Published 24 April 2026 · last updated 28 April 2026

Oil tanker

An oil tanker is a cargo vessel designed specifically to transport crude petroleum or refined petroleum products in bulk, using a system of segregated or divided tanks built into the ship’s hull. Oil tankers form the backbone of the global energy supply chain: an estimated two billion tonnes of oil and petroleum products move by sea each year, carried by a fleet that ranges from small coastal product carriers of 10,000 deadweight tonnes (dwt) to ultra-large crude carriers exceeding 320,000 dwt. Their design, construction, operational procedures, and environmental obligations are governed by a dense layer of international regulation - principally MARPOL Annex I and SOLAS Chapter II-2 - as well as national legislation such as the US Oil Pollution Act 1990. ShipCalculators.com hosts a dedicated suite of tanker compliance calculators covering everything from crude oil washing and inert gas supply to double-hull geometry and CII carbon intensity. The sector’s history is marked by catastrophic casualties that directly shaped the regulatory environment in place today, making the oil tanker one of the most thoroughly regulated vessel types in maritime law.

Contents

Origins and early history

The Zoroaster and the birth of purpose-built tankers

Petroleum had been transported by sea in wooden barrels and in converted sailing vessels before the 1870s, but the practice was cumbersome, expensive, and dangerous. The first purpose-built oil tanker is generally accepted to be the Zoroaster, a steam-powered iron vessel commissioned in 1878 by the Nobel brothers - Robert and Ludvig Nobel, whose Branobel company dominated kerosene production on the Caspian Sea. Designed by the Swedish engineer Sven Almquist and built by the Motala Verkstad yard in Sweden, the Zoroaster was 56 m in length and carried approximately 242 tonnes of refined oil in two iron tanks integrated into the hull. She entered service on the Caspian Sea between Baku (then part of the Russian Empire) and Astrakhan, replacing the barrel-fill method that had been causing enormous cargo losses and fire hazards. A series of sister vessels followed: the Nordenskiöld, the Tatarin, and the Darwin were all built to similar principles in the early 1880s.

The Nobel fleet demonstrated that cargo tanks built as an integral part of the ship’s structure, subdivided by transverse bulkheads, could carry liquid bulk cargo safely and economically. However, these early vessels were single-skinned river-and-coastal craft and bear only limited structural resemblance to modern deep-sea tankers.

The Glückauf and the modern tanker form

The first deep-sea tanker embodying the layout that would define the type for the next century was the Glückauf (meaning “good luck”, a traditional German miners’ salutation), launched in 1886 at the Armstrong Mitchell yard on the River Tyne, Newcastle upon Tyne, to a design attributed to Henry F. Swan in collaboration with British naval architect Colonel William Henry White. Built for the German-American Petroleum Company and measuring 97 m in length with a capacity of approximately 2,300 tonnes, the Glückauf introduced several features that became standard: cargo tanks occupying the full length and breadth of the double bottom and running to deck level, a cofferdam separating the cargo section from the engine room aft, a dedicated pump room, and a pipelined deck system for loading and discharging. Her machinery was placed aft to keep the entire midship section free for cargo - the configuration that all modern tankers still use.

The Glückauf grounded off Fire Island, New York, in 1893 and was lost, but by that time her design had been widely copied. The Standard Oil Company of New Jersey and the Anglo-American Oil Company (later Esso) quickly ordered fleets of similar vessels. By 1900 approximately 400 ocean-going tankers were in service, and the template set by the Glückauf - tanks occupying the full midship volume, pump room, expansion trunks on deck, machinery aft - remained essentially unchanged until MARPOL-driven double-hull requirements emerged a century later.

Growth through the twentieth century

The interwar period brought a rapid increase in tanker deadweight. Vessels of 12,000 to 15,000 dwt became common during the 1920s and 1930s. The Shell Group and Standard Oil’s successor companies invested heavily in fleet expansion to serve growing demand from motorised transport. During the Second World War, Allied tankers such as the T2 tanker (16,500 dwt, approximately 500 built in the United States) played a critical logistical role, though they suffered catastrophic losses - more than 50 T2s broke apart at sea due to low-temperature brittleness of the hull steel, a failure mode that directly drove post-war advances in steel quality standards and fracture mechanics.

Post-war reconstruction and the rapid expansion of the petrochemical industry drove a further leap in tanker size. The closure of the Suez Canal between 1956 and 1957 (Suez Crisis) and again between 1967 and 1975 forced operators to route tankers around the Cape of Good Hope, making much larger vessels economically attractive because the fixed cost of the long voyage was spread over a larger cargo. Tankers of 100,000 dwt (the “supertanker”) emerged in the late 1950s, vessels of 200,000 dwt by the mid-1960s, and by 1973 the Japanese shipbuilder Sumitomo had delivered the Globtik Tokyo at 477,000 dwt. The absolute peak was the Seawise Giant (later renamed Jahre Viking and then Knock Nevis), completed in 1979 and subsequently lengthened to 564,763 dwt - 458.45 m long, with a draught of 24.6 m fully loaded. She was so large that she could not transit the English Channel, the Suez Canal, or the Panama Canal, and was eventually moored permanently as a floating storage unit before being scrapped at Alang, India, in 2010.

Vessel types and size classification

The AFRA scale

The oil tanker market uses a sizing nomenclature derived from the Average Freight Rate Assessment (AFRA) system, introduced by Shell in 1954 as a mechanism for standardising freight rate assessments. The AFRA categories originally referred to the cost bands at which rates were averaged, but the size labels have persisted as industry shorthand long after the formal freight-rate mechanism became less relevant. The principal categories are listed below by approximate deadweight range, though boundaries shift over time as the market evolves.

General Purpose (GP): 10,000 to 25,000 dwt. The smallest ocean-going tankers, typically deployed on coastal and short-sea routes. GP tankers carry clean petroleum products, vegetable oils, or other liquid cargoes in smaller parcels. Many are equipped with stainless-steel or coated tanks to carry multiple grades simultaneously.

Medium Range (MR): 25,000 to 45,000 dwt. The most numerous size category in the global fleet. MR tankers serve as the primary workhorse of the refined products trade - carrying gasoline, jet fuel, diesel (gas oil), naphtha, and vacuum gas oil (VGO). Their draft typically allows access to most major product terminals without lightening. Many operate in pools such as the Handytanker, Torm, and Hafnia pools.

