Tanker Biofouling Management Guide
Overview
Understanding tanker biofouling
Oil tankers carry crude oil and refined petroleum products in bulk. Crude tankers move unrefined crude from extraction points to refineries; product tankers, generally smaller, carry refined products to consuming markets.
Classes follow deadweight tonnage (DWT). Ultra large crude carriers (ULCC, above 320,000 DWT) are now mostly floating storage; very large crude carriers (VLCC, 200,000–320,000 DWT) carry 2 million barrels from the Middle East to Asia. Suezmax (120,000–200,000 DWT) and Aframax (80,000–120,000 DWT) run Mediterranean, North Sea, and Caribbean trades, above the Long Range, Medium Range, and General Purpose product classes.
Tankers run at 12 to 15 knots and slow steam routinely. Non-biocidal foul-release coatings depend on hydrodynamic shear and are effective across roughly 15 and above, according to the US EPA review of underwater ship husbandry, so much of the fleet operates below the speed at which these coatings work as designed.
Tankers also spend long periods stationary: ballast waiting at anchorage, demurrage at congested terminals, and floating storage during oversupply. AIS data for ships over 500 GT, analysed by UMAS and UCL in 2024, found commercial vessels spend 15 to 22 days per year at anchor; floating storage can add weeks or months in warm water.
Marine biofilm, the first stage of biofouling, forms on all submerged surfaces within hours of immersion regardless of coating or speed, as described in a 2022 review in Nature Reviews Microbiology. According to the 2024 GESAMP Working Group 44 report on marine biofouling, a thin biofilm covering 50 per cent of a hull can increase fuel consumption by approximately 25 per cent; heavy macrofouling can raise it by up to 50 per cent.
Given the very large wetted surface areas of VLCC and Suezmax hulls, the absolute fuel and emissions penalties from tanker fouling are among the largest of any vessel type .Under the Carbon Intensity Indicator (CII) framework, hull condition is a significant variable in a vessel's rating.
Crude flows run from the Arabian Gulf through the Strait of Hormuz (around 20.9 million barrels per day) and across the Indian Ocean to Asia via the Strait of Malacca (around 23.2 million), warm, biologically productive waters that sustain year-round fouling.
A growing share of the ageing crude fleet operates as the so-called shadow fleet in sanctioned trades, with deferred maintenance, extended drydocking intervals, and limited access to class-approved coating and cleaning services, so fouling and its fuel, emissions, and biosecurity consequences run higher.
Global fleet
The tanker fleet at a glance
The global tanker fleet is among the largest merchant segments by deadweight tonnage, carrying approximately 2 billion tonnes of oil per year.
Ownership is fragmented: large independent owners, oil-major fleets, state-linked national oil companies, and, at the older end, the sanctioned shadow fleet.
Crude tanker ordering reached record levels in early 2026, driven by VLCC and Suezmax newbuild investment.
Active tankers (oil, crude and product)
24 978
Total active oil tanker fleet (2026).
Significance
Tankers pair the largest wetted surface areas in commercial shipping with a low-speed, high-idle profile in the warm Arabian Gulf, Strait of Hormuz, and Strait of Malacca crude corridors, and transit the Suez Canal, Bab el-Mandeb, Panama Canal, Turkish Straits, and Danish Straits between biogeographically distinct waters.
| Company | Vessels (approx.) |
| Sinokor Merchant | 152 |
| COSCO Shpg Energy | 120 |
| TORM A/S | 89 |
| Frontline | 80 |
| Scorpio Tankers | 72 |
| Dynacom Tankers | 67 |
| Intl Seaways | 63 |
| Hafnia Limited | 63 |
High risk regions
Where tankers face the highest fouling pressure
A handful of high-volume crude corridors route the tanker fleet repeatedly through the warmest, most biologically productive waters in commercial shipping.
A 2022 study in Frontiers in Marine Science found that in the Arabian Gulf, macrofouler coverage exceeded 86 per cent after six months immersion in southwestern Omani waters, with biomass more than triple that at cooler Kuwaiti sites.
A 2026 Marine Pollution Bulletin assessment across 35 European locations found Mediterranean sites showed the highest fouling intensity, with salinity the dominant driver rather than temperature alone. Ten principal regions are profiled below.
