SURF products

Balmoral CuNiClad™ marine antifouling system

Marine biofouling is an issue which must be seriously considered when installing any structure or launching any vehicle into an aquatic environment. Biofouling may be divided into microfouling - biofilm formation and bacterial adhesion - and macrofouling - attachment of larger organisms, of which, the main culprits are barnacles, mussels, polychaete worms, bryozoans, and seaweed.

 

Microfouling

The initial process in biofouling is the formation of a ‘conditioning film’ on the substrate surface. This film formation commences immediately upon immersion. The composition of the conditioning film varies with the composition of the water and the substrate but typically develops through the absorption of organic molecules such as humic substances, polysaccharides and proteins, and ions onto the substrate surface. Subsequent colonisation of the conditioning film by bacteria, fungi and microalgae to form the initial biofilm occurs within minutes, and is rapidly followed by the attachment of macroalgae and protozoa. Microbial cell adhesion follows the attachment of these microorganisms. These microbes produce proteins, glycoproteins and lipids, polysaccharides and other so-called ‘extracellular polymeric substances’ (EPS) to produce a matrix which binds together the microbes and other microfouling species. The EPS provides a substrate and nutrients for diatoms and algae which in turn facilitate the settlement of other fouling species.

Macrofouling

The presence of the EPS microbial film promotes the settlement and survival of drifting invertebrate planktonic larvae and early life forms of macro-organisms such as polychaetes, hydroids, molluscs and barnacles. The ‘hard fouling’ filter feeders form dense colonies with high growth rates and in appropriate environmental conditions, cause a particularly severe fouling problem.

The biological nature and degree of marine fouling is strongly influenced by environmental factors such as temperature, salinity, light levels and nutrient levels, as well as geographical location. In most marine service environments, ‘hard fouling’ in significant thicknesses only extends down to 30-50msw, with ‘soft fouling’ in progressively reducing thicknesses thereafter, down to typically around 100msw. There are however a significant number of locations worldwide where ‘hard fouling’, specifically by deepwater corals, is known to flourish down to extreme depths. The primary species, Lophelia pertusa has a main depth distribution band of 200–1000msw, particularly 200-400msw, although the species has been identified as deep as 3000msw. As Lophelia corallites grow 5-10mm pa, major accumulations can accumulate on marine installations during project lifetimes.

Accumulation of fouling organisms leads to increases in submerged weight but, far more importantly, such fouling significantly affects the hydrodynamic performance of subsea structures and equipment.

A range of coating systems and technologies has been developed to prevent marine biofouling to susceptible structures and equipment.

 

Recovered distributed buoyancy module fouled with Lophelia pertusa
Recovered distributed buoyancy module fouled with Lophelia pertusa

 

Types of marine antifouling coatings

Marine antifouling systems are of three generic types:

  1. Biocide release systems that are based on dissolution or hydrolysis of the binder. These systems release the biologically-active ingredient at a slow rate, the rate depending upon the identity of the binder system and the local environmental conditions. These are relatively new systems, with a somewhat restricted track record however the performance is often outstanding, providing long term resistance to all types of fouling organisms. Some of these treatments are now specified as being suitable for up to and potentially beyond 20 years untended service, however field evidence is not yet available to support these claims.
  2. External coatings with ‘low surface energy’ characteristics which prevent strong fouling adherence. When newly applied, the surface is so ‘slick’ that any initial adherence of fouling species is readily removed by the hydrodynamic drag resulting from vessel movement. However, there is no track record to support the use of such systems in permanent (eg 20+ years’) service and it would appear likely that general wear and tear of the thin, 150mic DFT, fouling resistant coating from continual fouling detachment will gradually degrade the surface coating. It is therefore standard practice for vessels that are antifouling-treated with these materials to be dry-docked and the ‘old’ coating removed and replaced at two year intervals with a maximum of 4-5 year periods being acceptable in some cases. Importantly, for effective operation the coating needs significant water movement across the surface (>2.5m/s) to sweep away initial deposits. Where this surface water velocity is not routinely achieved, as would be the case with tethered systems such as risers, periodic cleaning by water jetting or brushing using a ROV is essential. It should be noted that this cleaning operation may in itself degrade subsequent antifouling performance and coating longevity.
  3. Inherently toxic surfaces which inhibit the deposition or growth of the EPS pre-film essential for the deposition of all macro-fouling species. Standard systems of this type are based on copper metal or alloys which are inherently toxic to marine fouling organisms and are subject to ultra-slow dissolution into seawater; these properties deliver outstanding long term marine antifouling service.
    The primary antifouling systems using this technology are solid 90:10 CuNi alloy sheeting bonded to Neoprene-coated steelwork and systems using CuNi granules embedded in Neoprene sheet cold-bonded or vulcanised onto a neoprene coated substrate. Both CuNi sheeting and CuNi granule systems have massive track record, in many cases going back 25 years, for splashzone antifouling protection of platform jacket steelwork. Neither is suitable for use on complex shapes such as buoyancy modules nor on moulded polymer products such as VIV mitigation strakes.

