A platform is not a ship, and yet under MARPOL Annex I it is made to behave like one. Regulation 39, the single provision in Chapter 7 of the revised Annex I, is the bridge. It takes a structure that would otherwise sit outside the convention, a fixed production platform bolted to the seabed, a semi-submersible drilling rig, a floating production storage and offloading unit on a turret, and imposes a defined slice of the tanker-and-ship oil-pollution regime on it. The slice is narrow and the wording is hedged, but the obligations are real: fit the oily-water equipment, keep the record, discharge below 15 parts per million. This article is about that provision, not about the platforms themselves. For the engineering of a production unit, see the FPSO article; for the parent Annex, see MARPOL Annex I and the MARPOL convention as a whole.
What Regulation 39 says
The text is short. Regulation 39.1 sets the application, naming the units it reaches: fixed and floating platforms, including drilling rigs, when engaged in the exploration, exploitation, and associated offshore processing of seabed mineral resources. Regulation 39.2 then carries the operative command: those units shall comply with the requirements of Annex I applicable to ships of 400 gross tonnage and above other than oil tankers, so far as reasonable and practicable. That single sentence does the heavy lifting. It does not write a bespoke platform code; it reaches across to the ship rules already in Annex I and applies them by reference, with two qualifiers that an experienced surveyor reads carefully: the unit is treated as a 400 GT non-tanker, and the standard is “so far as reasonable and practicable” rather than absolute.
The choice of the 400 GT non-tanker reference point matters. Annex I splits its discharge regime between oil tankers, governed by Chapter 3 and the cargo-area rules such as Regulation 15 read with the slop-tank and crude-oil-washing machinery, and all other ships of 400 GT and above, governed by the machinery-space provisions. Regulation 39 puts the platform on the non-tanker side. A platform’s hydrocarbon inventory in its production tanks is not “cargo” in the tanker sense; the relevant Annex I exposure is the machinery-space oily water and the operational drainage, which is exactly the regime that applies to a 400 GT cargo ship’s engine room. So the cross-reference is not arbitrary. It selects the part of Annex I that fits what a platform actually discharges.
Regulation 39.2 then names three specific obligations that sit on top of the general cross-reference. They are the operative core of the provision.
The three specific obligations
First, the platform shall be equipped, so far as practicable, with the equipment required in Regulation 12 and Regulation 14 of the Annex. Regulation 12 governs the oil residue (sludge) tanks; Regulation 14 governs the oil filtering equipment, the 15 ppm separator and, where required, the alarm and automatic stopping arrangement. A platform that runs machinery, and they all do, generates oily bilge and sludge, so the convention wants the same separation and holding capability a ship of the same machinery class would carry.
Second, the platform shall keep a record of all operations involving oil or oily-mixture discharges, in a form approved by the Administration. This is the platform analogue of the oil record book that Regulation 17 imposes on ships. The form differs because a platform’s operations differ, but the principle is identical: every discharge, every transfer, every separator operation is logged so a port-state or coastal-state inspector can audit the unit’s compliance after the fact.
Third, subject to the provisions of Regulation 4 of the Annex, the discharge into the sea of oil or oily mixture shall be prohibited except when the oil content of the discharge without dilution does not exceed 15 parts per million. The 15 ppm figure is the same threshold Regulation 15 sets for a ship’s machinery-space discharge outside special areas. The phrase “without dilution” closes the obvious loophole: a unit cannot meet the limit by adding clean seawater to a 100 ppm stream and reporting the diluted concentration. The instrument measures the discharge as it leaves the separator, not as it mixes overboard. The Regulation 4 carve-out preserves the standing exceptions for securing the safety of the platform and for an emergency or an accidental discharge after all reasonable precautions, the same exceptions that protect a ship.
Alongside those three, the broader Annex I discharge regime that Regulation 39.2 pulls in carries an anti-circumvention rule of its own. The cargo-area discharge provision of Regulation 34 bars any discharge containing chemicals or other substances in quantities or concentrations hazardous to the marine environment, or chemicals or other substances introduced to circumvent the conditions of discharge. That clause is the same logic as “without dilution” extended to chemistry: an operator cannot dose the effluent with a demulsifier or a solvent that drops the measured oil reading while leaving the actual pollution load unchanged. It reaches a platform through the general 400 GT non-tanker cross-reference, not as a fourth obligation listed in Regulation 39 itself.
