolerances like that of a defined cone angle, it is

considered that the scatter in test data will be reduced and that the material factor can in principle

be reduced as well.

However, for failure modes that are not well defined and controlled, it is not advisable to reduce

the safety factor so that the utilisation level increases.

Where a more refined design procedure has been developed that can be based on more accurate

stresses derived from calibrated finite element analysis of the grouted connections, safety factors

from the design of offshore concrete structures, such as DNV-OS-C502 and Eurocode for the

design of concrete structures, can be used.

1.4.3

The partial safety factor formatThe partial safety factor format is in general explained in DNV-OS-J101 (2007).

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The design load effect (Sd) must be lower than the design capacity (Rd) for each failure mode:

dd RS (1)

The design load effect is obtained as described in DNV-OS-J101 (2007). The design capacity is

obtained by dividing the characteristic capacity (Rk) by a specified material factorm:

m

k

d

RR

=

(2)

m = material factor

m = 1.5 for the ultimate limit state, where the characteristic capacity is determined from accurate

finite element analysis of the grouted connection that has been calibrated/verified against testdata.

m = 2.6 for the ultimate capacity derived from test data, such as the capacity of grouted

connections with shear keys.

m = 1.0 for the ultimate limit state of torsion capacity, where a characteristic friction coefficient

= 0.40 is used in the design.

Reference is made to DNV-OS-C502 for safety factors for the fatigue limit state of the grout

material.

It is proposed to restrict the nominal contact pressure to 1.2 MPa to limit the consequence of the

abrasive wear failure mode. A load factor equal to 1.0 can be used to assess the abrasive wear.

Otherwise, the requirements as to workmanship and quality assurance described in DNV-OS-

J101 (2007) apply.

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2 CYLINDRICAL CONNECTIONS WITHOUT SHEAR KEYS2.1 Axial capacity2.1.1 Axial capacity due to steel surface irregularityDuring the work on this joint industry project, literature reaching more than 30 years back in

time has been reviewed. Unfortunately, no clear explanation of the physical capacity of grouted

connections without shear keys was found.

During the JIP, some more understanding of the actual physical behaviour has been gained based

on laboratory tests and reported settlements of a number of monopile grouted connections. This

experience has led to the following explanation of the axial capacity of cylindrical shaped

grouted connections without shear keys.

The axial capacity can be explained by resistance due to surface irregularity. Some definitions

are made in this respect:

Surface irregularity: Surface roughness in addition to surface tolerances in the tubularsections that form the grouted connection.

Surface roughness: Surface roughness from steel production and corroded surfaces. Tolerances: Ovality in the circumferential direction and undulation in the longitudinal

direction.

The axial capacity can be explained by different stages as shown in Figure 3 and Figure 4.During the first part of a loading, the capacity depends on a combination of surface roughness

and tolerances. This is illustrated as stage I in Figure 3. This stage may also be denoted as that

corresponding to bond capacity. At the end of this stage, the bond capacity is exceeded and a slip

occurs as illustrated in Figure 4. After this stage, the capacity depends mainly on tolerances. If

the tolerances are large, the capacity is also still large due to resulting pressure from the grout on

the steel owing to friction between the sliding surfaces. If the tolerances are small, the capacity

after the bond capacity is exceeded is also small, as indicated in Figure 4.

As soon as the bonds are broken, the capacity will depend on tolerances and radial stiffness as

indicated in Figure 4. This means that for a constant value of in Figure 3, the interface capacity

will be reduced by the increased radius of the grouted connection for a given constant radial

stiffness parameter (K), as indicated in Figure 5. It can be questioned whether the bond capacity

depends more on area and not so much on tolerances and the radial stiffness of the cylindrical

sections. However, axial test data from scaled specimens indicate that the capacity curve for the

breaking of bonds also follows a curve similar to that of Figure 5.

There are typically two circumferential welds in a monopiles grouted section. Fabrication is

always associated with some tolerances and it has been observed that just small tolerances at

these welds provide significant axial capacity. Thus, it is likely that the grouted connections have

a significant axial capacity immediately after installation, before the structures are subjected to

bending moments.

The axial capacity in cylindrical shaped grouted connections depends on fabrication tolerances

since larger tolerances increase the capacity. Minimum tolerances should be provided in

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fabrication standards in order to document a reliable capacity. However, such requirements have

so far not been specified.

Although the thickness of the monopile and transition piece is significant, the diameter to

thickness ratios of the monopile structures are relatively large and the structures may be defined

as shell structures or thin-walled structures. This means that the radial stiffness or flexibility of

the connections becomes an important design parameter.

The literature indicates that the design for the moment loading could be considered

independently from that for the axial load. Quoting ISO 19902 from 2007: "The representative

interface strength for axial force is thus not reduced by coexisting bending and shear". This

statement was based on test data presented in OTC paper no 5485 in 1986.

However, a bending moment is considered to lead to the ovalisation of the connection. This is

illustrated in Figure 6, where the moment is illustrated by an axial force couple. The axialcompression force leads to a radial displacement above the grouted connection due to Poissons

ratio of the steel. This leads to tensile stresses between the steel and grout which may lead to a

loss of bond capacity. As the bond capacity is lost, the displacement field moves into the

connection, with further loss of bond capacity as illustrated in Figure 7. As the direction of the

bending moment changes with time, the initial bond around the circumference may thus be lost

in the case of large bending moments.

The flexibility of the connections implies deformation during dynamic moment loading, leading

to sliding between the steel and grout surfaces as indicated in Figure 8. This sliding cannot be

resisted by a high friction coefficient or a high initial bonding strength between the steel and

grout due to radial deformations required to create reaction pressure in the grout that will resistthe bending moment.

Even if each relative sliding length during a load cycle between grout and steel is only a few

millimetres or a fraction of a millimetre, the accumulated relative sliding length may be large

after some years of service life due the significant number of dynamic load cycles. This depends

on the size of the alternating dynamic bending moment on the connection.

The accumulated sliding between the steel and grout surface is considered to lead to wear on the

grout surface and a reduced interface capacity with time. The accumulated sliding length for a

real structure is significantly larger than that which can be tested in a laboratory on scaled

specimens.

