By ShipCalculators.com · Published 14 June 2026

MARPOL Annex I Reg 27: tanker intact stability

MARPOL Annex I Regulation 27 sets the intact-stability floor that an oil tanker of 5,000 tonnes deadweight and above, delivered on or after 1 February 2002, must hold across every load case it works: laden, ballast, and the intermediate stages of cargo transfer. The criteria are the 2008 IS Code Part A general criteria carried into an environmental convention: GM0 corrected for free surface not below 0.15 m, area under the GZ curve at least 0.055 m,rad to 30 degrees and 0.09 m,rad to 40 degrees, and a righting lever of at least 0.20 m at 30 degrees of heel. The logic is direct: a tanker that capsizes or breaks is a pollution casualty, so a stability rule belongs alongside the double-hull and outflow rules.

Contents

MARPOL Annex I Regulation 27 carries the title “Intact stability.” It sits in Part A (Construction) of Annex I, in the chapter that holds the construction requirements for the cargo area of oil tankers, next to the double-hull rule (Regulation 19), the accidental-oil-outflow rule (Regulation 23), and the subdivision-and-damage-stability rule (Regulation 28). Reg 27 is the only Annex I regulation that governs the stability of the undamaged ship. It tells an oil tanker how upright and how stiff it must stay in normal service, before any collision or grounding is even contemplated.

This article is the MARPOL requirement, not the theory behind it. The general physics of transverse stability, the meaning of GM, the shape of the GZ curve and righting arm, and the derivation of the cross-curves of stability and KN tables are covered in the general intact stability article. Read that first if the terms below are unfamiliar. Here the question is narrower: which tankers does Reg 27 bind, exactly what numbers must they meet, why an oil-pollution convention reaches into stability at all, and how the master and surveyor verify it in service.

Application: which tankers, from when

Reg 27 applies to every oil tanker of 5,000 tonnes deadweight and above delivered on or after 1 February 2002. The “delivered on or after” wording uses the construction date as the trigger, so the rule reaches the newer tanker fleet and leaves pre-2002 ships under the older intact-stability provisions of their flag and class. The “as defined in regulation 1.28.7” cross-reference fixes the meaning of “delivered” against the Annex I definitions; it is the same dating mechanism Annex I uses for the double-hull phase-in, so a yard or owner reads one definition and applies it across the cargo-area rules.

The 5,000 t deadweight floor is a deliberate cut. Below it, a tanker is small enough that the operational and damage-stability regime is handled differently, and the larger pollution risk that Reg 27 targets does not bite as hard. Combination carriers, which can load oil in bulk in the same spaces they otherwise use for dry bulk, fall inside the definition of an oil tanker for this purpose, so an OBO or ore-oil ship carrying oil cargo is held to Reg 27 when it works as a tanker. A chemical tanker carrying oil as part of its cargo is also caught through the Annex I definition of an oil tanker.

The binding clause is broad on operating condition and narrow on slack. Reg 27 requires the tanker to meet the criteria “for any operating draught under the worst possible conditions of cargo and ballast loading, consistent with good operational practice, including intermediate stages of liquid transfer operations.” Two phrases carry weight. “Any operating draught” means the rule is not satisfied by checking the departure and arrival conditions alone; the whole envelope of working drafts must pass. “Including intermediate stages of liquid transfer operations” pulls the loading, discharge, and ballast-exchange sequences into scope, so the tanker has to stay stable while cargo is moving, not only when the tanks are settled. Under all conditions the ballast tanks are assumed slack, which is the harsh assumption: a slack tank carries a free surface that erodes stability, and Reg 27 makes the operator prove the ship survives that erosion rather than wishing it away with a pressed-up tank.

That last point is where Reg 27 differs in spirit from a simple “departure condition is fine” check. A tanker can leave port stable, discharge cargo at a terminal, take ballast into segregated tanks, and pass through a window where one large slack ballast tank and a partly emptied cargo tank both carry free surface at the same time. Reg 27 says that window has to pass the criteria too. The segregated-ballast-tank arrangement of Reg 18 shapes which tanks fill during that exchange, so the two rules interact: the SBT layout sets the geometry, and Reg 27 sets the stability floor the geometry has to clear.

