stage starts almost instantly upon immersion with the formation of a conditioning
layer of dissolved organic matter such as glycoproteins and polysaccharides.
Subsequently a so-called biofilm can be formed with colonizing bacteria and
micro-algae. Hours to days later a more complex community may form including
multicellular primary producers and grazers, for instance algal spores, marine fungi
and larvae of hydroids, bryozoans, and barnacles. If time and environmental conditions allow for, such communities may evolve to diverse and sometimes very thick
layers with both hard fouling organisms (barnacles, mussels, tube worms, corals,
etc.) and large populations of soft fouling such as ascidians, hydroids and macro
algae. However, it should be explicitly mentioned that in a natural environment the
biofouling process is very variable and never follows exactly this schematic representation. The process is influenced by many abiotic factors as well, such as salinity,
nutrient content, sunlight intensity and duration, currents, and temperature.
In existing wind farms, no antifouling techniques are currently applied on the
foundations. In this situation, the uncoated steel subsea zone and the coating system
on the transition piece are both susceptible to biofouling. Especially the boat
landing area (see Fig. 5.1b) is a substructure that for safety reasons may need extra
attention with regard to fouling prevention.
Biofouling on floating foundations as well as the tether ropes should be taken into
account when assessing the lifetime of the construction. Calculations of design loads
of offshore wind turbine foundations commonly apply a maximum biofouling layer
thickness of about 200 mm for extreme load conditions. A load calculation model
would also take into account weight and hydrodynamic loading (current and wave
load) by biofouling. At first glance, a value of 200 mm of maximum biofouling layer
thickness seems sufficient. However, in order to deduce a more reliable biofouling
layer thickness depending on the location, regular checks over a twenty year period
must take place. Biofouling on tether ropes can additionally influence the hydrodynamic behavior by the increased diameter of these tether ropes.
Biofouling can pose a risk to offshore wind foundations in the following cases:
• Increased drag load. The hydrodynamic profile of a biofouling layer strongly
deviates from that of the flat surface of a foundation. Extensive growth, in the
form of long trail-like colonies of mussels, algae and other soft elongated
macro-organisms that move along with the current, may sometimes result in
unexpectedly high drag loading. Biofouling may, however not necessarily pose
a risk to the mechanical load on the foundations in moderate tidal current
conditions.
• Influence on cathodic protection. Another effect of biofouling is coverage of
anodes, which affects the function of the cathodic corrosion protection system.
For visual inspection on site (weld inspection, wall thickness measurements) a
biofouling layer must be removed.
• Influence on MIC. Biofouling creates micro-environments encouraging
microbial corrosion (MIC). Knowledge on MIC processes inside monopile
foundations is still scarce and needs further elaboration for proper assessment of
risks on failure due to pitting corrosion.
5 Technical Risks of Offshore Structures
119
layer of dissolved organic matter such as glycoproteins and polysaccharides.
Subsequently a so-called biofilm can be formed with colonizing bacteria and
micro-algae. Hours to days later a more complex community may form including
multicellular primary producers and grazers, for instance algal spores, marine fungi
and larvae of hydroids, bryozoans, and barnacles. If time and environmental conditions allow for, such communities may evolve to diverse and sometimes very thick
layers with both hard fouling organisms (barnacles, mussels, tube worms, corals,
etc.) and large populations of soft fouling such as ascidians, hydroids and macro
algae. However, it should be explicitly mentioned that in a natural environment the
biofouling process is very variable and never follows exactly this schematic representation. The process is influenced by many abiotic factors as well, such as salinity,
nutrient content, sunlight intensity and duration, currents, and temperature.
In existing wind farms, no antifouling techniques are currently applied on the
foundations. In this situation, the uncoated steel subsea zone and the coating system
on the transition piece are both susceptible to biofouling. Especially the boat
landing area (see Fig. 5.1b) is a substructure that for safety reasons may need extra
attention with regard to fouling prevention.
Biofouling on floating foundations as well as the tether ropes should be taken into
account when assessing the lifetime of the construction. Calculations of design loads
of offshore wind turbine foundations commonly apply a maximum biofouling layer
thickness of about 200 mm for extreme load conditions. A load calculation model
would also take into account weight and hydrodynamic loading (current and wave
load) by biofouling. At first glance, a value of 200 mm of maximum biofouling layer
thickness seems sufficient. However, in order to deduce a more reliable biofouling
layer thickness depending on the location, regular checks over a twenty year period
must take place. Biofouling on tether ropes can additionally influence the hydrodynamic behavior by the increased diameter of these tether ropes.
Biofouling can pose a risk to offshore wind foundations in the following cases:
• Increased drag load. The hydrodynamic profile of a biofouling layer strongly
deviates from that of the flat surface of a foundation. Extensive growth, in the
form of long trail-like colonies of mussels, algae and other soft elongated
macro-organisms that move along with the current, may sometimes result in
unexpectedly high drag loading. Biofouling may, however not necessarily pose
a risk to the mechanical load on the foundations in moderate tidal current
conditions.
• Influence on cathodic protection. Another effect of biofouling is coverage of
anodes, which affects the function of the cathodic corrosion protection system.
For visual inspection on site (weld inspection, wall thickness measurements) a
biofouling layer must be removed.
• Influence on MIC. Biofouling creates micro-environments encouraging
microbial corrosion (MIC). Knowledge on MIC processes inside monopile
foundations is still scarce and needs further elaboration for proper assessment of
risks on failure due to pitting corrosion.
5 Technical Risks of Offshore Structures
119