Regulation 39.3 then adds the reception-facility hook. Where a platform’s oily mixture cannot be discharged into the sea in compliance with the regulation, it must be retained on board for later discharge ashore to a reception facility. This is the “no overboard, hold and land” backstop, identical in spirit to the ship-side regime under Regulation 38. The non-compliant slug never goes over the side; it goes to a supply boat and back to a port reception facility.
The renumbering: from Regulation 21 to Regulation 39
The provision is older than its number. The substance of Regulation 39 was carried for decades in the old Regulation 21 of the pre-2007 Annex I. When the Marine Environment Protection Committee revised and reorganized the whole Annex into a chapter structure, it adopted Resolution MEPC.117(52) on 15 October 2004, and that revised Annex entered into force on 1 January 2007. The reorganization grouped a handful of special-case provisions into Chapter 7, and the platform rule landed there as Regulation 39. The content moved across with only drafting changes; a 2004 surveyor working to “Regulation 21 platforms” and a 2026 surveyor working to “Regulation 39 platforms” are applying the same obligations. This matters for older approval documents and shipboard manuals, which may still cite the legacy number. An Administration’s approved record-of-operations form drawn up before 2007 under Regulation 21 remains the same instrument; only the citation changed.
The renumbering also tracks how Annex I as a whole was rebuilt. The same MEPC.117(52) revision created the modern double-hull rule at Regulation 19, the accidental-oil-outflow performance standard at Regulation 23, and the discharge-control architecture at Regulation 15. Regulation 39 sits at the far end of that same instrument, the last chapter, governing the units that are most ship-like in their machinery and least ship-like in their mission.
The “while engaged in exploration or exploitation” trigger
The whole of Regulation 39 hangs on a qualifier that is easy to read past and central to the regime: the platform falls under Regulation 39 only while engaged in the exploration, exploitation, or associated offshore processing of seabed mineral resources. Outside that activity, a different and stronger rule applies.
Think of a mobile offshore drilling unit on a long ocean tow between fields, or an FPSO that has disconnected from its turret and is steaming to a yard for a class-renewal docking. While it is on passage, it is not engaged in exploitation. It is a ship in transit. The qualifier in Regulation 39 lifts, and the unit becomes subject to the full Annex I that applies to any ship of its tonnage, with no “so far as reasonable and practicable” softening on the machinery-space discharge rules. Its Reg 14 oil filtering equipment must work to the ordinary ship standard, its SOPEP under Regulation 37 must be the standard shipboard oil pollution emergency plan, and its discharges must meet Regulation 15 outright. So the same physical unit toggles between two regulatory states: platform mode while it produces, ship mode while it moves.
This toggle is not a paperwork nicety. It changes which surveys apply, which certificate the unit carries, and which inspector has primary authority. In platform mode, the coastal state whose continental shelf hosts the unit is the dominant regulator, because UNCLOS gives the coastal state jurisdiction over installations on its shelf. In ship mode, the flag state’s IOPP certificate and the ordinary port-state control regime come to the front. A unit that has lived for fifteen years as a platform, inspected by the coastal-state offshore authority, finds itself on a single voyage subject to the IOPP regime and to PSC at the first port of call. Operators plan that transition deliberately, recommissioning the bilge and separator systems to ship standard before the tow.
Produced water versus machinery-space bilge: the central distinction
The most consequential thing about Regulation 39 is what it does not cover. A platform generates two very different oily-water streams, and the convention treats them differently.
The first stream is machinery-space bilge and operational drainage: the oily water from the engine room, the pump caisson, the drains around rotating equipment. It is the same kind of water a cargo ship produces, and it is exactly what Regulation 39’s 15 ppm limit and the Reg 12/14 equipment are aimed at. Treat it through the oily water separator, discharge below 15 ppm, log it. This is the bilge-and-ballast regime a marine engineer already knows.