The criteria: GM, GZ-curve area, and the lever maximum

Reg 27 specifies the intact-stability criteria in two short paragraphs, conventionally numbered 1.1 (in port) and 1.2 (at sea). The numbers are the 2008 IS Code Part A general criteria, which is why a naval architect who has run an IS Code Part A check already knows the targets. Each criterion is a floor, not a target; a tanker that lands exactly on a floor passes, and the practice is to design with margin above it.

The metacentric-height floor

The first criterion is the metacentric height. The initial metacentric height GM0, corrected for the free-surface effect of slack tanks and measured at zero heel, must not be less than 0.15 m:

GM_0 = KM - KG_{fluid} \geq 0.15\ \text{m}

Here $KM$ is the height of the transverse metacenter above the keel, read from the cross-curves of stability and KN tables or the hydrostatic data at the condition’s displacement, and $KG_{fluid}$ is the vertical center of gravity raised by the free-surface correction. The “fluid” subscript is the whole point: Reg 27 reads GM after the slack-tank penalty, not the solid GM. The same 0.15 m floor applies in port and at sea, so the in-port relaxation that some passenger-ship regimes allow does not appear here. A tanker that holds GM0 at 0.15 m has a small but positive initial stiffness; it returns toward upright from a small heel rather than lolling.

The free-surface correction is the part that bites a tanker hardest. A tanker is a ship of large, parallel-sided liquid tanks, and a half-full cargo or ballast tank has a wide free surface whose virtual rise in the center of gravity scales with the tank’s transverse moment of inertia:

\Delta GM_{fs} = \frac{\sum i \cdot \rho_{cargo}}{\Delta}

where $i$ is the transverse second moment of area of each slack tank’s free surface, $\rho_{cargo}$ is the density of the liquid in the tank, and $\Delta$ is the ship’s displacement. Reg 27 forces this correction in by assuming the ballast tanks slack, so the design has to either limit the number of simultaneously slack wide tanks or carry enough solid GM to absorb the loss. Longitudinal subdivision of the cargo block, the same bulkheads that limit accidental outflow under Regulation 23 and the deterministic Regulation 25 hypothetical outflow, also cuts the free-surface moment by narrowing each tank, so the pollution-prevention geometry and the stability geometry pull in the same direction.

The righting-lever curve

The GM floor controls only the slope of the righting-lever curve at the origin. Reg 27 then constrains the curve itself through four criteria on the GZ curve at larger angles, the dynamic-stability measures that decide whether the ship has the reserve to ride out a gust or a wave-induced heel.

The area under the righting-lever curve, the integral that measures the energy the ship can absorb before the lever runs out, must clear three floors:

\int_0^{30^\circ} GZ\, d\theta \geq 0.055\ \text{m}\,\text{rad}

\int_0^{40^\circ} GZ\, d\theta \geq 0.090\ \text{m}\,\text{rad}

\int_{30^\circ}^{40^\circ} GZ\, d\theta \geq 0.030\ \text{m}\,\text{rad}

The upper limit of the second and third integrals is 40 degrees or the angle of downflooding, whichever is smaller, so a tanker with low openings is measured only to the heel at which water would start to enter. The 30-to-40 band is the difference between the first two, and stating it separately forces the curve to keep climbing through the high-angle range rather than collecting all its area early and flattening.

Two more criteria fix the shape and height of the curve. The righting lever GZ must be at least 0.20 m at an angle of heel equal to or greater than 30 degrees:

GZ_{30^\circ} \geq 0.20\ \text{m}

And the maximum righting lever must occur at a heel angle preferably exceeding 30 degrees but not less than 25 degrees. The minimum-angle-of-maximum criterion is what keeps the curve from peaking early and collapsing: a ship whose GZ tops out at 18 degrees and falls away has poor dynamic reserve even if its initial GM looks healthy. The 25-degree floor on that peak is the criterion that a beamy, stiff tanker can struggle to meet, because high initial stiffness tends to push the peak to a lower angle.

These five numbers, the GM floor and the four curve criteria, are the whole of the Reg 27 intact-stability test. A tanker passes Reg 27 when every required loading condition, run with ballast tanks slack and the free-surface correction applied, clears all five. The GZ curve and righting arm article works through how the curve is generated from the cross-curves; the point for Reg 27 is that the same curve is read against these five floors for every condition in the booklet.