Arabian Gulf
Exposure:
High
Water temperature
~18 to 34 °C (yearly range)
Common fouling species
Amphibalanus amphitrite, Balanus sp., Hydroides elegans, Perna viridis, Saccostrea cucullata, tunicates, hydroids
Red Sea and Bab el-Mandeb
Exposure:
High
Water temperature
~22 to 32 °C (yearly range)
Common fouling species
Amphibalanus amphitrite, Saccostrea cucullata, Hydroides elegans, serpulid tubeworms, encrusting bryozoans
West Africa
Exposure:
High
Water temperature
~24 to 29 °C (yearly range)
Gulf of Guinea crude-loading region; warm equatorial waters, year-round settlement.
Common fouling species
Amphibalanus amphitrite, Crassostrea spp., Balanus spp., serpulid tubeworms, hydroids
Mediterranean
Exposure:
High
Water temperature
~14 to 29 °C (yearly range)
Highest fouling intensity among European regions, salinity the dominant driver (Kim et al., 2026); Aframax and Suezmax regional crude trades.
Common fouling species
Mytilus galloprovincialis, Amphibalanus amphitrite, Ciona intestinalis, bryozoans, Ulva spp.
North Sea and Baltic
Exposure:
Medium
Water temperature
~5 to 18 °C (yearly range)
Common fouling species
Semibalanus balanoides, Mytilus edulis, Ulva spp., diatom films
Strait of Malacca and SE Asia
Exposure:
High
Water temperature
~27 to 31 °C (yearly range)
Around 23.2 million barrels per day; principal gateway for Gulf crude to Asian refineries, concentrating traffic and anchorage.
Common fouling species
Amphibalanus amphitrite, Perna viridis, Hydroides elegans, colonial tunicates, hydroids
East Asia refining hubs
Exposure:
High
Water temperature
~15 to 30 °C (yearly range)
Primary discharge destination for long-haul crude; heavy anchorage queuing at import terminals.
Common fouling species
Amphibalanus amphitrite, Balanus reticulatus, Hydroides elegans, Perna viridis, bryozoans
US Gulf Coast
Exposure:
High
Water temperature
~18 to 31 °C (yearly range)
Major crude and product export region; warm, nutrient-rich coastal waters.
Common fouling species
Amphibalanus amphitrite, Crassostrea virginica, Hydroides elegans, hydroids, tropical algae
Caribbean and Latin America
Exposure:
High
Water temperature
~24 to 30 °C (yearly range)
Warm year-round waters serving Latin American crude exports; relevant to Brazilian terminal calls.
Common fouling species
Amphibalanus amphitrite, Hydroides elegans, calcareous algae, hydroids, colonial tunicates
US West Coast and North Pacific
Exposure:
High
Water temperature
~10 to 22 °C (yearly range)
Los Angeles, San Francisco, and Long Beach crude and product terminals operate under California's mandatory biofouling rules.
Common fouling species
Mytilus galloprovincialis, Balanus glandula, bryozoans, Ciona intestinalis
Vessel risk areas
High biofouling risk areas on tankers
Oil tankers have a simple appendage profile: a niche-area-to-wetted-surface-area ratio of approximately 8 per cent, among the lowest of the vessel types assessed in a 2017 Biological Invasions study of the global fleet.
On hulls this large, even that low proportion is a substantial absolute niche area, and the high-idle profile means the flat bottom and seawater systems foul readily.
For tankers, sea chests, the propeller and shaft, the rudder, and the extensive flat bottom matter most; stabiliser fins are not fitted and thrusters are uncommon.
Sea chests & gratings
Risk:
High
Why this area fouls
Sea chests feed raw seawater to the tanker's cooling, ballast, firefighting, and inert gas systems. Their interior surfaces see near-zero flow at any vessel speed, so fouling settles and develops undisturbed.
During ballast waiting and floating storage in warm anchorages, the seawater systems keep drawing water through the sea chests while the vessel is stationary. Angular grating geometry limits coating film thickness at edges, creating localised coating-failure points; the IMO Biofouling Guidelines single out corners, edges and grating bars for particular attention during coating application.
Mitigation
- Specialised coatings: biocidal formulations suited to near-zero flow; attend to film thickness at angular grating edges.