Laboratory tests show that the long-term resistance against sliding might be so much reduced

during service life that a reliable lower bound on resistance to be used in the design can hardly

be provided. This means that design solutions should be sought that can be documented to show

sufficient capacity with a resistance between the steel and grout surface equal to zero for

cylindrical shaped grouted connections.

Due to the low long-term axial capacity of cylindrical shaped grouted connections, it is believed

that the design of such connections in monopiles can no longer be recommended. The reason for

this is the:

Reduced interface shear capacity with the increasing diameter of the connections incombination with a low long-term effective resistance between the steel and grout surfaces

for loads that exceed the bond/sliding friction capacity.

Lack of requirements as to minimum tolerances.

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Stage I Bonds not broken Stage II Bonds are broken

Stage I Bonds not broken Stage II Bonds are broken

Figure 3 Different stages during the testing of axial capacity

Relative axial displacement

Axial

force/

capacity

Post bond capacity for significant tolerances

Post bond capacity for small tolerances

Bond capacity for small tolerances

Bond capacity for significant tolerances

Relative axial displacement

Axial

force/

capacity

Post bond capacity for significant tolerances

Post bond capacity for small tolerances

Bond capacity for small tolerances

Bond capacity for significant tolerances

Figure 4 Illustration of axial capacity as a function of surface irregularities (surface

roughness + tolerances)

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0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 300 600 900 1200 1500

Radius pile (mm)

Interfaceshearstrength(MPa)

Delta = 0.07 mm

Figure 5 Graph illustrating the interface shear strength as a function of pile radius after

bonds are broken

Transition

piece

Pile

Radial displacement outwards (mm)

of transition piece

N N N tensileN compressive

Radial displacement inwards (mm)

of transition piece

0.

0

0.

1

0.

2

0.

3

0.

4

0.

5

0.

6

0.

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M

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piece

Pile


of transition piece



of transition piece

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0

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piece

Pile


of transition piece



of transition piece

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0.

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0.

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0

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1500

M

Figure 6 Radial displacement in a large diameter connection subjected to an axial load

(from bending moment)

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0.

0

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piece

Pile


of transition piece



of transition piece

Loss of contact

500

0.

0

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of transition piece

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0

500

1000

1500

M

Figure 7 Radial displacement as contact is being lost in a large diameter connection

subjected to axial load (from bending moment)

v

M

v

M

Figure 8 Illustration of deformation and relative displacement between the transition

piece and monopile due to moment loading

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2.1.2 Prestressed flexible supports for the transfer of axial loadsBy using prestressed flexible supports between the pile and transition piece, a vertical

deformation of the transition piece relative to the pile top can be allowed without adding too

large stress ranges at the considered hot spots at the ends of bracket supports in the transition

piece. Some relative vertical displacement between the top of the pile and the transition piece

will occur when the grouted connection is subjected to a bending moment. Therefore some

flexibility of the supports is required in order to avoid a transfer of significant dynamic forces

through the flexible supports. Dynamic forces in the spring supports should be limited as these

forces will be transferred into the transition piece through the bracket supports that connect the

supports from the pile to the transition piece. The flexible supports should also allow sufficient

ovalisation of the connection so that the dynamic moment loading can be transferred by reaction

forces through the thickness of the grout from the transition piece to the pile without transfer ofsignificant horizontal dynamic forces through the spring supports.

A principle sketch of flexible supports for the transfer of axial loads is shown in Figure 9.

This designs concept and methodology have been developed by Statoil.

The spring supports should have low horizontal shear stiffness such that an opening between the

steel and the grout at the pile top will not introduce significant stresses into the transition piece.

The prestress to support the vertical weight should be assessed at an early design stage.

Spring

support

Spring

support

The spring supports are resting on the top of the pile.

Thus the vertical load is transferred from the transition

piece to the top of the pile.

Figure 9 A principle sketch of flexible supports for the transfer of axial load

2.2 Bending moment capacity2.2.1 Global structural behaviour of grouted connectionsThe axial stresses in the transition piece and the monopile due to the moment loading on the

grouted connections are usually much larger in a wind turbine structure than that from the

vertical permanent loading from the structure above the connection.

The moment loading is transferred from the transition piece to the monopile through horizontalcontact forces as indicated in the sketch in Figure 10.

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There will also be vertical friction forces due to the contact pressure that contributes to the

moment capacity of the grouted connection (red arrows in Figure 10).

The axial resistance to sliding in cylindrical shaped grouted connections is significantly reduced

as a function of the number of dynamic load cycles causing sliding in the connection. However,

it is found that a characteristic friction coefficient value of 0.40 between steel and grout can also

be applied to grouted connections subjected to long-term sliding.

As long as there is friction force between the steel and grout due to contact pressure, there will

also be vertical friction forces due to the surface irregularity (or fabrication tolerances) in the

connection. (Black arrows in Figure 10.) Reference is made to equation (7). This effect is not

recommended used in the design; however, it must be kept in mind when assessing laboratory

test data. If shear keys are installed around the circumference of the monopile and the transition

piece, these shear keys will also transfer vertical shear forces, contributing to the groutedconnections moment capacity.

The contact pressure shown in Figure 10 will act around most parts of the circumference. This

contact pressure will provide some horizontal shear resistance due to the friction between the

steel and grout. These horizontal shear forces shown in Figure 10 will also contribute to the

grouted connections moment capacity (green arrows in Figure 10).

The moment action will lead to a tension load in the circumferential direction of the grouted

connection that may exceed the grouts tension capacity. This will lead to the grout cracking as

indicated in Figure 11. Due to the relatively high local slenderness (diameter to thickness ratio)

of the pile and transition piece, ovalisation of the cylinders will also occur and a gap will open up

between the grout and the steel of these elements in the case of large moments. This will lead toa relative sliding between the steel and grout. One can thus assume that the main purpose of the

grout is to transfer pressure from the transition piece to the pile.

