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1
Corrosion Damage Effects on the Structural Integrity Assessment of
Offshore Structures
Sabina AGHASIBAYLI1, Prof Feargal BRENNAN2
1University of Strathclyde, Glasgow, United Kingdom, [email protected]
2University of Strathclyde, Glasgow, United Kingdom, [email protected]
Abstract
One of the biggest and most expensive challenges for the offshore wind power sector is corrosion
degradation of offshore structures. Many offshore wind foundations were installed with inadequate
corrosion protection and remedial management plans rely heavily on practices from the oil and gas
sector. These are not always appropriate given the differences in cost, damage tolerance and structural
reliability between the two sectors. Consequently, suboptimal corrosion protection and management
can result in unexpected failures that lower the design life, increase interventions and result in significant
adverse financial consequences.
This paper aims to provide a brief overview on corrosion of the offshore wind turbine foundations.
Corrosion types most commonly occurring in the offshore wind foundations and preferential locations
are discussed. The importance of in-depth corrosion study of welded structural carbon steel is
emphasized.
Keywords
Offshore wind turbine, carbon steel, S355, marine environment, artificial seawater, pitting, MIC, weld
2
Introduction
With the growing green energy demand, United Kingdom (UK) government announced a new
renewable target: by 2030 nearly third of all electricity to come from offshore wind power [1].
Thus for further development of offshore wind sector and achieving ambitious targets, it is
important to make the offshore wind sector favourable for investors and further decrease the
levelised cost of energy (LCOE).
LCOE largely depends on the capital expenditure (CAPEX), operational expenditure (OPEX)
and design life of an asset. Thus, appropriate alterations and optimisations in current
construction and installation, inspection and maintenance techniques would reduce the current
LCOE.
Majority of the offshore wind turbines are designed for operational life of 20 years with a
potential for life-extension. Naturally, it would be possible to prolong the life of the structure if
it is meeting reliability criteria and further operation of the turbine is financially feasible taking
into account additional operations and maintenance (O&M) costs. Competent design, timely
inspection and maintenance can make life-extension of the structure possible and ultimately
contribute to lowering of the LCoE.
Offshore wind turbine (OWT) foundations are subjected to aggressive marine environment and
experience enormous loads from wind and waves. The integrity of structures is at constant
threat. It requires millions of pounds yearly to control and mitigate the results of structural
degradation due to corrosion and fatigue. Consequently, corrosion degradation becomes one of
the biggest and most expensive challenges for the offshore wind power sector.
Figure 1. Industry challenges, gaps in current knowledge and consequences of the above
mentioned factors in OW industry
Unavailability of long-term data on corrosion and fatigue prevents from creating an explicit
prediction model causing over-design of structures. Many offshore wind foundations were
installed with inadequate corrosion protection and remedial management plans rely heavily on
practices from the oil and gas sector, which are not always appropriate given the differences in
3
cost, damage tolerance and structural reliability between the two sectors. Moreover, limitation
of inspections methods as well as a high cost puts the integrity of offshore structures in a big
risk.
Consequently, suboptimal corrosion protection and management can result in unexpected
failures that can pose both life and environmental threat, lower the design life, increase
interventions, negatively affect public opinion, cause energy supply problem and finally result
in significant adverse financial consequences.
This paper summarises essential information regarding corrosion aspects in offshore wind
turbine structure. Brief information is given on the following subjects:
Offshore Wind Turbines Structures (monopiles)
Corrosion of Offshore Wind structures: uniform, pitting and microbiologically
influenced corrosion (MIC)
Corrosion preferential locations and protection
Table 1. List of abbreviations and acronyms
BM Base metal
CAPEX Capital expenditure
CP Cathodic protection
DO Dissolved oxygen
HAZ Heat affected zone
HS High strength steel
LCOE Levelised cost of energy
MIC Microbial corrosion, microbiologically influenced corrosion, or microbially
induced corrosion
MP Monopile
NS Normal strength steel
O&M Operations and maintenance
OPEX Operational expenditure
OW Offshore wind
OWS Offshore wind structure
OWT Offshore wind turbine
PM Parent material
RNA Rotor nacelle assembly
SMYS Specified minimum yield strength
SRB Sulphate-reducing bacteria
WM Weld metal
WT Wind turbines
4
Literature Review
Offshore Wind Turbines Structures
Offshore wind turbines can be classified by the type of foundation: bottom-fixed and floating.
