Back to Ship Construction & Naval Architecture
Ship Construction & Naval Architecture

Bilge Holding Tank Cleaning & Safe Entry

Sound the tank, discharge legally via the 15 ppm OWS or slop tank, log the ORB, then clean only under full enclosed-space precautions.

7 min read
Intermediate
Ship Construction & Naval Architecture
Key Principles at a Glance 6 points
  • Cleaning begins by taking a sounding of the bilge holding tank, then informing the bridge before discharging water from the tank via the OWS, noting the ship position which must be outside a special area.
  • Discharge as much bilge water as possible overboard through the 15 ppm alarm; if that is not possible, transfer to the slop tank or send it to a shore facility.
  • Every discharge or transfer must be recorded as an entry in the Oil Record Book (ORB).
  • Before entering the tank to clean it, a full enclosed space entry must be completed: the space must be well ventilated and tested for sufficient oxygen and other gases, with O2 at 21% by volume.
  • A permit to work must be filled and authorised by the Master, valid only for a set period, with adequate lighting, signs and "men at work" boards, and the duty officer informed.
  • During cleaning the same enclosed space rules apply: standby man, lifeline, continuous oxygen analyser, limited persons, no ignition source, and rescue/resuscitation and hoisting equipment outside; after work the checklist is completed and the permit closed.

1. The Bilge Holding Tank & First Steps

The bilge holding tank collects oily bilge water from the machinery space bilges and other compartments before it is treated or transferred. Over time, sludge and sediment accumulate and the tank must be cleaned — a job that combines MARPOL discharge rules with enclosed space entry safety.

First Steps:

  • First, take a sounding of the tank to establish the quantity of bilge water and sludge present.
  • Now inform the bridge for discharging water from the tank via the OWS (Oil Water Separator), and note down the position of the ship — it should be outside the special area.
  • Discharge as much as possible of the bilge water overboard (subject to the discharge criteria).
Why Position Matters

MARPOL prohibits the discharge of oily bilge water inside designated Special Areas. Before any overboard discharge, confirm the vessel's position is outside a special area and that the 15 ppm equipment is operating correctly.

BILGE HOLDING TANK — SYSTEM & FIRST STEPS MARPOL ANNEX I 1. TANK SOUNDING & OVERBOARD PROCESSING Sound tank → Inform bridge (GPS check) → Process via 15 ppm OWS Surface Oil Layer Oily Water Mixture Heavy Sludge & Sediments Sounding OWS UNIT 15 ppm BILGE ALARM OVER- BOARD Recirculation (>15 ppm) 2. STATUTORY DISCHARGE CRITERIA MARPOL Annex I Regulation 14 / 15 rules 1. VESSEL MUST BE EN ROUTE Vessel proceeding on voyage with speed > 0 kn 2. 15 PPM LIMIT & AUTO-STOP Oil content ≤ 15 ppm • Auto 3-way divert valve 3. OUTSIDE SPECIAL AREAS Strict zero discharge in designated Special Areas (e.g., Med, Baltic, Black Sea, Red Sea, Gulfs) BRIDGE NOTIFICATION MANDATORY Bridge confirms GPS position before valve open INITIAL SOUNDING • Sound tank before starting • Calculate total oily water • Estimate bottom sludge volume • Log soundings in engine log • Prepares pumping plan BRIDGE & POSITION • Inform Duty Officer on bridge • Confirm ship is en route • Verify outside special area • Record GPS coordinates • Strict MARPOL compliance OWS OPERATION • 15 ppm monitor self-test • 3-way valve fail-safe check • Pump maximum water out • Recirculate if > 15 ppm • Leave minimal water for clean
Figure 1: Bilge Holding Tank First Steps. Sound the tank to estimate volume, inform the bridge to verify GPS position outside special areas and en-route status, and process bilge water through the 15 ppm OWS.

