What Regulation 18 requires, and what changed in the numbering
Regulation 18 of MARPOL Annex I carries the title “Segregated ballast tanks”. In the current consolidated Annex I, the one adopted by Resolution MEPC.117(52) on 15 October 2004 and in force from 1 January 2007, the segregated-ballast, protective-location and dedicated clean-ballast provisions are gathered under Regulation 18. In the pre-2004 consolidation the same material sat under the old Regulation 13 (with the protective-location sub-provisions in old Regulation 13E and the dedicated clean-ballast route in old Regulation 13A). A surveyor reading a class certificate issued before 2007 will still see the old Regulation 13 reference, and the two refer to the same legal content with the same thresholds.
The core duty is short to state and expensive to build. A tanker in scope must carry its ballast water in tanks that are physically and permanently separate from any tank that has ever held oil cargo or oil fuel. Those tanks have their own ballast pumps, their own ballast main and branches, and no cross-connection to the cargo system other than what the regulation narrowly permits for emergency use. The point is to stop the ship generating oily ballast in the first place. If the ballast water never touches a cargo surface, there’s no oily slop to discharge and no decanting operation to police.
That is the whole logic of the SBT requirement, and it’s why Regulation 18 reads as a construction-and-arrangement rule rather than a discharge limit. The discharge limits live in Regulation 15 and the oil-content criteria; Regulation 18 removes the discharge problem at source for the ballast leg of the voyage.
The deadweight thresholds: 20,000 dwt crude, 30,000 dwt product
Regulation 18.1 sets two thresholds, split by cargo type. Every new crude oil tanker of 20,000 tonnes deadweight and above must have SBT. Every new product carrier of 30,000 tonnes deadweight and above must have SBT. The higher line for product carriers reflects the late-1970s judgment that smaller product tankers, with cleaner cargoes and shorter, more varied trades, did not justify the same capital burden at the same size as crude carriers. You can step through the threshold test and the capacity criterion with the segregated ballast oil tankers calculator, which encodes the same deadweight gates.
The word “new” carries a defined meaning in the Annex I delivery-date taxonomy, and it’s the part that trips people up. A tanker is “new” for Regulation 18 if any one of the following conditions holds: the building contract was placed after 1 June 1979; or, in the absence of a building contract, the keel was laid or the ship was at a similar stage of construction after 1 January 1980; or the ship was delivered after 1 June 1982; or it underwent a major conversion for which the contract was placed after 1 June 1979 (or, absent a contract, work began after 1 January 1980, or it was completed after 1 June 1982). The 1978 Protocol drew those dates so that the SBT obligation would bite on the next generation of tankers ordered after the reform, not retroactively on hulls already on the water.
Existing tankers, the ones built before those dates, were handled separately and on a softer timeline. Crude oil tankers of 40,000 dwt and above that did not qualify as “new” could comply by fitting SBT, or by operating with crude oil washing under Regulation 33, or, as an interim measure, by running dedicated clean ballast tanks under the CBT provisions described below. That three-way choice for existing crude carriers is the historical heart of the regulation, and most of it has now expired because the single-hull fleet to which it applied has been phased out under the post-Erika double-hull rules.
A “crude oil tanker” and a “product carrier” are defined by the cargo trade, not by the hull form. A combination carrier (an OBO or O/O designed to alternate between oil and dry bulk) is treated as an oil tanker when it carries oil, and the 20,000 dwt crude threshold applies to it on an oil voyage. The deadweight figure used is the ship’s summer deadweight, the same figure shown on the load line and used across the Annex I size bands. For a quick orientation between deadweight and gross tonnage, which are not interchangeable and trip up applicability tests, the deadweight versus gross tonnage calculator lays out the difference.
The capacity criterion: draft, trim and propeller immersion
Regulation 18.2 does not fix the SBT capacity at a percentage of deadweight. It sets a performance test, and the tank volume falls out of meeting it. The SBT must have enough capacity that the ship can operate safely on any ballast voyage without recourse to the use of cargo tanks for water ballast, except in the heavy-weather case the regulation separately allows. “Safely” is then pinned to three measurable conditions that the ballast condition must satisfy, computed from the ship’s length.
