MARPOL Annex I Reg 28: tanker damage stability

Background: MARPOL 73/78 to consolidated 2025 edition

The damage stability rules for oil tankers entered the MARPOL framework through the 1973 International Convention for the Prevention of Pollution from Ships, which was overlaid by the 1978 Protocol following the Amoco Cadiz, Argo Merchant and Olympic Bravery casualties of the mid-1970s. The original Annex I, in force from 2 October 1983, contained subdivision and damage-stability text in Regulation 25 for new tankers above 5,000 deadweight tonnes (DWT) and prescribed deterministic damage extents that have, in their geometric form, persisted with only minor revision through every subsequent amendment cycle. The deterministic philosophy (pick a credible damage extent, apply it anywhere along the hull, demonstrate that the ship survives) was inherited from the 1966 International Convention on Load Lines (ICLL) and the 1960 SOLAS subdivision rules, and it was deliberately retained when probabilistic damage stability was introduced for passenger and dry-cargo ships in the 1990s.

Regulation 25 was supplemented in 1992 by the MEPC.51(32) amendments introducing Regulation 13F double-hull rules and the Regulation 13G phase-out programme for single-hull tankers, but the damage stability text itself remained largely unchanged. The 2003 acceleration of the single-hull phase-out under MEPC.111(50) following the Erika (1999) and Prestige (2002) casualties added political momentum for a full revision of Annex I. That revision was adopted at MEPC 52 by Resolution MEPC.117(52) on 15 October 2004 and entered into force on 1 January 2007. The MEPC.117(52) text consolidated and renumbered the entire Annex: the deterministic damage stability rule moved from Regulation 25 to Regulation 28 for tankers delivered on or after 1 February 2002 of 150 gross tonnage (GT) and above, with a transitional Regulation 25 retained for pre-2002 tankers and a Regulation 27 for intermediate tonnage.

The 2025 consolidated edition of MARPOL, published as IMO sales product IB520E, retains Regulation 28 in its MEPC.117(52) form with editorial amendments under MEPC.248(66) (Polar Code coordination) and MEPC.296(72) (IGF Code cross-references). The substantive deterministic damage extents, the Regulation 28.3 survival envelope and the Regulation 28.5 wave-bending-moment criterion are unchanged from the 2007 entry-into-force text. The 2008 MSC.281(85) Explanatory Notes coordinated SOLAS Chapter II-1 Part B-1 probabilistic damage stability with the MARPOL Annex I deterministic regime so that newbuild tankers subject to both regimes (a chemical-oil products tanker, for example) face one harmonised loading instrument and one set of approved damage cases.

The Regulation 28 regime applies to a tanker if any one of the following dates is on or after 1 February 2002: the building contract date, the keel-laid date (in the absence of a contract), or the delivery date. Tankers contracted before 1 February 2002, keel-laid before 1 August 2002, and delivered before 1 February 2005 fall under the legacy Regulation 25, which uses the same deterministic philosophy but slightly different damage extents and survival thresholds for tankers above 5,000 DWT and a less stringent regime below that threshold. Regulation 27 captures the intermediate tonnage band of small tankers (150 to 5,000 GT) delivered before 1 February 2002.

Damage extent and survival criteria: reference table

The table below collects the Regulation 28.1 damage extents and the Regulation 28.3 survival criteria in one place for quick reference, using $L$ for rule length in metres between perpendiculars, $B$ for moulded breadth in metres, and noting that every “whichever is the lesser” rule means the smaller of the two values is taken as the assumed damage.

Reg 28.1 deterministic damage extents: side damage

The side damage extent under Regulation 28.1.1 is defined by three orthogonal dimensions, applied at any point along the length of the tanker between forward and after perpendiculars. In every case the operative value is the lesser of the formula result and the capping absolute dimension: the regulation sets a maximum assumed damage, not a minimum.

Longitudinal extent: the lesser of $L^{2/3}/3$ or 14.5 m, where $L$ is the rule length in metres. For a 230 m Suezmax, the formula gives $230^{2/3}/3 = 37.2/3 = 12.4$ m, which is less than 14.5 m, so the assumed damage is 12.4 m. For a 320 m VLCC, the formula gives $320^{2/3}/3 = 46.4/3 = 15.5$ m, which exceeds 14.5 m, so the damage is capped at 14.5 m. The breakpoint where the formula and the cap are equal is at $L = (3 \times 14.5)^{3/2} = 43.5^{3/2} \approx 287$ m: every tanker above roughly 287 m between perpendiculars uses the 14.5 m flat cap.

Transverse extent: lesser of $B/5$ or 11.5 m, where $B$ is the moulded breadth in metres. For a 32 m Panamax tanker, $B/5 = 6.4$ m, which is less than 11.5 m, so the assumed transverse damage is 6.4 m. For a 60 m VLCC, $B/5 = 12.0$ m, which exceeds 11.5 m, so the cap governs at 11.5 m. The 11.5 m cap applies to hulls above $5 \times 11.5 = 57.5$ m breadth: in practice, Aframax, Suezmax, and Panamax tankers are governed by $B/5$, while the broadest VLCCs hit the cap.

