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THE VALUE OF HIGH STRENGTH STEELS, ENABLING LOW

CARBON ENERGY TECHNOLOGIES: OFFSHORE WIND FARMS

BY

Dr JITENDRA PATEL

Consultant, CBMM Technology Suisse S.A., Geneva, Switzerland

Director, International Metallurgy Ltd, England, UK

SYNOPSIS

As availability of sites for onshore wind power farms become gradually scarce and less

economically attractive, the offshore route is increasingly seen as the primary way to realise

low carbon energy generation targets. However, such developments bring technical and

economic challenges. One of which is the need for large quantities of steel for the tower and

foundation structure (averaging 800-1,000t/5MW), with special mechanical properties

regarding toughness (-40ºC), strength, weldability (thick plates, high heat-input) and now

fatigue. In comparison to onshore requirements, steel plates between 100 – 120mm thick are

increasingly being utilised with strengths approaching 600MPa.

The production of modern high-strength fine grained steels combine the advantages of high

strength properties with an optimum cost/performance ratio. They enable the use of

constructions with wall thickness reductions and further reduce the carbon footprint. This

article gives an overview on the production, properties and processing behaviour of high-

strength steels typically used in today’s monopole and jacketed structures. It provides an insight

to the role of micro-alloys such as niobium (Nb) as a key technology enabler in the

development of the desired steel microstructure to meet both technical and economic

challenges. The article also explores the value creation potential for the South East Asian steel

industry in the regional offshore market by the adoption of such high strength steels.

Keywords: Offshore, Wind, Steel, Strength, Niobium, Value, S355, S420, Plate.

1. OFFSHORE WIND ENERGY – SOUTH EAST ASIA

The push by several governments across the world to phase out nuclear power plants has

renewed the focus on renewable energies such as photovoltaics and offshore wind. According

to the latest statistics released by the International Renewable Energy Agency (IRENA), 2016

ended with a total of 14,081MW of offshore wind capacity in the world, of which 12,471MW

or 88.6% has been installed in Europe. Asia (including China and Japan) accounted for

1,581MW or 11.2% and from this only 41MW was accounted by others excluding China and

Japan. So, when compared to the rest of the world there is incredible growth opportunities in

South East Asia. For example, Taiwan alone plans to put into operation a highly ambitious

3,000MW of offshore wind capacity by 2025, i.e. over the next 7 years.

So, what does this mean for the steel industry? Taking the example of Taiwan, published

studies show that Taiwan possesses extraordinary advantages in developing offshore wind

farms, mainly due to the Taiwan Strait’s geography causing a channelling effect. In fact,

Taiwan’s west coast has full load-load hours potential that average 2,500 hours (above the

world average and that of most European farms). The bulk of Taiwan’s offshore wind farms

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will have to be installed in water depths from 20 to 50m. Assuming an average offshore turbine

rating of 5MW, at least 600 individual offshore towers and foundations will be required along

with supporting infrastructures such as substation (typically weighing 2,000t each). This large-

scale infrastructure development will require in the region of 800,000t of steel plate just to

cover the tower and subsea foundations.

2. OFFSHORE WIND TOWER

For offshore wind towers, the tower height will be governed by the size of the turbine generator

and the height of the rotor blades. A typical 5MW offshore turbine will have 60m long blades

requiring a tower height of at least 80m. The mass of the tower will range between 200-400t,

with almost 90% of the mass being steel plate and most of the rest flanges. The tower units are

uniformly tapered, with a top diameter of the order of 4-5m and a base of around 6m. The

design is driven by fatigue and extreme loading plus natural frequency requirements and

avoidance of buckling. The tower height is optimised for a given project with reference to

planning constraints and also by comparing additional costs for a taller tower with the

additional energy generated by accessing higher winds.

Towers are manufactured by cutting and rolling steel plate, welding to make on average 3m

“cans” then welding these to make a tower section of approximately 40m, with bolted flanges

at each end. Typically, steel plate of grade S355J2G3 M and/or NL are used at thickness

ranging from 10 to 70mm. The type of wind tower foundation selected is largely dependent on

the depth of the water and seabed conditions, but more comprehensively, the foundation

selection of a project will be influenced by the following factors:

Type of wind turbine / hub height that will be mounted onto the foundation

Soil conditions

Transportation / logistics

Material costs / manufacturing costs and limits

Installation limits with respect to crane capabilities, piling, drilling equipment etc.