The molded mean draft amidships, with the ship on an even keel or close to it, must be at least:
where is the ship’s length as defined in the load-line convention (measured in meters). The trim must not exceed:
And the propeller must be fully immersed in the segregated-ballast condition. Each of the three conditions has a physical reason. The minimum amidships draft keeps enough hull in the water that the ship handles in a seaway and isn’t thrown about by wind and wave on a light leg. The trim limit keeps the bow from riding so high that slamming and bow flare loads become a problem, and keeps the rudder and propeller properly submerged geometrically. The propeller-immersion condition stops the screw racing and ventilating, which wrecks propulsion efficiency and loads the shaft and engine with cyclic torque.
Worked through, the three conditions size the tanks. Take a Suezmax of m. The minimum molded mean draft is m, the trim limit is m by the stern, and the propeller must be covered at that draft and trim. The naval architect then solves for the ballast water mass that puts the ship at, or just above, that draft on an even-enough keel with the screw immersed, and lays out segregated tanks (typically the double-bottom and wing ballast spaces in a double-hull design) to hold it. On a modern double-hull crude tanker that ballast volume runs to roughly 40 to 45 percent of summer deadweight in tonnes of seawater, though the governing figure for any hull is the one in its approved trim and stability booklet, not a rule of thumb. The draft and trim arithmetic itself is standard hydrostatics; the mean draft calculator and the trim from loading calculator handle the underlying ship-condition computation.
| Symbol | Meaning | Unit |
|---|---|---|
| Segregated ballast oil tankers |
Source: IMO MARPOL I/18
Calculate Segregated ballast oil tankers →Regulation 18.3 allows one escape. In exceptionally severe weather, when the master judges it necessary for the safety of the ship to carry additional ballast water in cargo tanks, that additional water may be carried provided it’s handled under crude oil washing or is otherwise managed so that the resulting oily mixture is dealt with under the Regulation 34 discharge controls and recorded in the oil record book. It’s a genuine exception for heavy-weather survival, not a routine operating mode, and a port-state inspector who sees heavy-weather ballast in cargo tanks will look hard at the weather records and the ORB entries that should justify it.
Completely separate ballast piping and pumping
The “segregated” in segregated ballast tanks is about the system, not just the tanks. Regulation 18 requires the ballast water to be carried in tanks served by a ballast system that is independent of the cargo oil and fuel oil systems and permanently allocated to ballast. In practice that means a dedicated ballast pump room or dedicated ballast pumps, a dedicated ballast main running the cargo length, dedicated suctions and overboard discharges, and no permanent cross-connection through which cargo could enter a ballast tank or ballast water could enter a cargo tank.
The arrangement is reviewed at plan approval against the cargo and ballast piping general arrangement, and it’s the principal thing a surveyor checks for SBT compliance. The cargo and ballast systems must be drawn and built so that a single valve mis-operation can’t put oil into the segregated ballast. Where the regulation permits any interconnection at all, it’s narrow, for example a portable spool piece for emergency cargo-tank ballasting in the heavy-weather case, removed and stowed in normal operation, with the connection points blanked. A permanent open cross-connection between the cargo and segregated-ballast systems is a construction deficiency, and a class surveyor will write it up.
This complete separation is what makes SBT different from the older practice and from clean ballast tanks. With SBT the ballast water leaving the ship at the loading port has never been in contact with oil, so it can go straight overboard with no oil-content monitoring needed for that water. The vessel’s bilge and ballast system design therefore treats the segregated ballast as a clean system from the outset, separate from the oily-water handling that the cargo and machinery spaces still require. That clean-from-source character is also why SBT ballast water is governed by the Ballast Water Management Convention for its invasive-species risk rather than by the Annex I oil-content rules: it’s water, not oily mixture.