Vertical extent: unlimited from the baseline upward. The damage is taken to extend through the full depth of the side shell from the keel to the freeboard deck, so that any cargo, ballast or void space lying within the longitudinal-transverse footprint is treated as flooded.

The damage is applied at any longitudinal position along the cargo block and the engine room region, and the worst case for survival governs. In practice the worst side-damage cases for a typical double-hull tanker are at the midship region (where the largest cargo volumes flood through a single damage event), at the engine-room forward bulkhead (where machinery space flooding combines with cargo tank flooding through transverse permeability), and at the forepeak and afterpeak transitions (where free-surface effects in the void spaces are most severe).

The deterministic side-damage extent under Regulation 28 aligns with the damage extents historically used in SOLAS deterministic damage stability, and it is broadly consistent with the probabilistic damage extents assumed in the SOLAS Chapter II-1 Part B-1 probabilistic regime for non-tankers, though the application philosophy differs: SOLAS B-1 applies the damage probabilistically to each compartment with calculated p, s, and v factors, whereas MARPOL Reg.28 applies the damage deterministically at the worst location.

Reg 28.1 deterministic damage extents: bottom damage

The bottom damage extent under Regulation 28.1.2 is again defined by three orthogonal dimensions and applied at any longitudinal position. Bottom damage represents a grounding event rather than a collision, and the extents are smaller in the transverse and vertical directions than the side-damage values, reflecting the more limited lateral reach of a grounding contact.

Longitudinal extent: the lesser of $L^{2/3}/3$ or 14.5 m, identical to the side-damage longitudinal rule.

Transverse extent: lesser of $B/6$ or 10 m. For a 32 m Panamax tanker, $B/6 = 5.3$ m; for a 60 m VLCC, $B/6 = 10.0$ m and the cap kicks in at exactly $B = 60$ m. The $B/6$ ratio is narrower than the $B/5$ used for side damage and reflects the limited lateral spread of a grounding obstruction such as an isolated rock or submerged wreck.

Vertical extent: lesser of $B/15$ or 6 m, measured from the baseline upward. For a 32 m Panamax, $B/15 = 2.13$ m; for a 60 m VLCC, $B/15 = 4.0$ m; the 6 m cap applies at $B \geq 90$ m, a breadth reached by the largest VLCCs. The $B/15$ ratio was calibrated against the double-bottom height required for newbuild tankers under Regulation 19: a tanker with the minimum Reg.19 double-bottom height of $B/15$ will have the bottom damage extent penetrate the outer bottom but not breach the inner bottom, confining flooding to the double-bottom void or ballast space.

Regulation 28.1.3 separately requires that bottom damage in the forward 0.3L of the ship be assumed at any point along that forward region as a distinct damage case, with the same transverse and vertical extents but the longitudinal extent measured from the forward perpendicular aft. This forward bottom damage case models the “grounding while approaching” scenario. It is often the governing case for VLCCs and Suezmaxes because forepeak free-surface effects, combined with inner-bottom breach flooding through the forward cargo tanks, can drive a severe trim toward the bow that reduces forward freeboard below the survival margin.

Reg 28.3 post-damage stability survival criteria

Once the deterministic damage extent has been applied at the worst location, Regulation 28.3 requires the tanker to satisfy a four-part post-damage stability survival envelope. All four criteria must hold simultaneously at the equilibrium damaged condition for every damage case along the length of the ship and for every loading condition in the approved stability booklet.

The four criteria are:

Equilibrium positive metacentric height GM at or above 0.10 m: the residual transverse metacentric height in the equilibrium damaged condition, after free-surface corrections for all flooded spaces, must be at least 0.10 m. The 0.10 m floor is a deliberate margin above neutral stability (GM = 0) and ensures that small disturbances such as wind gusts, swell, or shifting consumables don’t cause the tanker to capsize. Class stability software corrects for free surface in flooded compartments using the MSC.281(85) Annex 4 method, which treats partially flooded compartments as having a free surface at the equilibrium waterline.

Range of stability at or above 20 degrees beyond equilibrium: the static GZ curve in the damaged condition must remain positive over a range of at least 20 degrees of heel angle, measured from the equilibrium angle to the angle of vanishing stability or the angle of progressive flooding, whichever is smaller. The 20-degree reserve provides capacity against transient heel from wave action, wind, and asymmetric flooding through openings.

Positive righting-lever area at or above 0.0175 m·rad: the integral of the GZ curve over the 20-degree positive range must be at least 0.0175 m·rad. This area criterion checks whether the dynamic energy available to right the ship from a transient heel is sufficient to absorb credible wave-induced heeling moments. The 0.0175 m·rad threshold matches the post-damage area criterion in SOLAS II-1 Part B-1 for cargo ships and represents the minimum safe dynamic stability margin established through the 1990s subdivision research programmes.