 

Distributed buoyancy modules with anti-fouling surface coating
Balmoral distributed buoyancy modules with anti-fouling surface coating

 

Balmoral CuNiClad marine antifouling system

Whilst copper-nickel alloy systems are unique in their combination of outstanding antifouling performance and extended service track record, neither of the current standard systems are suitable for use on complex shapes, due to their flat sheet form.

Like existing CuNi-based systems, Balmoral CuNiClad is based upon the use of marine grade 90:10 copper-nickel alloy, with the major advantage of being a spray-applied system. It is therefore ideally suited to application to complex shapes. The entire surface is coated with discrete CuNi granules so that there are no gaps in the protection onto which fouling can accumulate. As each granule is supported on, and in, a polyurethane matrix, despite the nobility of copper metal there is no possibility of galvanic corrosion of underlying steelwork and, additionally, no electrical continuity across the coated surface which, if applied to a CP-protected substrate, could otherwise interfere with the anti-fouling properties of the copper alloy.

Balmoral CuNiClad is cold-applied in a two-stage process, with the stages taking place sequentially. The application process is insensitive to local environmental conditions, with application temperatures across the 5-50°C range and relative humidities up to 90% being acceptable. A specially developed single pack/moisture-cured polyurethane binder is typically sprayed onto the clean substrate surface (roller and brush application are also possible) to a nominal 350 micron wet film thickness and the CuniClad copper nickel granules are then immediately applied in a low pressure spraying process to give a dense and tightly-bonded CuNi granule layer 150-200 mic thick in a final dry film thickness for the coating of nominal 350 microns.

Excess CuNi granules are applied to ensure the entire area of the PU primer is completely filled with granules; excess granules are recovered for later re-use. The entire coating process has minimal safety and environmental implications, with standard personal protective equipment such as respirators and hoods providing complete operator protection. The coating system is fully cured and ready for service 24 hours after application.

When first applied, the Balmoral CuNiClad antifouling system demonstrates the matt brown colour of the CuNi granules, however the coating will progressively turn green within weeks of immersion. The green colour may darken to near-black depending upon localised marine conditions.

 

 

SEM cross section of CuNiClad coating
SEM cross section of CuNiClad coating

 

SEM surface view of CuNiClad coating
SEM surface view of CuNiClad coating

Service life of Balmoral CuNiClad

The erosion/corrosion rate of 90:10 copper nickel alloy in seawater has been extensively studied since the introduction of the alloy over 50 years ago.

The erosion/corrosion resistance results from the formation of a thin protective film primarily of cuprous oxide but also containing nickel metal, iron oxide, cuprous hydroxychloride and cupric oxide. Film formation commences within days of immersion but may take 2-3 months to fully mature. This film formation has been monitored through its massive inhibiting effect on copper release rate. The copper content of the seawater effluent from a condenser with 90:10 CuNi piping was monitored for three months after startup. The initial release rate was observed to decline by 90% within 10 minutes and 99% within one hour. After three months, the concentration in the water flow was essentially identical to the background level of copper in the incoming seawater.