The physical layout follows from the separation. On a modern double-hull crude tanker the segregated ballast lives in the double-bottom tanks under the cargo block and in the wing tanks (the side spaces between the inner and outer shell). Each ballast tank has its own suction connected to the ballast main, and the main runs back to the ballast pump room, which on most tankers is the aft cargo pump room or a dedicated space with its own pumps. The ballast pumps are sized to fill or empty the full segregated capacity inside the cargo-handling window at the terminal, because a tanker discharges cargo and takes on ballast at the same time to keep the ship’s draft and stress within limits as the load comes off. A typical Suezmax ballast pump moves on the order of 2,000 to 3,000 cubic meters an hour, and the system runs two such pumps so the deballast keeps pace with a cargo discharge rate that can exceed 8,000 cubic meters an hour across the cargo pumps. The naval architect matches the ballast pumping rate to the cargo discharge rate at design so the loading-condition sequence never drives the hull-girder bending moment or the local draft out of bounds.
The separation also dictates the venting and sounding arrangements. Segregated ballast tanks vent independently of the cargo tanks, so a ballast-tank vent never connects to the inert-gas-protected cargo venting system, and the ballast tanks are sounded with their own gauging, separate from the cargo ullage system. Mixing the two would defeat the separation in a way a surveyor would catch at the gas-and-vent arrangement review. The one place the systems come close is the pump room, where cargo and ballast pumps may share a space; there the rule is arrangement and procedural separation, not a shared line, and the spool-piece-and-blank discipline applies to any emergency connection.
Protective location: SBT and void spaces as a collision and grounding shield
Regulation 18.12 to 18.15 adds the protective location (PL/SBT) requirement for the new tankers that need SBT. It’s a second job for the same empty tanks. Beyond holding ballast, the segregated ballast tanks and any cargo-length void spaces must be arranged along the ship’s sides and bottom so that they shield the cargo tanks. The idea is straightforward: if a striking ship’s bow or a reef hits an empty ballast tank instead of a loaded cargo tank, the breach lets in seawater rather than letting out oil.
The regulation turns that idea into a coverage test. The protectively located SBT and void spaces must cover, at minimum, a fraction of the side shell and bottom shell over the cargo tank length. The side and bottom protecting areas are computed from the ship’s principal dimensions. For the side, the required protecting area is expressed as:
where is the combined length of the cargo tanks, is the molded breadth and is the molded depth, with the regulation then prescribing the minimum fraction of that nominal area that the protectively located ballast and void spaces must occupy, and a minimum width and depth for each such space so that it actually absorbs penetration rather than being a token gap. The protecting spaces must also be distributed, not bunched, so the shield runs the length of the cargo block rather than protecting one tank and leaving its neighbor exposed.
Protective location was the precursor to the full double-hull idea. A protectively located SBT arrangement on a single-hull tanker put ballast tanks against the side and bottom shell along much of the cargo length, which already moved the outer cargo boundary inboard. The later Regulation 19 double hull generalized that concept into a continuous inner and outer shell over the whole cargo length, with the wing and double-bottom ballast spaces forming the void between them, so on a modern tanker the SBT, the protective location and the double hull are satisfied by one and the same set of wing and double-bottom tanks and the three requirements collapse into a single arrangement. The single-hull-versus-double-hull dimension of outflow performance ties into the damage-stability and oil-outflow regime under Regulation 28, which sets the subdivision and damage-stability criteria for the loaded condition.
Sizing the protective spaces in practice
The protective-location coverage test reads as geometry, but the working consequence is where the ballast goes and how wide and deep each protecting space has to be. The regulation doesn’t credit a token gap. Each protectively located ballast or void space has to meet a minimum transverse extent (the width of a wing tank inboard from the side shell) and a minimum vertical extent (the height of a double-bottom tank above the keel) so that it has the depth to absorb the penetration of a striking bow or the grounding contact of a reef before the inner boundary, the cargo tank wall, is reached. A wing tank one frame deep does nothing; a wing tank two or three meters wide takes the hit.