Within bottom fixed type of foundation, monopile and piled jacket foundations are the most
common types found in the offshore wind industry today (Figure 2).
Figure 2. Different types of support structures. Adapted from [2].
Figure 3. Basic components of offshore wind turbine with monopile foundation.
Conventional three-blade bottom-fixed offshore wind turbine (WT) comprises support structure
and rotor nacelle assembly (RNA). Support structure includes foundation, sub-structure
(transition piece) and tower (Figure 3). It provides structural integrity to the RNA and transfers
the aerodynamic and environmental loads acting on the structure down to the seabed. RNA
accommodates the power generating components and comprised of blades, hub and nacelle.
5
Monopile (MP) structures are favoured for its straightforward design and relatively simple and
inexpensive fabrication and installation. Subsequently, as the industry is pushing for larger
capacity turbines, deeper waters and lower LCOE, monopile structure remains a cost-effective
solution.
MP foundation is a single large diameter pile fabricated from multiple circumferentially welded
steel cans. Each can is made from cold-rolled thick carbon steel plates and longitudinally
welded.
Certainly, the choice of material is greatly affecting the degree of structural degradation of
OWT. Considering the size of turbines, the use of any corrosion resistant materials and alloys
in such quantities would be financially impractical. Currently dimensions of the monopile could
reach up to 10 m in diameter with plate thickness reaching 150 mm [3]. Thus, monopile support
structure for OWT is typically manufactured from inexpensive low alloy carbon steel grade
S355.
DNV-GL standard suggests a range of carbon steel grades with different strengths; and the
selection of material should be adequate to fulfil the requirements of a specific project [4]. The
low alloy carbon steel is divided into three categories depending on the strength of the steel:
Normal strength steel (NS) includes S235;
High strength steel (HS) contains S275 and S355;
And finally, the extra high strength steel has S420 and S460 [4].
The steel is designated according to the European standard where the letter represent the
application of the steel and the number represent specified minimum yield strength (SMYS) for
the smallest thickness in MPa [5].
The chemical compositions of S355 and the mechanical properties vary depending on the sub-
grade and the thickness of the product. The detailed chemical compositions and mechanical
properties of S355 are specified in the standard EN 10025 where the maximum allowable
carbon equivalent varies between 0.39% - 0.49% and the SMYS between 355 – 265 MPa
depending on the sub-grade and thickness [6],[7],[8],[9].
As the industry grows bigger, the use of higher strength steel types in the future is obvious.
However, it is important to note that these types of steels (SMYS>550 N/mm2) are prone to
hydrogen embrittlement, unlike S355 grade steel [10].
Corrosion of Offshore Wind Support Structure
Offshore wind turbines structures are subjected to the aggressive seawater environment,
temperature cycles, tidal fluctuations and variable cyclic load due to wave and wind impact.
Consequently, corrosion and fatigue damages are the potential causes of structural degradation
of the turbine. Untimely failures occur even with the application of corrosion protection
methods and performing regular inspections and maintenance.
Corrosion mechanism and degradation rates are greatly affected by the composition and
physical characteristics of the corrosive medium (seawater). Currently most of the offshore
wind turbines are installed in open sea or in coastal waters.
6
Natural seawater is a complex system consisting of a unique chemical combination of inorganic
and organic compounds and countless types of living organisms. Despite site-specific nature of
seawater composition, “ratios of the concentrations of the major constituents are remarkable
constant worldwide” (Error! Reference source not found.); and they “account for 99.95% of
the total solutes” [11]. Seawater is lightly alkaline with pH varying from 7.8 to 8.3, while
surface waters are usually more alkaline with pH greater than 8.
Chemical and biological profile of open seas and coastal water can significantly differ. Coastal
waters are often polluted due to human activities and become a more aggressive environment
for structures. Industrial, domestic and farming waste, and marine transport pollution introduce
heavy metal ions, nutrients, organic matter etc. in marine habitat [11]. Consequently, metal
degradation can occur through different corrosion mechanism.