The actual behaviour of the grouted connections subjected to a bending moment may be

simulated by a finite element analysis that accounts for compressive contact between the steel

and grout but without tensile contact stresses and with a proper friction coefficient where contact

pressure is present.

An analytical expression of the relationship between the contact pressure and bending moment

acting on the grouted connection can be derived based on certain assumptions concerning

pressure distribution. A constant pressure is assumed around half the circumference from b to d

in Figure 12. Then a linear pressure distribution is assumed from d to a and from b to a. (The

pressure distribution is considered to depend on the connections diameter thickness ratio. For a

low diameter-to-thickness ratio, a larger pressure is expected at position c than at other positions

around the circumference).

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Lg

Vertical shear force due to contact pressure

and friction

Horizontal shear

force

Contact pressure

p

D

p

M

HFHF

Lg

Vertical shear force due to contact pressure

and friction

Horizontal shear

force

Contact pressure

p

D

p

M

HFHFHFHF

Figure 10 Illustration of reaction forces in the grouted connection due to moment loading

M

HF

Opening between groutand steel

Sliding of grout against steel

Sliding of grout against steel

M

HF

Opening between groutand steelOpening between groutand steel

Sliding of grout against steelSliding of grout against steel

Sliding of grout against steelSliding of grout against steel

Figure 11 Behaviour of grouted connection in the case of a large bending moment

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x

x

x

x

x

x

x

pac

d

h

v

h

b

x

x

x

x

x

x

x

pac

d

h

v

h

b

x

x

x

x

x

x

x

pac

d

hh

vv

hh

b

x

x

x

x

x

x

x

pac

d

hh

vv

hh

b

Figure 12 Illustration of pressure distribution around the circumference

The moment action due to maximum contact pressure (blue arrows in Figure 10) is derived by

integrating the contact pressure around half the circumference from b to d in Figure 12 as

3

2

gp

p

LRpM =

(3)

where

p = maximum nominal pressure at the top and bottom of the grouted section as shown in

Figure 10.

Lg = height of grouted section.

The moment due to the horizontal friction force (green arrows in figures) is derived by

integrating the contact pressure within the stipulated red line from a to c in Figure 12 with

pressure 0.75p at position d as

2

gp

h

LRpM =

(4)

The moment capacity due to the vertical friction force (red arrows in Figure 10 and Figure 12) is

derived by integrating the contact pressure outside the stipulated line from a to c in Figure 12

with pressure 0.5p at position d as

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gpv LRpM2

= (5)

(The selection of pressure at position d to derive equations (4) and (5) is based on a comparison

with the results of a finite element analysis).

The total moment is derived as

vhptot MMMM ++= (6)

In addition to these moments, there is a friction moment due to surface irregularity as explained

above. This moment can be expressed as

gkf

2

ptyirregulariSurface LR4M = (7)

Reference is made to ref. /4/ for derivation.

The interface shear strength due to surface irregularity is significantly greater for small diameter

connections than for large diameter connections as shown in Figure 5. This means that the

friction moment due to surface irregularity becomes a significant parameter in planning testing

and assessing test data from scaled test specimens. However, for large diameter connections, the

friction moment is negligible compared with the other contributing moment capacities.

The friction resistance moment will also be reduced with time when sliding occurs between thesteel and grout.

It should also be noted that the friction moment due to surface irregularity is not included in the

finite element analysis normally performed by the industry as the surface irregularity is not

accounted for in the analysis models.

The contribution to the nominal contact pressure from the global shear force at the grouted

connection is considered to be small.

An estimate of the maximum nominal contact pressure is then derived from equations (3-6) as

gpgp

tot

LRLR

Mp

22 3)3(

3

++=

(8)

An estimate of the maximum opening between the steel and the grout at the top of the monopile

is derived as

TPpH += 3 (9)

Here, the compression of the grout is neglected as the contribution from this to the total

deformation is small for typical wind turbine connections.

p

p

ptE

Rp2

=

(10)

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TP

TPTP

tERp

2

=

The vertical relative deformation is derived as

g

p

HVL

R2 =

(11)

A significant sliding between the steel and concrete is expected to occur in the case of large

moments even with a high friction coefficient. Thus, it is not realistic to try to improve the

structural behaviour of the large diameter connections subjected to dynamic bending momentsby increasing the roughness of the steel surfaces.

2.2.2 Structural behaviour at the ends of grouted connections2.2.2.1 Discontinuity in geometry and force flowThere is a discontinuity in geometry and loading at the ends of the grouted section that may lead

to the grout cracking. The discontinuity in loading is related to the:

Axial load from self weight and from the bending moment resulting in axial stress in thetransition piece (considering the region at the top of the grout), and the pile (considering

the region at the bottom of the grout). Pressure loading due to moment action and due to pressure loading from settlements in

cone connections.

The effect of these discontinuities on local structural behaviour is explained in detail in the

following sections.

In finite element analyses of large diameter grouted connections, the regions at the grout ends

often show high stressed areas. These areas are at a corner where a singular stress field can be

expected based on a linear finite element analysis. Here the calculated stress is a function of

element size. However, high stresses in the grout are also expected at these areas in grouted

connections for large bending moments as explained in the following.

2.2.2.2 Discontinuity due to axial load in the transition pieceAn example of the structural behaviour of a large diameter connection is considered as follows:

Outer diameter of transition piece: 5400 mm. Thickness of transition piece: 70 mm. Thickness of pile: 80 mm.The axial stress in the transition piece due to the global bending moment equals 143 MPa. The

calculated radial displacement is proportional to the axial stress; thus, the displacement for a

lower axial stress can easily be deduced by a linear scaling.

Analysis results based on shell theory are shown in Figure 6. The analysis is performed for one

compressive loading and one tensile loading. For tensile loading there will be full contact

(horizontal pressure stress in the grout) at the top of the grout (as shown in the right part of

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Figure 6). For the compressive loading, there will be a split force in the transition piece that

attempts to separate the steel from the grout. If an opening between the steel and grout occurs,the supporting action from the pile is lost and the displacement field is shifted downwards as the

opening is extended downwards as indicated in Figure 7.