Long Range 1 (LR1): 45,000 to 79,999 dwt. LR1 vessels straddle the boundary between large product tanker and small crude carrier. Many are operated as clean product tankers carrying LR cargoes (naphtha, jet fuel) from the Arabian Gulf to Asia or Europe.

Long Range 2 (LR2) / Aframax: 80,000 to 159,999 dwt. This range contains two distinct commercial categories. LR2 tankers operating on clean products typically have fully coated cargo tanks. Aframax tankers (a term derived from AFRA, specifically from the 80,000 to 119,999 dwt band originally designated “AFRA Medium Large”) operate predominantly on dirty crude trades - North Sea crude to Europe, Caribbean and West Africa crude to the Gulf of Mexico, Mediterranean Urals grades, and Indonesian crude to Asia. The Aframax is particularly prevalent in the Baltic, Black Sea, North Sea, and Caribbean regional trades because its draft does not exceed the limitations of many second-tier terminals.

Suezmax: 120,000 to 200,000 dwt. Suezmax vessels are sized to transit the Suez Canal fully laden. The canal’s maximum permissible draft for laden tankers is approximately 20.12 m (66 ft), depending on current guidance from the Suez Canal Authority. Suezmax tankers dominate the West Africa to Europe trade and serve the US Gulf Coast from the Atlantic Basin.

Very Large Crude Carrier (VLCC): 200,000 to 319,999 dwt. VLCCs are the dominant vessel type for long-haul crude trades. The canonical route is from the Arabian Gulf (Ras Tanura, Kharg Island, Ruwais) to China, South Korea, Japan, and India via the Strait of Malacca or Lombok Strait. A loaded VLCC drawing 20 to 22 m cannot transit the Suez Canal fully laden and generally cannot call at many continental European or US Atlantic ports directly, requiring either lightering operations or purpose-built deepwater terminals. A typical VLCC carries 250,000 to 310,000 tonnes of crude oil in eight to 16 cargo tanks plus segregated ballast tanks, with a double-hull configuration mandated by MARPOL Annex I Regulation 19. The TI class vessels (Oceania, Africa, Europe, Asia) built at Daewoo Heavy Industries in 2003 each measure approximately 441,000 dwt and were among the last ULCCs constructed; they operate as VLCCs commercially.

Ultra Large Crude Carrier (ULCC): 320,000 dwt and above. Constructed primarily in the 1970s and early 1980s at the height of the supertanker era, ULCCs are now largely obsolete as trading vessels. Their extreme draft limits port access to a handful of purpose-built terminals such as the Louisiana Offshore Oil Port (LOOP) in the Gulf of Mexico and Antifer terminal near Le Havre, France. Most surviving examples are deployed as floating storage and offloading (FSO) units.

Ship structure and hull design

Double hull requirement

The contemporary oil tanker hull is defined structurally by the requirement for a double hull - a void space between the cargo tanks and the outer shell. The double hull serves two purposes: it protects the cargo from damage in a collision or grounding event before the outer shell is fully breached, and it eliminates ballast water from direct contact with cargo residues (the predecessor system used cargo tanks as ballast tanks, making ballast water contamination and sludge accumulation endemic).

The origins of the double-hull mandate lie in a series of casualties described in the section on marine pollution below. MARPOL Annex I Regulation 19 (formerly Regulation 13F, introduced as an amendment in 1992 following the Exxon Valdez disaster of 1989) requires all crude oil tankers of 20,000 dwt and above, and all product tankers of 30,000 dwt and above, ordered after 6 July 1993 to be fitted with a double hull. The minimum void space width is specified as a function of the vessel’s dimensions - typically the greater of 0.76 m or B/15 (where B is the vessel’s moulded breadth) for the side tanks, and the greater of B/15 or 2 m for the double bottom, subject to a maximum of 2 m in each case.

The double-hull area as a fraction of the total underwater hull can be estimated using the double-hull area calculator, which applies the Regulation 19 geometry formulae directly.

Single-hull tankers were phased out under a schedule established by the MARPOL Annex I Category 1/2/3 system introduced in 2001. Category 1 vessels (pre-MARPOL tankers, the most hazardous) were phased out by 2005; Category 2 and 3 vessels (MARPOL tankers with single hull but with separate ballast or mid-deck arrangements) were phased out by 2010 or, in some cases, by 2015 depending on flag-state and port-state consent under the Condition Assessment Scheme (CAS) of Regulation 20. For the practical geometry rules underpinning CAS compliance, see MARPOL Annex I Regulation 19 guidance.

Longitudinal framing and tank layout

Modern tankers use longitudinal framing - frames running fore and aft, with transverse web frames and bulkheads at wider spacing - rather than the transverse framing system common on general cargo ships. Longitudinal framing places more of the hull girder material at the extreme fibres (deck and bottom), giving superior resistance to sagging and hogging longitudinal bending moments, which are severe on long, heavily laden tankers.

Cargo tanks are arranged in a grid of longitudinal and transverse bulkheads. A typical VLCC has three longitudinal bulkheads (creating port, centre, and starboard tanks) and five or more transverse bulkheads, giving 15 or more separate cargo tank spaces. Segregated ballast tanks (SBTs) required by MARPOL Annex I Regulation 18 are distributed throughout the double hull void spaces and sometimes in dedicated wing or topside tanks. Regulation 18 requires SBT capacity sufficient to maintain an acceptable trim and metacentric height in all ballast conditions without any cargo tank being used for ballast, with the SBTs providing at least 30% of the deadweight capacity in most design interpretations.

Structural rules

Tanker structural design is governed by the International Association of Classification Societies (IACS) Common Structural Rules for Tankers (CSR-OT), which were merged with the corresponding rules for bulk carriers in 2014 to produce the Common Structural Rules for Bulk Carriers and Oil Tankers (CSR-BC&OT). These rules specify direct calculation methods - finite element analysis of the full three-dimensional hull structure - and prescriptive minimum scantling requirements based on the vertical wave bending moment at each cross-section, the still-water bending moment from the loading manual, local sea pressures, and dynamic hull girder loads. All classification societies belonging to IACS are required to apply CSR-BC&OT for vessels contracted on or after 1 July 2015.

