A critical requirement for LNG carriers is to arrive at their destination on time. Loss of speed due to hull and/or propeller fouling can result in not only delayed arrival times, but also increased engine wear and fuel consumption. This is where Foul Release Coatings starts to shine.
Fouling control is thus a vital part of vessel operating efficiency, but this has traditionally proved difficult on LNG vessels with even the best biocide containing antifoulings succumbing to severe fouling attack.
This article outlines a new solution to this problem. Revolutionary Foul Release coatings, which rely on high vessel speed and activity for their efficacy, are proving themselves to be the ideal solution for LNG hull and propeller fouling control. Not only can they can keep hulls and propellers smooth and free of macro-fouling for extended in-service periods of up to 60 months, but in addition, since they do not use biocides to control fouling, they can be an integral part of an LNG environmental management plan.
Late arrival of an LNG ship at its destination can result in major inefficiencies to the overall operation. “Tide and time wait for no man” is a well-known saying and missing the tide at some LNG berths can result in lengthy delays.
A primary cause of late arrival is through loss of speed due to fouling, either on the main hull or on the propeller, or on both. Once macro-fouling (weed or animal) becomes established on a hull it can reduce speeds by more than 1 knot, or increase fuel consumption by more than 10 %, as a result of the extra drag caused by the fouling. If fuel consumption is increased it not only results in extra cost but also increased greenhouse gas emissions. On LNG ships the main form of fouling experienced on the vertical sides is the green weed Ulva, commonly referred to as “marine grass”.
Analysis of the performance of different biocidal antifouling types (both with and without Tributyltin biocide), on a range of vessels, shows that of all vessel types it is LNG ships that suffer the greatest fouling challenge. Why this should be the case is not fully understood, although it has been speculated that the use of the “dump condenser” by steam turbine ships when in port may result in higher sea water temperatures around the hull when stationary, and thus higher fouling growth will result.
Foul Release coatings were introduced in 1999 as a novel alternative for control of fouling on fast (> 15 knots) and active ships, such as LNG’s. These coatings do not rely on biocides to control fouling but rather have a “non-stick” low energy surface to which fouling has great difficulty adhering. LNG ships, being both fast and active, have an ideal operational profile to maximise the efficacy of Foul Release coatings, and this article gives an overview of the results to date on the performance of this revolutionary new technology on LNG vessels.
Fouling Control on LNG Ships
On LNG ships the most commonly used antifoulings are the high performance Self-Polishing Copolymer (SPC) types. These are based on synthetic acrylic polymers that undergo a chemical reaction with sea-water (“hydrolysis”) at the surface. This results in a slow and steady dissolution (“polishing”) of the paint, thus providing controlled release of the biocides contained in the paint formulation, for up to 60 months in-service life. In addition, water flow over the hull results in a smoothing process whereby peaks of roughness are polished away preferentially, thus minimizing drag and maximising fuel efficiency.
Until 2003 SPC antifoulings were predominantly based on Tributyltin (TBT) acrylic polymers. Since 2003, as a result of the IMO “International Convention on the Control of Harmful Anti-fouling Systems on Ships”, TBT SPC systems have been phased out. They have mainly been replaced by acrylic systems based on either copper acrylate, zinc acrylate or silyl acrylate copolymers.
On LNG ships however the performance of both the TBT SPC and TBT-free SPC antifouling systems has not been entirely satisfactory, compared to the performance of the same paints on other scheduled ship types. Table 1 compares the performance of TBT SPC and Copper Acrylate (TBT-free) SPC technologies on a range of different scheduled ship types, using International’s “Dataplan” performance rating system.
“Dataplan” records the application and performance of coatings systems on over 30,000 vessels currently in service. At each docking, vessel details, including dry- docking interval and coating performance, are recorded. Antifouling coatings are assessed for the presence of fouling (type, severity and extent) and from this unique database, a Fouling Rating (on a scale from 0 to 100, where 0 = completely clean and 100 = totally fouled) for each particular vessel type can be calculated. Ships that dock with a Dataplan Fouling Rating of more than 10 are deemed to be unsatisfactory. A Fouling Rating of less than 10 generally indicates that only micro-fouling (slime) is present, with no significant quantity of macro-fouling (weed, animal).
Table 1 shows the % of vessels docking in an unsatisfactory condition for the two main SPC technology types:
|Table 1. Average % Unsatisfactory % of vessels with a Dataplan Fouling Rating > 10x vs SPC Type
|Copper Acrylate SPC
It can be seen that LNG vessels have a much higher failure rate than the other ship type, with 50 % or more of the LNG ships docking with a Dataplan fouling rating > 10, for both TBT and TBT-free SPC antifoulings.