For a global moment loading, one will have tensile stresses in the transition piece on one side

and compressive stresses at the opposite side as shown in Figure 6 and Figure 7. At the neutral

axis for moment loading, there is approximately zero stress in the transition piece. Thus the

radial displacement will vary around the circumference of a connection subjected to a bending

moment. This leads to ovalisation of the connection.

2.2.2.3 Discontinuity due to pressure loads acting between the pile and transition pieceAt the end of the grouted connection, there is a discontinuity in geometry and in transverse

loading (contact pressure between the steel and grout).

Even with a constant thickness over the length of the transition piece, there is a discontinuity in

geometry at the top of the pile when one considers the pile and the transition piece to behave

together as a composite structure with grout in between.

At the top of the pile there is also a discontinuity in the horizontal pressure acting on the

transition piece as there is no horizontal pressure acting on the transition piece above the top of

the pile.

A sketch of a grouted connection illustrating the deflection gradient in the transition piece for auniform loading from the grout is shown in Figure 13 for a high nominal pressure p = 3.0 MPa

(geometry as in section 2.2.2.2). From the deflection curve, it is understood that there will be a

significant local pressure at the top of the grouted connections. Reference is made to section

1.4.1 for a definition ofleTP and lep.

A similar assessment may be made for the bottom of the grouted connection.

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Transition

piece

Pile

lep

leTp

Potentialcrushing of

grout

-

0.

2

0.

0

0.

2

0.

4

0.

6

0.

8

1.

0

1.

2

1.

4

1.

6 -2000

-1500

-1000

-500

0

500

1000

1500

Distance

along

grout.

0is

attop

ofpile

Radialdisplacement(mm)

Radial displacement (mm) of

transition piece subjected to a

uniform compression field

along the grouted connection

pTransition

piece

Pile

lep

leTp

Potentialcrushing of

grout

-

0.

2

0.

0

0.

2

0.

4

0.

6

0.

8

1.

0

1.

2

1.

4

1.

6 -2000

-1500

-1000

-500

0

500

1000

1500

Distance

along

grout.

0is

attop

ofpile

Radialdisplacement(mm)

Radial displacement (mm) of

transition piece subjected to a

uniform compression field

along the grouted connection

p

Figure 13 Sketch of a grouted connection illustrating the deflection gradient in the

transition piece for a uniform loading from the grout

2.2.3 Assessment of grout capacity and design to avoid progressive grout crushingLocally, at grout ends, there may be a significant pressure load p between the steel and grout as

indicated in Figure 13. Depending on the friction coefficient, there may also be some shear stress

acting on the grout connection during dynamic loading, with sliding occurring between the steel

and grout. The shear stress may act both upwards and downwards during a loading cycle, which

in principle may lead to cracking in different directions as indicated in Figure 14.

The maximum and minimum principal stresses in the grout are derived as

( )( )2

2

4112

4112

+=

++=

LocalII

Local

I

p

p

(12)

The directions of principal stresses depend on the actual friction coefficient and on the sign on

shear stress. Therefore, cracking due to tensile loads normal to minimum principal stress

direction will also depend on the friction coefficient.

It is important to account for the stress increase due to the discontinuity at the ends of the grout

as illustrated in section 2.2.2. This can be presented in terms of stress increase by a stress

concentration factor such that the local contact pressure becomes related to the nominal contact

pressure as

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NominalLocal pSCFp=

(13)

An analytical analysis of the stress concentration factor at the top of the pile is given as

2/3

TP

TP

t

R025.01SCF

+=

mm100tmm50

mm2750Rmm2250

Tp

TP

(14)

An analytical analysis of the stress concentration factor at the lower end of the transition piece is

given as

2/3

p

p

t

R025.01SCF

+=

mm100tmm50

mm2750Rmm2250

p

p

(15)

The nominal contact pressures are assumed to be derived from the procedure in section 2.2.1with a friction coefficient of = 0.40. This friction coefficient is assumed for the global

behaviour of the connection. A friction coefficient = 0.40 is considered to be a lower value (or

characteristic value) to be used for a design where a low value provides a low capacity (or here a

high pressure load which is used to calculate the load effect). Due to scatter in tested friction

coefficients, it is also likely that the friction coefficient may be much larger. It may also vary in a

connection depending on local surface irregularities. Therefore the use of a larger friction

coefficient is recommended in equation (12) depending on the consequences of grout cracking.

Based on the combined axial and dynamic bending tests, it is proposed to limit the contact

pressure between the steel and grout. A simple design approach is suggested, based on analytical

considerations. A more refined design approach based on linear elastic finite element analyses ofthe grouted connection is recommended for more detailed design analyses.

Based on present knowledge, it is proposed to limit the nominal design contact pressure to 1.5

MPa when using a high capacity grout. (A laboratory test showed cracking of the edge with a

nominal contact pressure equal to 2.0 MPa.) The nominal design contact pressure is understood

to be the pressure derived from an analytical calculation of the pressure excluding the stress

increase from end discontinuities (ref. section 2.2.1). If the nominal design contact pressure is

derived from finite element analysis, stress increase from end discontinuities should be excluded.

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Outside of

transition piece

Inside ofpile

Potential grout cracking under

reversed dynamic loading

pLocal

Localp=

Outside of

transition piece

Inside ofpile

Potential grout cracking under

reversed dynamic loading

pLocal

Localp=

pLocal

Localp=

Figure 14 Potential cracking in grout under reversed dynamic loading

2.2.4 Assessment of local pressure on ends of grouted sectionsBy following the procedure for calculating the nominal contact stress in section 2.2.1, the local

design contact stress is obtained from the nominal design contact stress as

dNominal,d,Local pSCFp = (16)

Alternatively, this local design stress can be derived directly from a linear finite element

analysis. A finite element analysis is recommended used to calculate the local pressure to

document a final design.

The design tensile stress is derived as

)411(2

2,

Local

dLocal

d

p +=

(17)

where Local is a local friction coefficient representative for the contact area at the edges of the

grout connection. A large upper value for the friction coefficient should be used in the design.