Take the same Suezmax of m with a molded breadth m and a molded depth m, and a cargo tank length of around 215 m. The nominal protecting-area expression comes out at square meters, and the regulation requires the protectively located spaces to cover at least the prescribed fraction of that nominal figure, distributed along the cargo length. On a double-hull hull that coverage is met automatically because the inner-shell-to-outer-shell void runs continuously along both sides and across the bottom over the whole cargo block. On the older protectively-located single-hull designs it had to be engineered tank by tank, and the designer had to choose which ballast tanks sat against the shell and how the cargo tanks nested inboard of them.
The distribution rule is what stops a designer gaming the coverage. It isn’t enough to put a large protecting space at one end and leave a stretch of cargo tank against the bare shell elsewhere; the protecting spaces have to run the length of the cargo block so the shield is continuous. That distribution requirement is the bridge between protective location and the later double hull: a continuous inner shell is just the limiting case of fully distributed protecting spaces, and it’s why the double-hull rule could absorb and supersede protective location without changing the underlying intent. The outflow performance that the arrangement buys is then quantified under the damage and oil-outflow criteria of Regulation 28.
Dedicated clean ballast tanks: the interim route for existing crude tankers
Regulation 18.10 and 18.11 preserve the dedicated clean ballast tanks (CBT) provisions, an interim compliance route created for existing crude oil tankers of 40,000 dwt and above that were built before the SBT obligation applied. Retrofitting full segregated ballast (new piping, new pumps, new tank dedication with structural and system separation) onto an existing single-hull tanker was costly, and for a hull near the end of its life it made little sense. CBT was the cheaper middle path.
Under CBT, certain existing cargo tanks were dedicated solely to the carriage of clean ballast water. “Clean ballast” has a precise Annex I meaning: ballast water carried in a tank that, since oil was last carried in it, has been so cleaned that effluent from it discharged in clean conditions would not produce a visible sheen and would meet the oil-content limit. The CBT tanks were cleaned to that standard and then kept for ballast only, so in operation they behaved like segregated tanks even though the piping wasn’t fully separated. The arrangement and operating procedures were written up in a Dedicated Clean Ballast Tanks Operations Manual approved by the Administration, and the vessel had to carry oil-content monitoring and the means to verify the ballast was clean before discharge.
CBT was always framed as interim. The regulation set time limits, and the route closed as those ran out and as the single-hull phase-out removed the hulls that relied on it. A naval architect or surveyor working a modern fleet will rarely meet a live CBT vessel today; the provision survives in the consolidated text as the historical record of how the existing fleet bridged the gap to either full SBT or scrapping. Its operational logic, dedicate clean tanks, verify cleanliness, monitor the discharge, still informs how a tanker treats any tank it uses for clean ballast under the heavy-weather exception.
The SBT plus COW plus IGS package that replaced load on top
To understand why Regulation 18 exists in the shape it does, you have to see it as one part of a three-part package, and you have to know what it replaced. Before the package, tankers managed ballast and tank cleaning through load on top (LOT).
Under LOT, a tanker on its ballast leg carried seawater ballast in some of its cargo tanks. It also water-washed other cargo tanks to remove the clingage left by the last cargo, sending the oily washings to a slop tank. The oily ballast and the slop were allowed to settle by gravity over the voyage, the cleaner water underneath was decanted overboard under the discharge rules of the day, and the oil that floated to the top was retained. At the next load port, fresh cargo was loaded “on top of” that retained oil and slop. LOT was a real improvement over simply pumping oily ballast into the sea, and through the 1960s and 1970s it cut operational oil pollution. But it depended entirely on careful decanting, the oily water it discharged still carried oil, and a casualty or a careless operation could put a large slug of oil into the sea.