This being the case, a better way to overcome the severe fouling challenge on LNG ships is required, and the revolutionary new Foul Release coatings have provided this.
Foul Release Coatings
Foul Release technology was first discovered in the early 1970’s but it is only in recent years that commercial products based on the technology have been used on Deep Sea vessels. Most commercially available Foul Release coatings use a PDMS (polydimethylsiloxane) cross-linked polymer as the main film forming material. This gives a very smooth, low energy surface, which is elastomeric (rubbery) in nature. Studies have shown that it is a combination of both low surface energy and elastic modulus that imparts the fouling release properties.
Towing tank and boundary layer studies have demonstrated that the increased smoothness of Foul Release coatings compared to biocidal antifoulings can give reduced hydrodynamic drag, in non-fouling fresh water conditions. Once immersed in seawater however this reduced drag is generally cancelled out by the ubiquitous slime fouling that always forms on Foul Release coatings. This thin slime layer cannot be removed by increasing speed since it exists within the viscous sub-layer of the turbulent boundary layer.
Over 60 full ship applications of the Foul Release coating “Intersleek 700” have been carried out on Deep Sea ships, including 11 LNG full ships. Of these LNG ships 4 have now re-docked, with only light slime fouling in each case, and no macro-fouling at all, giving a Dataplan Fouling Rating of 0, as shown in Table 2.
|Table 2. LNG ships re-docking with Intersleek 700
Picture 1 shows the typical in-docking condition of one of these ships, after low pressure fresh water washing has commenced on the vertical sides. Two areas can be seen where the light slime fouling has been washed off revealing the original glossy surface, ready for the next in-service period without any further work necessary.
In addition to providing outstanding control of fouling, Foul Release coatings also have a number of other significant environmental and operational advantages for LNG ships, compared to biocidal antifoulings, as follows:
- Durable and long lasting;
- No change in colour over time;
- Reduced paint usage/less waste;
- Lower M&R costs at future drydockings:
- Easy and quick to wash down;
- Wash water not treated as toxic waste;
- Only 1 coat re-coat required after 60 months;
- Draft marks do not generally need re-painting;
- Shorter drydock time.
- Higher solids/Lower solvent content (VOC).
- No biocides released into the sea.
The environmental benefits above can form an integral part of improving environmental compliance and competitiveness, and the operational benefits can result in significant savings at future maintenance and repair dockings.
The fouling of propellers can cause significant loss of power or speed. Despite this fact, some ship operators only clean propellers at routine maintenance drydockings. Others carry out regular in-water cleaning at ~ 6 to 9 month intervals, using divers, since they see a noticeable performance improvement each time.
The use of biocidal antifouling coatings on propellers has generally not been found to be successful since they do not last long enough under the high shear conditions. By contrast Foul Release coatings work best under high shear conditions, and the first successful trial on a propeller was reported in 1993. Since then the Foul Release coating Intersleek 700 has been applied to the propellers of over 80 large ships, including LNG’s.
Studies have been recently carried out at Newcastle University to examine the effect on performance of coating a propeller with Intersleek 700. Speed trials using the University Research vessel “Bernicia” showed that coating the propeller with Intersleek 700 did not of itself give any performance enhancement compared to the uncoated clean and smooth condition (Rubert Scale “A”). However the trial did show that the propeller was kept totally free of all macro-fouling throughout the year, where previously it had become extensively covered in tubeworms and barnacles.
This ability to prevent fouling on propellers has been confirmed from the Deep Sea ship applications carried out. Around the edges of the blades, where cavitation occurs, the complete coating system is removed, down to the underlying bronze surface. This effect is generally worst on the suction (forward) side, but no fouling can grow in the extreme conditions in these areas. For the remainder of the propeller, where the coating system is not removed, the Intersleek 700 remains fully intact and without any fouling. Thus the propulsive efficiency of the propeller is maintained, and there is no requirement for in-water cleaning throughout the in-service period.
Foul Release coatings on the underwater hulls and propellers of LNG ships are a revolutionary new way to improve efficiency. They not only give improved environmental compliance, since they do not use biocides to prevent fouling, but they have also been shown to provide better antifouling protection than SPC antifoulings. This results in increased fuel efficiency and reduced maintenance costs. Foul Release coatings are also very long lasting, and do not need to be continually re-painted at every dry-docking. They can also keep fouling off propellers, thus maintaining propulsive efficiency and removing the need to carry out in-water cleaning during the in-service period.