2. Legal Discharge: 15 ppm, Slop Tank, Shore & ORB

If the bilge water cannot be discharged overboard within the permitted limits, it must be dealt with in another legal way:

  • Via the 15 ppm alarm testing — discharge is only permitted when the oily water separator output is within the 15 ppm limit.
  • If discharge overboard is not possible, transfer into the SLOP tank.
  • Or send it to a SHORE facility (reception facility) for disposal.
  • Note down the entry into the Oil Record Book (ORB).
Oil Record Book — Non-Negotiable

Every transfer, discharge, and disposal of oily residues must be recorded in the Oil Record Book. Missing or inaccurate ORB entries are a serious MARPOL violation and a frequent cause of port state detention.

BILGE RESIDUE DISCHARGE & TRANSFER DECISION TREE MANDATORY ORB 1. MARPOL ANNEX I DISCHARGE ROUTES 15 ppm OWS test pass → Overboard • Failure → SLOP tank or Shore Bilge Holding Tank Contents ≤15 ppm & Outside SA? YES DISCHARGE OVERBOARD Via approved OWS NO OPTION A Transfer to SLOP Tank OPTION B Send to SHORE Facility Shore connection: standard MARPOL flange (215 mm OD, 6 slots) 2. OIL RECORD BOOK (ORB PART I) Mandatory official legal document ORB PART I — MACHINERY SPACES Code (D) / (I) Discharges & Transfers: • Date & operational code entered • Start & stop GPS coordinates • Total quantity discharged (m³) • Identity of tank (Bilge Holding) • Disposal route: OWS / Slop / Shore Signatures Required: ✓ Officer in charge of operation ✓ Chief Engineer countersignature PSC AUDIT PRIORITY — ZERO ERRORS 15 PPM OVERBOARD • En route outside special area • Approved 15 ppm monitor • Auto-stop 3-way divert valve • Oil content strictly ≤ 15 ppm • Overboard valve sealed shut SLOP & SHORE ROUTE • Transfer to ER slop tank • Decant or incinerate residues • Discharge to shore reception • Standard MARPOL flange used • Retain receipt from terminal ORB PART I LOG • Mandatory for all movements • Log start & stop coordinates • Exact quantity in cubic metres • Signed by Officer & C/E • Port state control check
Figure 2: Discharge Decision Flow. If the 15 ppm alarm test permits, discharge overboard; otherwise transfer to the slop tank or send to a shore reception facility. Every operation is recorded in the Oil Record Book.

3. Preparing the Tank for Entry

Once the bilge water has been transferred, the tank itself must be cleaned — and that means people must enter it. Before cleaning the tank after transfer, a full enclosed space entry must be completed first.

  • The space must be well ventilated.
  • The space should be checked for sufficient O2 level and other gases. The O2 level should be 21% by volume; a percentage less than that is not acceptable and more time for ventilation should be given in such circumstances.
  • Enough lighting and illumination should be present in the enclosed space before entering.
  • A proper permit to work has to be filled out and the checklist checked to prevent any accident that could endanger life.
  • The permit is valid only for a certain time period; if it expires, a new permit must be issued and the checklist filled out again.
  • The permit has to be checked and permitted by the Master.
  • Proper signs and "Men at Work" sign boards should be provided so no one starts equipment, machinery or operations in the space, endangering the people working.
  • The Duty Officer has to be informed before entering.
  • The checklist has to be signed by the person involved in the entry and also by a competent officer.
BILGE TANK PRE-ENTRY ISOLATION & TESTING MANDATORY ENCLOSED SPACE PERMIT TO WORK PROTOCOL ENGINE ROOM DECK / TANK TOP BILGE HOLDING TANK INTERIOR MANHOLE OPEN BLOWER Fresh Air Fresh air purged to tank floor BLANK SPADE Line Isolated SAFE LAMP <24V Intrinsically safe 1. GAS SAMPLING TEST O2 LEVEL 21.0% Safe LEL (GAS) < 1% LEL TOXIC (CO) < 25 ppm H2S GAS 0.0 ppm Nil 2. CONTROLS & MASTER PERMIT Master Sign-Off Permit authorised in writing Strict time validity window Bridge OOW Notified Duty officer logged entry Comms frequency established "MEN AT WORK" BOARD Posted at starter panels No machinery energised DUAL SIGNATURES Signed by Entrant person Signed by Competent Officer TANK ISOLATION & VENT • Forced air fan ducted to floor • Continuous positive airflow • Pipe blanking / lines spaded • Suction valves locked shut • Safe lamp: <24V certified torch GAS TESTING LIMITS • Oxygen strictly 20.9% to 21% • Flammable gas < 1% LEL • CO < 25 ppm / H2S nil • Sample top, middle, and bottom • Calibrated 4-gas detector used PERMIT & NOTICES • Master signature required • Time-limited valid window • Duty Officer on bridge advised • "Men at Work" lock signs hung • Entrant + Officer co-signature
Figure 3: Tank Entry Preparation. Before cleaning, the tank must be positively isolated with blanks, continuously ventilated from the bottom floor, tested with a calibrated 4-gas analyser (O2 21%), illuminated with safe low-voltage lamps, and authorised by the Master under a time-limited Permit to Work.