Angle of heel at equilibrium no greater than 25 degrees: the equilibrium heel angle measured from upright must not exceed 25 degrees. The limit relaxes to 30 degrees if the deck edge does not immerse before the angle of progressive flooding, as stated in Regulation 28.3.3. The 25-degree cap protects crew evacuation, deck operations, and the integrity of weathertight closures during the damaged condition. Regulation 28.3.4 adds that no opening which could lead to progressive flooding of additional compartments may be submerged at any point on the GZ curve up to the required range of stability.

These criteria apply at all permeabilities prescribed in Regulation 28.4: 0.95 for void spaces, 0.85 for stores and machinery, 0.95 for accommodation, and 0.0 for full cargo tanks (the cargo displaces the incoming water). Empty cargo tanks and ballast tanks use 0.95.

Damage extents collected as equations

The sections above give each Regulation 28.1 extent in words and work the numbers for several hull sizes. Collected as inline equations, with $L$ the rule length in metres between perpendiculars and $B$ the moulded breadth in metres, the three orthogonal damage extents are:

For the longitudinal extent (shared by side and bottom cases):

For the side-damage transverse extent:

The side-damage vertical extent is unlimited from the baseline upward.

For the bottom-damage transverse and vertical extents:

The Regulation 28.3 post-damage survival envelope, which must be satisfied at equilibrium under every governing damage case:

Here $\text{GM}_{\text{eq}}$ is the equilibrium metacentric height in the damaged condition with free-surface correction; $\theta_{\text{range}}$ is the range of the positive GZ curve from equilibrium to the angle of vanishing stability or progressive flooding; $A_{\text{righting}}$ is the area under the GZ curve over that range; and $\theta_{\text{eq}}$ is the equilibrium heel angle.

Wave bending moment and shear force criterion (Reg 28.5)

Regulation 28.5 introduces a longitudinal-strength check unique to the MARPOL Annex I tanker regime and not paralleled in the SOLAS damage stability framework. After damage and free-surface flooding, the tanker’s vertical bending moment and vertical shear force distribution along the hull must remain within the still-water plus wave-induced design envelope from the IACS Common Structural Rules for Bulk Carriers and Oil Tankers (CSR-BC&OT), or the equivalent class-society longitudinal strength rule for tankers built under earlier rules.

The damage stability flooding alters the buoyancy distribution along the hull length. A flooded midship cargo tank, for example, increases local downward weight and reduces the local buoyancy reserve, redistributing the still-water bending moment. The class-society rule envelope must accommodate this redistribution; if the resulting damaged-condition bending moment or shear force exceeds the rule envelope at any longitudinal station, the tanker fails Regulation 28.5 even if the Regulation 28.3 survival criteria are met.

In practice, Regulation 28.5 is checked by the class-society stability software alongside the Regulation 28.3 GZ curve calculation. The post-damage condition output feeds into the longitudinal strength module, which compares the resulting bending moment and shear force at each frame station against the still-water-plus-wave envelope. The tanker passes Regulation 28.5 only if every station is within envelope. This criterion has caused several newbuild design iterations during contract design review where the as-drawn cargo tank arrangement produced acceptable Reg 28.3 survival but exceeded the longitudinal strength envelope under midship damage; the standard remedy is to add a transverse cofferdam to limit the flooded volume of the affected cargo block.

Relationship to Reg 23 accidental oil outflow probabilistic methodology

Regulation 23 introduces an accidental oil outflow performance parameter that operates on a probabilistic basis rather than the deterministic Reg 28 basis. Whereas Reg 28 asks “after this prescribed damage, does the ship survive?”, Reg 23 asks “across the credible distribution of damage events, what is the expected oil outflow?”. The two regulations are complementary: Reg 28 ensures that the tanker does not sink after a defined collision or grounding, while Reg 23 ensures that the cargo loss into the sea, integrated over the damage probability distribution, remains below a threshold tied to the deadweight of the ship.

Regulation 23 applies a probabilistic damage location and extent distribution derived from the IMO casualty database, calculates the cargo outflow from each combination of damaged compartments, and integrates the result to produce a mean oil outflow parameter O_M, a bottom outflow parameter O_MB and a side outflow parameter O_MS. The acceptance criterion ties these parameters to the deadweight in tonnes through a non-linear scaling formula. Regulation 23 does not replace Regulation 28; both must be satisfied independently for a newbuild double-hull tanker. The probabilistic methodology is documented in the MEPC.122(52) Explanatory Notes and was the technical basis for the Regulation 12A oil fuel tank protection Regulation 12A.10 probabilistic equivalent route for non-tanker bunker tanks.

Relationship to MARPOL Annex II Reg 16 chemical-tanker damage rules

MARPOL Annex II governs noxious liquid substances carried in bulk on chemical tankers, and its Regulation 16 prescribes a deterministic damage stability regime closely analogous to MARPOL Annex I Regulation 28 but tuned to the IBC Code ship-type classification (Type 1, Type 2, Type 3). Type 1 chemical tankers (the most hazardous cargoes such as fuming acids and reactive monomers) face the strictest damage extents and a survival envelope that uses the same Reg 28.3 GM, range, area and angle thresholds applied to a more demanding damage location set. Type 2 and Type 3 tankers face progressively relaxed extents.