Mean sea level and variations in water depths

Dynamic wind and wave loading especially with respect to fatigue

Seabed stability

Decommissioning strategy

The most common foundations types are, monopole, concrete gravity-base structures, suction

caisson/bucket, steel jackets and tripods. Traditionally, offshore jacket designs have been used

at water depths between 30m and 50m and monopiles for shallower water depths. However, in

recent years this has started to be extended to permit jacket depth options extend to towards

60m. At such depths, a larger structure is required and longer construction times extending

more than a week, which is typical for a 40m jacket structure. Additionally, the fabrication

sites would need to be located close to loading areas on the coast as transporting such structures

by road or inland waterways would be limited. Nevertheless, the greatest advantage of the

jacket when compared to the monopole is the weight. For a 5MW turbine at 20m water depth,

indicative mass for a jacket (including pin piles) and monopole (including transition piece) is

around 600t. At 30m water depth, the jacket mass is likely to be around 800t with the monopole

mass significantly higher, and at this depth it is much easier to design a stiffer jacket structure

for large turbines to meet the natural frequency requirements, giving such structures the edge

over monopoles.

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3. STEEL SPECIFICATION FOR OFFSHORE WIND TOWER

Steels applied in construction of offshore wind farms are often required to perform the

opposing goals of high safety / reliability and economical efficiencies. Due to their excellent

strength-to-weight ratio and wide availability, the family Grade of 355 structural steels are

predominantly employed in offshore structures. In the fabrication of such structures, it is the

weld that is designated the weakest part where cracks may originate and grow. Therefore,

reducing the number of welds using wider plates and lengths is often considered advantageous.

However, the market availability of very wide plate at heavy thicknesses is often limited and

even more so when normalised plates are required. Therefore, and where permissible, the use

of higher strength Grade S420 structural steel is being sought and applied increasingly to

offshore wind farms where applicable (for light-weighting), but the main stay remains offshore

grades of S355.

Steels for offshore structures are generally categorised using a shorthand notation as: (1)

Special; (2) Primary; or, (3) Secondary, depending upon their specific use as part of the overall

structure. Type 1 – special high strength steels are used for jacket legs, braces and joints,

concentration of pad-eyes / lifting trunnions, piles and pile sleeves. Type 2 – primary high

strength steels are used for mudmats, J-tubes, caissons etc. Type 3 - secondary high strength

steels used for ladders, handrails, stairs, grating etc. Table 1 below provides further details of

the steel types and quality for a typical jacket sub-structure.

Table 1: Steel types and quality for a typical jacket sub-structure

Standard Category Steel Quality Example of structural

element

EN 10225 Special /

Primary

Plates: S355G9+N/G9+M

Sections: S355G11+N/G11+M

Welded hollow sections: S355G13+N

Seamless hollow sections: S355G14+Q/G14+N

Jacket leg joints

Concentration of –pad-

eyes

Deck legs

Steel with requirements for improved deformation

properties in z-direction, EN10164 quality class

Z35

Plate: S355G9+N/G9+M+Option 13

Sections: S355G11+N/G11+M+Option 13

Welded hollow sections: S355G13+N+Option 13

Seamless hollow sections:

S355G14+Q/G14+N+Option 13

Deck legs

Jacket tubular

Pile to jacket connections

Deck tubular

Plate >25mm thick

Beams with flanges

>25mm thick

Plate: S420G1+Q/G1+M

Sections: S420G3+M

Welded hollow sections: S420G5+Q

Seamless hollow sections: S420G6+Q

Steel with requirements for improved deformation

properties in z-direction, EN10164 quality class

Z35

Plate: S420G1+Q/G1+M+Option 13

Sections: S420G3+M+Option 13

Welded hollow sections: S420G5+Q+Option 13

Seamless hollow sections: S420G6+Q+Option 13

EN 10025 Secondary S235 to S275

Ladders

Hand rails

Stairs

The most important properties are strength and toughness as defined by the steel designation.