Table 2. Concentrations of major constituents in seawater (35‰ salinity; 1023 kg/m3 density,
temperature 25˚C) [11]
Concentration
mmol kg-1 g kg-1
Cat
ions
Na+ 468.500 10.7700
K+ 10.210 0.3990
Mg2+ 53.080 1.2900
Ca2+ 10.280 0.4121
Sr2+ 0.090 0.0079
Anio
ns
Cl- 545.900 19.3540
Br- 0.842 0.0673
F- 0.068 0.0013
HCO3- 2.300 0.1400
SO42- 28.230 2.7120
B(OH)3 0.416 0.0257
Artificially simulated seawater is often used for corrosion testing in laboratory. Synthetic
solution is prepared from “inorganic salts in proportions and concentrations representative of
ocean water” [12]. Artificial seawater, though chemically comparable with natural seawater,
lacks any living organisms and organic agents. In some cases, organic matter and independently
cultivated microorganisms can be added separately into synthetic solution [13]. In addition, the
composition of calcareous deposits on the metal surface in synthetic seawater deviate from that
forming in natural marine environment [14]. Regardless of methodology and solution used,
artificial seawater does not fully represent the aggressive marine environment, especially for
long-term corrosion degradation.
While the use of transported natural stored and/or recirculated seawater brings the corrosion
testing a step closer to recreating the real-life conditions offshore, it is important to note that
corrosion data obtained in such solution may significantly differ from the in-situ testing [11].
With time, the characteristics of stored seawater are probable to transform, e.g. pH values,
concentration of dissolved oxygen (DO), temperature profile, living organisms. In addition,
recirculated seawater may cause continuous accumulation of rust in the system, affecting the
test results.
In a word, replicating marine environment for corrosion testing is a challenging task.
Fluctuations in seawater composition due to geographical location, depth, season, pollution etc.
even further complicates the corrosion prediction and modelling. Attention should be given to
7
corrosion data collected in various mediums, e.g. synthetic seawater, natural stored seawater
etc., before extrapolation to long-term prognosis of corrosion in offshore conditions.
There are different types of corrosion occurring on carbon steel surface in marine environment.
They are typically classified by attack mechanism or visual appearance. Most common
corrosion forms are listed below (Table 3). However, this paper is focusing on uniform, pitting
and MIC corrosion types.
Table 3. Corrosion types in OWT
Uniform corrosion Uniform dissolution of the metal in the environment (atmosphere,
seawater, soil etc.)
Pitting corrosion
Type of a localised attack when local metal dissolution leads to
formation of small cavities on the surface of the metal. In extreme
cases, pitting corrosion may cause through thickness perforation.
Microbially induced
corrosion (MIC)
Type of corrosion influenced by the products of bacterial
metabolism, e.g. hydrogen sulphide production by sulfate-reducing
bacteria (SRB). Form of attack is often pitting.
Corrosion fatigue
and cracking
Material deterioration occurs due to the effects of fatigue and
corrosion attack. Fatigue is the material degradation due to cycling
loading. Fatigue is associated with crack initiation and propagation,
preferentially from the pits acting as a stress concentration areas.
Crevice corrosion
Type of localised corrosion that occurs due to formation of
gaps/crevices. This leads to development of differential aeration
cells that trigger metal dissolution.
Galvanic corrosion Also called bimetallic corrosion. Attack occurs due to
electrochemical reaction between dissimilar metals in electrolyte.
Erosion corrosion Material thinning occurs due to abrasive action of waves, solid
particles, wind.
Currently, the most successful model describing corrosion progression is proposed by Robert
E. Melchers (University of Newcastle, Australia). The model is applicable to uniform
dissolution in terms of mass loss and localised corrosion in terms of pit depth, and it is valid for
corrosion in seawater and soils [15], [16]. Figure 4 illustrates the basic concept of the model.
Each phase is described in Table 4.
8
Figure 4. Graphical representation of the Melcher's bi-modal model [15].
Table 4. Corrosion phases in Melcher's bi-modal model [15], [17], [18].
Phase 0
Kinetic controlled
oxidation and
MIC
Short phase following near-linear function. This phase is
controlled by the kinetics of oxidation. Initiation of rust
layer and biofilm growth.
Phase 1
Concentration
controlled
oxidation
Corrosion in this phase depends on the amount of available
oxygen near the surface.