Why an environmental convention sets a stability rule

MARPOL is a pollution-prevention convention. SOLAS is the safety-of-life convention, and stability has historically lived there. So the question is fair: why does Annex I, an environmental annex, carry an intact-stability rule at all?

The answer is that a tanker that loses stability is a pollution event. A ship that lists past the angle of vanishing stability and capsizes, or that takes a large list and works its deck and tank-top welds until the hull fractures, spills its cargo into the sea. The casualty record behind the double-hull push, the same record that produced Regulation 19, includes ships that broke or rolled rather than ships that simply holed a single bottom. Keeping the oil inside the hull is the object of Annex I, and intact stability is one of the conditions that keeps the hull intact and upright long enough for the oil to stay where it belongs. Reg 27 is pollution prevention by the stability mechanism, in the way that Reg 19 is pollution prevention by the structural mechanism and Reg 23 is pollution prevention by the outflow-geometry mechanism.

The drafting choice also closes a gap. SOLAS intact-stability requirements through the 2008 IS Code apply to cargo ships of 24 m in length and over, so a large tanker is already covered by the IS Code as a cargo ship under SOLAS regulation II-1/5. Putting the same criteria into Annex I gives the pollution-prevention regime its own hook into a tanker’s stability, independent of the SOLAS certificate. A port-state control officer inspecting under MARPOL can check Reg 27 compliance against the International Oil Pollution Prevention Certificate supplement and the approved stability information, rather than having to reach across to the SOLAS regime to verify that the tanker is stable. The two regimes carry the same numbers but give two enforcement routes, which matters for a convention whose whole purpose is keeping oil out of the water.

There is a quieter reason too. MARPOL Annex I cargo-area rules push tank arrangements toward configurations that, taken alone, can hurt stability. Segregated ballast tanks under Reg 18 add wide wing and double-bottom tanks; the double-hull rule under Reg 19 adds a continuous double bottom and wing void or ballast space. Each of these adds slack-tank free surface during ballast operations. Reg 27 is the counterweight: the same annex that mandates the tank arrangement also mandates that the tanker stay stable while those tanks fill and empty. Without Reg 27, an owner could argue that the pollution-prevention arrangement was complete once the geometry was in place. With it, the arrangement is complete only when the geometry passes the stability floors over the full transfer sequence.

Relationship to the 2008 IS Code and SOLAS

Reg 27 does not invent its own criteria. It carries the general intact-stability criteria of the International Code on Intact Stability, 2008, adopted by resolution MSC.267(85) on 4 December 2008 and in force from 1 July 2010. The five floors above, the 0.15 m GM, the 0.055 and 0.09 and 0.03 m,rad areas, the 0.20 m lever at 30 degrees, and the 25-degree minimum angle of maximum lever, are the IS Code Part A general criteria, the criteria the IS Code itself draws from the earlier work codified in resolution A.749(18). A tanker that meets IS Code Part A over its MARPOL load cases meets Reg 27 by construction, because the numbers are identical.

The 2008 IS Code became mandatory under SOLAS regulation II-1/5, which means a cargo ship of 24 m and over already has to meet IS Code Part A under SOLAS. Reg 27 layers the same requirement into MARPOL for the specific population of tankers it names. The practical effect is that a 5,000 t deadweight crude carrier delivered in 2015 has two independent regulatory hooks into the same stability calculation: SOLAS II-1/5 through the IS Code, and MARPOL Annex I Reg 27. The naval architect runs the calculation once and the approved stability information serves both certificates.

The IS Code structure matters for reading Reg 27. The IS Code splits into Part A (mandatory criteria) and Part B (recommended provisions and guidance). Reg 27 incorporates the Part A general criteria; the Part B guidance on, for example, the weather criterion or operational measures for combination carriers, sits in the background as recommended practice rather than a hard MARPOL floor. The explanatory notes to the IS Code, issued as MSC.1/Circ.1281, explain how the criteria are applied, and a class society reads them alongside the Code when it approves a tanker’s stability booklet. The point of citing the resolution number, MSC.267(85), rather than “the IMO stability code,” is that the criteria have a documented version: a tanker approved against the 2008 Code is approved against that text, not against the 1993 Code it replaced.