For an oil-chemical product tanker carrying cargoes covered by both Annex I and Annex II, the more stringent of the two regimes applies at each location along the hull. The IBC Code Section 2.6 cross-references MARPOL Annex II Regulation 16 and the analogous SOLAS II-1 probabilistic methodology for non-IBC ships. In practice, the class-society loading instrument under MSC.337(91) runs both Reg 28 and IBC Code damage cases in a single batch and reports the governing case. The harmonised approach was codified by the 2008 MSC.281(85) Explanatory Notes that bridged the SOLAS, MARPOL Annex I, and MARPOL Annex II / IBC Code damage stability regimes for ships subject to multiple instruments.

SOLAS Chapter II-1 Part B-1 probabilistic damage stability for non-tankers

SOLAS Chapter II-1 Part B-1 introduces a probabilistic damage stability regime for cargo ships above 80 m and passenger ships of any length, in force for newbuilds delivered on or after 1 January 2009 under the MSC.216(82) amendments. The probabilistic regime calculates an attained subdivision index A by summing across all credible damage cases the product of three factors: p (probability the compartment is damaged), s (probability of survival given the damage), and v (probability the damage extent is vertically limited). The attained index A must equal or exceed a required subdivision index R calculated from ship length, deadweight, and complement.

The probabilistic regime under SOLAS Part B-1 does not apply to oil tankers, which remain under the deterministic MARPOL Annex I Regulation 28. The two regimes use different mathematical machinery, different damage location distributions, and different survival criteria. A common design-review error is to confuse the two: a tanker designer who applies the SOLAS B-1 attained-index methodology produces an answer with no regulatory standing under MARPOL, and a non-tanker designer who applies the deterministic Reg.28 extents will overconstrain the cargo arrangement.

For the rare ship subject to both regimes (an oil-chemical-products carrier that also carries dry containers on deck, for example), the loading instrument runs each regime independently and reports both compliance certificates. The harmonised MSC.281(85) explanatory notes coordinate the input data so that the same hull-form geometry, the same loading conditions, and the same approved damage extents feed both calculations, and the surveyor verifies both at IOPP and SOLAS renewal surveys.

Class society implementation: DNV, LR, ABS, BV, KR, NK, RINA, CCS, RS, IRS

The major class societies implement Regulation 28 through dedicated tanker rule chapters that translate the IMO text into detailed engineering rules and approval procedures:

• DNV: Rules for Classification of Ships, Pt.5 Ch.8 Tankers, supplemented by DNV-CG-0145 Damage Stability class guideline, which prescribes the loading instrument acceptance procedure, the approved damage case library and the class survey checklist for IOPP renewal.
• Lloyd’s Register: Rules and Regulations for the Classification of Ships, Part 4 Chapter 9, with worked side-damage and bottom-damage examples and the loading instrument approval workflow under MSC.337(91).
• American Bureau of Shipping (ABS): Rules for Building and Classing Steel Vessels, Part 5C Chapter 1 for tanker subdivision and damage stability, with the harmonised SOLAS-MARPOL loading instrument procedure and the IOPP renewal-survey verification list.
• Bureau Veritas (BV): NR467 Rules for the Classification of Steel Ships, Part D Chapter 7 for oil tankers, including the Reg 28.3 survival criteria and Reg 28.5 wave bending moment check in a single approval workflow.
• Korean Register (KR): Rules for the Classification of Steel Ships, Part 7 Chapter 1 for tankers, with the Korean shipyard-specific design templates that integrate Reg 28 and Reg 23 in the contract design phase.
• Nippon Kaiji Kyokai (ClassNK): Rules for the Survey and Construction of Steel Ships, Part R, with the IOPP renewal-survey damage stability documentation checklist used at the major Japanese shipyards.
• Registro Italiano Navale (RINA): Rules for the Classification of Ships, Part E Chapter 7, integrating Reg 28 with the Mediterranean-region oil pollution preparedness rules.
• China Classification Society (CCS): Rules for the Classification of Sea-Going Steel Ships, Part 6 Chapter 1, with the Chinese shipyard newbuild approval workflow for VLCCs and Suezmaxes built at the Bohai Bay yards.
• Russian Maritime Register of Shipping (RS): Rules for the Classification and Construction of Sea-Going Ships, Part V, with the ice-class tanker damage stability supplements for the Northern Sea Route.
• Indian Register of Shipping (IRS): Rules and Regulations for the Construction and Classification of Steel Ships, Part 5 Chapter 7, with the coastal tanker fleet implementation rules.

The International Association of Classification Societies (IACS) publishes Unified Interpretations in the MPC (MARPOL Pollution Convention) and SC (SOLAS Convention) series that harmonise the interpretation of Regulation 28 across member societies. The current IACS UI MPC and SC papers cover the deterministic damage extents, the probabilistic Reg 23 methodology, the loading instrument acceptance procedure under MSC.337(91), and the IOPP renewal-survey documentation list. Member societies are bound to apply the IACS UIs in their own rule implementations, producing a high degree of consistency across class societies in practice.