Cargo systems

Tank coatings and cargo compatibility

The distinction between crude tankers and product tankers is largely a function of tank coatings and the resulting cargo compatibility. Crude oil contains sulphur compounds, asphaltenes, and wax, but because crude trades typically involve a limited range of grades, contamination between grades is less critical and some operators use bare steel tanks with a corrosion allowance. Product tankers, by contrast, must carry petroleum products - gasoline, kerosene, jet fuel, naphtha, gas oil, and VGO - that are sensitive to contamination from previous cargoes or from tank scale.

Product tanker tank coatings fall into two main categories. Epoxy coatings (pure epoxy or modified epoxy) are the most widely used for clean petroleum products; a properly applied and cured epoxy lining resists permeation by aromatic hydrocarbons and provides a smooth surface that retains fewer residues after stripping. Zinc silicate coatings are harder and more resistant to abrasion but require careful curing and cannot be used with all cargo types. Tank coating condition is assessed during drydocking surveys and affects a vessel’s ability to trade in premium cargo markets.

The product tanker EEDI/CII reference calculator and crude tanker CII reference calculator both incorporate the MEPC ship-type reference lines that apply respectively to product and crude tankers under MARPOL Annex VI.

Cargo pump systems

Tanker cargo discharge depends on fixed pump systems rather than crane-and-hook handling. The conventional arrangement for large crude tankers uses centrifugal deepwell pumps or centrifugal pumps mounted in a dedicated pump room at the forward end of the machinery space. A typical VLCC has three or four main cargo pumps each rated at approximately 3,000 to 4,000 m³/h at 8 to 10 bar discharge pressure, giving a total discharge rate of 12,000 m³/h or more. Auxiliary stripping pumps (reciprocating positive-displacement type) or eductors driven by the main pump discharge pressure are used to strip the last residues from the tank bottoms after the main pumps lose suction.

Product tankers more commonly use deep-well electric-motor-driven pumps, one per tank or one per group of tanks, which avoid the need for a pump room with its associated ventilation, gas-detection, and entry hazard requirements. The main cargo pump sizing calculator applies the hydraulic power, NPSH available, and specific speed relationships for vertical centrifugal cargo pumps. The stripping pump calculator covers reciprocating displacement stripping operations.

The cargo loading rate and topping-off control is covered by the tanker loading rate and topping-off calculator, which also accounts for SOLAS II-2 overflow prevention requirements.

Crude oil washing

Crude oil washing (COW) is a technique in which hot crude oil itself is used as the washing medium to clean tank surfaces during discharge, removing wax and asphaltenic deposits that would otherwise accumulate as clingage on tank walls and floors. COW machines - rotating nozzles driven by the discharge pressure of the main cargo pumps - are fixed installations in each cargo tank, typically with three nozzles per tank providing full angular coverage.

COW is mandated by MARPOL Annex I Regulation 33 for crude oil tankers of 20,000 dwt and above and for product tankers of 30,000 dwt and above that carry crude. The regulation specifies that COW must be carried out on a sufficient number of tanks each voyage to keep overall tank sludge accumulation within limits. A full COW of all tanks is required at intervals specified in the COW manual.

The IMO has published detailed COW guidelines in resolution A.446(XI) as amended. Calculating the time required for a bottom wash cycle, a top wash cycle, and a complete COW cycle can be performed using the crude oil washing time calculator, the COW tank cycle calculator, and the COW efficiency calculator. For the regulatory detail of MARPOL Annex I Regulation 33, see the COW regulation formula page.

Inert gas system

The inert gas system (IGS) is the primary safety installation on all tankers above 8,000 dwt. Its function is to maintain the atmosphere in cargo tanks at an oxygen concentration below the explosive range of hydrocarbon vapour - the lower flammable limit for most petroleum vapours is approximately 1% oxygen by volume, but MARPOL Annex I Regulation 28, reinforced by SOLAS II-2 Chapter 4 Regulation 4.5, requires the oxygen content in an inerted tank to be kept below 8% by volume, and in practice operators target below 5%. Positive pressure (100 to 1,400 mm water gauge) is maintained to prevent atmospheric air from being drawn in when cargo is discharged and the vapour space expands.

The three sources of inert gas on crude tankers are: the flue gas from a boiler or inert gas generator (oil-fired), the exhaust gas from the main engine (less commonly, on motor tankers with suitable exhaust quality), or dedicated inert gas generators (N₂ based, more common on product and chemical tankers where hydrocarbon-contaminated boiler flue gas is unsuitable). For crude tankers with boiler-derived inert gas, the gas is scrubbed, cooled, and passed through a deck water seal before entering the cargo main; a pressure-vacuum breaker valve prevents both excessive positive or negative pressure in the tanks.

Calculating the supply requirements of the inert gas system - the volume flow rate needed to inert tanks during loading, maintain pressure during voyage, and purge during tank cleaning - is covered by the IGS supply calculator. The Lloyds Register COW/IGS interaction rules are handled by the LR COW Rule 1 calculator and LR IGS calculator. The underlying MARPOL Annex I Regulation 28 formulae are at /docs/formulas/marpol-i-28.

Vapour emission control

Volatile organic compounds (VOCs) evaporated from cargo tanks during loading and on passage represent a cargo loss, a source of greenhouse gas emissions, and a health hazard. MARPOL Annex VI Regulation 15 requires VOC management plans for crude oil tankers visiting ports or terminals where VOC recovery systems are installed, and flag states may require VOC management plans for tankers in their fleet regardless of port requirements. The VOC tanker emissions calculator estimates cargo vapour losses under the MARPOL Annex VI Regulation 15 methodology. The VOC management plan calculator supports the preparation of Annex VI-compliant documentation.

Cargo heating

Many crude grades - particularly heavy crudes such as Basrah Heavy, Bonny Light with high wax content, and Venezuelan Merey - require heating during the voyage to prevent solidification or excessive viscosity that would impair pumping on arrival. Heating coils, either steam or thermal-oil based, are fitted at the bottom of the tanks. The required duty depends on the thermal properties of the cargo, the ambient sea and air temperature on the loaded passage, and the target delivery temperature. The cargo heating duty calculator models the steady-state heat balance across the tank boundary.