The value may depend on the consequence of the grout cracking. This should involve an

assessment of the long-term capacity of the grout. Local = 0.70 can be considered used to assess

the local tensile stress. To calculate the nominal pressure, a friction coefficient of = 0.40 can be

used.

The design capacity is calculated as

mctcd /= (18)

where

ct = characteristic tensile capacity of the grout (5 percentile value of the test data).

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m = material factor = 1.5.

Then the design criterion reads

cdd (19)

For finite element analysis, see also section 6.

2.2.5 On improvement of local structural behaviour2.2.5.1 Reduction of contact pressure by increasing the size of the connectionA simple way of reducing local stress in the grout is to extend the length of the grouted section.

2.2.5.2

Local softening of structureThe compressive stresses at the grout ends can be reduced by reducing the radial stiffness of the

end members (pile at the top of the grouted connection and transition piece at the bottom of the

connection) as indicated in Figure 15. Reference is made to section 1.4.1 for a definition of leTP

and lep. A similar radial displacement in the pile and transition piece can be achieved by such a

design. However, there will still be bending stress in the grout that might lead to horizontal

cracking. It can be discussed if such cracking is acceptable or not. The design must be such that

the cracked pieces cannot fall out. Some cracking of the grout is considered acceptable provided

it does not lead to a progressive degradation of the connection.

A finite element analysis is recommended to document a final design.

Transition

piece

Pile

lep

leTp

tp

0.5tp

0.5tTP

tTP

Transition

piece

Pile

lep

leTp

tp

0.5tp

0.5tTP

tTP

Figure 15 Softening of grout ends by end rings with half the thickness of the ending

member

2.3 Torsion capacityThe capacity with respect to a torsional moment depends on the resistance to sliding between the

grout and steel and the actual tolerances in the cylindrical shaped connections.

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The laboratory axial sliding tests showed a rather low resistance after some cycles. Similar

behaviour may be expected for alternating long-term dynamic torsional moments.

Due to this observation, it may be recommended for structures using cylindrical shaped

connections to design mechanical locks to transfer the torsional moment from the transition piece

to the pile. Alternatively, inspection of the relative rotation is recommended. Even if the

resistance factor is low, the friction coefficient is still considered to be significant such that some

torsional moment can be transferred in the connection if there is a contact pressure present, e.g.

from an external bending moment. Also, a cone section will provide pressures that will result in

a significant torsion capacity, see also section 4.3.

If the vertical forces are transferred by spring supports, the resistance against torsion will be less

due to lower contact pressure between the steel and grout as explained in ref. /4/. Appropriate

capacity against torsion may be achieved by installing supports that can transfer torsionalmoment but that are flexible in a vertical direction in order not to increase the vertical spring

stiffness. The supports need to be fixed to the pile and transition piece.

2.4 Friction coefficientThe results of laboratory friction tests show significant scatter.

A characteristic friction coefficient value of 0.40 is recommended as a characteristic value for

design.

A mean value of the friction coefficient equal 0.70 can be used to assess the actual behaviour of

existing structures.The friction coefficient value can also be used as a parameter for tuning an FE analysis with

measured data.

2.5 Abrasive wear of contact surfacesThe abrasive wear of sliding contact surfaces between the steel and grout is a failure mode that

needs to be considered in the design. The rate of wear may be considered to be proportional to

the contact pressure and sliding length. The sliding length is also proportional to the alternating

bending moment and thus to the contact pressure. Based on the performed bending tests, it is

proposed to limit the nominal contact pressure to 1.2 MPa to limit the consequence of this failure

mode. Abrasive wear is due to long-term loading of a similar form as used for fatigueassessment; therefore, a load factor equal to 1.0 can be used.

It is recommended to perform testing of abrasive wear of the grout used for design of grouted

connections used in wind tower structures for documentation of long term durability.

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3 CAPACITY OF GROUTED CONNECTIONS WITH SHEAR KEYS3.1 Capacity for static loadingGrouted connections with circumferential shear keys subjected to axial static loading and

dynamic loading in one direction (or a high static loading) have been used in a number of

offshore platforms without any known negative experience.

The capacity of rather large diameter grouted connections with shear keys can be considered

documented for static axial loading. Reference is made to design equations in DNV-OS-J101

(2007) and ISO 19902 (2007).

It is considered a complex task to separate moment resistance from shear keys from that due to

ovalization (contact pressure between steel and grout). An alternative is to assume that both the

axial force and the moment have to be resisted by the shear keys. This can be performed by

calculating an equivalent load that includes the static axial force and the static moment. This can

be performed by increasing the axial load by the following stress increase factor

pRP

M

21SIF

+=

(20)

Presently, there is no finite element analysis method/programme readily available to the industry

that can be used for reliable analysis of the ultimate capacity of these connections.

3.2 Capacity for dynamic loadingThe capacity of large diameter grouted connections with shear keys has not been properly

documented for use in large diameter grouted connections subjected to axial load combined with

alternating dynamic moment loading.

Another joint industry project involving a test programme of grouted connections with shear

keys subjected to alternating dynamic loading has been proposed to assess the long-term

capacity.

ISO 19902 (2007) gives some guidance on alternating dynamic loading in jacket structures. For

structures with connections subjected to alternating dynamic loading, it should be documentedthat the axial capacity, as calculated without shear keys, will not be exceeded in more than one

direction.

For fatigue assessment the axial load can be increased due to bending moment by a stress

increase factor (for similar reasons as for static loading) if it is assumed that the shear keys

should be designed also to transfer the bending moment

pRP

M21SIF +=

(21)

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4 CAPACITY OF CONICAL SHAPED GROUTED CONNECTIONS4.1 Axial capacity4.1.1 BackgroundThe purpose of this chapter is to provide some basis for the design of conical shaped grouted

connections subjected to axial loading.

Introducing a cone angle is a way to introduce requirements as to well defined minimum

fabrication tolerances such that one can be certain that the fabrication is in agreement with the

design assumptions. By using a well defined cone, it is believed that settlements due to axial

loading can be limited.

A small cone angle is assumed such that the moment on the connection can be considered to be

transferred as compression in the grout similar to that in connections with a cylindrical shaped

connection (pile and transition piece).