 

Seawater corrosion of copper from 90:10 CuNi piping

Seawater corrosion of copper from 90:10 CuNi piping

 

Once a good protective layer has formed, the erosion/corrosion rate will continue to decrease over a period of several years and to exhibit the classical parabolic protection growth rate of protective layers. For this reason, it is impossible to predict the service life of copper-nickel alloy systems based on short term testing. For ‘worst case’ calculations, corrosion rates of 2-20 microns per year are assumed, however in the definitive 14 year study at LaQue Corrosion Services, the corrosion rate of 90:10 CuNi alloy stabilised after six years at 1.3 microns per year.

 

Corrosion rate over time for 90-10 Copper-Nickel (in quiet, flowing (0.6m/s) and tidal sea water)

Corrosion rate over time for 90-10 Copper-Nickel (in quiet, flowing (0.6m/s) and tidal sea water)

 

The erosion/corrosion rate of the copper-nickel alloy remains low with increasing seawater velocity, due to the resilience of the protective film, with velocities of 3m/sec having minimal effect(6). For copper-nickel alloys used on vessel hulls, velocities up to 12m/sec have been shown to give minimal erosion(7). The copper-nickel thickness in Balmoral CuNiClad is nominal 175 microns. Even allowing for the reduced volume fill of copper-nickel in CuNiClad vs. solid alloy, the service life of the Balmoral CuNiClad system in typical seawater velocities of <3m/sec is anticipated to easily exceed 30 years.

 

 

Performance in field trials

The antifouling performance of Balmoral CuNiClad has been monitored in marine field trials.

By the Institute of Marine Science, University of Portsmouth, on a trials raft in Langstone harbour, UK. The CuNiClad was applied onto orange and yellow LLDPE sheets.

Orange sheet samples before immersion
Orange sheet samples before immersion

 

Untreated orange and yellow sheets after one year
Untreated orange and yellow sheets after one year

 

Half-treated sheets after one year
Half-treated sheets after one year

 

Fully-treated sheets after one year
Fully-treated sheets after one year

 

Environmental conditions at the test site were as follows:

 

Mean daily water temp (°C) Langstone Harbour 5 year period 2009-2013

Mean daily water temp (°C) Langstone Harbour 5 year period 2009-2013

 

At Weymouth harbour under the supervision of the Port Authority. After two years of immersion, the antifouling performance of PU sheets was as below (CuNiClad on left).

 

Fully-treated sheets after one year
Fully-treated sheets after one year

 

Coating adhesion

The standard substrates used in marine polymer products are rotationally moulded polyethylene (buoyancy module shells) and cast polyether polyurethane (pipe/cable protection products and buoyancy module coatings).

Adhesion testing has been performed on sheet samples of each substrate.

 

Substrate
Test
Standard
Result
Comment
Polyethylene
X-Cut
ISO 16276-2:2007
Level 0 (no coating removal)
Pass
Polyethylene
Dolly pluck adhesion
In-house
Av. 4.4MPa (Range 3.2-5.8MPa)
Adhesive failure- binder to substrate
Polyethylene
X-Cut
ISO 16276-2:2007
Level 0 (no coating removal)
Pass
Polyethylene
Dolly pluck adhesion
In-house
Av. 2.7MPa (Range 1.8-3.1MPa)
Adhesive failure- binder to substrate

 

All of the dolly pluck adhesion test failures for both the polyethylene and polyurethane substrates were adhesive between the binder and the substrate. In order to determine the bond strength between CuNi granules and binder, the CuNiClad system was applied directly onto blasted steel and the dolly pluck adhesion tests repeated.

 

Substrate
Test
Standard
Result
Comment
Blasted carbon steel
Dolly pluck adhesion
In-house
Av. 11.8MPa (8 tests)
(Range 10.0-14.0MPa)
Part adhesive, part cohesive failures