Relationship to Regulation 28 damage stability

Reg 27 is the intact half of a pair. Regulation 28 is the damaged half: it sets the subdivision and damage-stability requirements a tanker must meet after assumed side or bottom damage, the deterministic damage extents and the post-damage survival criteria on residual GZ, range, and equilibrium heel. The two regulations describe the same ship in two states. Reg 27 governs the undamaged ship in normal service; Reg 28 governs the ship after a hole has been opened in it. A tanker has to pass both, and the two calculations share inputs: the same loading conditions, the same lightship and tank data, the same free-surface treatment of slack tanks.

The relationship is sequential in a casualty. A tanker first has to stay intact-stable to avoid capsizing in normal operation, which is Reg 27; if it is then holed, it has to stay afloat and upright enough to limit the spill, which is Reg 28. A failure of either is a pollution event, which is why Annex I carries both. The intact criteria are stiffer at the origin (the 0.15 m GM and the early curve area) and the damage criteria are about survival at larger heel and reduced buoyancy. The naval architect who builds the approved loading conditions runs each condition through both checks, so a condition that passes Reg 27 intact but fails Reg 28 damaged is not an allowed loading.

Reg 28 also adds the verification machinery that ties both back to the master. Amendments adopted by resolution MEPC.248(66) on 4 April 2014, in force from 1 January 2016, require oil tankers to be fitted with a stability instrument capable of verifying compliance with intact and damage-stability requirements, approved by the Administration against the IMO performance standards. So while the criteria live in Reg 27 and Reg 28, the onboard means of checking them against an actual loading lives in the Reg 28 instrument requirement. A tanker built after 1 January 2016 carries an approved loading instrument that runs the Reg 27 intact check and the Reg 28 damage check on the planned condition before the master signs off the departure.

The stability booklet and the loading instrument in practice

The criteria are abstract until a master has to load cargo against them. The practitioner tools are the approved stability information, the marine stability booklet and loading computer. The booklet is the approved document that records the lightship, the tank capacities and centroids, the cross-curves, and a set of standard loading conditions each shown to pass Reg 27 and Reg 28. The loading instrument is the onboard computer that lets the master build a condition that is not in the booklet and check it against the same criteria.

A master planning a load enters the cargo and ballast distribution into the instrument. The instrument computes the displacement, draft, and trim, reads the cross-curves for the GZ curve, applies the free-surface correction from the slack tanks, and reports GM0 and the four curve criteria against the Reg 27 floors. If GM0 comes out at 0.12 m, the condition fails, and the master moves cargo down or presses up a slack tank to lift GM above 0.15 m before loading. The instrument is the practical embodiment of Reg 27: the regulation states the floors, and the instrument is what tells the master, condition by condition, whether the floors are cleared.

The approval chain matters here. The loading instrument’s stability software is type-approved against the IMO guidelines for the approval of stability instruments (MSC.1/Circ.1229 and its successors), and the ship-specific data file is checked against the approved stability booklet so that the instrument and the booklet agree. A port-state-control officer verifying a departure condition compares the instrument output against the approved booklet, which is the standard cross-check: the booklet is the reference approved by the Recognized Organization, and the instrument has to reproduce it. When the two disagree, the booklet governs, and the instrument data file is suspect.

Intermediate-condition checks during cargo operations

The clause that distinguishes Reg 27 from a static departure check is the requirement to meet the criteria through the intermediate stages of liquid transfer. A tanker rarely fails Reg 27 at the settled departure or arrival condition; the risk sits in the middle of a load or discharge, when several tanks are part-full at once and the free-surface penalty peaks.

Consider a crude tanker discharging at a terminal and taking segregated ballast to keep the hull immersed for the next leg. Early in the discharge, the cargo tanks are nearly full, the ballast tanks nearly empty, and few tanks are slack. Late in the discharge, the cargo tanks are nearly empty and the ballast tanks nearly full, again with few slack tanks. The dangerous window is the middle, where cargo tanks are half-discharged and ballast tanks are half-filled, and the free surface from both adds up. A tanker that passes Reg 27 at the start and end of the operation can dip below the GM floor in that middle window if the operation is run without checking it. The regulation closes that gap by requiring the intermediate stages to pass.