For example, for plate grades S355G3 and G6 to G11 a minimum impact energy requirement

of 50J at -40ºC exists and for grades S420G1 to G3 a minimum impact energy requirement of

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60J at -40ºC. Whilst at first this may not be viewed as too demanding, as maximum plate

thicknesses are 150mm for S355 and 100mm, for S420 grades this can become challenging to

meet in many mills.

4. NIOBIUM METALLURGY: HIGH STRENGTH, HIGH TOUGHNESS

It has been well established that the crystalline grain structure plays a vital role in determining

such properties of steels in terms of yield strength and toughness, which manifests itself by the

ductile-brittle transition temperature. The yield strength can be expressed by the well-known

Hall-Petch relationship [1-2]:

y = i + kyd-1/2

Where y is the yield strength, i and ky are constants (independent of the grain size) and d is

the grain size (in mm). This basic equation was subsequently further extended to consider the

strengthening effect of several alloying elements. Within the literature there are many

published semi-empirical relationships for yield strength, tensile strength and impact transition

temperature. Those presented by Gladman and Pickering in the 1950’s and 60’s is still often

used today:

YS (MPa) = 53.9 + 32.3%Mn + 83.2%Si + 354%Nf + 17.4d-1/2

TS (MPa) = 294 + 27.7%Mn + 83.2%Si + 3.85% Pearlite + 7.7d-1/2

For low carbon steels the ductile-brittle transition temperature can also be expressed as a

function of the grain size [3]:

Impact Transition Temperature (ºC) = -19 + 44%Si + 700(√%Nf) + 2.2(Pearlite) – 11.5d-1/2

Where Nf and Pearlite are the respective weight percentages of free nitrogen and pearlite in the

microstructure. The one overwhelming and undisputable factor in these relationships is the

power of the final grain size. Figure 1 compares the effects of various strengthening

mechanisms of low carbon steels on the ITT produced by a 15MPa increase in strength by

various strengthening mechanisms [7].

Figure 1: Change in ITT produced by a 15MPa increase in strength

by various strengthening mechanisms [7].

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Applying the above equations, this 15MPa (YS) is achieved by refining the grain size by around

2.7µm and consequently results in a lowering of the ITT by -10ºC. So, a very minor refinement

in the final grain size can generate significant benefits. Such minor refinement can be achieved

by accelerated cooling of the as-hot rolled product and is relatively straight forward to achieve

in strip steels (typically 2-18mm thickness) and plate steels (typically 8-20mm thickness).

However, it becomes increasingly challenging to produce even this level of minor grain

refinement in thicker products by accelerated cooling alone. Therefore, to have meaningful

grain refinement in thicker (heavy) plates, >20mm, and to ensure a reasonable degree of

through thickness homogeneity in the grain size a very small addition of niobium (Nb) is

required such that thermo-mechanical controlled rolling can be undertaken.

During thermomechanical rolling, strain-induced precipitation of microalloying elements such

as niobium play a vital role in controlling the austenite microstructure. The basic principles of

Nb-metallurgy are well established and presented in several seminal publications and so will

not be repeated herein. EN10025 Grade S355 (and S420) plate steels can be made via a

traditional hot-rolled normalized (N) route, a nomalized-rolled (NR) route, or more effectively,

via a thermo-mechanically controlled processed route (TMCP). The normalized-rolled (NR)

route incorporates an element of lower temperature rolling thereby removing the need for a

normalizing heat treatment, however, it is important that the final plate has a microstructure

and properties similar to steels produced via the heat treatment route.