- MGPS: Marine Growth Prevention System; copper and aluminium ion dosing protects continuously regardless of vessel speed, essential during long stationary periods.
- Inspection and documentation: record sea-chest condition in the biofouling record book.
Flat bottom
Risk:
High
Why this area fouls
A crude tanker's flat bottom is an exceptionally large horizontal surface; on a VLCC it spans a 60-metre beam for much of the vessel's length, one of the largest continuous settlement surfaces of any vessel type.
Fouling intensity across the flat bottom correlates with time spent stationary, and slow steaming plus ballast waiting and floating storage maximise that exposure. Barnacle adhesion force increases exponentially with organism base diameter, so fouling left to establish during long waiting periods becomes progressively harder to remove.
Mitigation
- Antifouling coatings: the primary drydock focus; on a VLCC flat bottom, specification and application quality translate into very large absolute fuel savings.
- Ultrasonic antifouling: transducers bonded to the inside of the hull plating, with no hull penetration, carry ultrasonic energy along the plates to discourage settlement across the flat bottom while the vessel is stationary or slow steaming; it runs continuously and supplements the coating rather than replacing it.
- Proactive grooming: hull-crawling robots suit the large flat expanse; in a 54-month Florida Institute of Technology trial, weekly grooming sustained a fouling rating of zero with no non-indigenous species on groomed surfaces.
- Reactive in-water cleaning: brush and cavitation jet systems for established fouling; must conform to local capture and containment requirements.
Propeller & shaft
Risk:
High
Why this area fouls
The propeller and shaft are performance-critical despite low service speed: the very large power demand of crude carriers converts any efficiency loss into significant absolute fuel and emissions penalties. During slow steaming, ballast waiting, and floating storage the propeller sits stationary in warm water and fouls.
AA 2023 CFD study in Polish Maritime Research found that at medium calcareous fouling, thrust can fall 26 per cent and torque rise 7 per cent, cutting propeller efficiency 30.9 per cent; an unpainted propeller may need 57.8 per cent more power after four years versus 2.7 per cent with foul-release coating. The greatest proportional loss occurs from clean surface to light slime.
Mitigation
- Propeller coating: foul-release coating limits degradation: 2.7% versus 57.8% additional power over four years (Zinati et al., 2023).
- Propeller polishing: scheduled in-water polishing restores surface smoothness between drydockings.
- Ultrasonic antifouling: transducer positions at the stern can extend coverage to the propeller area in line with the system design, limiting biofilm on the blades during port calls and anchorage; it complements propeller coating and polishing.
- Performance monitoring: ISO 19030 metrics and slip-ratio tracking flag propeller fouling before penalties compound.
Rudder
Risk:
Medium
Why this area fouls
Crude tanker rudders sit in the turbulent propeller slipstream in transit, which accelerates coating erosion on leading and trailing edges.
During long stationary periods the rudder, its fittings, and recesses offer sheltered surfaces where fouling establishes, and its position in the propeller wash makes coating condition hard to inspect during underwater surveys.
Mitigation
- Coating specification: abrasion-resistant formulations suit leading edges better than soft self-polishing systems.
- Drydock inspection: inspect the rudder and fittings specifically at drydock, where failures hidden by propeller wash in-water can be identified.
Internal seawater systems
Risk:
High
Why this area fouls
Tankers run extensive seawater circuits for engine cooling, ballast, firefighting, and inert gas generation: continuously wetted internal surfaces invisible to hull inspection and unprotected by hull coatings.
These systems keep drawing warm seawater during ballast waiting and floating storage, sustaining internal fouling pressure while the vessel is stationary. The IMO Biofouling Guidelines (MEPC.378(80)) identify MGPS downtime as a risk parameter to log in the biofouling record book.
Mitigation
- MGPS: copper and aluminium ion dosing is the primary proactive tool; anode sizing and positioning relative to circuit flow govern effectiveness.
- Monitoring and logging: anode consumption and strainer inspection records evidence biosecurity compliance; log MGPS downtime in the biofouling record book.