The practical control is the approved loading sequence. For a tanker on a repeating trade, the operator builds a loading and discharge sequence that has been checked at each intermediate step and stays above the Reg 27 floors throughout. The loading instrument is the tool that builds and verifies that sequence, and the master follows it rather than improvising the order in which tanks are worked. Where a tanker is on a dedicated service with a small number of loading permutations, all of which have been approved in the stability information, the Reg 28 instrument requirement can be waived under the conditions set out in the Annex I exemption-and-waiver provisions, because the approved sequences already cover every condition the ship will see. That waiver does not relax Reg 27; it accepts the approved sequences as the proof that Reg 27 is met.

The free-surface assumption drives the sequence design. Because Reg 27 assumes the ballast tanks slack, the operator cannot rely on pressing up a ballast tank to remove its free surface unless the tank is genuinely full at that stage of the sequence. A sequence that presses up the after ballast tanks early, then works the cargo tanks, keeps fewer tanks slack at the worst moment than a sequence that fills ballast and discharges cargo together. The loading instrument lets the operator test these orderings before the operation rather than discovering a low-GM window during it.

Combination carriers and the operational dimension

Combination carriers sit at the awkward edge of Reg 27. An ore-oil or OBO carrier loads oil in bulk in spaces that also carry dry bulk, and its stability character changes with the cargo. Carrying oil, it is a tanker and Reg 27 binds it. Carrying ore, it is a bulk carrier with a high, dense cargo and a very stiff stability character. The IS Code recognizes that combination carriers can need supplementary operational procedures rather than pure design measures to meet the criteria, because the range of conditions is wider than a dedicated tanker’s.

The operational dimension is the recognition that not every Reg 27 condition can be solved by the hull form. A dedicated crude carrier meets the criteria by design across its load envelope. A combination carrier with a more variable cargo can lean on approved operational procedures, the loading-sequence controls in the stability information, to keep the intermediate and slack-tank conditions inside the floors. The regulation allows this because the object is a stable ship in service, not a hull form that passes in the abstract; an approved procedure that keeps the ship above the floors is as good as a hull that never needs the procedure.

This is also where Reg 27 connects to SOLAS Chapter II-1 construction, subdivision, and stability. The combination carrier’s intact-stability approval under the IS Code, mandatory through SOLAS II-1/5, and its MARPOL Reg 27 approval are the same calculation read twice. The operational procedures that satisfy Reg 27 are the procedures recorded in the SOLAS-approved stability information. A surveyor checking either certificate is checking the same approved booklet and the same loading instrument, which is the practical benefit of the two regimes sharing the IS Code Part A numbers.

A worked intermediate-condition check

A concrete example shows how the floors bite. Take a product tanker of 45,000 t deadweight discharging at a terminal. The fully loaded departure condition cleared Reg 27 comfortably, with a free-surface-corrected GM0 of 0.95 m, because a laden tanker carries its cargo low and the tanks are pressed full with little free surface. The arrival-ballast condition also cleared, with GM0 of 0.78 m, because the segregated ballast tanks are full and the cargo tanks empty, again with few slack tanks. The question Reg 27 asks is what happens in between.

At the half-discharge point, six of twelve cargo tanks sit half-empty while three pairs of segregated ballast tanks fill to maintain immersion. Suppose the cargo tanks are 12 m wide and 30 m long at the part-full level, giving each a transverse free-surface moment of inertia of roughly $i = \frac{l \cdot b^3}{12} = \frac{30 \cdot 12^3}{12} = 4{,}320\ \text{m}^4$ . Six such tanks slack at once, with cargo of density 0.84 t/m^3, raise the virtual center of gravity by:

\Delta GM_{fs} = \frac{\sum i \cdot \rho_{cargo}}{\Delta} = \frac{6 \cdot 4{,}320 \cdot 0.84}{40{,}000} \approx 0.54\ \text{m}

against a displacement of about 40,000 t at that stage. The three filling ballast tanks add their own free surface while they are slack; say they contribute another 0.18 m of virtual rise at the midpoint of their fill. The solid GM at this displacement might be 1.20 m from the hydrostatics, but after the 0.54 m cargo correction and the 0.18 m ballast correction, the corrected GM0 falls to about 0.48 m. That still clears the 0.15 m floor, so this sequence passes; but the example shows how a condition that starts at 0.95 m and ends at 0.78 m can dip far below both in the middle. A poorly ordered sequence, one that lets nine cargo tanks go slack at once while ballast also fills, can push the same ship under the 0.15 m floor.