IMO Hull Damage Risk Tool for novel hull forms

For novel hull forms that do not fit the standard tanker geometry assumed by Regulation 28 (a trimaran tanker, a SWATH tanker, a wing-in-ground-effect cargo carrier), the IMO maintains a Hull Damage Risk Tool that allows designers and flag administrations to verify an equivalent level of safety on a probabilistic basis. The tool draws on the IMO casualty database, applies a Monte Carlo simulation of damage location and extent, and calculates the resulting outflow and survival statistics. The output is compared against the equivalent statistics for a standard tanker meeting Regulation 28 deterministically; if the novel hull form matches or exceeds the standard tanker’s safety profile, the flag administration may grant an equivalent arrangement approval under MARPOL Annex I Regulation 5.

The Hull Damage Risk Tool has been used in fewer than ten newbuild approvals to date, primarily for research vessels and specialised offshore tankers. The mainstream double-hull tanker fleet relies on the deterministic Regulation 28 route because the cost of preparing a probabilistic equivalent submission, including the Monte Carlo damage simulation and the casualty-database benchmarking, exceeds the cost savings achievable from a non-standard tank arrangement. The tool nonetheless provides a regulatory pathway for genuinely novel designs and is referenced in the MEPC and MSC explanatory notes as the recommended method for equivalent arrangement verification.

ICLL freeboard interaction

The 1966 International Convention on Load Lines (ICLL), as amended by the 1988 Protocol and the MSC.143(77) revisions of 2003, prescribes the assigned summer freeboard as the maximum permissible loaded draught for a given hull form, taking into account the hull’s reserve buoyancy, deck wetness margin, and longitudinal strength envelope. The ICLL freeboard interacts with MARPOL Regulation 28 because the post-damage condition must be evaluated starting from the draught corresponding to the assigned freeboard, not from any deeper or shallower draught chosen by the designer.

A tanker operationally ballasted to a draught below the ICLL summer mark will, in the damaged condition, sink deeper as flooding progresses, and the equilibrium waterline used for the Reg 28.3 check is the post-flooding equilibrium rather than the pre-flooding operational waterline. The ICLL summer mark sets an upper bound on the intact draught used in the damage stability calculation, and the ballast draughts and partial-load draughts between the ballast mark and the summer mark are each evaluated as separate loading conditions in the Regulation 28 worst-case search. This is why the approved trim and stability booklet for a VLCC typically contains 8 to 14 conditions, and a product tanker 10 to 16, covering the full operational draught range.

Master’s loading manual and on-board verification

The loading manual is the on-board document, approved by the flag administration through the recognised organisation, that contains the intact and damaged stability calculations for every loading condition the master may use during the tanker’s service life. Under Regulation 28 the loading manual must:

• List every standard intact loading condition (homogeneous full load, partial load patterns, ballast departure and arrival, intermediate fuel consumption states), with the resulting GM, trim, longitudinal bending moment, and shear force.
• Present, for each intact condition, the damaged condition for every governing damage case along the length of the tanker, with the resulting equilibrium GM, range of stability, righting-lever area, angle of heel, and progressive flooding angle.
• Confirm that every damaged condition meets the Regulation 28.3 survival envelope and the Regulation 28.5 wave bending moment criterion.
• Provide the master with an operational envelope of permissible cargo distribution, ballast distribution, and consumable consumption states that, by interpolation between calculated conditions, guarantees compliance with Regulation 28.

In addition to the printed loading manual, every newbuild tanker delivered after 1 January 2014 under the MSC.337(91) revision carries an approved stability instrument: a class-approved loading computer that runs the damage stability calculation in real time on the master’s bridge or cargo control room console. The instrument lets the master verify a non-standard loading condition (a partial cargo load, an unusual ballast pattern during tank cleaning, a heavy-weather ballast configuration) against Regulation 28 before it is executed. The stability instrument approval procedure under MSC.337(91) recognises three types: Type 1 (intact stability only), Type 2 (intact plus deterministic damage), and Type 3 (intact plus deterministic damage plus probabilistic Reg 23 outflow). Tankers under MARPOL Annex I Regulation 28 must carry at least a Type 2 instrument; most modern newbuilds carry Type 3 to support both Reg 28 and Reg 23 verification.

MEPC approval procedures: MSC.281(85), MSC.292(87), MSC.337(91)

Three IMO Maritime Safety Committee resolutions govern the approval procedures for damage stability calculation methods and on-board stability instruments used to verify Regulation 28 compliance:

MSC.281(85), adopted 4 December 2008, Explanatory Notes to the SOLAS Chapter II-1 Subdivision and Damage Stability Regulations: this resolution coordinates the SOLAS B-1 probabilistic regime with the MARPOL Annex I deterministic Regulation 28 regime, harmonising input data, loading conditions, and permeability assumptions for ships subject to both. The Annex 4 free-surface treatment, the Annex 5 progressive flooding methodology, and the Annex 6 wind heeling moment definition are referenced by class societies in their Reg 28 approval workflows.