Risk profile
Summary of biofouling risk for tankers
How hull geometry, operating profile, niche-area share, regional trading, the shadow fleet, and biosecurity exposure shape tanker biofouling risk: a qualitative rating anchored in performance exposure, recognising compliance and biosecurity implications too.
| Dimension | Tanker characteristic | Why it matters |
|---|---|---|
| Hull geometry | Very large wetted surface area, particularly on VLCC and Suezmax hulls; exceptionally large flat bottom. | The absolute fuel and emissions penalties from tanker fouling are among the largest of any vessel type; biofilm on 50% of a hull can raise fuel use ~25%, heavy macrofouling up to 50% (GESAMP, 2024). |
| Operating profile | Low service speeds (12–15 knots) with routine slow steaming; extended ballast waiting, demurrage, and floating storage leave the hull stationary for long periods. | Foul-release coatings effective at 15–30 knots (US EPA, 2011) often operate outside their designed envelope; commercial vessels average 15–22 days/year at anchor, higher for tankers in floating storage (UMAS, 2024). |
| Niche areas | 8% niche-area-to-WSA ratio (Moser et al., 2017), but very large absolute niche area; sea chests and propeller are highest-consequence areas | Propeller fouling at medium calcareous condition can cut efficiency 30.9%; an unpainted propeller may need 57.8% more power after four years vs 2.7% with foul-release coating (Zinati et al., 2023). |
| Regional trading | Crude corridors route the fleet repeatedly through the Arabian Gulf, Strait of Hormuz, and Strait of Malacca, among the highest fouling-pressure waters in commercial shipping. | Arabian Gulf macrofouler coverage exceeded 86% after six months in Omani waters (Muthukrishnan et al., 2022); the Mediterranean shows the highest fouling intensity in European waters (Kim et al., 2026). |
| Shadow fleet dimension | A significant proportion of the ageing crude fleet operates as the sanctioned shadow fleet with deferred maintenance and extended drydock intervals. | Deferred hull husbandry on ageing tonnage is associated with higher fouling accumulation and correspondingly higher fuel, emissions, and biosecurity consequences. |
| Biosecurity exposure | Calls at New Zealand, Australian, and California ports on Pacific crude and product trades. | Between 56% and 70% of established coastal non-indigenous species are attributed to transport via biofouling (GESAMP, 2024); non-compliant hulls can face denied port entry. |
| Performance & compliance | Hull condition is a significant variable in CII calculations; steady speeds on defined routes support meaningful performance normalisation. | Thin biofilm covering 50% of a hull can increase fuel consumption by ~25%; heavy macrofouling can raise fuel use by up to 50% (GESAMP, 2024), impacting CII ratings. |
Management practices
Recommended biofouling management for tankers
Antifouling coatings (SPC, CDP, hybrid, foul-release, silicone-hydrogel, biocide-containing foul-release), ultrasonic and MGPS systems, in-water cleaning and ROV (remotely operated vehicle) hull grooming, drydock cleaning, and performance and condition monitoring for the low-speed, high-idle tanker profile.
-
Ultrasonic and MGPS systems
Tankers spend long periods stationary in warm anchorage and floating storage waters, where coatings protect least and fouling takes hold fastest. Active systems add a layer of defence proportional to idle time.
- Ultrasonic antifouling — complementary layer that disrupts settlement during ballast waiting and floating storage.
- MGPS for sea chests and strainers — copper and aluminium ion dosing to suppress accumulation in continuously running seawater circuits.
-
Antifouling coating selection for tankers
Tankers combine 12 to 15 knot service speeds with long stationary periods in warm anchorages and floating storage, a duty cycle largely below the speed envelope at which conventional foul-release systems work.
- Self-polishing copolymers (SPC) — flow-driven biocide release; polishing rate slows at low tanker speeds.
- Controlled depletion polymers (CDP) — passive depletion less speed-dependent; limited in warm tropical anchorages.
- Hybrid coatings (SPC/CDP blends) — well matched to the slow-steaming, high-idle tanker profile.
- Silicone foul-release — depends on 15–30 knot shear; a poor match for most tankers.
- Fluoropolymer foul-release — same speed-dependent limitation; potential durability advantage for longer dockings.
- Silicone-hydrogel hybrids — hydration layer resists settlement during low-speed and stationary conditions.
- Biocide-containing foul-release — adds low-level biocide for the stationary periods that dominate the tanker profile.