The second stream is produced water: the formation water that comes up the well with the oil and gas, separated out on the topsides before the crude goes to storage. A mature offshore field can produce far more water than oil by volume; a unit can handle hundreds of thousands of barrels of produced water a day. This water carries dissolved and dispersed hydrocarbons, but it is not machinery bilge, and it is not “an oily mixture” in the Regulation 39 sense.
The IMO Unified Interpretation 67 to Annex I draws the line explicitly. Under UI 67, produced water, offshore processing drainage, and displacement water are not treated as oily mixtures under Regulation 39. The 15 ppm limit applies to the machinery-space and contaminated-seawater operational discharges, not to the produced-water stream. So a platform can be in full compliance with Regulation 39 on its bilge and still be discharging produced water at a concentration the 15 ppm number would never permit, because produced water is regulated by a different instrument entirely.
That other instrument is the coastal state’s offshore-discharge regime. MARPOL Annex I deliberately leaves produced water to the regional and national regulators who set the dispersed-oil limits, sampling protocols, and reporting for it.
Where produced water actually goes: the national and regional regimes
In the North East Atlantic, produced water is governed by the OSPAR Convention and its offshore oil and gas measures. The controlling instrument is OSPAR Recommendation 2001/1 on the management of produced water from offshore installations, amended by Recommendation 2006/4 and Recommendation 2011/8. It sets a performance standard of 30 milligrams per liter of dispersed oil in produced water discharged to the sea, the figure operators on the UK and Norwegian shelves work to. The standard is calculated as a flow-weighted average over a period, not an instantaneous reading, and from 2007 the dispersed-oil concentration is determined by the OSPAR reference method, a gas-chromatography flame-ionization method that replaced the older infrared methods. That 30 mg/l number is an OSPAR and coastal-state standard, not a MARPOL number, and it sits alongside Regulation 39 rather than inside it. A North Sea FPSO meets Regulation 39 on its machinery bilge and the OSPAR 30 mg/l standard on its produced water.
The OSPAR regime carries more than the concentration cap. Recommendation 2001/1 layers on a best-available-techniques & best-environmental-practice duty, so an installation that meets 30 mg/l can still be pushed to do better where BAT allows it, and the recommendation set a target to cut the total quantity of oil discharged with produced water by 15% by 2006 against the 2000 baseline. The chemicals dosed into the topsides treatment, the demulsifiers & scale inhibitors, run through OSPAR’s Harmonised Mandatory Control System under Decision 2000/2, a separate consent track from the oil-in-water number. None of that is MARPOL. It is the body of regional law that fills the space Regulation 39 leaves empty for produced water. An installation exceeding 30 mg/l on an annual basis must file a BAT and BEP evaluation showing the measures planned to bring it back into line.
On the US outer continental shelf the split is between two federal agencies. The Bureau of Safety & Environmental Enforcement, BSEE, regulates the safety & operational side of the installation, while the Environmental Protection Agency regulates the discharges under the Clean Water Act through the National Pollutant Discharge Elimination System, NPDES, general permits for the Gulf of Mexico and other offshore regions. Produced-water discharge limits, the daily-maximum & monthly-average oil-and-grease values, live in the NPDES permit, not in MARPOL. The western Gulf general permit, GMG290000, sets a 42 mg/l daily-maximum and 29 mg/l monthly-average oil-and-grease limit on produced water, drawn from the effluent guidelines at 40 CFR Part 435 Subpart A, numbers that sit close to the OSPAR figure but are reached through a different statute and a different sampling method. The US implementation of MARPOL Annex I in 33 CFR Part 151 also distinguishes a fixed or floating drilling rig or other platform engaged in seabed mineral operations from one that is not, mirroring the Regulation 39 trigger.