Phase 2
Diffusion
controlled
oxidation and
polarisation
As rust and biofilm starts to thicken on the surface of the
metal, the concentration of oxygen that can penetrate
through the barrier becomes restricted. Thus, the oxidation
process becomes restrained by the diffusion of oxygen. The
corrosion rate is gradually reducing.
Phase 3 Bacterial
influence
Just as the layer becomes thicker, oxygen depleted zones
start to develop inside the deposit. Corrosion rates are first
sharply increasing, due to microbiological activity (e.g.
SRB).
Phase 4 Steady state
With time, corrosion rates are gradually decreasing
establishing a steady dissolution. The function is near-
linear.
The point “ta” on the graph (Figure 4) represents a change from mainly aerobic mode (Phases
0-2) to predominantly anoxic mode (Phases 3 and 4). Corrosion progression in phases 0-2 can
take years in low temperature waters, e.g. about 3 years in North Sea. While in tropical regions
phase three starts in less than a year [19]. It is important to note, that in case of large seasonal
temperature change the corrosion rates should not be directly extrapolated for long-term
corrosion (Figure 5).
9
Figure 5. Melcher's model: corrosion loss vs time showing effect of seasonal temperature on
corrosion. [17]
The scientific term “pitting corrosion” is associated with a local rupture of metal’s passive film
in aggressive environment and formation of a pit due to metal dissolution inside, while the rest
of the surface remains free of corrosion [20]. For carbon steel, which does not form a protective
oxide layer, “pitting” is a localised corrosion attack that takes a form of a cavity on the metal
surface. These pits can be originated from pitting corrosion, MIC or erosion corrosion.
According to [18] pits initiate and propagate at different moments in time; the propagation rate
also varies from pit to pit; and pitting mechanism may differ with time due to local change of
conditions, e.g. biofilm and rust layer development, micro-environment formation, etc.
The mechanism of pitting attack is still widely uncertain. However, the fact that Melcher’s
model also works for corrosion in fresh distilled water, raises an interesting point that
degradation of metal occurs according to bi-modal behaviour regardless of availability of
microorganisms and chlorides in water. In abiotic water during oxygen-controlled mode, a non-
uniform rust layer is forming on the surface causing formation of oxygen-depleted zones within
the layer. Pitting corrosion in these anoxic regions is forming due to MnS inclusion [21]. In
case of biologically active waters, the localised corrosion is worsen by anaerobic
microorganisms flourishing in anoxic nooks [15]. Moreover, research by [22] suggests that pits
form in areas with high tensile residual stresses that act as anodic regions.
Figure 6 illustrates the growth model of pits, described in detail in [23]. Pits typically start with
growing in depth, later coalescing with neighbouring pits eventually forming wide shallower
pits (macro pitting). New pits now start to develop at the bottom of joined pits (micro-pits). The
rate of pit growth is proportional to time following the law: pit size ~ t1/3 [24]. This formation
is largely influenced by the products of bacterial metabolism in anaerobic conditions. It is
reasonable to state, that for offshore structures such “stepped bench” topography are very
dangerous when paired with fatigue degradation [25].
10
Figure 6. Proposed model of pitting growth. [23]
The significant effect of marine biofouling and bacterial activity on the severity of corrosion is
long known. Attachment of living marine organisms, such as oysters, mussels, barnacles, algae,
seaweeds etc., on the surface of immersed structures is called macro-fouling. Micro-fouling
refers to formation of slimes due to bacteriological activity. The metal degradation occurs due
to damaging effect of attachment and/or products of bacterial metabolism.
Bacteria, most commonly held responsible for corrosion of carbon steel in marine environment,
are sulphate-reducing bacteria (SRB). In oxygen depleted zones, e.g. under deposits, in anoxic
waters, SRB flourish reducing sulphur compounds (sulphates, sulphites etc.) to hydrogen
sulphide (H2S). Introduction of such aggressive compounds in the system leads to formation of
pits [26], [27].
Corrosion preferential locations and protection
The choice of corrosion protection method completely depends on the exposure area of the
structure. These areas on offshore structure can be classified into the following zones:
atmospheric, splash, tidal, submerged and buried zone (Figure 7).
Figure 7. Relative loss of metal thickness of unprotected steel on offshore wind turbine
structure in seawater. Adapted from [28].
