Clean the hull when resistance increases 10% - GreenSteam

Jan 07 2021

Data analysis by GreenSteam found that the best time to de-foul the hull is when the resistance increases by 10% – and some other useful insights.

Hull fouling is world shipping’s biggest preventable cause of excess fuel consumption and avoidable greenhouse gas emissions.


GreenSteam’s recent whitepaper covers three key topics, including what rules owners and operators should follow to optimise hull cleaning using vessel data, whether regularly timed hull cleaning is enough to prevent runaway growth, and the cost-benefit of hull cleaning optimisation. This article is a summary of the white paper.


The study was originally performed on behalf of a customer who wanted a better system to decide when to schedule in-water hull cleaning.


The customer suggested a strategy of cleaning the vessel’s hull when it reached a threshold frictional resistance due to fouling relative to the resistance at the time of the last re-coating (when the hull had no fouling at all).


However, this strategy would require a system for tracking the growth and acceleration of the fouling, indicating a need for heightened vigilance.


To meet this challenge, GreenSteam built a model to calculate “fouling performance”. This model is based on daily fuel waste due to fouling, which could be expressed in tonnes of fuel wasted, or its dollar cost. The model also assessed how other KPIs such as speed can compound fuel wastage due to fouling.


GreenSteam also wanted to see how hull performance improved after “in water” cleaning. This was calculated in terms of the level of fouling resistance, and the pace of fouling growth before cleaning, with a particular focus on the pace of fouling development after cleaning.


Two simple rules were developed as a result of this analysis.


The first is that the hull should be cleaned before the fouling resistance reaches 10 per cent (calculated as hull resistance with fouling compared to hull resistance with a new coating).


The second rule was that if the rate of growth of fouling resistance exceeds 10 per cent a year, the vessel owner should be in a state of heightened vigilance and carefully monitor fouling to prevent runaway growth.


Fouling growth tends to follow a common pattern, where some initial slime growth precedes more substantial plant and animal fouling, which attaches more firmly to the coating. This is followed by harder, more resilient plant and animal organisms which colonise and deeply penetrate the coating.


The study found that a 10 per cent increase in resistance due to fouling marks a transition from simple slime growth to more substantial and more firmly bonded marine organisms.


Up to this point, the coating can be cleaned with light cleaning methods with no damage to the coating. After this point, harsher cleaning methods are needed, increasing the likelihood that the coating will be damaged.


GreenSteam performed a cost-benefit analysis of different cleaning strategies, accounting for fuel wastage, cleaning rates, risk of damage to the hull, and lost revenue due to layups, and found that more frequent but gentler cleaning is the most useful strategy to keep operational and capital expenditure low.


An analysis of 50 vessels using GreenSteam’s platform showed that around half the vessels were only cleaned after this 10 per cent milestone had passed.


This may be because shipping companies associate hull cleaning with damage to the hull coating, which is not surprising if these companies typically only clean hulls at the level of fouling that requires harsh and potentially damaging cleaning methods.


To determine the specific point where the growth of marine life anchored to the hull changes, it is important to monitor the pace of change of fouling resistance. Fouling will happen faster if the vessel is stationary, and if it is in tropical, coastal waters.


Building on its first two rules, GreenSteam suggests two further rules to help the industry make decisions around hull cleaning with the bottom-line in mind.


Rule 3: The hull should be cleaned when the cost of cleaning is less than the value of the wasted fuel due to the fouling – including lost earnings during the cleaning.


Rule 4: The hull should also be cleaned before the excess fuel consumption due to fouling exceeds the “5 per cent allowance” in the charter party for fuel consumption.


The break-even point for hull cleaning does not always happen before the hull reaches 10 per cent resistance – it depends on the vessel speed. However, according to Rule 3, this means that companies are taking on additional costs for more extensive cleaning and potential coating damage.


Following these four rules, combined with automated push warning notifications, allows vessel operators to reduce fouling-derived fuel wastage by at least 40 per cent.


This is based on a simulation for 3 vessel types, a MR tanker, a handysize bulker, and a capsize bulker, over 1.4 to 3 years, with one simulation calculating accumulated bunker waste from fouling, and another simulation following these rules.


Are periodic inspections enough?

Many shipping companies might prefer to rely on hull cleaning at planned intervals or do it when they sense that the vessel’s performance is deteriorating.


However, the data suggests that simple periodic cleaning is not a sensible substitute for a monitored, condition-based approach.


GreenSteam analysed 50 vessels with a wide range of coating types and trading patterns, chosen at random from its customer base in different sectors. GreenSteam found that half of these vessels’ hulls were cleaned after the 10 per cent additional resistance threshold had been passed (Rule 1).


GreenSteam also found that a third of the 50-vessel sample passed the 10 per cent point in under 6 months, and another third took between 1.5 and 3 years to reach this point.


Some shipping companies rely on periodic inspection of the hull, getting a snapshot of the hull’s condition at a certain point in time. However, in addition to being expensive, this method does not take into consideration the cost impact of fouling before being detected by this method.


GreenSteam customers can use the GreenSteam system to get automated notifications around Rule 1, warning them when fouling goes above a certain threshold.


To prove the cost-benefit of its rules, GreenSteam also looked at financial data for the handysize bulk carriers, MR tanker, and capsize bulker analysed above. Each vessel’s hull fouled at a different rate, and operators took a different approach to timing of hull cleaning.


GreenSteam calculated the ROI for following Rule 1, using simple assumptions on time charter equivalent (TCE) rates, days out for cleaning and cost of hull cleaning.


Following Rule 1 reduced total vessel fuel consumption by 4 per cent, and even after accounting for extra cleaning costs, time out of service and GreenSteam’s fee, resulted in a payback of around $20k per vessel per year.


If the service was used on 50,000 vessels in the world fleet, the shipping industry would save $1bn a year.


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