The practitioner takeaway is that a platform compliance file has two distinct halves. One half is the MARPOL Annex I and Regulation 39 half, governing bilge and operational drainage to 15 ppm, audited by the flag state and the coastal state. The other half is the produced-water half, governed by OSPAR, BSEE & EPA, or the equivalent national regime, with its own permit, its own limits, and its own sampling. Confusing the two, applying the 15 ppm machinery number to produced water, or assuming MARPOL governs the topsides water treatment, is a recurring error in audits. The two limits are not interchangeable: the 15 ppm Regulation 39 number is an instantaneous oil-content reading at the separator outlet, while the OSPAR 30 mg/l number is a flow-weighted dispersed-oil average measured by a prescribed laboratory method on a much larger and chemically different stream.
How an FPSO is treated: ship for some purposes, platform for others
The FPSO is the cleanest illustration of the dual nature Regulation 39 creates, and the reason this provision needs its own article alongside the general FPSO overview. The general article describes what an FPSO is: the spread-moored and turret-moored and disconnectable subtypes, the converted-tanker versus purpose-built hulls, the topsides modules, the contractors and operators. This article is about how the law catches it.
An FPSO is a vessel by construction, often a converted VLCC, with a hull, ballast tanks, an engine room, and a marine crew. It is a platform by function, sitting on a single field for twenty years processing well fluids. The regulatory regime splits along that seam. For its construction and structural integrity, the unit answers to classification-society rules written for floating production installations, the ABS FPI Rules, DNV-OS-D203, Bureau Veritas NR 459, read with the relevant IACS recommendations. Those rules cover hull-girder strength, mooring, fatigue, & the topsides structure. Regulation 39 does not touch any of that; it is not a construction standard.
For its pollution discharges while on station and producing, the FPSO is a floating platform under Regulation 39. Its machinery-space bilge is held to 15 ppm; its produced water is held to the coastal-state offshore standard. Its emergency planning is the platform plan, coordinated with the coastal-state response system, not the shipboard SOPEP. And the coastal state, exercising shelf jurisdiction under UNCLOS, is the primary inspecting authority.
For its movements, the FPSO is a ship. The moment it disconnects and goes on passage, the Regulation 39 trigger lifts, the full Annex I applies, the IOPP certificate and the flag-state survey regime govern, and port-state control attaches at the first port. A disconnectable FPSO in a cyclone basin, off North Australia or in the Bay of Bengal, can make this transition several times in a season, weathervaning off and reconnecting, so its compliance team has to manage the toggle in real operational time.
The IMO FPSO and FSU guidelines
The IMO did not leave the ship-versus-platform line to each Administration’s reading of Regulation 39. It wrote a dedicated interpretation. The first version was MEPC/Circ.406, issued in 2003, the Guidelines for the application of the MARPOL Annex I requirements to FPSOs and FSUs. When the revised Annex I came in under MEPC.117(52), the Committee re-cut those guidelines to the new chapter-and-regulation numbering and adopted them as Resolution MEPC.139(53) at its fifty-third session. The current text is the 2018 version, adopted as Resolution MEPC.311(73) on 26 October 2018, which folds in the Annex I amendments up to and including MEPC.276(70). A surveyor working an FPSO or FSU file reads Regulation 39 through that resolution, not in the abstract.
The guidelines do the practical work Regulation 39’s terse wording leaves open. They state which Annex I requirements apply to a unit on station, how the oil record book and the record of oil discharges are to be kept, and how the IOPP certificate or the equivalent statutory document is to be issued and surveyed for a unit that does not behave like a trading tanker. They make a defined departure from the standard survey regime in Resolution A.1049(27), the survey-and-certification guidelines, allowing in-water surveys for the hull where dry-docking a permanently moored unit is impractical, on the condition that the departure does not compromise safety or pollution prevention. That is the reasonable-and-practicable valve made concrete: the environmental and structural result is preserved while the literal ship procedure is set aside for a unit that will sit on one field for two decades.
The guidelines also reserve flexibility to the flag administration. Member governments may depart from the guidelines where a particular unit needs it, and are asked to report that experience back to the Organization so the text can be amended. So the regime around Regulation 39 is layered: the regulation sets the obligation, MEPC.311(73) interprets it for production and storage units, and the flag administration applies it to the specific hull, with the coastal state’s offshore regime sitting alongside for the produced water the regulation does not reach.