11
Zone 1
Atmospheric corrosion
Atmospheric zone is associated with little corrosion caused by
seawater droplets containing marine salts, i.e. seawater spray.
This area is protected by coating.
Corrosion rates 0.050-0.075 mm/year [29].
Zone 2
Splash zone
Splash zone is prone to severe corrosion degradation due to
continuous wetting and drying processes and wave effect. Pits
formed in this area are typically deep [30]. The external area is
always protected by coating and corrosion allowance; internally
– by corrosion allowance and optionally by coating.
Corrosion rates 0.20-0.40 mm/year [29].
Zone 3
Tidal zone
This area as well is experiencing recurring wetting and drying
processes and wave action. It is usually protected by coating,
corrosion allowance and cathodic protection when immersed.
Corrosion rates 0.05-0.25 mm/year [29] with localised corrosion
rates up to 0.50 mm/year [31].
Zone 4
Submerged zone
The external areas of the offshore structure must be protected by
means of cathodic protection. Internally the surface should be
protected by corrosion allowance and/or cathodic protection.
Coating can be adopted if necessary.
Pits in immersed zones are usually broad and shallow with
growth rates 0.20-0.30 mm/year [30]. Uniform corrosion rates
0.10-0.20 mm/year [29].
Zone 5
Buried zone
Areas of structure buried in seabed. Uniform corrosion with low
dissolutions rates is assumed, however it has been found that this
zone is prone to localised corrosion at the mudline. Currently
there are no guidelines for protection of buried areas, though CP
may be used if adequately designed [32].
Corrosion rates of 0.06-0.10 mm/year are expected [29],
however [31] reports possible pitting rates up to 0.25 mm/year.
According to DNVGL-RP-0416 Standard, the minimum corrosion rate for submersible part is
0.10 mm/year for internal surfaces and 0.30 mm/year for external (applicable to North Sea
area). While minimum corrosion rates in warmer regions of the globe, e.g. sub-tropical and
tropical, are expected to be 0.20 mm/year for internal and 0.40 mm/year for external surfaces
[33]. Although some report corrosion values of 2.5 mm/year for carbon steels [34].
Initially it was believed that internal compartments are perfectly sealed and airtight, preventing
ingress of oxygen and aerated seawater. Consequently, all available oxygen would be
eventually consumed causing uniform metal dissolution with low corrosion rates and, at last,
the environment would become anaerobic. Thus, in earlier version of DNVGL-RP-0416
Standard no corrosion protection was applied for internal parts of the offshore wind turbine.
However practically, J-tube seals and grout connections fail and airtight compartments are
systematically accessed. Hence, internal compartments are experiencing excessive corrosion,
making earlier corrosion estimations no longer valid [35]. Moreover, during the installation,
12
foundations are often left in seawater without any corrosion protection (except corrosion
allowance) for up to 12 months. While, fatigue life cycle calculations on the design stage are
not taking into account a free-corrosion period of the structure.
Figure 8 illustrates some current problems arising in the OWT structure, areas suffering from
corrosion and types of metal degradation with each case being discussed below.
Figure 8. Types of corrosion and its preferential locations in OWT. Adapted from [36]
Splash zone Prone to severe material degradation due to continuous wetting and drying.
Always protected by coating (externally).
Waterline Uniform and localised corrosion (MIC, pitting) appears on the surface of
metal due to formation of macrogalvanic element (i.e. differential aeration
cell), especially in case of stagnant water condition. In comparison, lower
parts of the monopile usually suffer less from the corrosion.
13
Weldments Welding process alters the microstructure, introduces defects and residual
stresses in the material. Weld and heat affected zone (HAZ) are prone to
corrosion pitting and fatigue cracking. Weldments are considered weak
points of the structure and often are roots of failure.[37]
Unprotected
surface
As a result of uncoated surfaces (external and internal) and often insufficient
cathodic protection, unprotected metal is suffering from uniform and
localised attack. Free-corrosion conditions.
J-tube cable
entry
Imminently with time, the cable entry seal fails and aerated seawater enters
the structure, increasing the overall corrosion rate. The water replenishment
changes the chemical and biological environment inside the monopile. Tidal
variations may occur inside the monopile, changing the water level inside on
a regular basis. Significant increase in oxygen levels is expected at the cable
entrance.