Tank ullage, volume, and VEF

Oil quantity measurement on tankers is performed by manual or automated ullage gauging - measuring the distance from the tank reference point to the cargo surface. Converting an ullage reading to a gross observed volume requires the tank calibration table. Correcting for vessel trim and list uses the wedge formula or the ship’s trim correction tables. Temperature and density correction to standard conditions follows ASTM/IP Table 54B (petroleum liquids) or equivalent ISO 91-1 procedures.

The ullage-to-gross-observed-volume calculator handles the multi-step conversion from raw ullage to GOV, TOV, and GSV. The Vessel Experience Factor (VEF), used to reconcile ship-figure measurements with shore-figure measurements accumulated over a series of voyages, is computed by the VEF calculator.

Environmental regulation

MARPOL Annex I - oil pollution prevention

MARPOL Annex I, which entered force on 2 October 1983, is the foundational international instrument for preventing oil pollution from tankers. Its key structural requirements for tankers are:

Regulation 18 - segregated ballast tanks: All new crude tankers of 20,000 dwt and above and new product tankers of 30,000 dwt and above must be fitted with SBTs of sufficient capacity to maintain safe operation without using cargo tanks for ballast.
Regulation 19 - double hull: Mandatory for the vessel categories and size thresholds described above, with the geometry requirements described under hull design. The MARPOL Annex I Regulation 19 formula page details the wing-tank width and double-bottom height calculations.
Regulation 20 - condition assessment scheme (CAS): Single-hull tankers retained in service beyond the Phase I/II phase-out dates must pass a CAS survey demonstrating that structural condition is adequate. CAS requirements were tightened substantially after the Erika and Prestige casualties.
Regulation 21: Specifies retention of Category 1 and Category 2 tankers beyond their phase-out dates, governing the CAS application procedures.
Regulation 28 - inert gas systems: As described above.
Regulation 33 - crude oil washing: As described above.
Regulation 34 - control of operational discharge: The 15 ppm oil-in-water limit for machinery space bilge discharges and the prohibition on cargo-related oily water discharge within 50 nautical miles of land (or in a Special Area, entirely). The Oil Record Book Part I (machinery spaces) and Part II (cargo/ballast operations) must be maintained to demonstrate compliance.

For detailed regulatory formulae and worked examples on Regulation 18 SBT sizing, see /docs/formulas/marpol-i-18. The Regulation 34 oil-in-water check is supported by /docs/formulas/marpol-i-34.

MEPC.117(52) oil outflow performance

MEPC resolution 117(52), adopted in October 2004, established the oil outflow performance standard for tankers as an alternative compliance pathway for new construction. The standard calculates a probabilistic mean oil outflow (MOO) based on the vessel’s tank layout, and requires the MOO to be below specified limits for accidental collision and grounding scenarios. The MOO is expressed as a fraction of the cargo capacity. The MEPC.117(52) oil outflow calculator implements the full probabilistic calculation procedure of Regulation 23 of MARPOL Annex I, and the formula methodology is documented at /docs/formulas/mepc-117-oil-outflow.

OPA 90 - US Oil Pollution Act 1990

The US Oil Pollution Act 1990 (OPA 90), enacted by Congress in August 1990 in the direct aftermath of the Exxon Valdez disaster, imposes requirements on tankers operating in US waters that go beyond MARPOL. OPA 90 mandates double-hull construction for all tankers entering US waters from 1 August 1993 for new builds, with a full phase-out of single-hull tankers by 2015. It established a US$1 billion Oil Spill Liability Trust Fund (OSLTF) financed by a per-barrel fee on domestic oil production and imports. Unlimited liability was placed on responsible parties in cases of gross negligence or wilful misconduct. OPA 90 also introduced requirements for response plans and contingency planning that have influenced IMO’s Shipboard Oil Pollution Emergency Plans (SOPEP) requirements now found in MARPOL Annex I Regulation 37.

MARPOL Annex VI - greenhouse gas measures

Tankers are subject to the full suite of MARPOL Annex VI energy-efficiency measures. The Energy Efficiency Design Index (EEDI), introduced by MEPC.212(63) in 2012, requires new tankers to meet progressively tightening carbon intensity reference lines. By Phase 3 (vessels contracted from 1 April 2022 onwards), new tankers must be at least 30% more efficient than the Phase 0 baseline for their size category. The EEDI attained calculator and EEDI required calculator implement the full EEDI methodology for tankers.

For ships in service, the Energy Efficiency Existing Ship Index (EEXI), entering force in January 2023 under MEPC.328(76), requires a demonstrated attained EEXI below the required EEXI for the vessel’s type and size. The EEXI attained calculator supports compliance verification for existing tankers.

The Carbon Intensity Indicator (CII), introduced by MEPC.337(76) and effective from 1 January 2023, imposes an annual carbon intensity rating (A through E) based on the vessel’s actual transport work and fuel consumption. A tanker that receives a D or E rating for two or three consecutive years must submit a corrective action plan to its flag state administration. The CII attained calculator, CII required calculator, and CII rating calculator are directly applicable to crude and product tankers. For the detailed CII and EEDI methodology on tanker ship-types, see the what is CII and what is EEDI articles.

Notable casualties and regulatory consequences

Torrey Canyon, 1967

The tanker Torrey Canyon, a 120,000 dwt Liberian-registered vessel chartered by British Petroleum, grounded on Pollard Rock on the Seven Stones reef between the Scilly Isles and Land’s End, Cornwall, on 18 March 1967. The grounding was caused by navigational error: the master, under schedule pressure, took an inside passage and failed to correct in time. Approximately 119,000 tonnes of Kuwaiti crude oil spilled over several days, contaminating over 190 km of Cornish coastline and 80 km of Brittany beaches. The United Kingdom government ordered the wreck bombed with aviation fuel and high-explosive bombs in an attempt to burn the remaining cargo; the effectiveness of this measure was limited. Total cleanup costs and compensation were estimated at approximately £8 million at contemporary values. The Torrey Canyon disaster was the proximate cause for convening the 1969 Brussels CLC (Civil Liability Convention) and 1971 Fund Convention, and created the political impetus that ultimately produced MARPOL.