Table 2: Chemical composition of S355 through to S690 steel grades

Grade S355J2G3 S355G8+N S355ML 355EMZ S460N S460ML 450EMZ S690Q

Route N N TMCP TMCP N TMCP TMCP QT

Thickness 25mm 50mm 25mm 25mm 50mm 50mm 25mm 25mm

C 0.160 0.14 0.08 0.08 0.150 0.07 0.08 0.09

Si 0.410 0.20 0.35 0.35 0.40 0.25 0.29 0.41

Mn 1.36 1.47 1.53 1.45 1.50 1.55 1.24 1.42

P 0.012 0.014 0.012 0.009 0.012 0.012 0.015 0.015

S 0.007 0.004 0.005 0.003 0.004 0.004 0.002 0.002

Nb 0.030 0.025 0.025 0.023 0.040 0.040 0.020 0.035

V -- -- -- -- 0.12 0.04 0.020 0.050

Ti 0.001 0.004 0.005 0.005 0.012 0.015 0.005 0.038

Mo -- -- -- -- -- -- 0.140 ---

Ni -- -- -- 0.40 0.55 0.25 0.40 0.020

Cu -- -- -- <0.02 0.55 -- <0.02 0.012

Cr 0.010 -- -- <0.02 -- -- 0.020 0.023

CEV 0.389 0.41 0.335 0.354 0.497 0.353 0.351 0.343

Pcm 0.311 0.227 0.231 0.354 0.206 0.220 0.250

YS (MPa) 380 >355 395 431 >460 >460 478 789

UTS (MPa) 555 555 555 523 >560 >560 566 834

YS/UTS 0.685 -- 0.712 0.824 -- -- 0.845 0.946

As seen from Table 2, TMCP S355 and S420 steels have lower CEVs than the normalized steel

grade of the same yield strength and have excellent toughness behavior as illustrated in Figure

2 [8] where the brittle and fracture toughness behavior is significantly reduced by niobium (Nb)

microalloying and the use of TMCP rolling.

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Figure 2: CVN for a conventional normalised and a TMCP steel (both 350MPa) [8]

With conventional steels, due to the higher carbon content there is a risk of hydrogen induced

cracking at the heavier gauges and therefore such steels are pre-heated prior to welding.

However, due to its low carbon content TMCP rolled steels do not require as higher preheating

temperature, thus saving time and money at thicknesses. As shown in Figure 3 [8] even a S460

rolled steel does not require any preheating at 30mm, and therefore this allows for significant

cost savings both in terms of material weight and less welding consumables (i.e. less steel used)

but also time. Therefore, where permissible under the guidelines for offshore wind tower

structures, TMCP plates should be employed for all thickness limits.

Figure 3. CVN for conventional normalized steels and a TMCP steel (both 460MPa) [8]

5. ROLLING OF HEAVY PLATES

As raised earlier, for heavy offshore plates, low temperature toughness is often the Achilles

heel for many mills. The following section focuses on a key aspect of hot rolling to help achieve

this; during the high temperature roughing process the rolled slab is expected to recrystallize

completely after deformation, through the process of classical static recrystallization. However,

this is dependent on the amount of deformation (strain) and the temperature at deformation.

During hot rolling it is an accepted fact that a gradient of strain and temperature develops

20-120 -100 -80 -60 -40 -20 0

350

0

50

100

150

200

250

300

Tem perature (D eg C )

Charpy-V impact energy (J)

S355J2G 3

N b - TM C P

S355M L

1000 20 40 60 80

200

0

20

40

60

80

100

120

140

160

180

Plate Thickness (m m )

Preheating Temperature (Deg C

)

N orm alised

TM C P

C om parison of S460N vs S460M L

Q = 2 kJ/m m

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throughout the thickness and this cannot be prevented. Consequently, the situation which

presents itself is where recrystallization occurs through the slab thickness, as this will be

dependent on the temperature at a specific point and whether the critical strain necessary for

recrystallization has been exceeded. Although the rolled slab is above the non-recrystallisation

temperature, the solute microalloys will still play a role in retarding the recrystallization

process in comparison to a plain carbon-manganese (CMn) steel (along with the influence of

other solutes).

This effect is aptly demonstrated in Figure 4 below [9]. It highlights that for a CMn steel the

critical strain and thermal energy is readily achieved throughout during hot rolling to develop

a relatively uniform microstructure through the thickness. In comparison, for a CMnNb

(0.055%Nb) steel the strain is focused near the surface and quarter-thickness where the

temperatures are lower and grains do not recrystallize. As rolling continues and the temperature

drops then enough strain is accumulated to cause recrystallization even under a lower thermal

driving force. This will first occur at the surface where the accumulated strain will be the

highest. Therefore, even during rough rolling care must be exercised to ensure that the optimum

conditions are applied per pass.