MSC.292(87), adopted 21 May 2010, Amendments to the procedures for approval of damage stability calculation methods: this resolution introduces the direct calculation method for damage stability and prescribes the verification process for software packages used by class societies and shipyards. The major stability software platforms (NAPA, GHS, AVEVA Marine, BV Mars, DNV Nauticus Hull, ABS Eagle Edge) are approved under this resolution to perform the Reg 28 calculation.

MSC.337(91), adopted 30 November 2012, Revised guidelines for the approval of stability instruments: this resolution sets the Type 1 / Type 2 / Type 3 classification of on-board stability instruments and prescribes the approval testing procedure, documentation package, and surveyor verification at delivery and each periodic survey. All MARPOL Annex I tankers built after the resolution’s application date carry a stability instrument approved under MSC.337(91), and the instrument’s installation and software version are checked at every IOPP renewal survey.

Class survey and IOPP renewal-survey verification

The International Oil Pollution Prevention (IOPP) Certificate under MARPOL Annex I Regulation 7 is issued for a maximum of five years and renewed at a renewal survey within three months before the certificate’s expiry date. At the renewal survey, the class surveyor (acting as the recognised organisation under the flag administration’s authority) verifies:

• The loading manual is on board, current, and contains the approved damage stability calculations for every standard loading condition.
• The stability instrument under MSC.337(91) is operational, the software version matches the approved version, and the instrument’s input data (lightship, capacity tables, free-surface tables) matches the as-built hull condition, allowing for any surveyed lightship verification.
• The damage stability documentation issued by the class society at delivery (the trim and stability booklet and the damage stability calculation report) is on board and corresponds to the current ship configuration, with any modifications since the last survey reflected in updated calculations and revised approved versions.
• The subdivision arrangement matches the as-built drawings, with all watertight bulkheads, watertight doors, weathertight closures, and progressive flooding boundaries in good condition and tested at the survey.

The renewal survey’s damage stability check is integrated with the broader MARPOL renewal survey under Regulation 6 covering the oil filtering equipment, the discharge control, and the oil record book. Failure of the damage stability check at renewal results in a statutory deficiency, and the IOPP certificate is not renewed until the deficiency is rectified, which in extreme cases (a substantial conversion or structural modification that invalidates the original damage stability calculation) requires a full re-approval of the loading manual and the stability instrument software.

Newbuild design impact: subdivision and cargo tank arrangement

Regulation 28 imposes a substantial constraint on the cargo tank arrangement of newbuild tankers because the damage extents prescribed in Reg 28.1 must be survived at any longitudinal position. The classical response, refined through three decades of double-hull tanker design, is the transverse cofferdam between cargo blocks that limits the volume of cargo flooded by any single damage event. A typical Suezmax cargo block contains six pairs of cargo tanks divided by transverse cofferdams at approximately 30 to 40 m intervals, limiting each Reg 28 damage case to flooding two adjacent cargo tanks plus the wing ballast space.

The longitudinal double-hull bulkhead required by Regulation 19 interacts with Regulation 28 because the damage extent transversely ($B/5$ for side, $B/6$ for bottom) generally penetrates the outer hull but stops at or just beyond the longitudinal bulkhead of the double hull, depending on the breadth and chosen wing-tank width. Designers tune the wing tank width and the double-bottom height to balance Reg 19 protective volume requirements against Reg 28 damage stability survival. A wider wing tank improves Reg 19 outflow performance but moves more cargo volume into centreline tanks, which can increase the post-damage free-surface contribution from a single damaged centreline tank.

The freight efficiency penalty of Regulation 28 is small for the modern double-hull tanker fleet because the deterministic damage stability regime has been integrated into the standard newbuild templates of the major tanker shipyards (Hyundai Heavy Industries, Samsung Heavy Industries, Daewoo Shipbuilding & Marine Engineering, Mitsui E&S, Imabari Shipbuilding, Japan Marine United, China State Shipbuilding Corporation, China Shipbuilding Industry Corporation). The standard VLCC, Suezmax, Aframax, Panamax, and Handysize templates each have a baseline cargo volume approximately 5 to 8 percent below the geometric maximum that would be permitted without Reg 28, with that volume returned in the form of wing ballast tanks, transverse cofferdams, and the Reg 19 double-hull protective volume.

Intact vs damage stability: distinction and governing regime

Intact stability governs the safety of the tanker in its undamaged operational state and is regulated by the 2008 IS Code (the Intact Stability Code) adopted as Resolution MSC.267(85), made mandatory through SOLAS Chapter II-1 Regulation 5-1 and MARPOL Annex I implicit cross-reference. The IS Code prescribes the upright GM, the GZ curve area, the angle of maximum righting lever, and the wind-and-rolling severe-weather criterion that the tanker must satisfy before any cargo is loaded. The IS Code is the floor: if the intact ship doesn’t meet the IS Code, the loading condition isn’t permissible regardless of damage stability performance.