The flag-state and coastal-state jurisdiction split
The jurisdiction split is the practical engine behind the FPSO’s dual status. A ship flies a flag, and the flag state issues its statutory certificates and conducts its surveys under MARPOL and SOLAS. An installation on a continental shelf is subject to the coastal state, which under UNCLOS Article 60 and Article 80 has jurisdiction over installations and structures on its shelf, including for the protection of the marine environment.
An FPSO sits across both. Many flag administrations and coastal states resolve this through a memorandum or a national offshore framework that defines, for that unit on that field, which authority does what. The flag state typically retains the seafarer and ship-systems side; the coastal state takes the production, the well, the produced water, and the offshore safety case. Regulation 39’s “so far as reasonable and practicable” wording gives the two authorities room to agree which Annex I requirements apply in full to a unit that will never again behave like a trading ship. This is also why the offshore support and marine operations regime around an FPSO, the supply boats, the standby vessels, the subsea and offshore installation work, sits in a different regulatory channel from the production unit itself: the support fleet are ships in the ordinary sense, fully under Annex I and flag-state control, while the platform they serve is in Regulation 39 platform mode.
UNCLOS gives the coastal-state side its teeth. Article 60, applied to the continental shelf through Article 80, gives the coastal state the exclusive right to construct and authorize installations on its shelf and exclusive jurisdiction over them, including jurisdiction with regard to laws and regulations for the protection of the marine environment. The same article lets the coastal state declare a safety zone of up to 500 meters around an installation, measured from its outer edge, and a vessel must respect that zone. So when an FPSO discharges produced water on a state’s shelf, the state regulating that discharge is exercising a treaty power that does not depend on the flag the unit happens to fly. The flag state’s writ over the produced water is thin; the coastal state’s is direct. That allocation is what makes the OSPAR and NPDES regimes, not MARPOL, the operative law for the produced-water stream.
The 500-metre and nearest-land geography
Regulation 39 does not float free of geography; it ties into the Annex I definitions of “from the nearest land” and into the offshore safety zone. The 15 ppm discharge condition is the Annex I machinery-space condition, and the surrounding Annex I machinery applies the “nearest land” measure when a special area or a distance limit is in play. The revised Annex I, in its application provisions, also reaches ships operating in the immediate vicinity of a platform: a ship alongside or within 500 metres of a platform engaged in exploitation, while it is engaged in operations associated with that exploitation, is brought within the platform discharge regime rather than the ordinary ship regime for those operations.
That 500-meter coupling is the standard offshore safety-zone radius and it has a clear purpose. A supply boat sitting on the 500-meter zone, taking displacement water or slops from the platform, should not be able to discharge under a more permissive ship rule simply because it is a separate hull. While it is doing platform-associated work inside the zone, the platform’s discharge conditions follow it. Step outside the zone and resume ordinary trading, and the ship reverts to its own Annex I status. This is the same boundary logic the “while engaged in exploitation” trigger applies to the platform, projected onto the vessels working around it.
Relationship to OPRC and to the platform emergency plan
Regulation 39 governs routine and operational discharge. It does not govern the response to a spill. That is the domain of SOPEP under Regulation 37 for ships and of the OPRC Convention for offshore units, and the distinction is sharp.
The International Convention on Oil Pollution Preparedness, Response and Co-operation, OPRC 1990, was adopted on 30 November 1990 and entered into force on 13 May 1995. Its Article 3 requires that operators of offshore units under a Party’s jurisdiction have oil pollution emergency plans coordinated with the national response system. OPRC defines an offshore unit as any fixed or floating offshore installation or structure engaged in gas or oil exploration, exploitation, or production activities, or in the loading or unloading of oil, which is the same population of units Regulation 39 catches. So an FPSO carries an offshore-unit emergency plan under OPRC Article 3, coordinated with the coastal-state response framework, while a trading tanker carries a SOPEP under Regulation 37. When the FPSO goes on passage and becomes a ship, the SOPEP regime attaches for that voyage, the same toggle again.