Amoco Cadiz, 1978

The Amoco Cadiz, a 228,000 dwt VLCC carrying 223,000 tonnes of mixed crude oil from the Arabian Gulf, suffered a steering failure off Brittany, France, on 16 March 1978. Salvage operations were delayed by disagreement over the terms of a salvage contract. The vessel grounded near Portsall on the Finistère coast and broke apart, releasing its entire cargo plus approximately 4,000 tonnes of bunker fuel. The spill of approximately 223,000 tonnes remains one of the largest in history. It contaminated 360 km of Breton coastline, devastated the local fishing and tourism industry, and caused long-term ecological damage documented in scientific studies over the following decade. The Amoco Cadiz disaster accelerated IMO work on traffic separation schemes, vessel traffic services, and enhanced MARPOL discharge controls.

Exxon Valdez, 1989

The Exxon Valdez, a 213,000 dwt VLCC operated by Exxon Shipping Company, grounded on Bligh Reef in Prince William Sound, Alaska, on 24 March 1989 after the officer of the watch failed to return the vessel to the traffic lane after an ice-avoidance diversion. Approximately 37,000 to 50,000 tonnes (estimates vary) of North Slope crude oil were discharged, contaminating approximately 2,000 km of coastline and killing an estimated 250,000 seabirds, 2,800 sea otters, 300 harbour seals, 250 bald eagles, and up to 22 killer whales. The US National Transportation Safety Board investigation identified excessive workload, alcohol use by the master (who was below deck at the time of grounding), and inadequate vessel tracking by the US Coast Guard as contributing factors. Exxon Corporation eventually paid approximately US$1 billion in criminal fines and civil damages, and the civil litigation over punitive damages ran in US courts for 20 years.

The direct regulatory consequences were profound. OPA 90 was enacted within 16 months of the grounding. At the IMO, the 1992 MARPOL amendments (MEPC.36(63)’s predecessor amendment series) introduced Regulation 13F mandating double hulls for new tankers. The Exxon Valdez is thus the single event most directly responsible for the contemporary double-hull fleet.

Erika, 1999

The Erika, a 25-year-old Maltese-registered single-hull tanker of 37,000 dwt, broke apart in heavy weather approximately 60 nautical miles off the coast of Brittany on 12 December 1999, spilling approximately 20,000 tonnes (variously reported as up to 30,000 tonnes) of heavy fuel oil. The spill contaminated approximately 400 km of the Atlantic coast of France from Vendée to Finistère. Subsequent investigation revealed that the vessel’s structural condition had been seriously misrepresented in classification survey records - a scandal that triggered a fundamental review of the quality of classification society surveys.

The European Commission responded with three packages of legislation (Erika I, II, and III) that accelerated the MARPOL phase-out schedule for single-hull tankers in European waters and established the European Maritime Safety Agency (EMSA). The Erika case also produced a landmark French Supreme Court ruling in 2012 holding the cargo owner (Total) liable for ecological damage under French tort law - the first time a major oil company was held criminally liable for a shipping pollution incident.

Prestige, 2002

The Prestige, a 42,820 dwt single-hull Bahamas-registered tanker carrying 77,000 tonnes of heavy fuel oil, suffered structural failure off Cabo Fisterra, Galicia, Spain, on 13 November 2002 and broke in two, sinking on 19 November. The resulting spill contaminated approximately 2,000 km of coastline across Galicia and northern Portugal, caused massive damage to the fishing industry, and generated widespread public anger across Europe and Spain. The Prestige disaster accelerated the IMO decision to advance the MARPOL single-hull phase-out for Category 1 tankers from 2007 to 2005, and for Category 2 and 3 vessels on an accelerated schedule.

Hebei Spirit, 2007

The Hebei Spirit, a 146,000 dwt crude oil tanker anchored in a designated anchorage area off Taean, South Korea, was struck by a barge under tow on 7 December 2007. The collision punctured three of the vessel’s tanks, releasing approximately 10,500 tonnes of crude oil. The spill was the largest in South Korean history and severely damaged the tourism and aquaculture industries of the Taean Peninsula. The incident reinforced the importance of anchor watch procedures and the hazards of congested anchorages, and was subsequently used in officer training materials worldwide.

Fire safety and foam systems

Oil tankers carry large volumes of flammable cargo and are therefore subject to the most detailed fire protection requirements in SOLAS Chapter II-2 and the IGS requirement of MARPOL Annex I. The main fire protection systems on a crude or product tanker include:

Fixed foam system: A fixed deck foam system covers the entire cargo tank deck from a foam compound storage unit and proportioner. For VLCCs and large Aframax/Suezmax vessels, the system typically uses high-expansion foam or medium-expansion foam applied via foam monitors and outlets. The design application rate and foam concentrate quantity are calculated using the SOLAS II-2/MSC.1/Circ.1479 methodology. The tanker fire foam system calculator implements the relevant sizing rules.
Fixed dry powder system: Required by SOLAS II-2 Regulation 10.8 for tankers, the dry powder (potassium bicarbonate or monoammonium phosphate) system is designed to suppress fires at manifold connections and pump room openings.
Inert gas system: As described above, the IGS is the primary fire-prevention (rather than fire-fighting) system, keeping tank atmospheres outside the flammable range.
Fire main: A pressurised seawater main runs the full length of the vessel with hydrant connections throughout the cargo area.

The interaction between the IGS and COW systems means that a detailed safety analysis must address the transition from loaded to ballast condition, the purging and gas-freeing sequence before hot work, and the confined-space hazards of tank entry. The confined space entry gas check calculator, gas-free certificate calculator, and hot work permit calculator support onboard safety management.

Commercial and chartering framework

Tanker charter parties

The commercial employment of oil tankers is governed by standardised charter party forms developed by the oil majors, P&I clubs, and shipowner associations. The three most widely used for crude oil and dirty product trades are:

ASBATANKVOY: Published by the Association of Ship Brokers and Agents (ASBA), ASBATANKVOY is a voyage charter form widely used in the US Gulf, Caribbean, and Atlantic crude trades. It specifies laytime and demurrage provisions in detail, addresses the allocation of cargo heating costs, and incorporates the standard NOR (notice of readiness) tendering and acceptance procedure.