4. Cleaning Safely & Closing Out the Permit

While cleaning the bilge holding tank, the same enclosed space safety rules apply throughout:

  • One person must always be kept standby to communicate with the person inside.
  • The person may carry a lifeline inside with them.
  • The person should carry an oxygen analyser, on at all times, to monitor the oxygen content; if the level drops, it sounds an alarm and the space is evacuated without delay.
  • No source of ignition may be taken inside unless the Master or competent officer is satisfied.
  • The number of persons entering is limited to those actually needed.
  • Rescue and resuscitation equipment — including breathing air apparatus and spare charge bottles — must be present outside, and means of hoisting an incapacitated person must be available.
  • After finishing the work, when the person is out of the enclosed space, the after-work checklist has to be filled and the permit to work closed.
Exam Answer Structure

Answer in two clear phases: (1) Before entry — sound the tank, inform the bridge, discharge via the 15 ppm OWS or transfer to slop/shore, log the ORB, then complete enclosed space entry (ventilate, test O2 21%, light, permit). (2) During/after — standby man, lifeline, oxygen analyser, rescue equipment, then the after-work checklist and permit closure.

TANK CLEANING SAFETY & STANDBY RESCUE POST-CLEANING VERIFICATION & PERMIT CLOSE-OUT MANHOLE RIM STANDBY WATCH Continuous watch & radio TRIPOD ENTRANT Active continuous O2 meter BRONZE / BRASS TOOLS Non-sparking, rubber buckets SCBA RESCUE GEAR • Breathing apparatus • Spare charge cylinder 1 PERSONNEL EVACUATION & HEADCOUNT All entrants 100% confirmed out of tank coaming. Standby man tallies crew. 2 TOOL TALLY & ROSE BOX INSPECTION All scrapers, buckets, lamps removed. Bilge strummer/rose box clean. 3 MANHOLE BOX-UP & GASKET SECURING Brand new oil-resistant gasket fitted; bolts torqued diagonally and watertight. 4 AFTER-WORK CHECKLIST & PTW CLOSE-OUT Permit to Work formally closed by Master & C/E. Bridge informed. "Men at Work" lock tags cleared; system returned to service. STANDBY WATCH RULES • Dedicated watch at coaming rim • Never leave post or enter tank • Continuous verbal or radio link • Sound alarm if comms lost • Tripod hoist rigged and ready EQUIPMENT & RESCUE • Active chest O2 monitor on • Full body harness & lifeline • Spark-proof brass scrapers • SCBA set + spare bottle outside • Evacuate instantly on gas alarm PERMIT CLOSE-OUT • 100% crew headcount verified • All tools & gear accounted for • Fresh neoprene gasket torqued • Checklist signed by entrant & C/E • Master cancels PTW in logbook
Figure 4: Cleaning Safety & Close-Out. Throughout cleaning, a dedicated standby man, lifeline, continuous chest oxygen analyser, non-sparking bronze tools, and external SCBA rescue equipment protect the entrant. After work, headcount and tools are verified, a new gasket is secured, and the permit is formally closed by the Master.