The two instruments are complementary, not redundant. Regulation 39 and Annex I prevent the routine, day-to-day oil load from machinery and operations. OPRC prepares for the non-routine event, the blowout, the riser failure, the loss of a produced-water treatment train, and ties the unit’s plan into the national and international response machinery. The wider OPRC and HNS framework, including the 2000 HNS Protocol, extends that preparedness to hazardous and noxious substances. An offshore permit and emergency response regime on a given shelf usually bundles the OPRC plan, the discharge permit, and the safety case into one consent package, so the operator deals with a single coastal-state authority across all three.
The record of oil discharges and the 15 ppm discipline
The record obligation is easy to underrate and central to enforcement. Regulation 39 wants the platform to keep a record, in a form the Administration approves, of every operation that involves a discharge of oil or oily mixture. That record is the platform analogue of the ship’s oil record book under Regulation 17, and it serves the same evidentiary purpose: a coastal-state or flag-state inspector cannot watch the separator run for a year, so the law makes the unit write down what it did. The entries cover the separator operations, the quantities discharged overboard, the quantities held & landed ashore, the disposal of sludge & oily residues, and the times and positions of those operations. An inspector reconciles the record against the separator’s own continuous monitoring trace and against the receipts from the supply boats that landed the slops. A gap, a discharge logged at a time the oily water separator was down for maintenance, or a sludge balance that does not close, is the kind of finding that turns a routine inspection into a detention or a prosecution.
The 15 ppm number governs what may leave the side from the machinery and operational streams. The instrument that proves it is the 15 ppm bilge alarm required by Regulation 14, set to monitor the discharge continuously and, where required, to trigger an automatic three-way valve that recirculates the effluent back to the bilge holding tank the moment the oil content climbs above 15 ppm. So the limit is not a sampling target the crew aims for; it is a hard interlock the equipment enforces. The “without dilution” wording means the reading is taken on the stream as it comes off the separator, before any clean seawater can be introduced, so the unit cannot meet the number by volume rather than by treatment. A platform that cannot get a particular stream below 15 ppm has only one lawful route under Regulation 39.3: hold it on board and land it ashore to a reception facility. The non-compliant slug never goes over the side.
The “so far as reasonable and practicable” qualifier sits on the equipment, not on the 15 ppm limit. Read the provision carefully: the platform is to be equipped, so far as practicable, with the Reg 12 and Reg 14 fittings, but the discharge prohibition itself, except below 15 ppm, is flat. A unit can argue to its Administration that a particular ship-standard fitting does not map onto its layout and propose an equivalent arrangement; it cannot argue that the 15 ppm number should be relaxed because the fitting was hard. The practicability valve buys flexibility on the means, never on the discharge concentration. That distinction is what an inspector tests when an operator claims an arrangement is the reasonable-and-practicable one for the structure.
This record-and-equipment regime dovetails with the platform’s emergency planning rather than overlapping it. The record of oil discharges documents the routine, permitted discharges; the SOPEP or, on a production unit, the OPRC offshore-unit emergency plan documents what happens when a discharge is not routine. An inspector reads the two together. The record shows the unit’s normal operating envelope; the emergency plan shows that the unit is ready when something leaves that envelope. A platform that keeps a clean record but cannot produce a current, coordinated emergency plan fails the inspection on the preparedness side even though its routine discharges are in order.
A produced-water and bilge arrangement that satisfies Regulation 39
Consider a converted-VLCC FPSO on a UK North Sea field, spread-moored, processing 90,000 barrels of oil a day with a 70% water cut, so roughly 210,000 barrels of produced water a day rises with the crude. The unit has to satisfy two regimes at once on two physically separate water streams, and a well-built arrangement keeps them apart by design rather than by paperwork.
The machinery and operational stream, the engine-room bilge, the pump-caisson drainage, the deck drains around rotating equipment, collects in a bilge holding tank and feeds a marine oily water separator sized to the unit’s machinery class. The separator discharges through the Reg 14 15 ppm bilge alarm; the alarm’s three-way valve recirculates back to the holding tank whenever the reading exceeds 15 ppm. Every separator run, every overboard discharge, every transfer to the holding tank is logged in the approved record of oil discharges. That stream is the Regulation 39 stream, and it is held to 15 ppm, full stop. When the machinery side cannot make the number, for example during a separator overhaul, the bilge is held and landed ashore by supply boat under a reception-facility receipt that closes the record’s mass balance.
