20 | Global Contact | december 2012

offShore wInd energy

Improving wind turbine support structures

monoPiles are currenTly The mosT common foundaTion for offshore wind Turbine sTrucTures. as The diameTers and Technical knowledge increase, design sTandards have To be uPdaTed now and

Then. esPecially, seTTlemenTs in The grouTed connecTions have challenged The indusTry To come uP wiTh new soluTions and To uPdaTe The offshore winds leading indusTry sTandard. The new design

feaTures conical shaPed connecTions and now connecTions wiTh shear keys.

TexT marJoleIn roggen

The most well-known offshore wind turbine

support structures are monopile, gravity

based, jackets, and tripod structures.

A monopile a steel pile driven into the

seabed is the most widely used design.

Grouted connections are used to connect

the transition piece to the monopile.

As wind energy is a fast developing

industry, DNV KEMAs Claus Fridtjof

Christensen says, we have to ensure the

widely used standard for the design of

offshore wind turbine structures DNV-

OS-J101 is based on the most recent

technical knowledge, methodology,

experience and test results. An unintended

force transfer through the temporary

supports as a result of settlement in some

grouted connections has led to concern

about fatigue cracking in the structures

which would lead to repair needs. During

a thorough review of the standard in 2009,

we discovered that some scale effects were

not properly accounted for in this standard

or in other standards for similar types of

connections. The existing design practices

did not properly describe the physical

behaviour of the grouted connections.

Therefore, we contacted the industry

players and initiated a joint industry

project to look further into the issue

with the aim of solving an industry-wide

challenge, DNVs Inge Lotsberg adds.

The joint industry project has concluded

that a cylindrical shaped design of

grouted connections without shear keys

or additional support arrangements

for axial load is not recommended for

large diameter grouted connections.

Together, we have come up with an

improved solution. Solutions for

existing installations, including support

arrangements were also made available.

In fact, there are two solutions for new

designs. In 2011, the joint industry

project launched a recommended design

procedure for grouted connections,

recommending the use of conical shaped

connections. Later, a second phase of

the joint industry project was initiated to

assess the physical behavior of grouted

connections with shear keys.

Shear keyS Shear keys are circumferential weld

beads on the outside of the monopile and

the inside of the transition piece in the

grouted section. The shear keys purpose is

to increase the sliding resistance between

the grout and steel so that no settlement

occurs. The existing design standards for

such connections were based on limited

test data on small diameter connections,

for alternating dynamic loading.

Therefore, before this solution could

be recommended, a design practice for

shear keys is needed to be developed and

properly incorporated in a revised design

standards, says DNVs Andreas Lervik.

PhySICal teStIngWhat did we do? The collaboration

between DNV and owners, operators, grout

producers, university, certifying body and

designers has involved carrying out physical

testing in DNVs laboratory in addition to

structural analyses, field monitoring and

the sharing of experience. The laboratory

testing is intended to obtain more reliable

data for design and guidance on the use

of shear keys in monopile solutions. After

the testing a design methodology has been

developed to account for the dynamic

bending moment and the vertical force in

connections with shear keys. The results

have been compared with laboratory test

data from simulated capacity of large

diameter connections, Lotsberg says. It is

found that the design procedure provides

design data that are in good agreement

with the measured laboratory data.

The revised design standard is expected to

be launched early 2013.

Det Norske Veritas AS. All rights reserved. 2011

Transition piecewith angled surface

Monopile withangled surface

Grout

Brackets for temporary support

before grouting

Det Norske Veritas AS. All rights reserved. 2011

Transition piece

Monopile

Gaps and stresses caused by deformation in the transition piece

and monopile*

*please note that deformation and gap sizes shown have been significantly enlarged for illustration purposes only and are not true to scale to actual events.

Grout

Brackets fortemporary support

before grouting

Global Contact | december 2012 | 21

More information:

contact@dnvkema.com

T +31 26 356 35 00

www.dnvkema.com/windenergy

Grouted connections A grouted connection connects the transition

piece to the monopile. A transition piece is

placed on top of the monopile, resting on

temporary supports. During installation,

the transition piece is then jacked up before

grouting. After curing, the jacks are removed,

leaving a gap of a few centimetres between

the temporary supports and the monopile.

Settlement down to the temporary supports may

result in a different force flow in the structures.

Conical shaped connectionThe monopile and transition piece are

fabricated with a small cone angle in the

grouted section. If the bonds between the steel

and grout are broken, some slight settlement

of the transition piece will occur. This will

introduce compressive contact stresses between

the steel and grout which, together with some

friction, will provide sufficient resistance

against further settlement.

Shear keysShear keys are circumferential weld beads on

the outside of the monopile and the inside of

the transition piece in the grouted section. The

purpose is to increase the sliding resistance

between the grout and steel to avoid settlement.