Carl Barnes details the need for protective coatings to maintain the integrity and value of LNG carriers throughout their trading lifetimes.
Liquefied Natural Gas (LNG) is natural gas that has been temporarily converted into a liquid making it easier to store and easier to transport by ship where pipelines are not available.
A refrigeration process is used to condense natural gas into LNG by cooling it to a temperature of minus 260 degrees Fahrenheit (-162oC) and typically 610 ft3 (17 m3) of natural gas can be converted into a single cubic foot (0.02m3) of LNG.
LNG is now an area of significant focus as a replacement for bunker fuel for ships as well as a growing interest in clean energy onshore, that is fuelling this demand further.
The main LNG export countries include Qatar, Australia, Malaysia, USA, Russia, Indonesia, Algeria, Nigeria and UAE with import regions including Japan, South Korea, China, India and Europe.
Transporting LNG globally via pipelines is not practical therefore the only viable method of transporting LNG globally is via LNG carriers. The LNG is transported in specially insulated tanks aboard these purpose-built vessels.
78,000 DWT LNG carrier
The LNG carrier fleet has been growing significantly and as of 2020 the fleet surpassed 590 vessels representing approximately 48m DWT. This is an increase in vessel numbers of more than 40% from 2010 levels. With regards to orders for new vessels, as of May 2020, the global orderbook stood at a healthy 11.7m DWT, however deliveries are down significantly compared to this time last year[1].
LNG carrier vessels are high value assets and whilst the newbuilding price, based on a 170,000 m3 LNG carrier, is down from US$202m in 2011 to US$186m as of April 2020, it has been relatively steady. Large LNG carriers represents the most expensive investment for a prospective owner compared to the other major cargo carrying vessel types. As of November 2019, mid and large LNG carriers accounted for 27% of the investment at global newbuilding and 14% of the total fleet (both orderbook and delivered vessels). The total fleet was estimated at more than US$90,000 million at the end of May 2020[2]. However, recent developments have impacted investor sentiment and as a result large LNGc represent about 3% of investment by ship type as of May 2020[1].
High value assets and long lifetime expectations
With the significant investment required for an LNG carrier, the average vessel lifetime expectation can be up to 40 years. Approximately 20% of the current fleet is greater than 15 years old, with a further 7% of the fleet aged 25 years plus.
Corrosion prevention
Coatings therefore play a critical role in protecting the integrity and maintaining the value of the LNG carrier throughout its trading lifetime. With the required prolonged life time, one of the main key requirements of the coating system is to provide long term corrosion protection, particularly in areas such as the water ballast tanks which are critical to the structural integrity of the vessel and due to their complexity, are difficult and costly to repair and maintain.
Water ballast tanks – complex structures
Choosing a high performance proven anticorrosive primer is vital in maintaining the vessel value whilst also maintaining the vessel integrity and safety of operations.
Typically, a pure epoxy anticorrosive primer, pigmented with aluminium flake should be considered. The lamellar (plate like) shape of the aluminium pigmentation helps to provide increased barrier properties to the coating by reducing the rate of water and oxygen transport through the coating.
Glass flake and mica pigments will also provide these barrier properties; however, a further advantage of aluminium flake is if the coating is damaged exposing the steel substrate. The aluminium pigmentation reacts with the hydroxide ions produced at the cathode in the corrosion cell (area of damage), reducing the pH at the coating – steel interface and decreasing the rate of cathodic disbonding (under-film corrosion).
High performance anticorrosive coatings will also help control future maintenance costs as well as limiting the downtime in conducting expensive and time-consuming repairs to water ballast tank coatings that will have a significant impact on chartering and trading.
Corrosion in water ballast tanks has been found to significantly increase after 10 – 15 years in service which is particularly relevant to LNG carrier vessels with the expected prolonged service lifetime.
Fouling control at newbuilding
Fouling control requirements for LNG carrier vessels starts at the new building stage before the vessel enters service. As these are complex vessels, the fitting out period after launch can be in excess of 12 -18 months. This is significantly longer than the other main vessels types such as tankers, bulkers and container vessels, which typically will have fitting out periods of no more than three to four months.
It is during the fitting out period that the vessel will sit static alongside the quay with a significant area of the hull exposed to the high fouling challenge found in the coastal waters around major South Korean new-build shipyards, responsible for constructing the majority of large LNG carrier vessels.
The fouling control products applied to the underwater hull for fouling protection once the vessel enters service are designed to work under dynamic conditions and are not designed to provide fouling protection under the extended static conditions of the fitting out period.
The underwater hull therefore needs specific fouling protection during the fitting out period and this is in addition to the fouling control system for when the vessel enters service under dynamic conditions.
The solution chosen for the fitting out period will depend very much on the choice made for the in-service fouling control system. Essentially there are three main types of fouling control technology available for vessels in service, these detailed as follows:
Biocide free Foul Release Coatings:
These products, typically based on a silicone matrix are characterised by their low surface energy and elastomeric nature which, along with water flow when the vessel is underway, prevents and / or removes fouling.
During the extended static period at fitting out, regular underwater cleaning would be required to keep the underwater hull clean from fouling growth. As these systems are silicone-based care should be taken to ensure the cleaning methods used do not scratch and damage the coating as this will degrade the performance of the product once the vessel enters service.