The 1978 Protocol replaced LOT with a structural fix in three coordinated parts: segregated ballast tanks (Regulation 18, the subject here) took the ballast water out of the cargo tanks, crude oil washing (Regulation 33) replaced water washing with washing in the cargo crude itself, and a mandatory inert gas system kept the tank atmosphere below the flammable range so COW could run safely. SBT is the leg that closed the ballast-water route to oily slop; the COW and IGS legs are worked through in their own articles. Built since the early 1980s, the world crude tanker fleet has carried all three as standard, and the protective-location and later double-hull rules folded the SBT tanks into the hull structure itself, so a modern Aframax, Suezmax or VLCC meets Regulation 18 through the same wing and double-bottom spaces that form its double hull. The Regulation 18.2 capacity test is the part that stays distinctively reg-18 even on those hulls: the segregated volume is sized to the molded-draft, trim and propeller-immersion conditions, not to the double-hull geometry, so a designer who has the double-hull box still has to confirm the ballast it holds satisfies on an even keel with the screw covered, and may add ballast above the structural void to do so.
Why the 1978 Protocol introduced SBT: the casualties and the phase-in
The SBT obligation didn’t arrive in the original 1973 MARPOL Convention. It came two years before that convention even entered force, bolted on by the 1978 Protocol, and the reason was a run of tanker casualties in the mid-1970s that made the original text look inadequate before it took effect. The 1973 Convention had set discharge limits and required some ballast control, but it left the structural question (where the ballast water sits and where the oil sits relative to the shell) largely open.
The casualty that crystallized the reform was the Amoco Cadiz, the 233,690 dwt VLCC that lost steering off Brittany on 16 March 1978 and grounded on Portsall Rocks, breaking up over two weeks and releasing roughly 230,000 tonnes of crude along about 320 km of coast. The Argo Merchant grounding off Nantucket on 15 December 1976, which released around 27,000 tonnes of fuel oil, had already put tanker safety on the political agenda in the United States the year before. The international response was the Conference on Tanker Safety and Pollution Prevention (the TSPP Conference) in February 1978, which adopted the 1978 Protocol to MARPOL. That Protocol introduced the mandatory SBT requirement for new tankers above the thresholds, brought in protective location, made the inert gas system mandatory for the larger tankers, and tightened the slop and discharge provisions. MARPOL 73/78, the Convention as modified by the Protocol, entered force on 2 October 1983.
The phase-in followed the delivery-date logic already described. New tankers, ordered or delivered after the 1979 to 1982 dates, got full SBT and protective location from the keel. Existing crude tankers of 40,000 dwt and above got the three-way SBT-or-COW-or-CBT choice on a transitional timetable, which let an aging hull comply cheaply through dedicated clean ballast tanks rather than a costly retrofit. The protective-location requirement then sharpened over time into the full double hull, the later casualty record (Exxon Valdez in 1989, Erika in 1999, Prestige in 2002) and the single-hull phase-out it drove being the story of Regulation 19 double hull rather than of Regulation 18; the phase-out closed off the CBT route in practice by removing the hulls that used it. Each step kept the Amoco Cadiz lesson: put empty space, not oil, against the part of the shell most likely to be breached.
How Regulation 18 sits with the rest of Annex I
Regulation 18 is a construction and arrangement rule, and it interlocks with the operational and outflow rules around it. It removes oily ballast at source; Regulation 15 and the oil-content discharge criteria still govern any oily mixture the ship does generate, from machinery spaces and from the cargo system, on the rare occasions cargo-tank ballast is carried. The oil record book Part II records ballast and cargo operations, and the heavy-weather cargo-tank-ballast exception under Regulation 18.3 must show up there with the weather justification. The slop and residue handling that SBT reduces is built and sized under the oil residue (sludge) tanks provisions, and the fuel-oil-tank protection idea, keeping bunkers away from the shell, parallels the protective-location concept on the cargo side and is set out in Regulation 12A.
The damage-stability and oil-outflow performance that protective location and the double hull deliver is assessed under Regulation 28, which sets the subdivision and stability criteria for assumed collision and grounding damage. So the empty ballast tanks do two regulatory jobs: Regulation 18 sizes and locates them, and Regulation 28 confirms the loaded ship survives the damage they’re meant to take. Ship-to-ship transfer of cargo, where ballast handling and draft management matter operationally, is governed by Regulation 41, and the shipboard pollution emergency plan under Regulation 37 covers the response if the shield is breached after all. Pollution liability for any oil that does escape is allocated through the 1992 Civil Liability Convention.