A design with a small cone angle could also have been denoted a grouted connection with

defined minimum fabrication tolerances. However, a conical shaped grouted connection is

considered to be a more practical notation which explains the connections physical behaviour

with respect to axial capacity.

A cone angle in the range 1-3o can be recommended used.

4.1.2 Equation for calculating the axial capacityIt is assumed that the grout material will not transfer significant tensile stress in the hoop

direction. The grout may also crack in a radial direction when subjected to a significant bending

moment (similar to that of a cylindrical connection). One can thus assume that the main purpose

of the grout is to set up a pressure between the transition piece (sleeve) and the pile if the

transition piece tries to slide downwards relative to the pile.

For a vertical settlement equal to v there will also be a horizontal displacement equal to asshown in Figure 16

tanv

= (22)

The design should allow for a vertical settlement equal to v.

The pressure between steel and grout can be derived as

pRK

Ep

=

(23)

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++=

pg

g

TP

TP

p

p

REEt

tR

tRK

(24)

Pile Grout Sleeve

CL

v

Pile Grout Sleeve

CL

v

p

Pile Grout Sleeve

CL

v

Pile Grout Sleeve

CL

v

p

Figure 16 Conical shaped grouted connection

For equilibrium for vertical weight, Fg, ref. Figure 17:

gFPP =+ sincos (25)

Where P is the total reaction force to be transferred through the grout.

Pressure loading acting on the outside grout area is

coneAPp /= (26)

When the pressure is known, the resulting value can be calculated from equation (23), and theresulting settlement can be derived from equation (22).

The outer area in Figure 18 of the cone is obtained as

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sin22

gtpgcone LRLA += (27)

Where the parameters are defined in Figure 18.

P

Fg

P

P

Fg

P

Figure 17 Illustration of the force equilibrium from vertical weight

Lg

Rpt

Lg

Rpt

Figure 18 Sketch showing calculation of area of outside cone

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4.2 Moment capacity4.2.1 Global behaviourFor a small cone angle (lower than say 4o) the moment capacity in a grouted conical connection

is considered to be similar to that of large diameter cylindrical connections as described in

section 2.2.

A conical connection using the same amount of steel as in a cylindrical grouted connection will

show much the same flexibility with respect to a large bending moment as that of a cylindrical

section. Thus the amount of relative sliding due to a bending moment in a conical connection

will also be expected to be similar to that of a cylindrical connection. Therefore, a similar

reduction in the resistance between the steel and grout during sliding by a reduction of surface

irregularities may be expected.Even if the surface irregularities are reduced during sliding, the friction coefficient between the

steel and grout is still larger than 0.40, such that resistance against sliding in the axial direction

will still be present due to the contact pressure between the contact surfaces and the resulting

settlements are expected to be small.

4.2.2 Local capacity at the ends of a grouted connectionThe local loading at the grout ends will depend on the design.

For a small cone angle, the local behaviour of a grouted conical connection is considered to be

similar to that of large diameter cylindrical connections as described in section 2.2.2 if there is acone along the full height of the grouted section. For this geometry, there will also be some

additional compressive stresses due to the settlement of the transition piece. However, with a

friction coefficient equal to 0.40, the local pressure on a cone connection end will not be

significantly larger than that on a cylindrical connection.

The additional stresses from settlements may be avoided by using a short cylindrical section at

the termination of the grouted connections as shown in Figure 19. By using a short cylindrical

section of height in the order of one elastic length or less, one will achieve rings that are

compressed due to the pressure on the cone section such that these rings will act as softening

rings with respect to moment action too, ref. Figure 19.

If required, rings may also be softer following the principles of reduced thickness as explained insection 2.2.5.2.

A short cylindrical section at the top of the pile is proposed in some new designs. At the bottom

of the connections, some designers have used a section with reduced thickness at the region of

the grout packer. This geometry has not normally been part of the analysis as it has been

assumed that this part will not transfer significant forces.

It has been questioned if the grout will crack at transitions in geometry. However, the tests

carried out on conical connections (with a conical angle of 2.0 degrees), including a 200 mm

long cylindrical connection at the top of the cone, did not show any such cracking.

Concern about cracking of the grout as discussed above is avoided if a continuous cone is usedwithin the grouted area.

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N

Outside of

transition

piece

Inside of

pile

Pressure due

to settlement

Deflection due to

pressure from

settlement

N

Outside of

transition

piece

Inside of

pile

Pressure due

to settlement

Deflection due to

pressure from

settlement

Figure 19 Loading on cone sections with cylindrical rings at pile and transition piece ends

4.3 Torsion capacityThere will be a permanent contact pressure from the grout to the steel surfaces in a cone

connection due to the self weight of the tower above the grouted connection. This results in atorsional moment capacity derived from the following equation

)sin3

LsinLRR(Lp2M

2

2

gpt

2

ptgt

g ++= (28)

where

p = contact pressure between steel and grout due to self weight

Rpt = radius of connection on top of pile

Lg = length of grouted section

= cone angle

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The largest torsion capacity is achieved for the smaller cone angles as this geometry also

provides the largest contact pressure from the grout to the steel surface.

An additional resistance from the overall bending moment can be considered together with the

design torsional moment to assess the capacity. However, this also depends on the correlation

between different types of loads.

This means that a conical connection can transfer a significant torsional moment due to the

contact pressure between the steel and grout combined with a relevant friction coefficient. The

consequence of some rotation due to torsion in a conical shaped connection is not considered to

be significant and, based on experience from the laboratory torsion test performed in this project,

it is likely that some resistance against rotation will be achieved from surface irregularity (most

likely ovality tolerances). Thus, for conical connections one may consider using a load and

material factor equal to 1.0 when a characteristic friction coefficient = 0.40 is used for thedesign with respect to torsional moment.

4.4 Abrasive wear of contact surfacesThe abrasive wear of sliding contact surfaces between the steel and grout is a failure mode that

needs to be considered in the design in the same way as for cylindrical shaped connections, ref.

section 2.5.