The lesson the example carries is the value of the approved sequence. The operator who works the cargo tanks in pairs, emptying one fully before starting the next, keeps fewer tanks slack at the worst moment and holds GM well above the floor throughout. The operator who opens all tanks together to speed the discharge collects the full free-surface penalty at once. Reg 27 does not prescribe the order; it sets the floor and leaves the loading instrument to prove that the chosen order stays above it. The arithmetic above is illustrative of the mechanism rather than a specific ship’s approved figures, which come from that ship’s hydrostatics and tank geometry.

How Reg 27 developed and where the numbers come from

The intact-stability numbers in Reg 27 are older than the regulation. The 0.055, 0.09, and 0.03 m,rad area criteria, the 0.20 m lever at 30 degrees, the 25-degree minimum angle of maximum lever, and the 0.15 m GM trace back to the statistical analysis of ship casualties carried out for the IMO in the development of resolution A.167 in the 1960s and consolidated through resolution A.749(18), the Code on Intact Stability adopted in 1993. The 2008 IS Code, resolution MSC.267(85), updated and made mandatory the criteria that A.749(18) had carried as recommendation. So when Reg 27 incorporates the criteria, it incorporates a set of numbers with a long casualty-analysis pedigree, not figures invented for tankers.

The MARPOL side is newer. Reg 27 in its current form belongs to the revised Annex I that entered force on 1 January 2007, the consolidation that renumbered and reorganized the annex into its present chapter structure. The 1 February 2002 delivery trigger predates that renumbering: it marks the construction date from which the newer tankers, the ones designed under the double-hull and SBT regime, were brought under the dedicated intact-stability criteria. Pairing the 2002 trigger with the criteria means the tankers most shaped by the pollution-prevention tank arrangements are the ones held to the matching stability floor, which is the design logic behind the date.

The verification machinery came later still. The stability-instrument requirement under Regulation 28, adopted by resolution MEPC.248(66) on 4 April 2014 and in force from 1 January 2016, closed the last gap: it put an approved onboard means of checking the Reg 27 and Reg 28 criteria into the hands of the master, with a defined phase-in for existing tankers reaching to 1 January 2021. Before that amendment, a tanker met the criteria through its approved booklet and standard conditions; after it, a tanker built from 2016 carries an instrument that checks any condition, which tightened the in-service verification of Reg 27 from a booklet lookup to a live calculation.

The currency point for a practitioner is the version. A tanker’s stability approval references a specific edition of the IS Code and a specific consolidated edition of Annex I, and the paragraph numbering inside Reg 28 shifted across amendments, with the stability-instrument paragraph renumbered as the annex was revised. When citing the requirement for a particular ship, the construction date fixes which consolidated text applies, which is why the delivery date is not just an application trigger but the key to reading the right version of the rule.

Verification, survey, and port-state control

Reg 27 is verified at two points: at approval, when the tanker’s stability information is built and checked, and in service, when the loaded condition is verified against the approved information. At approval, the Recognized Organization acting for the flag Administration reviews the stability booklet, confirms that the standard loading conditions meet the Reg 27 floors with the free-surface correction applied and the ballast tanks assumed slack, and approves the booklet and the loading instrument data file. The approval is recorded so that the in-service checks have a reference.

In service, the master verifies each departure condition against the approved information using the loading instrument, and retains the calculation. The record-keeping is part of the regime: after verifying that the vessel meets the applicable stability requirements, the master retains the draft, trim, and supporting stability calculations on board for at least the duration of the voyage. A port-state control officer inspecting under MARPOL can ask for the approved stability booklet, the loading instrument output for the current condition, and the retained calculations, and can compare them. A mismatch between the instrument and the booklet, or a current condition that the instrument shows below the Reg 27 floors, is a detainable deficiency, because it means the tanker is working outside its approved stability envelope.

The verification chain is why the exact identifiers matter. A surveyor checks the tanker against the approved booklet that the RO signed, against the IS Code Part A criteria carried into Reg 27, and against the loading instrument approved under MSC.1/Circ.1229. Each link is a documented approval with a version. “The tanker is stable” is not a finding; “the departure condition shows GM0 of 0.34 m corrected for free surface and clears all four curve criteria against the approved booklet” is.