SHELLVOY 6: Shell’s proprietary voyage charter form, used for Shell-fixed crude and product cargoes. It contains detailed clauses on weather routing, port risk allocation, and war risk deviation, and is notable for its structured demurrage and dispatch calculation methodology.

BPVOY 5: BP’s voyage charter equivalent to SHELLVOY 6. BPVOY 5 contains similar operational clauses and is particularly detailed on vessel inspection requirements, the OVID (Oil Majors’ Vessel Inspection Data Exchange) system, and the SIRE (Ship Inspection Report Exchange Programme) database operated by OCIMF (Oil Companies International Marine Forum).

Time charter contracts for tankers commonly use the Intertanko or BIMCO TANKERTIME form. The CII voyage charter clause calculator and the lifecycle fuel TCO calculator are tools for evaluating the carbon cost implications of charter party terms under the current CII regime.

Trade flow patterns

The global crude oil trade moves in a set of established patterns determined by the geography of production, refining capacity, and import demand:

Middle East Gulf to Asia: The dominant VLCC trade lane. Saudi Arabia, Iraq, Kuwait, the UAE, and Iran collectively export approximately 15 to 17 million barrels per day by sea. Approximately 70% of this volume is directed to China, India, Japan, South Korea, and other Asian importers, traversing the Strait of Malacca (maximum permissible draft approximately 21 m) or the Lombok Strait for deep-draft VLCCs.

West Africa to Europe and Asia: The dominant Suezmax trade lane for crude, supplemented by VLCCs on the long-haul runs from Nigeria, Angola, and the Republic of Congo to China and India. European refiners in Italy, Spain, France, and the Netherlands import West African crudes both for their low sulphur content and as Brent-indexed alternatives.

US Gulf to the Far East: Surging since 2016 as US shale oil (light tight oil grades such as WTI Midland, West Texas Light, and Eagle Ford) became export-eligible after the lifting of the US crude export ban in December 2015. VLCC loading at the Louisiana Offshore Oil Port (LOOP) or via lightering at anchorage off the Texas and Louisiana coast serves Asian refineries.

North Sea and Baltic to Europe: The dominant Aframax trade. North Sea grades (Brent, Forties, Oseberg, Ekofisk, Buzzard) move from Sullom Voe, Flotta, Mongstad, and Rotterdam on Aframax tankers to refineries around the North Sea rim. Russian Urals crude from Primorsk, Ust-Luga, and Novorossiysk moves similarly.

P&I insurance and the IOPC Fund

Oil tanker operators insure their liability for oil pollution through Protection and Indemnity (P&I) Clubs, which are mutual insurers owned by their shipowner members. The International Group of P&I Clubs (IG Clubs), comprising 13 major mutual clubs, collectively insure the great majority of the world’s ocean-going tanker tonnage. Under the 1992 Civil Liability Convention (CLC 1992) and the 1992 Fund Convention, the liability for oil pollution from tankers is channelled first to the shipowner (up to the CLC limit, which for large vessels is a function of gross tonnage, approximately SDR 89.77 million for the largest vessels) and then supplemented by the International Oil Pollution Compensation (IOPC) Fund up to approximately SDR 203 million in aggregate. The 2003 Supplementary Fund Protocol provides a third tier of compensation for states that have ratified it, raising the total available per incident to approximately SDR 750 million.

Inspection, survey, and certification

Enhanced Survey Programme

Tankers are subject to the IACS Enhanced Survey Programme (ESP), which is more rigorous than the standard five-year class survey cycle. ESP requires close-up surveys and thickness measurement of all structural members in all tanks on a rolling basis, with enhanced attention to areas known to be prone to corrosion - the deckhead and underdeck structure in ballast tanks (where condensation and repeated wetting and drying accelerates corrosion), the lower strakes of cargo tanks, and the floor plating at the tank bottom.

The ESP cycle for tankers involves: an annual survey (general examination), an intermediate survey at 2.5 years, and a special survey at five-year intervals. At the special survey, all cargo tanks, ballast tanks, and void spaces must be cleaned, gas-freed, and made safe for entry so that the surveyor can conduct close-up examination of specified areas.

Port state control and SIRE

Tankers are subject to port state control inspections under the regional MOU (Memorandum of Understanding) system - the Paris MOU in European waters, the Tokyo MOU in Asia-Pacific, the US Coast Guard under OPA 90 in US ports, and other regional bodies worldwide. Oil majors and major trading companies additionally require vessels to hold a valid SIRE (Ship Inspection Report Exchange Programme) report, conducted by an accredited OCIMF inspector. The SIRE 2.0 system, launched in 2021, uses a digital platform with a standardised questionnaire of over 600 questions covering safety management, structural condition, navigational equipment, and cargo system integrity.

Vessels rated as high risk under port state control targeting matrices face more frequent inspection and may be detained for deficiencies. The Paris MOU publishes a white list, grey list, and black list of flag states based on the detention rate of their vessels - a powerful incentive for flag state administrations to maintain survey standards.

Classification and CSR compliance

All commercially operated tankers trading internationally are required by flag state law to maintain class with a recognised classification society. The major classification societies for tankers include Lloyd’s Register, Bureau Veritas, DNV, American Bureau of Shipping, ClassNK, Korean Register, and RINA. For tankers built under CSR-BC&OT rules (contracted from 1 July 2015), the classification survey must verify compliance with the CSR structural standards as built and assess the condition of the structure over its service life, applying corrosion margins and renewal criteria when measured thicknesses fall below the renewal thickness. Class notation symbols on the certificate indicate optional notations for notation groups such as COW, IGS, ESP, and Condition Monitoring that go beyond the standard survey regime.

Emissions and energy efficiency

CO₂ and carbon intensity

An oil tanker’s CO₂ emissions are a function of the fuel consumed, the fuel’s carbon conversion factor (CF), and the distance and cargo transported. Under MARPOL Annex VI CII, the annual efficiency ratio AER = CO₂ / (cargo capacity × distance) is compared to the required CII for the ship’s type and size. For a VLCC of 300,000 dwt, the required CII reduces by approximately 2% per year through 2030, meaning the fleet must either slow-steam, load more fully, or adopt lower-carbon fuels to maintain an acceptable rating. The voyage fuel and CO₂ calculator and the CO₂ from fuel consumption calculator are standard tools for voyage-level emissions estimation available through ShipCalculators.com. For the well-to-wake perspective, the HFO well-to-wake calculator and VLSFO well-to-wake calculator provide lifecycle emission factors consistent with the IMO LCA guidelines adopted under MEPC.377(80).