Damage stability governs the safety of the tanker in the postulated damaged state and is regulated by MARPOL Annex I Regulation 28 for oil tankers, by SOLAS Chapter II-1 Part B-1 for non-tankers, and by the MARPOL Annex II Regulation 16 / IBC Code Section 2.6 for chemical tankers. The damage stability regime sits on top of the intact stability regime: a loading condition must satisfy the IS Code first as an intact condition, then satisfy Reg 28.3 as a damaged condition under each prescribed damage extent. A tanker can fail damage stability while satisfying intact stability when, for example, the GM is large in the upright condition but post-damage free-surface effects in a flooded centreline tank reduce the residual GM below 0.10 m.

The class-society stability software runs both calculations on the same loading condition and presents the master with both compliance certificates. The loading manual lists the IS Code criteria and the Reg 28 criteria side by side for each standard condition, and the stability instrument under MSC.337(91) verifies both regimes in real time on the bridge.

Origin of the damage-extent and survival values

The $L^{2/3}/3$ longitudinal extent traces to the 1966 ICLL subdivision research, which correlated recorded damage length from the IMO casualty database with the cube-root-square scaling of typical collision energies. The 14.5 m cap was set at MEPC 32 in 1992 after the database was reanalysed to capture residual minor-collision damage that scales with the bow geometry of the striking ship rather than the length of the struck tanker. The $B/5$ side and $B/6$ bottom transverse ratios approximate the penetration of a striking bow into a struck side and the lateral spread of a grounding contact. The $B/15$ bottom vertical extent is calibrated to penetrate a single bottom but stop at the inner bottom of a Regulation 19 double-hull tanker; the 6 m cap reflects the maximum credible vertical penetration before structural arrest from the inner bottom plating.

The survival envelope was calibrated through the 1990s subdivision research programmes (the HARDER EU project, the IMO SLF subcommittee work, the IACS longitudinal strength studies) to give a balanced post-damage survivability margin across the casualty range. The 0.10 m GM floor matches the SOLAS II-1 Part B-1 post-damage GM threshold, the 20-degree range is the established minimum safe transient-heel reserve, and the 0.0175 m·rad area is the established minimum dynamic-energy reserve.

The equilibrium waterline behind every GZ check is found iteratively: trim and heel are adjusted until the buoyancy of the intact volume equals the displacement of the intact ship plus the displaced cargo, fuel, and ballast. The GZ curve is then built from that waterline with progressive heel, carrying the free-surface effect of every flooded compartment. The Regulation 28.5 longitudinal-strength check rides on the same flooded condition, comparing the damaged-condition bending moment and shear force at each station against the IACS Common Structural Rules envelope for tankers built under CSR-BC&OT, or the equivalent class rule for older tankers.

Worked example: Suezmax side damage

Take a Suezmax double-hull tanker with rule length $L = 270$ m, moulded breadth $B = 48$ m, and depth $D = 23$ m. The Regulation 28.1 extents are:

• Longitudinal: $270^{2/3} \approx 42.0$, so $42.0/3 = 14.0$ m; $\min(14.0,\ 14.5) = \mathbf{14.0}$ m.
• Transverse, side: $\min(48/5,\ 11.5) = \min(9.6,\ 11.5) = \mathbf{9.6}$ m.
• Transverse, bottom: $\min(48/6,\ 10) = \min(8.0,\ 10) = \mathbf{8.0}$ m.
• Vertical, bottom: $\min(48/15,\ 6) = \min(3.2,\ 6) = \mathbf{3.2}$ m.

The class stability software applies these extents at every frame station along the cargo block, the engine-room region, and the forward 0.3L section. For each case it solves the equilibrium waterline, the GM after free-surface correction, the GZ curve from equilibrium to vanishing stability or progressive flooding, the range, the positive area, and the heel angle.

Say the worst case is a side damage at midship flooding the No. 3 starboard cargo tank and the adjacent wing ballast tank to a combined permeable volume of 12,500 m³. The equilibrium gives a damaged draught of 17.8 m, a heel of 8.5 degrees, and a trim of 0.6 m by stern. The free-surface-corrected GM at equilibrium is 0.42 m, above the 0.10 m floor. The GZ curve stays positive from 8.5 degrees equilibrium heel to 32 degrees vanishing stability, a range of 23.5 degrees, above the 20-degree floor. The positive righting-lever area is 0.0265 m·rad, above the 0.0175 m·rad floor. The 8.5-degree equilibrium heel sits under the 25-degree cap. The Regulation 28.5 check confirms the damaged midship bending moment stays within the CSR-BC&OT envelope. This loading condition and this damage case pass Regulation 28 on all five counts.

Common application errors

The recurring mistakes at design review and at IOPP renewal survey are specific and avoidable. Confusing Regulation 28 with SOLAS B-1 is the first: Regulation 28 is deterministic and SOLAS B-1 is probabilistic, they use different machinery, and a tanker is governed by Regulation 28 while a non-tanker cargo ship is governed by SOLAS B-1. The oil-chemical-products tanker is governed by both, with the more stringent applying at each location.