The produced-water stream never touches that equipment. The 210,000 barrels a day comes off the first-, second-, and third-stage separators on the topsides and passes through a dedicated produced-water treatment train, typically hydrocyclones for bulk dispersed-oil removal followed by a degassing or flotation vessel and, on tighter installations, a polishing stage such as a compact flotation unit or a nutshell filter. The treated water is monitored by an online oil-in-water analyzer and sampled to the OSPAR reference method, and it is held to the OSPAR Recommendation 2001/1 standard of 30 mg/l dispersed oil as a flow-weighted average, not the 15 ppm MARPOL number. The chemicals dosed into that train, the demulsifier upstream and the scale inhibitor in the well, are consented separately through OSPAR’s Harmonised Mandatory Control System under Decision 2000/2. The produced-water records, the daily oil-in-water averages & the chemical-use returns, go to the coastal-state regulator, not into the MARPOL record.
So the same hull runs two parallel compliance files on two parallel pipes. An auditor who confuses them, who looks for the produced water in the MARPOL record or expects the bilge to meet 30 mg/l, has misread the regime. The clean arrangement is the one where the two streams are kept physically separate from the bilge-and-ballast system through to overboard, each with its own treatment, its own monitor, its own limit, & its own record, so that neither regulator has to take the operator’s word that cross-contamination is not happening. When the unit disconnects and goes to a yard, the produced-water train shuts down with the wells, the toggle flips to ship mode, and only the machinery stream remains, now held to full Annex I without the platform softening.
The drilling-rig case
A mobile offshore drilling unit is the clearest test of the Regulation 39 trigger, because a rig spends its life moving between activities. While it is drilling a well, exploring or exploiting seabed mineral resources, it is a platform under Regulation 39: 15 ppm on its machinery bilge, the platform record of operations, the Reg 12/14 equipment so far as practicable. The offshore drilling and wells operation it is performing is exactly the “exploration or exploitation” the regulation names.
When the well is done and the rig moves off location under tow or under its own power to the next prospect, it is on passage. The trigger lifts. For that move it is a ship of its tonnage, and the full Annex I machinery-space discharge regime applies without the “so far as reasonable and practicable” softening. A semi-submersible under tow across an ocean is, for MARPOL purposes during the tow, a ship, and its bilge discharges must meet Regulation 15 outright. Rig operators manage this by maintaining the separator and bilge systems to ship standard at all times, since the unit can flip into ship mode on short notice and an inspector at the destination port will judge it as a ship.
The drilling rig also shows why “so far as reasonable and practicable” exists at all. A jack-up or a semi-submersible was not designed around the same engine-room and bilge architecture as a 400 GT cargo ship. Some Annex I machinery-space fittings simply do not map onto a rig’s layout. The qualifier lets the Administration accept an arrangement that achieves the environmental result, the oily water disposed of in accordance with the Regulation 15 options or landed ashore, even where a literal ship fitting would not fit the structure. It is a practicability valve, not a loophole; the anti-circumvention and no-dilution clauses keep it honest.
How Regulation 39 sits within Annex I
Regulation 39 is deliberately a referring provision, not a self-contained one. It draws almost all of its substance from the rest of Annex I and applies it to a special class of unit. So understanding it means understanding the rules it points to.
The 15 ppm number and the discharge architecture come from Regulation 15, the machinery-space discharge control for ships of 400 GT and above. The equipment to achieve it comes from Regulation 14, the oil filtering equipment and the 15 ppm bilge alarm, and the holding side comes from Regulation 12, the oil residue tanks. The record obligation parallels the oil record book of Regulation 17. The hold-and-land backstop runs to the reception facilities of Regulation 38. The operational-requirement inspection hook for port-state control sits in Regulation 11, the PSC operational-requirements provision. Regulation 39 binds these together for the platform population. It is less a new rule than an instruction on which existing rules to apply, to whom, and how hard.