The regulatory interaction between CII ratings and charter party terms is explored in slow steaming and CII.

Fuel types

The majority of VLCCs and large crude tankers burn heavy fuel oil (HFO) or very low sulphur fuel oil (VLSFO, maximum 0.50% S) in a slow-speed two-stroke diesel main engine. Product tankers and smaller crude tankers may use marine gas oil (MGO) or ultra-low sulphur fuel oil (ULSFO) when operating in Emission Control Areas (ECAs). LNG dual-fuel tankers are a small but growing sub-sector, with orders placed by major tanker owners from approximately 2018 onward. The MARPOL IMO 2020 sulphur cap article covers the 0.50% global sulphur limit and ECA requirements in detail. The properties of the primary bunker grades used by tankers are described in the heavy fuel oil and marine gas oil articles.

Mooring and ship-to-ship operations

Terminal mooring arrangements

Large crude tankers are moored at a variety of terminal types depending on the port infrastructure available. Conventional jetty berths with parallel approach and breasting dolphins are the standard arrangement at most refinery terminals and export loading jetties. For VLCCs at open-sea terminals where water depth prevents berthing alongside a fixed jetty, two alternatives are common: single-point mooring (SPM) and conventional buoy mooring (CBM).

A single-point mooring (SPM) - also called a single buoy mooring (SBM) - consists of a large buoy anchored to the seabed by a chain system and connected to the subsea pipeline by a riser and swivel assembly. The tanker moors to the buoy at its bow and is free to weathervane around it in response to wind and current, maintaining a more stable heading than would be possible at a fixed berth in exposed conditions. SPM systems are found at terminals on the Arabian Gulf (Ju’aymah, Ras Tanura), the West African coast, and offshore storage terminals worldwide. The SPM mooring load calculator models the environmental load distribution on SPM systems.

Conventional buoy mooring (CBM) systems use a pair of bow and stern buoys to hold the vessel in a fixed orientation while cargo transfer takes place through flexible hoses from the buoys to the vessel’s manifold. The CBM mooring load calculator and the multi-buoy mooring calculator address the load calculations for CBM and MBM configurations respectively.

Ship-to-ship transfer

Ship-to-ship (STS) transfer is used to lighten VLCCs at anchorage when port draught restrictions prevent a fully laden vessel from proceeding to a berth, or to transfer cargo between vessels for commercial reasons. In a typical STS operation, the two vessels moor alongside each other using a combination of breast lines and spring lines, with fendering between the hulls. Transfer rates are limited by the receiving vessel’s pump capacity and manifold pressure rating. OCIMF’s Ship to Ship Transfer Guide provides the industry-standard procedures, and the STS mooring load calculator implements the recommended environmental load calculations.

The MARPOL Annex I requirements for STS operations - specifically the notification requirements to the nearest coastal state - are addressed by Regulation 40A.

Ballast water and slops management

Segregated ballast operations

When a tanker discharges its cargo and proceeds to the next loading port in ballast, the segregated ballast tanks are filled with seawater to provide adequate draught for propeller immersion and sufficient metacentric height for stability. A typical VLCC in ballast condition draws approximately 10 to 12 m compared with 20 to 22 m fully laden. The ballast capacity of MARPOL-compliant tankers must be sufficient to achieve a summer load line freeboard plus an adequate metacentric height without any cargo tank being used - the requirement of MARPOL Annex I Regulation 18. The ballast exchange volumetric calculator and D-2 discharge standard check under the Ballast Water Management Convention apply to ballast water management on tankers.

Slops handling and the Oil Record Book

After cargo discharge and crude oil washing, residual oily water (slops) accumulates in the tanks designated as slop tanks - typically one or two tanks at the after end of the cargo block. Slops are allowed to settle, with the water phase decanted overboard when the oil content is verified to be below the discharge limits of MARPOL Annex I Regulation 34 (15 ppm for machinery space discharges; a complete prohibition on cargo-related oil discharge except where the rate does not exceed 30 litres per nautical mile, the total quantity discharged does not exceed 1/30,000 of the cargo carried, the vessel is not in a Special Area, and the vessel is more than 50 nautical miles from the nearest land). The slops handling calculator assists with the volume accounting and discharge rate check. All slops operations must be recorded in the Oil Record Book Part II, which must be retained on board for three years and be available for inspection by any authorised officer.

Fleet economics and market structure

Freight rate benchmarks

The oil tanker freight market uses the Worldscale (WS) flat rate system, published annually by Worldscale Association (London) Ltd and Worldscale Association (NYC) Inc. The WS flat rate for a route is expressed in US dollars per metric tonne and represents the notional freight required to achieve a defined return for a standard 75,000 dwt vessel on the specific voyage. Actual market rates are quoted as a percentage of the WS flat rate - thus “WS 70” on the TD3C route (Arabian Gulf to China) means 70% of the published Worldscale flat rate for that voyage. When tanker supply is tight relative to demand, rates can spike to WS 200 or above; in oversupplied markets they may trade below WS 50.

The Baltic Exchange publishes daily tanker rate indices: the Baltic Dirty Tanker Index (BDTI) covering crude and dirty products routes, and the Baltic Clean Tanker Index (BCTI) covering clean product routes. These are widely used as financial benchmarks and as the underlying reference for freight derivatives (Forward Freight Agreements, FFAs).

Tanker lifecycle and scrapping

Modern double-hull VLCCs and Suezmax tankers have a design life of 25 years, though the economic life depends heavily on drydocking costs, steel renewal required at special surveys, and the cost of meeting increasingly stringent environmental standards. Tankers are typically scrapped at yards in Bangladesh (Chittagong), India (Alang), Pakistan (Gadani), and Turkey (Aliaga). The Hong Kong Convention on safe and environmentally sound ship recycling, which entered into force in June 2025, imposes mandatory inventory of hazardous materials and requires recycling only at approved facilities - requirements that directly affect tanker operators planning vessel disposal.