The water in the segregated ballast tanks raises a separate, non-oil issue once it’s clean: it can move invasive species between ports. That risk is governed not by Annex I but by the Ballast Water Management Convention and handled aboard by the vessel’s ballast water management systems and exchange or treatment procedures. SBT solves the oil problem and creates the room in which the biological problem then has to be managed; the two regimes apply to the same water at the same time, for different reasons.
Commercial and operational consequences across the fleet
SBT is a construction rule, but it shapes how a tanker is operated and chartered every day. The clearest operational consequence is the ballast-and-deballast sequence at the load and discharge terminals. Because the ballast can’t share the cargo tanks, a tanker arrives at the load port full of segregated ballast water and pumps that water out as cargo comes in, keeping the hull-girder stress and the local drafts within the loading-condition limits at every step. The chief officer plans this sequence against the ship’s loading computer, and the segregated ballast tanks are the variable that makes the sequence work without ever putting water in a cargo tank. The reverse runs at the discharge port: ballast in as cargo comes off. The ballast pump capacity that lets this happen in the terminal’s window is a design decision driven directly by Regulation 18.
The capacity that SBT consumes is the commercial price of the rule. On a modern double-hull tanker the segregated ballast volume sits in the double-bottom and wing spaces, which are structure the ship needs anyway, so the marginal cargo penalty is small. On the older protectively-located single-hull designs the ballast tanks ate into volume that could otherwise have carried cargo, and that lost capacity was the cost the 1978 Protocol accepted to keep oil away from the shell. The deadweight thresholds, 20,000 dwt for crude and 30,000 dwt for product, drew the line where that trade-off was judged worth making. Below the lines, a smaller tanker on a shorter trade could carry its ballast more flexibly without the same capital and capacity burden, though it still faced the discharge controls of Regulation 15.
For the chartering and vetting side, SBT compliance is assumed, not negotiated, on any tanker in international crude or product trade today, because the fleet is double-hull and double-hull tankers satisfy SBT and protective location as a by-product of their structure. Where the requirement surfaces commercially is at survey and at port-state control: a ship with a cross-connected ballast system, a missing protective-location coverage, or a draft-and-trim shortfall that forces cargo-tank ballast carries a certificate condition that takes it out of trade until rectified. The cost of that off-hire dwarfs the maintenance of the separation, which is why the discipline holds in practice. The deadweight figure that drives the threshold test is the summer deadweight, and the relationship between deadweight and the gross tonnage used for other regulatory bands is set out in the deadweight versus gross tonnage calculator.
Survey, certification and the IOPP record
SBT compliance is verified through the same survey cycle that issues and renews the International Oil Pollution Prevention (IOPP) Certificate. The certificate’s supplement, IOPP Form B for oil tankers, records the SBT arrangement, the protective-location particulars where applicable, and any CBT or COW endorsement. The survey work follows the harmonized scheme and is delegated by most flag States to a Recognized Organization, in practice an IACS member class society acting under the IACS Unified Requirements UR Z survey procedures.
At the initial survey before the ship enters service, the surveyor confirms the segregated ballast tanks, pumps and piping match the approved plans, that the ballast system is genuinely separate from the cargo and fuel systems, and that the protective-location arrangement meets the coverage and minimum-dimension criteria. The capacity criterion is confirmed against the approved trim and stability booklet, which must show a compliant segregated-ballast departure condition meeting the molded-draft, trim and propeller-immersion limits. At renewal survey every five years and at the annual and intermediate surveys in between, the surveyor confirms nothing has been modified to defeat the separation, that no unauthorized cross-connection between cargo and ballast has been added, and that the ballast system remains in working order.