Biocide containing Foul Release Coatings:
Recently, a different approach has been adopted incorporating a biocide into the foul release formulation. This approach is claimed to reduce the problems associated with slime attachment and provide improved performance under extended static periods. However, cleaning would still be required for the extended static period during fitting out.
Biocidal antifouling coatings:
A biocidal antifouling coating comprises a soluble, or partly soluble, resin system that contains a mixture of biocide(s) effective against a broad range of fouling organisms. They are designed to dissolve or ‘polish’ away over time, delivering the biocide under dynamic conditions. These products are not designed to function under extended static periods and will eventually foul during the long fitting out period encountered by LNG carrier vessels.
Unlike foul release coatings, cleaning is not really a viable option for several reasons. Cleaning will inevitably deplete the thickness of the antifouling system each time cleaning is carried out, which is undesirable as these products are applied at a specific thickness which is related to the design speed and activity of the vessel. Furthermore, as these products have no foul release properties any fouling that does attach will adhere to the hull and be significantly more difficult to remove compared to foul release systems. This is specifically relevant for barnacle fouling; after a period of time the barnacle shell will cut through the coating and eventually penetrate to the steel substrate. At this point, removal via underwater cleaning without significantly damaging the antifouling coatings, tie coat and anticorrosive coating is not possible.
Heavy barnacle fouling
Therefore, the solution is to apply an extra coat of antifouling as the final coat, specifically designed to protect against fouling during fitting out. Typically, these fitting out antifouling products are specifically designed to polish rapidly under static conditions and/or may contain biocides that specifically target barnacle fouling. The fitting out antifouling will likely be almost polished away at the end of the fitting out period, exposing the main antifouling system for when the vessel enters service.
Whilst these technologies are the main options for fouling control, new and innovative fouling control technology continues to be developed, such as a polishing system that is biocide free to other methods that include UV lights, ultrasonic methods and hull aeration. Performance under static conditions is clearly an area that would benefit from further advancements in fouling control.
In-service operational efficiency
Once the LNG carrier vessel is in active service, their typical trading routes often present a notoriously difficult fouling challenge, including static periods in high fouling locations such as west coast of Africa, Middle East, South East Asia and Australia.
The settlement of marine species on the hull of an LNG carrier vessel will result in significant economic penalties and the potential to cause environmental damage. Accumulation of biofouling on the hull leads to the following:
- An increase in hull roughness, which has a direct impact on fuel consumption and consequently the emission of air pollutants from the vessel – which the IMO has adopted regulations to address.
- Increased risk of translocating non-native, potentially invasive species.
The most significant financial penalty for the shipping industry is the increase in fuel consumption due to the adverse effects on hydrodynamic performance as shown below in Table 1.
| Hull condition | Additional shaft power to sustain speed (%) |
| Freshly applied coating | 0 |
| Deteriorated coating or thin slime | 9 |
| Heavy slime | 19 |
| Small calcareous fouling or macroalgae | 33 |
| Medium calcareous fouling | 52 |
| Heavy calcareous fouling | 84 |
Table 1: Roughness and Fouling Penalties – Adapted from Schultz, 2007[3].
Typically for a large LNG carrier burning 150 – 170 tpd of fuel and 70% activity profile, the annual fuel bill will be in the range of US$15 million – US$17 million, assuming MGO fuel price of US$ 400/t, which is currently low. Therefore, hull fouling, particularly animal fouling (calcareous) will have significant consequences on the annual fuel bill and subsequent emissions.
Therefore, along with anticorrosion protection, fouling control is a vital requirement for LNG carrier to minimise fuel consumption, reduce engine wear and ensure on time scheduled arrival at their destination port.
For these reasons careful consideration should be taken when specifying the fouling control scheme and the following factors should be considered before making any choices:
- Trading route, including typical port locations.
- Vessel activity / speed.
- Expected locations of extended static periods.
- In service period, i.e. time between dry dock (e.g. three years, five years).
- Historical fouling control product performance on similar trades / vessel types.
- The potential for hull cleaning / grooming in service.
- Hull performance expectations.
This information allows for a functional specification to be built that is specific to the individual vessel rather than typical generic industry specifications. The functional specification allows the paint suppliers to provide a product(s) that is best suited to unique requirements for fouling control required by LNG carrier vessels.
Summary and conclusions
Careful selection of the underwater hull and anticorrosive coatings for LNG carrier vessels will have a significant positive impact on the main priorities for owners / operators. Helping in controlling corrosion, protecting their high-value asset and maintaining a clean underwater hull to ensure maximum operating efficiency and subsequent emission control.
Whilst the cost for coating an LNG carrier vessel is minimal, typically 2 – 3% of the assets value, the value delivered by the coatings is clearly significant and informed product selection is vital to ensure optimal protection and performance throughout an expected 40-year lifetime.
Given the wide range of anti-corrosion and fouling prevention products and technologies available in the market, it is ever more critical to ensure that a robust and effective selection process is put in place to ensure the selected coatings are best suited to the new-build and in service needs of the vessels.
[1] Clarkson Research Services Limited (“Clarksons Research”), 2020. World Shipyard Monitor, Vol 27, No 5. ISSN:1358-8788.
[2] VesselsValue, 2020.
[3] Schultz, Michael P. (2007) Effects of coating roughness and biofouling on ship resistance and powering, Biofouling, 23:5, 331-341, DOI: 10.1080/08927010701461974
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The Author
Carl Barnes