Build-up of
gasses and
acidifying
Presence of certain bacteria, e.g. sulphate-reducing bacteria (SRB), in
seawater and seamud does not only induce the corrosion rates and cause
severe pitting, but also causes formation of hydrogen sulphide gas (H2S). The
oxygen depletion inside the monopile can trigger excessive production of
hydrogen sulphide [38]. Moreover, inadequate CP installation can cause
hydrogen (H2) gas accumulation in the internal parts of the WT putting the
whole asset in danger of explosion. Usage of sacrificial aluminium anodes
and formation H2S may promote acidification of the water inside the
monopile. To avoid that, replenishment of water and venting is required
inside the monopile. [10]
Grouted
connection
The gap between the monopile and transition piece is normally filled with
cement grout. In course of time due to constant variable loading, inadequate
grouting design, high friction, settlement of TP a number of OWT have
shown to experience grouting failure, such as cracking. This issue poses a
risk of possible ingress of oxygen and aerated seawater. Current projects have
adopted different design approach to avoid this problem [36].
Stagnant
water
Stagnant water conditions are formed in many OWT monopile foundations.
It causes development of unique environment, which prevents any adequate
predictions of the corrosion mechanism and rates to be made.
Insufficient
cathodic
protection
In the majority of projects, galvanic (sacrificial) anode based cathodic
protection is used for external and internal parts of the WT. The common
problems with CP are usually the following: erroneous current demand
calculations, current drainage, lack of experience and knowledge, distance
from anodes and coverage of a large structure, premature anode dissolution
due to harsh environment and inaccurate design.[39]
14
Mud zone Mud line zone is expected to have low general corrosion rates of ~0.015
mm/year. However, this area is at high risk of macrogalvanic element (i.e.
differential aeration cell) formation, as well as MIC due to biologically active
soil and seawater, i.e. localised corrosion [40]. Consequently, development
of pits induces the possibility of fatigue cracking development. Mudline
corrosion while posing a high risk to structural integrity has received little
attention in scientific research and industry practice. This area is almost never
inspected in real-life. [32]
Current ways of corrosion mitigation in OW industry include corrosion allowance, application
of cathodic protection (CP), coating/painting as well as regular inspection and, if possible,
monitoring [29]. Nowadays, companies become more invested in using monitoring systems,
such as SHM, as a way to replace/reduce inspection [47].
Albeit many advantages of the impressed current cathodic protection, the use of galvanic
sacrificial anodes is a most preferred choice in industry [41]. Standard practices for CP for
external parts of offshore structures are well proven by decades of experience in oil and gas
sector. The use of CP for monopile internal compartment poses a number of challenges.
Inadequate CP installation and confined space are major reasons for acidification of water
column, gases build-up, and accelerated corrosion [42]. It has been found, that replenishment
of water inside the monopile is essential for internal cathodic protection to satisfactorily
function [43].
Inspection offshore is restricted by the high cost, harsh environment, safety regulations and
limitations of current techniques. Much of OWT sections do not get an adequate attention, some
are inaccessible or require special handling. So, mudline of the structure is hardly inspected if
ever. The reliability of this section is fully relying on satisfactory service of cathodic protection.
Typical areas of inspection in offshore almost entirely focus on welds. Despite being an
essential joining technique in fabrication of offshore structure, welding process changes the
microstructure of the welded region, introduces residual stresses and various defects [44].
Welds and specifically HAZ are prone to severe deep pitting [45] [46] that act as stress raisers
and cause fatigue cracking. However, pits are hard to detect, the most effective identification
techniques today are replying on destructive testing methods, which obviously are not
applicable to functioning structures. Consequently, better understanding of pitting formation in
welds is required.
Conclusion
A concise overview of current corrosion state in offshore wind industry has been addressed in
this paper. Respectively, a major question is arising: “What can be done to improve the current
status of corrosion in OWT?”. After all, improved corrosion prediction will result in safe long-
lasting asset exploitation beyond its designed life-span. This will directly lead to reduction in
levelised cost of energy. Knowing when to expect the problem, where to search for it and what
to anticipate, will transform the current inspection schedules, ensure the reliability of asset and
overall improve the O&M. Consequently, it is important to deepen our understanding of
corrosion progression of welded structural carbon steel in seawater under fatigue load in critical
areas of the wind turbine structure.
15
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