Figure 4: Austenite microstructures of CMn (left) ad CMnNb

steel developed during hot rolling x200 magnification [9]

To produce heavier gauge plates the challenge clearly lies in having enough thickness in the

incoming material that will permit sufficient reductions to the final product dimensions. The

benefits of this are well established, as most producers will operate according to a set of

guidelines developed with experience. Some employ a minimum total reduction ratio of 2.4:1

or 3.2:1 (and even greater) to develop satisfactory internal quality both in terms of soundness

(no harmful porosities/voids) and of mechanical properties even in the core (especially

toughness and z-directional properties). As described earlier, the ideal process is to impart

larger strain per pass during the roughing process where complete recrystallization is expected

to occur. The primary aim is to impart strain at the core (mid-thickness) of the material. The

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efficiency of this is often described by the rolling shape factor, m, as shown in Figure 5 [10].

The factor depends on the radius of the work roll and the total reduction per pass and it

expresses the size of the compressive stress zone in the material in the roll gap. Clearly the

heavier the reduction per pass the higher will be the shape factor resulting in improved z-

directional properties. The rolling shape factor, m, can be calculated by the following equation:

m = 2 √(R(H1 – H2)) / H1 + H2

where R is the roll radius, H1 and H2 are the material stock thickness before and after the

rolling pass.

Figure 5: Influence of per-pass reduction on deformation and properties [10]

For some offshore and heavy structural plates such as platforms this becomes a key process

requirement where the percentage reduction in area in the z-direction (through thickness) is

part of the final material specification. Figure 5 highlights the benefits in employing rolling

with a high shape factor and also greater levels of total reduction [10]. The added advantage in

adopting this strategy is that it also increases the likelihood of complete recrystallization of the

austenite thereby promoting through thickness grain size homogeneity. Early studies [11] had

established that a rolling shape factor of 0.8 was necessary to ensure that local reduction at the

core was equal to the total reduction applied.

However, amongst other considerations, realistically, high shape factor rolling is dependent on

having enough thickness in the starting stock, enough roll force and torque within the rolling

stand and being able to maintain the rolled product dimensions between passes. As mentioned

in earlier, these factors are to some extent limited at many mills where the starting continuously

cast slab thickness is either 200 or 250mmm, irrespective of the final product gauge (so the

total reduction ratios will be smaller for heavier gauge end products), and mills having rolling

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load and torque limitations. Therefore, applying such heavy drafts will prove challenging, as

for example, targeting a 0.8 rolling shape factor will mean a single pass reduction of 23% for

a slab thickness of 230mm to 177mm (53mm reduction) with a roll diameter of 1,000mm.

Furthermore, most slabs will require pre-sizing to make the starting width before the actual

rolling process gets underway. Consequently, the actual slab thickness at the start of rolling is

reduced and this again limits the amount of high shape factor rolling that can be undertaken

and the amount of total reduction that can be afforded in the recrystallization temperature zone

and that below the non-recrystallisation temperature.

To help mitigate some of the limitations, and where feasible, cooling or chilling of the stock

material surface can help distribute the strain more to the core. This is shown in Figure 6 [12]

where the strain in the thickness at both mid and quarter width positions at the core is higher

than the surface and higher in comparison to a non-surface chilled slab. Alternatively, the

material stock can be allowed to be cooled and then rough-rolled using reasonable amounts of

descaling sprays to bring down the surface temperature. Nevertheless, individual pre-sizing

reduction passes can range from 2-10% and therefore the shape factor rolling can be as low as

0.2. Aside from this, the application of such light individual passes can unintentionally lead to

the development of a coarse mixed ferrite grain size in the final product which will deteriorate

the low temperature impact properties. Even if the stock material experiences repeated cycles

of recrystallization during rough rolling and sufficient reduction below the non-

recrystallisation temperature, the detrimental effects of light individual passes at the very

beginning of rolling may still be reflected in the final mechanical properties.