Limitations

Reg 27 is an intact-stability rule and nothing more. It does not address the ship after damage, which is the separate domain of Regulation 28 and, for the broader fleet, of the probabilistic damage-stability framework in SOLAS. A tanker can pass Reg 27 in every intact condition and still fail Reg 28 after assumed damage; the two checks are independent and a loading condition has to clear both.

The criteria are static floors, not a dynamic seakeeping assessment. The five numbers measure initial stiffness and the area and shape of the righting-lever curve, which correlate with resistance to capsize, but they do not model parametric roll, broaching, dead-ship behavior in a beam sea, or the second-generation intact-stability phenomena the IMO has addressed through later guidelines. A tanker that meets Reg 27 has adequate static reserve; it can still be vulnerable to a specific dynamic failure mode that the general criteria do not capture. The second-generation intact-stability criteria sit outside Reg 27 as guidance for ships whose hull form flags a vulnerability.

The 5,000 t deadweight floor and the 1 February 2002 delivery date leave a population outside Reg 27. A 4,000 t deadweight product tanker, or a tanker delivered in 1998, is not bound by Reg 27 and is held instead to the intact-stability requirements of its flag and class, typically the IS Code as mandatory under SOLAS for ships of 24 m and over. The pollution-prevention hook that Reg 27 gives a port-state officer does not reach those ships through Annex I; the safety hook through SOLAS still does.

The free-surface assumption is conservative by design and can be more severe than the real condition. Reg 27 assumes the ballast tanks slack under all conditions, which penalizes a tanker that in practice presses up its ballast tanks. The regulation accepts that conservatism to guarantee a floor that holds even when tanks are worked badly. The cost is that a tanker may carry more design GM than a less conservative assumption would require, which is a deliberate trade of cargo flexibility for a guaranteed margin.

Finally, Reg 27 verifies the ship as approved, not the ship as it has aged. The booklet rests on the lightship weight and center of gravity from the inclining experiment at delivery, and on tank geometry as built. Steel renewals, added equipment, sediment in tanks, and modifications shift the real lightship away from the approved value over a tanker’s life. The lightweight check at periodic survey and any required re-inclining keep the approved data honest, but between those checks the Reg 27 margin is computed against the as-approved lightship, not the current one. A tanker whose actual KG has crept up since delivery can be closer to the floors than its booklet shows. The discipline of lightweight versus deadweight accounting and periodic deadweight surveys is what keeps that drift bounded.

Frequently asked questions

Which oil tankers must meet MARPOL Annex I Regulation 27?

Every oil tanker of 5,000 tonnes deadweight and above delivered on or after 1 February 2002, as defined in regulation 1.28.7. The tanker must satisfy the Reg 27 intact-stability criteria at any operating draft under the worst conditions of cargo and ballast loading, including intermediate stages of liquid transfer.

What is the minimum GM under Regulation 27?

The initial metacentric height GM0, corrected for free-surface effect and measured at zero heel, must not be less than 0.15 m. The same 0.15 m floor applies in port and at sea.

What are the GZ-curve area criteria in Reg 27?

The area under the righting-lever curve must be at least 0.055 metre-radian to 30 degrees of heel, at least 0.09 metre-radian to 40 degrees (or the downflooding angle if smaller), and at least 0.03 metre-radian between 30 and 40 degrees. The righting lever must be at least 0.20 m at 30 degrees, and the maximum lever must fall at a heel preferably above 30 degrees and not below 25 degrees.

Why does an environmental convention set a stability rule?

A tanker that loses transverse stability and lists, capsizes, or breaks up is a pollution casualty. Reg 27 keeps the cargo inside the hull, which is the same purpose served by the double-hull (Reg 19) and accidental-outflow (Reg 23) rules. Stability is pollution prevention by another mechanism.

How does Reg 27 relate to the 2008 IS Code?

Reg 27 incorporates the general intact-stability criteria of the 2008 IS Code (resolution MSC.267(85)), which is also mandatory under SOLAS regulation II-1/5. The numeric thresholds in Reg 27 are the IS Code Part A general criteria, so a tanker that meets the IS Code Part A floor over its MARPOL load cases meets Reg 27.