The definitional foundation is Regulations 1 and 2 of Annex I, which define oil, oily mixture, oil tanker, and the scope of the Annex, and which set the platform population in the first place. The parent MARPOL Annex I article carries the wider definitional overview. The result is that Regulation 39 is best read as the last link in a chain that starts with the Annex I definitions, runs through the machinery-space discharge and equipment rules, and ends by projecting all of it onto the platforms that the offshore industry runs.
Limitations
This article states the discharge and equipment obligations of Regulation 39 and the regulatory architecture around it. It does not reproduce the verbatim treaty text, and an operator drafting a compliance plan must work from the current consolidated MARPOL Annex I as issued by the IMO and as transposed into the flag state’s national law, together with the relevant Unified Interpretations. The Unified Interpretation 67 treatment of produced and displacement water is summarized here; the precise scope of what UI 67 excludes, and any later interpretation, must be checked against the current MEPC.1/Circ. interpretation set.
The produced-water limits cited, the OSPAR 30 mg/l dispersed-oil performance standard and the US NPDES permit values, are coastal-state and regional standards that sit outside MARPOL. They change on their own schedules. The OSPAR figure is set by Recommendation 2001/1 as amended by 2006/4 and 2011/8 and is a flow-weighted average measured by the OSPAR reference method, not an instantaneous reading; OSPAR revises its measures periodically. The US offshore oil-and-grease limits live in EPA general permits such as GMG290000 that are reissued on a multi-year cycle, drawn from the effluent guidelines at 40 CFR Part 435. This article gives the structure of those regimes and the principle of the MARPOL-versus-national split, not the live permit numbers for any specific field. An operator must read the current permit for the actual block, and must confirm the current OSPAR reference method and any later amendment before relying on the 30 mg/l figure.
The IMO interpretation cited for FPSOs and FSUs is the current one, Resolution MEPC.311(73) of 26 October 2018, which superseded MEPC.139(53) and the original MEPC/Circ.406. That resolution incorporates Annex I amendments only up to MEPC.276(70); any later amendment to Annex I must be read against it, because the Committee re-issues these guidelines when the underlying Annex changes. An operator working a specific FPSO or FSU must confirm that the version in hand is the one its Administration applies, since a flag may transpose a particular edition into national law.
The flag-state and coastal-state jurisdiction split described here is the general UNCLOS-based pattern. The exact allocation for a particular FPSO on a particular shelf is set by that coastal state’s offshore framework and, often, by a specific arrangement between the flag and coastal administrations. There is no single global rule for who certifies what on a production unit; the article gives the framework, not the answer for a named vessel. The classification-society treatment of FPSO construction is covered in the FPSO article and the offshore class rules, not here, because Regulation 39 is a pollution-prevention provision and does not address structural integrity.
Finally, the “so far as reasonable and practicable” qualifier means that two platforms on different shelves, under different Administrations, can satisfy Regulation 39 with materially different equipment arrangements. The standard is one of demonstrated environmental result accepted by the Administration, not a single prescribed fit. Any statement here about what equipment a platform “must” carry is the default ship-class expectation; the actual approved arrangement for a given unit is whatever its Administration has accepted as reasonable and practicable for that structure.
See also
- MARPOL Annex I: the parent annex on prevention of pollution by oil
- MARPOL convention: the framework convention and its six annexes
- FPSO: Floating Production, Storage and Offloading vessel: the engineering and classification overview of the production unit
- MARPOL Annex I Regulations 1 and 2: definitions and application
- MARPOL Annex I Regulation 12: oil residue (sludge) tanks
- MARPOL Annex I Regulation 14: oil filtering equipment
- MARPOL Annex I Regulation 15: machinery-space discharge control
- MARPOL Annex I Regulation 17: oil record book
- MARPOL Annex I Regulation 37: SOPEP
- MARPOL Annex I Regulation 38: reception facilities
- Oily water separators and bilge water treatment
- OPRC 1990 and the HNS Protocol
- Offshore drilling and wells
- Offshore support and marine operations
- Port-state control