Published on 19 Jun 2023
Published on 19 Jun 2023
The IMO has set targets of a 70% reduction in vessel carbon emissions by 2050.
One of the most direct ways to lower emissions is to reduce fuel consumption, which in turn cuts bunker costs. It is common to cut fuel consumption by reducing ship speed but a far more effective alternative is to reduce fuel consumption via a well-maintained hull. A well-maintained hull will not only help in reducing emissions to protect the environment but will also save fuel costs.
Hull management is the maintenance of the ship’s hull in a smooth and foul-free state. Records show that a considerable amount of energy is required to overcome the frictional resistance between the submerged hull and water. This frictional drag impairs the ship’s speed, hence driving energy demand for propulsion and ultimately, fuel consumption.
This issue is worsened by hull roughness. Damage or decay to the hull structure causes hull roughness. This leads to uneven coating and hull plate corrosion, which affects the hull’s surface profile. The hull is most prone to such damages during docking and dry-docking, where paint and coating are most susceptible to abrasions. The growth and accumulation of marine creatures such as barnacles, algae or slime on the hull results in hull fouling.
These factors contribute to the effects of drag and diminish ship performance and energy efficiency- increasing carbon emissions.
Undertake both technical and operational measures to maintain a smooth hull surface, and ultimately reduce fuel consumption. These measures include regular hull inspections, maintenance and reducing time at anchorage to limit hull fouling.
Planned maintenance and hull cleaning
The most fundamental measure of hull roughness management is through the Planned Maintenance System (PMS). Hull maintenance includes a complete survey of the hull in dry dock twice in 5 years. Such dry-docking maintenance allows the inspection, repair, and cleaning of the underwater hull, which is not accessible during ship’s operations.
Hull cleaning is necessary in mitigating hull’s biological roughness. Proper cleaning of light slime can reduce fuel consumptions by 7% to 9%, while heavy slime cleaning reduces that up to 18%. Removal of macro fouling caused by barnacles can account for significant fuel savings of about 20% to 30%.
Time-based maintenance is insufficient given the impact hull roughness has on dynamic CII ratings. It is prudent to continuously monitor ship performance with the help of data intelligence, hence condition-based maintenance is preferred. This involves sensors and Artificial Intelligence (AI) to detect and monitor performance levels, which indicate when cleaning is required.
Performance-based systems track change in fuel consumption levels and main engine power through data collection to identify degrading hull conditions and timely maintenance. Consider the following when undertaking hull maintenance:
This system of monitoring allows effective evaluations to achieve continuous improvement.
Through such measures, operators can proactively manage structural defects and anomalies early to mitigate risks and avoid more costly repairs. These fuel savings satisfy decarbonization objectives and help in improving all CII ratings from E to D and D to C etc.
Anti-fouling paint
Applying a coat of antifouling paint is another approach in preventing hull fouling and preserving its smoothness. At present, many environmentally friendly products such as low friction coatings and metal-free antifouling coatings are available. These high-quality paints keep the hull polished with minimum fouling for a longer period.
Some advanced anti-fouling paints provide an ultra-smooth surface and can save up to 10.8% fuel as compared to a conventional anti-fouling paint. An alternative type of coating using silicone or fluoro-silicon base is also available. This paint utilizes nonstick properties to mitigate adhesion of bio-organisms through its extremely smooth surface, shedding any micro and macro growth during the ship’s motion.
Another type of coating addresses other causes of hull surface imperfections. This covers and smoothens welding seams present on the hull through coating with a special modified epoxy. This reduces turbulence and drag to obtain 2.5% fuel savings.
With the rising need for energy-savings and low emission targets, advancement in anti-fouling coating technology is bound to progress. However, the degree of fouling reduction will vary with trading pattern and operational profiles, regardless of the coating type used. It is important to ensure regular maintenance and repainting of these coatings due to damage and deterioration over time.
Apart from the technical aspects, good operational practices must be adopted to minimize hull fouling. Ships trading in warm waters and subjected to prolonged idle periods in ports or at anchorage are most vulnerable to fouling. Therefore, to reduce the risk of hull fouling, coordinated efforts between operators, charterers, and terminals are crucial to reduce the ship’s idle time. This includes providing a more accurate estimated time of arrival, timely loading / unloading and departure, and efficient berth planning.
Conduct efficient ballast operations to achieve lighter displacement while ensuring the stability of the ship and adequate immersion of the propeller. This minimizes the hull surface submerged which lowers the water resistance reducing fuel consumption by up to 7%.
In terms of hull design, key features of hydrodynamic optimization include lightweight constructions and the inclusion of the bulbous bow with other design extensions.
In newbuilds, light construction materials must be used in the manufacturing of the ship to reduce weight of the ship. A weight reduction of 20% has been seen to reduce engine requirements by approximately 5%. The addition of the bulbous bow, minimizes the interference caused by the incoming wave system to the bulb, improving the flow of water around its hull. This reduction in drag allows up to 20% less fuel consumption, significantly increasing the hull efficiency.
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