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5 FATIGUE CAPACITY OF GROUTED CONNECTIONS IN AIR ANDSEAWATER ENVIRONMENTS

5.1 Design S-N curves for grout materialThe fatigue of grouted connections in wind tower structures has not been considered to be a

failure mode that has been governing for designs using S-N curves for an air environment.

Until now there has not been fatigue test data available for grout in seawater.

Tests of grout permeability with respect to water indicate that the grout behaves better than

concrete structures with respect to the ingress of water into material without cracks.

It is realised that vertical cracking of the grout will occur during ovalisation of the large diameter

connections subjected to large moments. An opening between the grout and steel will occur for

large bending moments. Thus water will likely enter the grouted connection unless special

measures are taken to avoid water ingress. This depends on the quality of the grout seal.

The fatigue capacity of concrete in water is reduced when the concrete is subjected to stress

ranges in compression. If the stress cycles are going from compression to tension, the capacity is

reduced further.

It may be difficult to avoid compressive to tensile stresses during a loading cycle in a grouted

connection in a monopile subjected to a dynamic bending moment. Therefore, a long-term

sealing of the connections should be aimed for in order to achieve a reliable connection duringservice life.

Reference is made to DNV-OS-C502 for S-N curves for the capacity of concrete in air and in

seawater. These design curves may be used for fatigue design of grout material until more

specific curves for grout material have been derived. Further tests on grout material are

recommended.

It is recommended to use the safety factor stated in DNV-OS-C502 for the fatigue limit state for

the grout material.

5.2 WatertightnessDue to some uncertainty about the long-term fatigue capacity of grout material, it might be

recommended to aim for a proper sealing of the grout in connections subjected to large dynamic

long-term loading.

The pumping of water in and out of cracks and opened gaps in the grouted connection during

dynamic cycling should be avoided as this might squeeze out loose grout particles and cracked

pieces from the connection.

This means that it would be preferred to place the grouted connections in an air environment or

alternatively reliable seals to prevent water ingress for proper long-term structural behaviour

may be developed.

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5.3 Grout sealsThe grout seal at the bottom of the connections may stop the ingress of water from below.

However, it may be questioned if the grout seals used today can show sufficient durability with

respect to wear such that they will remain tight during their service life as they are mainly

intended for temporary use. This would require further assessment and testing for documentation

of long-term functionality.

Grout seal /packer

Transition piece

Monopile

Grout seal /packer

Transition piece

Monopile

Figure 20 Sketch of a typical (temporary) grout seal/packerused in grouted connections in

monopiles

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6 FINITE ELEMENT ANALYSES OF GROUTED CONNECTIONS6.1 Requirements as to analysis programsThe finite element analysis program should as a minimum be documented for linear elastic

material and properties that represent a contact problem involving friction between the grout and

steel.

If a finite element program is used to analyse the ultimate capacity, taking the non-linearity of

the grout into account, the program should first be assessed against a relevant comparable test to

document that it provides reliable results.

6.2 Finite element modellingFor fatigue analysis of the steel structure, it is in general recommended to use 20-node

isoparametric elements according to DNV-RP-C203. It is sufficient to have one element over the

thickness of the monopile and the transition piece to represent a linear stress distribution.

However, for the region with a contact area between the grout and steel, the use of 1 st-order shell

elements in conjunction with 1st-order solid elements for the grout to steel is recommended.

Three elements are recommended used over the thickness of the grout. The aspect ratio of the

element in the other directions should be limited to 4. Thus the size of the element in the

circumferential direction should be limited to 4 times that of the thickness.

For the first 3 elements at the edge of the grout, it is recommended to use an aspect ratio equal to1. Larger elements can be used further away from the edges of the connection.

If a non-linear material model is used to analyse the ultimate capacity of grouted connections, it

is recommended to reduce the capacity of the stress strain relations used as input to the analysis

by a material factor ofm = 1.5.

6.3 Acceptance criteriaReference is made to sections 1.4.3 and 2.2.4 for acceptance criteria with respect to the ultimate

limit state.

Reference is made to section 5 for acceptance criteria with respect to the fatigue limit state.

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7 REQUIREMENT AS TO THE IN-SERVICE INSPECTION ANDMONITORING OF NEW DESIGNS

It has previously not been required to perform inspections of the grouted connections during

service life. However, recent experience shows that some in-service inspection may be useful to

ensure the reliable operation of the structures.

For new types of connections where the moments are still transferred through the grouted

connections, it is considered important to check that the grout will not crack and be lost at the top

or bottom of the connections.

For grouted connections with a conical shaped geometry, it is also recommended to check that

the amount of settlements is as expected. From the amount of settlements it may be possible to

deduce the mean friction and wear on the connection and also predict the amount of further

settlements.

It is considered to be difficult to plan inspection/monitoring for more than one year ahead. It may

be sufficient to inspect/monitor a limited number of structures if it is found that the behaviour is

as intended for all structural parts of those inspected. If this is not the case, the inspection should

be extended to a larger number of structures.

Findings from the first year of operation should form the basis for inspection/monitoring in the

second year.

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8 MITIGATION OF EXISTING STRUCTURES WITH LOW AXIALCAPACITY

8.1 Inspection for settlementAll wind turbine structures with large diameter grouted connections designed according to the

industry design practice until 2009 should be inspected for settlements of the transition piece

onto the pile or onto temporary supports.

Contact points around the circumference should be noted for further assessment.

8.2 Inspection to reveal cracking of the groutIt would be useful to achieve as much information as possible regarding the observed cracking of

grout at the top and bottom of the grouted section in order to assess the capacity based on

experience.

The most heavily loaded structures should be selected for inspection.

8.3 Inspection to reveal potential fatigue cracksThe moment loading on grouted connections in 14 different wind tower structures shows a

significant scatter. This also means that the stress ranges in the steel in the towers show a

corresponding scatter as the geometries are similar from one tower to another. The consequenceof settlements on temporary supports with respect to fatigue cracking can therefore be very

different from one wind farm to another. Thus, the consequences of settlements need to be

assessed separately for each wind farm. (However, the long-term stress range distribution for

fatigue may show different characteristics to those of the ultimate bending moments).