The second error is applying the damage extents as “greater of” rather than “lesser of” for the capped dimensions. Regulation 28.1 sets maximum assumed damage values: the longitudinal extent is the lesser of $L^{2/3}/3$ or 14.5 m, not the greater. Applying the “greater of” rule to a 320 m VLCC would assume 15.5 m of longitudinal damage instead of the correct 14.5 m cap, producing an unnecessarily onerous test that doesn’t match the regulation.

The third error is omitting the Regulation 28.5 wave bending moment check. Some class stability software predating the MSC.292(87) revision didn’t run Regulation 28.5 by default, so the surveyor must confirm the loading manual prints bending moment and shear force for each damaged condition, not only the GZ curve. The fourth error is using the operational draught instead of the ICLL summer draught: the intact draught fed into the Reg 28 calculation must cover the full operational range up to the assigned summer freeboard.

The fifth error is ignoring the forward 0.3L bottom damage of Regulation 28.1.3, which is often the governing case for VLCCs and Suezmaxes and is checked separately from the general bottom damage. The sixth is failing to update the loading manual after a structural change: any modification to lightship, capacity tables, watertight subdivision, or progressive-flooding boundaries voids the existing calculation and forces a re-approval before the next IOPP renewal.

Limitations

Regulation 28 is a deterministic regime, not a probabilistic one, and that is its first limitation. It asks whether the ship survives one prescribed damage extent applied at the worst location; it doesn’t weigh the probability that such damage occurs. The probabilistic counterpart for cargo loss, Regulation 23, runs alongside it on a separate basis, and the two answer different questions: Reg 28 settles survival, Reg 23 settles expected oil outflow across the casualty distribution. Neither replaces the other, and a result under one carries no standing under the other. A tanker designer who applies the SOLAS B-1 attained-index method to a Reg 28 problem produces a number with no regulatory force under MARPOL.

The damage model itself is a calibrated approximation, not a physics simulation. Flooding is taken as instantaneous and complete: the prescribed volume floods to the equilibrium waterline at once, with no credit for inflow rate or for subdivision finer than the documented watertight boundaries. The permeabilities in Regulation 28.4 are fixed values (0.95 for void spaces and accommodation, 0.85 for stores and machinery, 0.0 for full cargo tanks, 0.95 for empty cargo tanks) rather than measured properties of the ship. Real flooding permeability varies with cargo, outfitting, and the inflow path, so the standardised figures are a conservative convention. This is a limitation worth stating plainly to the master: the calculated survival margin is an estimate built on standardised inputs, and an unusual loading or an undocumented subdivision change can move the true margin away from the booklet value.

Several boundary conditions sit outside the base calculation and need separate treatment. Asymmetric flooding that doesn’t equalise within 10 minutes invokes Regulation 28.3.5, which forces the worst intermediate condition rather than the equalised final one. Progressive flooding through engine-room ventilators, accommodation doors, or tank vents truncates the GZ curve at the first opening to immerse, so an opening the booklet didn’t flag becomes a hidden limitation on the usable range. A partial cargo load with cargo in some tanks and not others can beat the homogeneous full load as the governing case, because an asymmetric centre of gravity cuts the post-damage GM. Ice-class tankers under Polar Code Part II-A carry an added damage-stability overlay whose bow extent can exceed the standard forward-0.3L case.

The scope is tanker-only and date-bounded. Regulation 28 governs oil tankers of 150 GT and above delivered on or after 1 February 2002; pre-2002 tankers fall under Regulation 25 with the same deterministic philosophy but slightly different extents and a less stringent envelope below 5,000 DWT, and small tankers below 5,000 GT delivered before 2002 fall under Regulation 27. A novel hull form that doesn’t fit the standard tanker geometry can be approved instead through the IMO Hull Damage Risk Tool under the Regulation 5 equivalent-arrangement route, a path used in fewer than ten approvals to date. The intact-stability floor of the 2008 IS Code (MSC.267(85)) always governs first: a condition must pass the IS Code as an intact ship before Regulation 28 is tested as a damaged ship, and a tanker can clear intact stability yet fail Reg 28 when free-surface effects in a flooded centreline tank pull the residual GM below 0.10 m.

See also

• MARPOL Annex I oil pollution prevention
• MARPOL Annex I Regulation 12 oil residue tanks
• MARPOL Annex I Regulation 12A oil fuel tank protection
• MARPOL Annex I Regulation 14 oil filtering equipment
• MARPOL Annex I Regulation 15 discharge control
• MARPOL Annex I Regulation 17 oil record book
• MARPOL Annex II noxious liquid substances
• MARPOL Convention
• MARPOL Annex VI
• Probabilistic damage stability
• Damage stability
• ShipCalculators.com calculator catalogue

Related calculators

• MARPOL Annex I/28 - Subdivision damage stability
• Damage Stability - Multi-Draft A
• MARPOL Annex I/27 - Intact stability
• MARPOL Annex I/18 - Segregated ballast oil tankers
• MARPOL Annex VI/7 - Duration of certificate
• MARPOL Annex VI/6 - IAPP certificate
• MARPOL Annex VI/5 - Survey and certification
• MARPOL Annex VI/28 - CII