"Tankers spend so much of their life slow steaming or sitting stationary in warm water that coating choice, niche-area protection, and proactive grooming have to be planned around idle time, not nominal service speed."
Cathelco · Evac Group editorial -
In-water cleaning, grooming, and drydock
Tankers call at a defined set of terminals and idle at nearby anchorages, where in-water cleaning access varies: in GloFouling’s 2022 review of 138 biofouling provisions, 14 per cent did not allow in-water cleaning at all, 37 per cent required prior permission, and 28 per cent required or recommended debris capture.
In-water cleaning is 35–65% less expensive than drydocking (US EPA, 2011), and the stationary periods that drive tanker fouling are also windows for scheduled grooming.
- Proactive grooming — particularly well matched to the tanker profile; ballast-waiting and floating-storage periods are grooming windows.
- Brush-based hull-crawling — suits the extensive flat bottom and vertical sides; tankers are strong candidates for efficient robotic coverage.
- Cavitation waterjet — 50–250 m²/h coverage; effective on harder calcareous macrofouling.
- High-pressure waterjet — effective on established fouling; risk of coating removal where paint is already breaking down.
- Drydock cleaning — five-year class survey cycle with intermediate dockings; full hull preparation and coating renewal.
-
Performance, documentation, and condition monitoring
Tankers run steady speeds on defined routes, which supports meaningful normalisation of hull and propeller performance data. Monitor propeller fouling separately, given the non-linear sensitivity of propeller efficiency to early-stage fouling.
- Biofouling record book — log ballast-waiting and floating-storage periods, water temperature, MGPS downtime; framework defined in MEPC.378(80).
- Biofouling management plan (BFMP) — inspection and cleaning triggers tied to stationary duration; required by California rules.
- Hull and propeller performance monitoring — ISO 19030 KPIs; directly feed CII calculations.
- AI, machine learning, and in-situ sensing — emerging tools including CNNs, Gaussian splatting, and biosensors.
Regulations
Compliance framework
IMO biofouling guidelines (MEPC.378(80)), New Zealand CRMS, Australian biosecurity requirements, US EPA Vessel General Permit and California State Lands Commission rules, and Brazil's NORMAM-401/DPC.
Between 56% and 70% of established coastal non-indigenous species are attributed to hull biofouling transport, according to the 2024 GESAMP Working Group 44 report, driving increasingly prescriptive mandatory frameworks where tankers call.
IMO Biofouling Guidelines
2023 guidelines (MEPC.378(80)) for managing ships' biofouling, with a 0 to 4 fouling rating scale and risk parameters including extended idle periods and MGPS downtime.
Voluntary today; at MEPC 83 (April 2025) IMO agreed to develop a legally binding framework for entry into force in 2029 at the earliest. The extended-idle parameter bears directly on floating storage.
New Zealand CRMS
Craft Risk Management Standard administered by MPI, one of the two most comprehensive mandatory frameworks in force globally. Build compliance into pre-arrival planning, especially after recent floating storage or extended anchorage.
Australian Requirements
Biosecurity Act 2015 and Biofouling Management Regulations 2021 (in force 15 June 2022). Comply via a BFMP and record book aligned to IMO guidance, cleaning all biofouling within 30 days before arrival, or a pre-approved alternative method.
US EPA & California
EPA Vessel General Permit hull husbandry requirements, plus California SLC rules for vessels of 300 GRT or more able to carry ballast water: vessel-specific BFMP, annual record book, Annual Vessel Reporting Form 24 h before first California arrival, and management of seven enumerated niche areas.
The 45-day extended residency provision applies to tankers in floating storage off California.
Brazil NORMAM-401/DPC
Ordinance DPC/DGN/MB No. 180/25 (June 2025), full enforcement from 10 June 2026; applies to vessels >24 m in Brazilian jurisdictional waters.
BFMP and record book aligned to MEPC.378(80); arrival at IMO fouling rating 1 or below; extra requirements when transiting between Brazil's three marine biogeographic regions. Brazil is a significant crude exporter.
Download the full biofouling management guide + more
Enter your email and we'll send a full management guide, complementary management plan and editable record book. These are helpful tools we've shared with thousands of our customers, and come at no cost to you.