A finding that the ballast system has been cross-connected to the cargo system, or that the ship can’t meet the draft and trim criteria without cargo-tank ballast, is an SBT deficiency that goes on the certificate as a condition and restricts the ship from trading until rectified. Port-state control inspectors look at the same arrangement and at the oil record book entries for any sign that cargo tanks are being used for routine ballast outside the heavy-weather exception, which is a detainable item.
Limitations and practitioner notes
Regulation 18 is a strong source-control rule with clear edges, and the caveats below matter more in practice than any claim that it covers the whole ballast problem. The first limitation is that the capacity criterion is a performance test, not a number. Regulation 18.2 gives no fixed percentage of deadweight; it gives the molded-draft, trim and propeller-immersion conditions, and the tank volume falls out of meeting them for a specific hull. The 40-to-45-percent-of-deadweight figure quoted for modern double-hull tankers is a representative range for orientation, not a regulatory value, and the governing figure for any ship is the segregated-ballast condition in its approved trim and stability booklet. Don’t design or audit to the rule of thumb.
A second limitation is that segregated does not mean the ballast water is environmentally free. SBT water is clean of oil because it never touched cargo, so the Annex I oil-content rules don’t constrain its discharge. But it carries the invasive-species risk that the Ballast Water Management Convention exists to control, and a ship in full SBT compliance can still be in breach of the BWM Convention if it discharges untreated ballast where treatment or exchange was required. The two regimes are independent; passing one says nothing about the other.
A third limitation is the heavy-weather exception and its abuse. Regulation 18.3 lets the master carry additional ballast in cargo tanks when it’s needed for the safety of the ship in severe weather, but that water becomes an oily mixture subject to the Regulation 34 discharge controls and crude oil washing, and it must be justified in the oil record book against the actual weather. Routine cargo-tank ballasting dressed up as heavy-weather ballast is a common port-state-control finding and a detainable one; the exception is for survival, not for trimming convenience.
A fourth limitation is that the dedicated clean ballast tank route is effectively historical. CBT was an interim measure for existing crude tankers of 40,000 dwt and above, it ran on time limits, and the single-hull phase-out has removed nearly all the hulls that used it. Treating CBT as a live design option for a new build is an error; it was never available for new tankers, which always needed full SBT. The provision survives in the consolidated text as the record of how the legacy fleet complied, and its cleanliness-and-monitoring logic still informs how any tank used for clean ballast is treated.
A final limitation concerns the worked numbers. The Suezmax draft and trim figures, the 40-to-45-percent capacity range, and the side-protective-area expression are illustrative; the governing values for any vessel are in its approved stability booklet and its IOPP supplement, and the exact protective-location coverage fractions and minimum dimensions are those in the current Regulation 18.12 to 18.15 text read with the ship’s class rules. The regulation number used here is the current consolidated Annex I Regulation 18 (formerly Regulation 13), title “Segregated ballast tanks”, as adopted by Resolution MEPC.117(52) and in force 1 January 2007 as amended. A pre-2007 certificate citing old Regulation 13, 13A or 13E is referring to the same content. The regulatory basis here is MARPOL Annex I Regulation 18 in the current consolidated text, read with the 1978 Protocol that introduced the SBT obligation, Regulation 28 for the damage-stability and outflow performance, Regulations 33 and the inert gas requirements for the COW package, and the IACS UR Z survey procedures.
See also
- Segregated ballast oil tankers calculator (MARPOL Annex I/18)
- MARPOL Annex I: Regulations for the Prevention of Pollution by Oil
- MARPOL Annex I Regulation 33: Crude Oil Washing
- MARPOL Annex I Regulation 28: Damage Stability
- MARPOL Annex I Regulation 15: Discharge Control
- MARPOL Annex I Regulation 17: Oil Record Book
- MARPOL Annex I Regulation 12: Oil Residue (Sludge) Tanks
- MARPOL Annex I Regulation 12A: Oil Fuel Tank Protection
- Marine Inert Gas Systems
- Marine Bilge and Ballast Systems
- Ballast Water Management Convention
- MARPOL Convention