The end regions of brackets in temporary attachments at the transition piece are considered to be

the most critical areas with respect to fatigue crack initiation. These areas will likely attract

significantly larger stress ranges if the attachments transfer vertical forces due to dynamic

moment action as compared with non-loaded attachments that were intended in the design.

The fillet welds at the lower part of the attachments may be the most critical areas with respect to

potential fatigue cracking depending on the local design. Depending on the main loadingdirection, these connections may be subjected to a severe loading in the horizontal direction if an

opening between the transition piece and pile occur due to ovalisation. The horizontal force P

produces stresses in the weld as shown in Figure 21.

The engineering shear stress derived for the fillet weld in Figure 21 can be derived as

La

Pw

2=

(29)

i.e. the engineering shear stress is simply obtained as the force divided by weld throat area.

The parallel shear stress in Figure 21 is similarly derived as

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LaT

2// =

(30)

where the notations are illustrated in Figure 21.

The calculated combined stress range can be combined with S-N curve W3 in DNV-RP-C203

Fatigue Design of Offshore Steel Structures.

Th

t

a

L

PTh

t

a

L

P

Throatsection

Figure 21 Sketch of fillet weld connection showing stress components

8.4 Mitigation with respect to axial forceIf it is not acceptable that the forces are transferred through the temporary attachments,

mitigation has to be designed with the purpose of removing this force flow.

A mitigation that is considered feasible is the use of flexible supports as described in chapter

2.1.2. The size of the required flexible supports will depend on the actual moment acting on the

considered wind tower structure.

A mitigation should be made in such a way that the temporary attachment becomes unloaded as

assumed in the design.

8.5

Mitigation with respect to fatigue cracksEven if the force flow from loaded attachments is reduced to that of a non-loaded attachment,there will be a stress range in the axial direction of the transition piece at the position of the

attachment. Thus, it will likely be necessary to remove fatigue cracks that might have been

initiated. This may be performed by grinding out the crack into the base material at the hot spot

region. A significant grinding at hot spots can be performed without a need for rewelding.

Reference is made to Norsok N-006.

Potential fatigue cracks in the fillet weld will most likely initiate from the weld root. If the

attachment is no longer loaded, the force P in Figure 21 is removed and the fatigue crack growth

will likely stop. Thus, potential cracks in the fillet welds may be left as is after a mitigation

where the attachments again become unloaded.

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9 REFERENCES/1/ DNV-OS-J101 Design of Offshore Wind Turbine Structures. October 2007.

/2/ ISO 19902 Fixed Steel Offshore Structures. 2007.

/3/ Proposal to a Joint Industry Project on Grouted Connections. Capacity of Grouted

Connections in Wind Turbine Structures. Dated 11 November 2009. Rev. 02.

/4/ Lotsberg, I.: Background for Recommendations on Design of Grouted connections in

Offshore Wind Turbine Structures. DNV report No. 2010-0650. Rev. 01. July 2010.

/5/ Hamed, A., Serednicki, A., Lotsberg, I, Fjeldberg, A. and Ulle, H.: Testing of Grouted

Connections subjected to Static Axial Load and Dynamic Sliding. DNV Report No2010-0651.

/6/ Berg Larsen, M. Finite Element Analysis of a Grouted Conical and a Grouted

Cylindrical Connection. DNV Report Copenhagen, dated April 2010.

/7/ Harwood, R. G, Billington, C. J., Buitrago,. J., Sele, A. and Sharp, J. V.: Grouted Pile

to Sleeve connections: Design Provisions for the New ISO Standard for Offshore

Structures. OMAE 1996, ASME.

/8/ Ingebrigtsen, T., Lset, . and Nielsen, Sren, G.: Fatigue Design and Overall Safety

of Grouted Pile Sleeve Connections. Paper presented at Offshore Technology

Conference, 22, OTC Paper No 6344, Houston, Texas, 7-10 May 1990./9/ Klose, M., Faber, T., Schaumann, P. and Lochte-Holtgreven, S.: Grouted Connections

for Offshore Wind Turbines. Proceeding ISOPE 2008.

/10/ DNV-OS-C502 Offshore Concrete Structures. April 2007.

/11/ DNV-RP-C203 Fatigue Design of Offshore Steel Structures. April 2010.

/12/ Lamport, W. B., Jirsa, J.O. and Yura, J. A.: Grouted Pile-to-Sleeve Connection Tests.

OTC paper no 5485 from 1986.

/13/ Norsok N-006 Assessment of Structural Integrity for Existing Offshore Load-bearing

Structures. Revision 01, March 2009.

/14/ Lotsberg, I.: Fatigue Capacity of Load Carrying Fillet Welded Connections subjected

to Axial and Shear Loading. Journal of Offshore and Arctic Engineering 2009. Vol.

131, Iss. 4.

/15/ DNV Report 2010-3620. Testing of Conical Shaped Grouted Connections subjected to

Axial Load and Dynamic Bending Moment. January 2010.

/16 Skjolde, M.: Factors of Safety for Grouted Connection Phase II the Impact of Cyclic

Load on Connections with High h/s Values. DNV Report No. 94-3243. DNV August

1994.

/17 OMAE Paper 2011-49169 On the structural capacity of grouted connections in offshore

structures. To be presented at OMAE in Rotterdam 19-24 June 2011.

/18 Proposal to a Joint Industry Project on Generation of Test Data for Documentation of

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Design on Capacity of Grouted Connections with Shear Keys subjected to Alternating

Dynamic Loading. Dated 12 January 2011.

/19 DNV Report 2010-3369 Compressive test of grouted tubular connection. Rev 1. Dated

2011-01-28.

Liability

DNV does not guarantee the correctness of the information provided in this document.

Liability for losses, claims and liabilities related to or arising from the use of information provided in this document suffered by any Party to therelevant JIP Agreement is governed by the JIP Agreement.

For any party which is not a Party to the JIP Agreement, DNV does not assume any liability for any losses, claims and liabilities related to or

arising from the use of information provided in this document.

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