Figure 6: Local reduction through thickness of a model

steel slab showing influence of surface chilling [12]

The cause of this phenomenon is due to strain-induced grain boundary migration which occurs

when a light rolling pass reduction imparts less than the critical amount of strain required for

partial recrystallization. During strain-induced grain boundary migration a few coarse grains

can develop which are much larger than the initial austenite grain size. Figure 7 highlights for

a given steel composition this phenomenon, where it can be observed that at higher

temperatures the application of light rolling pass reduction will result in grain coarsening. Most

researchers agree that once these large grains are formed they will persist through the remainder

of the rolling sequence resulting in a final ferrite mixed grain structure. However, their absence

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in Nb-Ti microalloyed steels has been reported by some researchers, which indicates the

beneficial role of both Nb acting as a solute during the high temperature rough rolling and Ti

as fine TiN precipitates.

Figure 7: Recrystallized austenite grain size during pre-rolling

The early work of Tanaka et al. [13] reported that for repeated passes, the rolling pass reduction

is greater than the critical amount for partial recrystallization then the amount of material

recrystallized will increase with more rolling passes. In this case austenite grains recrystallized

in the earlier pass will remain the same whilst the unrecrystallized grains will preferentially

recrystallize. Thus, when complete recrystallization is attained the microstructure should be

refined and uniform in nature. However, this was different to other reports [14-15] which

suggested that when both recrystallized and unrecrystallized grains are present and rolled again

the imparted strain will concentrate in the softer recrystallized grains and these grains will

preferentially recrystallize. In either case, there is a critical amount of reduction (imparted

strain) required for partial and complete recrystallization [13]. If this is not feasible, then

consideration should be given to rolling at the lowest possible strain level to avoid the peak

strain resulting in the maximum abnormal grain size, as well as chilling the surface of the

material. Alternatively, the application of a dual Nb-Ti microalloyed steel has been observed

to limit if not eliminate the phenomenon. In any case the practice of steadily increasing the

reduction per pass in the roughing sequence has shown by several workers (such as Stalheim

et al., [16]) to yield better through thickness properties.

6. NORMALISED vs TMCP

A recent publication [17] evaluated low and medium carbon Nb-microallyed plate steels for

wind towers and found that to comply with the normalized rolling definition of EN 10025-2, a

medium carbon microalloyed steel should be used if the material is to be normalising heat

treated. Otherwise a low carbon Nb-microalloyed TMCP steel should be used, which affords

improved impact toughness and fracture toughness as compared to medium-carbon steels. The

paper evaluates in depth the mechanical, fracture toughness and uniaxial fatigue properties of

the types of steels that are typically available for wind tower applications. Plates ordered with

low temperature toughness often specify a requirement for “normalized rolling” (e.g. +N

delivery condition). Structural steel plates produced in the normalized rolling condition will

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require control of the hot rolling process such that the final deformation is accomplished over

a temperature range which is intended to produce a microstructure and properties equivalent to

that obtained in a normalizing heat treatment. Furthermore, steel plates supplied in the

normalized rolling condition are, by definition, required to maintain the specified mechanical

properties following a normalizing heat treatment. However, the ASTM specification does not

have this normalized heat treatment conditional. Thus, when the end user specifies the ASTM

A572/A709 Grade 50 specification (equivalent to S355), a low carbon (less than 0.10%C) steel

chemistry design is often applied.

An evaluation was made of a typical low-carbon A572/A709 Grade 50 to comply with the

normalized rolling definition of EN 10025-2 as part of the collaborative research project. It

was determined that these low-carbon HSLA steels, which achieve their excellent balance of

strength and toughness principally via ferrite grain refinement and a reduced carbon level, did

not have sufficient capability to retain the specified tensile properties after normalizing. Based

on these results as well as similar studies, it was determined that to comply with EN 10025-2

normalized rolling definition, a medium-carbon steel must be employed to ensure that the

specified minimum as-rolled mechanical properties are retained after normalizing. However,

this situation seriously deteriorates the fatigue and fracture toughness performance of higher

carbon normalized rolled and/or normalize-heat treated plate steels compared to as-rolled low

carbon Nb-microalloyed plates. Due to the normalizing heat treatment option, higher carbon

and manganese levels are necessary due to the softening effect during heat treatment and

another drawback of the higher carbon grade is increased steelmaking cost compared to a low

carbon Nb-microalloying approach, where castability and slab surface quality is significantly

improved in the form of increased casting speed and lower rejects for defects since the higher

carbon peritectic issues are eliminated.

