Robotic hull cleaning: biofouling, fuel and the rules in port

Colonisation starts within minutes of immersion. Dissolved organic molecules adsorb onto the coating and form a conditioning film, bacteria settle on it, and a slime layer develops. Only after that do macro-organisms follow: barnacles, tubeworms, hydroids, algae and mussels. The two stages are technically different problems. A slime layer is a roughness problem. An established calcareous layer is a roughness problem plus a mass, adhesion and coating problem.

The mechanism is straightforward and does not need a headline number. Growth increases the hydraulic roughness of the wetted surface. Roughness thickens the turbulent boundary layer and raises frictional resistance, which on a full-bodied merchant hull is the dominant share of total resistance. Higher resistance means more delivered power for the same speed, or less speed for the same power. Fuel consumption and emissions follow directly.

What does not follow is a universal percentage. The figure of up to 40 percent additional consumption, routinely attributed to the International Maritime Organization, is not an IMO statement; it originates in a magazine article from 2009 and has been copied ever since. No official IMO percentage exists. The honest answer for an operator is that the penalty is vessel-specific and has to be measured on the vessel, through speed and power analysis, shaft power meters or hull performance monitoring against a clean-hull reference. That is also the only form of evidence that survives a discussion with a charterer. Anyone building a business case on a borrowed number is building it on sand; a defensible case starts with the operator's own performance data, which is also the basis for any cost comparison against the status quo.

The regulatory picture is frequently overstated in vendor material. The IMO biofouling guidelines are recommendations. They are not a MARPOL annex and they do not carry the binding force of the Ballast Water Management Convention.

• MEPC.207(62) from 2011 was the first set of guidelines on the control and management of ships' biofouling. It has been repealed.
• MEPC.378(80), adopted on 7 July 2023, replaced it. It is the current reference and it is voluntary. It sets out the expectation of a biofouling management plan and a biofouling record book, but as guidance rather than as a certified requirement.
• MEPC 83 in April 2025 did not adopt a binding instrument. It agreed a work item to develop one, with a target horizon around mid-2029. Any text presenting a mandatory biofouling regime as already in force is wrong.
• MEPC.1/Circ.918, issued in April 2025, is the guidance specifically addressing in-water cleaning. It is the document to read before planning a cleaning call.
• The GloFouling Partnerships project ended in May 2024. Its successor structure is the GIA for Marine Biosafety. Citing GloFouling as an ongoing programme is out of date.

The practical consequence is that compliance pressure on biofouling today comes less from IMO law than from two other directions: the port or coastal state where you want to clean, and the fuel and carbon instruments that make a rough hull expensive.

The efficiency instruments are binding, unlike the biofouling guidelines. MEPC.328(76) introduced the Energy Efficiency Existing Ship Index and the Carbon Intensity Indicator. The amendments entered into force on 1 November 2022 and apply from 1 January 2023. The CII review, phase 2, runs from spring 2026 to spring 2028, so the rating mechanics are under active revision rather than settled.

On the European side, two instruments bite directly. The EU Emissions Trading System was extended to maritime transport by Directive (EU) 2023/959, applicable from 1 January 2024. FuelEU Maritime, Regulation (EU) 2023/1805, has applied since 1 January 2025 and targets the greenhouse gas intensity of the energy used on board. Both convert avoidable resistance into a monetised cost rather than an abstract inefficiency.

What is not in force is the IMO Net-Zero Framework. The extraordinary session of 14 to 17 October 2025 closed without adoption and was adjourned to roughly October 2026. It is a proposal, not a rule, and should be described as such.

For hull husbandry this changes the decision logic. As long as fuel was the only cost driver, cleaning intervals could be set opportunistically around dry dockings. With a rating and a carbon price attached, hull condition becomes a documented parameter that has to be managed between dockings, which is precisely the interval that in-water work addresses. The same argument drives the wider move towards recurring robotic condition capture in the subsea and marine asset segment.

The single most useful document in this field is not an IMO resolution. It is the Industry Standard on In-Water Cleaning with Capture, version 1.0, published in 2021 by BIMCO and the International Chamber of Shipping, with an associated certification procedure that was updated in September 2023. Where the IMO texts describe intentions, this standard describes acceptance criteria that a port, a charterer or a class surveyor can actually check.

Its logic rests on containment. Material removed from the hull is not released into the harbour basin. It is captured at the tool, routed through a filtration and treatment stage, and the solids and biocide-loaded residues are retained for disposal ashore rather than discharged. Around that core sit the elements an operator has to verify before contracting a cleaning call:

• Capture of the removed fouling at the point of removal, rather than dispersal into the water column.
• Treatment of the collected effluent before any water is returned, covering particulates, heavy metals and biocide residues from the antifouling coating.
• Independent testing and certification of the equipment rather than a self-declaration by the cleaning contractor.
• Protection of the coating during the operation, since aggressive removal of an established layer can strip antifouling and shorten the interval to the next intervention.
• Documentation of the run that is good enough to be produced to a port authority afterwards.