New building market

Tanker newbuilding prices are driven by steel costs, berth availability at major shipbuilding countries (China, South Korea, Japan), and the cost of compliance with current and anticipated future environmental standards. Orders placed since approximately 2022 have increasingly specified LNG-ready or methanol-ready propulsion systems to hedge against the uncertainty of future fuel regulations under the IMO 2030 and 2040 carbon intensity targets. Ammonia-fuelled tanker designs are at the conceptual stage, with several classification societies having issued approval-in-principle for ammonia-fuelled VLCC concepts. The full suite of tools for evaluating the commercial and carbon cost implications of alternative fuels on tanker operations is available at the ShipCalculators.com calculator catalogue.

Sanctions evasion and the shadow fleet

Origins and structure

From 2022 onwards, sanctions imposed by G7 nations and the European Union on Russian oil exports following Russia’s invasion of Ukraine created a structural divergence in the tanker market. Operators and traders seeking to move Russian crude oil from Baltic Sea ports (Primorsk, Ust-Luga) and Black Sea ports (Novorossiysk) and from the Pacific terminal at Kozmino assembled a fleet of older tankers - predominantly Aframax-sized vessels - operating outside the normal commercial and insurance frameworks. This grouping, commonly referred to in market commentary as the “shadow fleet” or “dark fleet,” is estimated by various shipping intelligence services to have comprised several hundred vessels by mid-2023, though exact counts are contested because ownership opacity is a defining characteristic.

Shadow fleet vessels typically share several features: registration under flags of convenience in jurisdictions with limited regulatory oversight, insurance from non-International-Group P&I clubs or from state-run entities, opaque beneficial ownership through multiple layers of single-purpose companies, and in some cases the disabling of AIS transponders during sensitive transits. Many vessels exceed 20 years of age, and have been assessed by European port state control authorities and independent analysts as carrying above-average structural risk relative to the mainstream double-hull fleet.

Ship-to-ship transfer and cargo laundering

Shadow fleet operations frequently involve ship-to-ship (STS) transfer at anchorages outside territorial waters, particularly in the waters off the Laconian Gulf and Cape Matapan in Greece, in the Strait of Gibraltar approaches, in the Malta channel, and in the Gulf of Oman and northern Arabian Sea. STS transfer serves to obscure the origin of sanctioned crude by commingling it with other grades or by changing the vessel that ultimately delivers the cargo to the end buyer. MARPOL Annex I Regulation 40A requires prior notification to coastal state authorities of STS transfers within 200 nautical miles of their coast, but compliance with this provision is inconsistent among vessels associated with these operations.

Iranian and Venezuelan programmes

The practice of sanctions-evading tanker shipping predates the Russian programme. Iran has operated a large fleet of state-controlled and third-party tankers to export crude oil in defiance of US sanctions since approximately 2012, with activity intensifying after the re-imposition of sanctions from 2018. Vessels carrying Iranian crude routinely use ship-to-ship transfers, AIS manipulation, flag changes, and cargo blending to obscure origin. Venezuela’s state oil company PDVSA similarly relied on sanctions-evading tanker arrangements from approximately 2019, using STS transfers off the east coast of Venezuela, in the Caribbean, and at Malaysian anchorages.

Regulatory responses and spill risk

The European Union included provisions targeting shadow fleet tankers in successive sanctions packages on Russia, including prohibitions on providing insurance, brokerage, and technical assistance to vessels carrying Russian crude above the G7 price cap (set at US$60 per barrel for crude from December 2022). Port state control authorities in the Paris MOU and Tokyo MOU regions have applied heightened scrutiny to vessels associated with these operations, and several have been detained for structural and safety deficiencies. The concentration of ageing, poorly-maintained tankers in high-traffic areas conducting STS operations represents a material pollution risk - a concern reinforced by the December 2024 incidents involving shadow fleet tankers in the Kerch Strait, which resulted in an oil release near the Russian coast and triggered an international response from environmental agencies.

References

International Maritime Organization. MARPOL: Articles, Protocols and Annexes. 2011 consolidated edition. IMO, London. Annex I, Regulations 18, 19, 20, 21, 28, 33, 34.
IMO Resolution MEPC.117(52), Revised guidelines on technical and operational measures for tankers. 2004.
International Maritime Organization. Crude Oil Washing Systems, IMO resolution A.446(XI) as revised by A.497(XII). IMO, London.
US Congress. Oil Pollution Act 1990 (OPA 90), 33 USC 2701 et seq. Enacted 18 August 1990.
IACS. Common Structural Rules for Bulk Carriers and Oil Tankers (CSR-BC&OT). Effective 1 July 2015. International Association of Classification Societies, London.
National Transportation Safety Board. Marine Accident Report: Grounding of the US Tankship Exxon Valdez on Bligh Reef, Prince William Sound, Alaska, March 24, 1989. NTSB/MAR-90/04. Washington DC, 1990.
IMO. Report of the Marine Environment Protection Committee, 36th Session, MEPC 36/22. Amendment adopting double-hull requirements. 1992.
Arnould, J. P. Y. and Whitehead, H. “The impact of the Amoco Cadiz oil spill.” Marine Pollution Bulletin, vol. 14, no. 8, 1983.
European Commission. Communication on a Second Set of Community Measures on Maritime Safety following the Sinking of the Oil Tanker Erika (Erika II package). COM(2000) 802 final. Brussels, 6 December 2000.
IMO. MARPOL Annex VI and NTC 2008 with guidelines for implementation. 4th edition. IMO, London, 2013.
Corkhill, Michael. “The world’s biggest ship: Seawise Giant/Jahre Viking/Knock Nevis.” Lloyd’s List, 14 January 2010.
Stopford, Martin. Maritime Economics. 3rd edition. Routledge, London, 2009. Chapter 14, “The tanker market”.

External links

IMO - MARPOL - International Maritime Organization official MARPOL pages
OCIMF - Oil Companies International Marine Forum, publisher of SIRE and tanker safety guidance
IOPC Funds - International Oil Pollution Compensation Funds, details of 1992 Fund and 2003 Supplementary Fund
EMSA - European Maritime Safety Agency, CleanSeaNet satellite oil-spill detection service
Paris MOU - Port state control organisation with inspection and detention statistics