Table 3. Mechanical property comparison [18]

Steel YS

(MPa)

UTS

(MPa)

-50oC

CVN (J)

Upper Shelf

CVN (J)

Fracture

Toughness

KIC (MPa.m1/2)

Fatigue

Endurance Limit

Se (MPa)

Low

C – Nb 448 524 366 380 412 303

Medium

C – Nb 441 565 41 163 258 269

Medium

C-Nb Normalized 393 531 108 217 275 241

7. FATIGUE AND FRACTURE TOUGHNESS IMPLICATIONS

The improved fatigue and fracture toughness properties of the as-rolled low carbon Nb-

microalloyed compared to the normalized rolled and heat treated medium carbon Nb-

microalloyed are remarkable on industrially produced heats as shown in Table 3 [18]. There

are several implications as it relates to future wind tower designs involving specifications, the

selected carbon level and steelmaking/rolling practices. Therefore, considering future ofshore

engineering challenges and design trends of taller wind tower structures and thus larger

foundation requiremensts, several specification and chemistry issues need to be addressed. For

example, based on these results the implications of specifying the EN 10025 normalized rolling

delivery condition are clear: the steel chemical composition constraints imposed by the EN

10025 normalized rolling requirement result in wind turbine tower plates with reduced

weldability, toughness measured as Charpy V-Notch and KIC. The second implication involves

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cost. The cost of steelmaking and welding in low carbon steel are reduced compared to the

medium 0.15%C steel.

8. CLOSING REMARKS

Over the last decade, the power generation of individual offshore wind towers has increased

from 2MW to >8MW and in some case now approaching 10MW. This has been through use

of larger generation units which require larger rotor blades and consequently taller towers. The

result is the topside structure is much heavier and thus requires heavier / more robust, sub-sea

foundation structures. As in some locations these structures are also exposed to extremely low

temperatures, the steels used need to be thicker and tougher. As has been well demonstrated

from other market sectors such as construction, oil and gas pipeline systems, the use of higher

strength steels (≥355MPa YS) can afford several advantages resulting in significant cost

savings. Higher strengths would allow a reduction in the tower shell thickness, which would

directly lead to lower material costs, less fabrication and welding costs as well as reduced

weight required for the foundation. Although a reduction of the weight of the topside itself is

possible, for wind turbines, maintaining the loading capacity given by the strength and safety

of the construction is of prime importance (e.g. resistance against buckling). Recent studies

[RFCS-CT-2006-00031, 2007] have investigated the use of high strength steels with minimum

yield strength of 690MPa for the tower application and found that such steels can be applied to

topside wind towers, and for the lower section of the tower. This could lead to a reduction of

nearly 65% of the original shell thickness. However, it should be noted that whilst the use of

higher strength steels for the lower tower section is feasible, construction of the entire tower

with e.g. S690 steels would also lead to a reduction in the dead weight of the tower and

consequently the Eigenfrequency of the tower (i.e. the normal modes at which the tower will

vibrate).

When considering the anticipated market size for offshore wind towers: (1) Taiwan 3,000MW

by 2025; (2) Vietnam 6,200MW by 2030; (3) Philippines 2,000MW by 2020; (4) South Korea

2,500MW by 2022; (5) Thailand 1,800MW by 2025; (6) Indonesia 500MW by 2025, the

Southeast Asia (excluding Japan and China) region could easily require a total of 20 million

tonnes of steel plate of Grades 355 over the next 10 years. Therefore, to reduce the overall steel

consumption and costs, steels with higher strengths at S420 must be considered to keep wind

power generation economically feasible. The role of Nb-microalloying in the production of

high strength steel plates (family Grade of S355) for wind towers is well established in today’s

market giving excellent mechanical properties, low temperature impact performance and

improved weldability, but with the adoption of Grade S420 type steels (also microalloyed with

niobium) to selected structural elements further 10% weight saving could be achieved resulting

in savings of 2 million tonnes for these planned installments and thereby further adding to the

green credentials of Nb-microalloyed High Strength Steels.

9. REFERENCES

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