The distinction matters commercially because it, not the shape of the machine, decides where a system can work. A cleaning device without capture is a maintenance tool. A cleaning device with certified capture is a tool that can be scheduled in a port that regulates the activity.

There is no EU-wide regime for in-water cleaning. The field is fragmented across national and port-level rules, and the honest description of the situation is a patchwork rather than a framework. Several claims that circulate in industry articles do not hold up when checked against primary sources, so they are worth stating negatively.

• Germany: there is no nationwide ban and no federally uniform permit requirement. Bremen has operated a permit guidance document since 17 November 2021, developed out of the DBU-funded CLEAN project. That is the concrete German reference point.
• Denmark: the rule that entered into application on 1 July 2025 concerns discharge of scrubber washwater, not hull cleaning. The two are routinely confused in trade coverage.
• Assertions about blanket regimes in other ports and coastal states are, on inspection, either secondary or contradictory. They should not be relied on when planning an operation.

The operational conclusion is unglamorous but firm. Permission is established per port, in writing, before the vessel arrives, on the basis of the specific system, its capture and treatment chain and its certification status. A general statement that in-water cleaning is allowed or forbidden in Europe is not a usable input for a voyage plan. The second reason for the caution is biosecurity: the concern behind most restrictions is the translocation of non-indigenous species on the hull, which is why release of viable material, not the cleaning itself, is what regulators target.

Most hull-cleaning robots in commercial use are crawlers that adhere to the steel shell, either magnetically or through a negative pressure generated by their own thrusters, and are operated from the surface. The cleaning head is either a set of rotating brushes or discs, or a water jet arrangement, in both cases combined with a suction path that carries the removed material away from the tool. That suction path is what makes capture possible; without it, the debris simply enters the water column.

The second design axis is the fouling stage the machine is built for. Proactive grooming addresses the slime layer at short intervals with low mechanical aggression and is intended to prevent macrofouling from establishing at all. Reactive cleaning removes an established layer, requires more energy at the surface and carries a materially higher risk to the antifouling coating. The two approaches produce different scheduling logic and different waste streams, and a system optimised for one is rarely convincing at the other.

As market examples, the field divides along the capture criterion rather than along nationality or robot format:

• ECOsubsea, Fleet Robotics (Delft, Netherlands), HullWiper and CleanSubSea operate with retention of the removed material, which is the prerequisite for working under the BIMCO and ICS standard.
• The Jotun HullSkater follows the proactive grooming route and has no capture stage. Independent water measurements around its operation recorded copper concentrations of up to 30.04 micrograms per litre against a background of 3.70 micrograms per litre. The programme is nevertheless active rather than discontinued, with a Lloyd's Register release in June 2025 and a DNV verification dated 21 May 2026.

Neither group is universally right. A vessel on a fixed liner rotation calling at ports that permit grooming has a different optimum from a tramp trader that needs a certified capture system it can book anywhere. The comparison belongs in the procurement file alongside the rest of the robot classes used on marine assets, not in a slogan.

Hull cleaning and in-water survey are separate activities that share a work site, and conflating them causes real problems in tendering. Cleaning is maintenance. Survey is a class activity that produces a certificate, and the certificate does not come from the robot.

The notations are frequently misquoted, so precision helps. DNV does not operate a notation called IWS. It has BIS and IW for bottom survey afloat, set out in DNV-RU-SHIP Pt.7 Ch.1 Sec.5. Lloyd's Register is the society with *IWS, in LR Rules Part 1, Chapter 3, Section 4. On the IACS side, Recommendation No. 42 covers remote inspection techniques, UR Z7 covers hull classification surveys, and UR Z17 governs service suppliers, with Section 3 addressing in-water survey in lieu of docking survey carried out by diver or ROV and Section 16 addressing providers of remote inspection techniques.

The commercially decisive point sits in that last reference. Market access in this segment runs through recognition as a service supplier under IACS UR Z17, not through owning a vehicle. A well-built robot without that recognition cannot produce a survey record that class will accept, while a recognised supplier can deploy several different vehicles. Anyone evaluating cleaning and inspection providers should ask for the recognition first and the datasheet second. The same distinction applies to ROV and AUV inspection work more generally: the machine delivers the record, the qualified surveyor delivers the assessment.

werob is a manufacturer-independent systems integrator for service robotics and a brand of CITO GmbH in Hamburg. It does not build hardware and does not carry out cleaning or survey operations. Its contribution in this field is the unglamorous part of the decision: sorting systems by capture and certification status, by the permit situation in the ports a vessel actually calls at, by service coverage along the trade, and by how the resulting data has to be recorded so that it is usable in a class or authority discussion later.