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Ship Construction & Naval Architecture

Sounding Pipe Regulations & Safety Features

SOLAS rules on sounding pipes, the 32 mm and 65 mm diameter limits, striking plates and self-closing sounding caps.

6 min read
Intermediate
Ship Construction & Naval Architecture
Key Principles at a Glance 6 points
  • SOLAS applies to sounding pipes for bilges, cofferdams and double bottom tanks situated in the machinery space.
  • All sounding pipes must extend to a position above the ship's bulkhead deck which is at all times accessible.
  • The sounding pipe diameter should generally be not less than 32 mm.
  • Where sounding pipes pass through refrigerated spaces, to allow for icing, a minimum diameter of 65 mm is required where the temperature is 0°C or less.
  • Sounding pipes for the bilges of insulated holds must themselves be insulated and not less than 65 mm in diameter.
  • Safety features include a thick steel doubling or striking plate securely fixed below each pipe for the sounding rod to strike, and a self-closing parallel plug cap that opens on being loaded and closes automatically when released.

1. Purpose & SOLAS Application of Sounding Pipes

A sounding pipe is a vertical pipe fitted to a tank or compartment by which its liquid level can be measured with a sounding rod or tape. It allows the crew to determine whether a bilge, cofferdam or tank contains liquid — and how much — without opening the compartment.

Where SOLAS Requires Sounding Pipes:

The SOLAS regulation applies to sounding pipes for:

  • Bilges — to detect accumulated water and leakage.
  • Cofferdams — the narrow void spaces between bulkheads or around tanks.
  • Double bottom tanks situated in the machinery space.
Why Sounding Matters

Regular sounding is the primary means of detecting flooding, leakage and unequally distributed liquids. A prompt sounding can reveal a breached tank or hull before it escalates into a stability or pollution emergency.

SOLAS REG. II-1/12 | SOUNDING PIPE MANDATORY APPLICATION BULKHEAD DECK Engine Room Floor Plates TANK TOP PLATING KEEL & BOTTOM PLATING 1. BILGE WELL / SUMP 2. DOUBLE BOTTOM (DB TANK) 3. COFFERDAM (Void Space) Sounding Tape & Bob 1. BILGE WELLS • Detects engine water ingress • Located at lowest aft wells • Early warning for hull breach Flood & Leak Warning 2. DOUBLE BOTTOM TANKS • Heavy fuel, diesel & ballast • Daily bunker consumption audit • Essential for trim & stability Fuel & Ballast Sounding 3. COFFERDAMS • Void isolation safety space • Separates oil from water • Verifies dry integrity Dry Void Verification
Figure 1: Sounding Pipe Application. Sounding pipes are fitted to bilges, cofferdams and double bottom tanks in the machinery space so that liquid levels can be measured from an accessible position.

2. Extension & Accessibility Rules

SOLAS requires that all sounding pipes shall extend to a position above the ship's bulkhead deck which shall at all times be accessible.

  • Above the bulkhead deck: The pipe opening must terminate above the bulkhead deck, so that sounding can be carried out even if the compartment below is flooded or inaccessible.
  • At all times accessible: The terminal end must not be blocked by cargo, stores or fittings — the crew must be able to reach the sounding position in any condition.
Why Accessibility is Critical

If a sounding pipe opening is buried under cargo or behind a structure, a flooding bilge or tank may go undetected. Accessibility is therefore a surveyor inspection point during safety construction surveys.

SOLAS CHAPTER II-1 | EXTENSION & ACCESSIBILITY MANDATE BULKHEAD DECK LEVEL SAFE ACCESSIBLE CREW WALKWAY Permanent access 24/7 • Never obstructed by cargo Self-Closing Plug FLOODED MACHINERY OR CARGO COMPARTMENT Space completely flooded and sealed below bulkhead deck Striking Plate Continuous Sounding Safe sounding despite flood 1. TERMINAL POSITION • Extends above bulkhead deck • Free from floodwater ingress • Watertight deck gland seal Above Flood Level 2. 24/7 ACCESSIBILITY • Never blocked by cargo/stores • Accessible in all sea states • Surveyor primary survey check Always Reachable 3. SAFETY IMPLICATION • Inaccessible pipe hides leak • Allows early bilge pumping • Prevents sudden vessel capsize SOLAS Core Rule
Figure 2: Extension & Accessibility. Sounding pipes must terminate above the bulkhead deck and remain accessible at all times, so that liquid levels can be checked even when the space below is flooded or closed off.

3. Diameter Requirements: 32 mm & 65 mm

The minimum diameter of a sounding pipe depends on its location and the risk of icing or condensation:

  • General requirement: The sounding pipe diameter should not be less than 32 mm.
  • Refrigerated spaces: Where sounding pipes pass through refrigerated spaces, to allow for icing, the minimum diameter is 65 mm where the temperature is 0°C or less.
  • Insulated holds: Sounding pipes for the bilges of insulated holds shall be insulated and not less than 65 mm in diameter.
Why Bigger Pipes in Cold Spaces?

In refrigerated spaces, moisture in the pipe can freeze. Ice buildup in a small-bore pipe could block it completely. A 65 mm minimum bore gives enough clearance for the sounding rod and resists ice blockage.

GENERAL MINIMUM: ≥ 32 mm Standard Machinery, Bilges & Double Bottoms ≥ 32 mm STANDARD SPECIFICATIONS • Minimum inner diameter: 32 mm • Standard for bilges, tanks & cofferdams • Allows smooth sounding rod descent General Marine Mandate COLD SPACES: ≥ 65 mm + INSULATION Refrigerated Holds (≤ 0°C) & Insulated Bilges ≥ 65 mm COLD SPACE PROTOCOL • Temperature ≤ 0°C: Minimum 65 mm bore • Prevents frost/ice from choking pipe • Insulated hold pipes must have insulation Ice Allowance Protection
Figure 3: Diameter Requirements. General sounding pipes are not less than 32 mm, but where they pass through refrigerated spaces at 0°C or less, and for the bilges of insulated holds, a minimum diameter of 65 mm is required, with insulation on the pipe.

4. Safety: Striking Plate, Self-Closing Cap & Location

Striking Plate (Doubling):

  • A thick steel doubling or striking plate shall be securely fixed below each sounding pipe for the sounding rod to strike upon.
  • Without it, repeated sounding would hammer and perforate the tank bottom, causing leakage.

Self-Closing Sounding Cap:

  • The pipe is fitted with a parallel plug with an arrangement which opens on being loaded and gets automatically closed when released.
  • This avoids damage in case a person forgets to close the sounding cap — preventing water or oil from entering or escaping through an open pipe, and preserving the watertight integrity of the compartment.

Location:

  • The sounding pipe is always located close to the suction pipe, so as to get the correct sounding for the pump to take suction.
SAFETY ENGINEERING | STRIKING PLATE & SELF-CLOSING PLUG A. SELF-CLOSING PLUG Counterweight Failsafe Gravity Action Snaps closed when released B. STRIKING PLATE Sounding Rod Bob Protects Bottom from Piercing C. SUCTION PROXIMITY Sounding Suction True Pump Suction Datum 1. SELF-CLOSING CAP • Counterweight / spring cock • Auto closes on hand release • Stops vapor & oil overflow Failsafe Auto-Close 2. STRIKING PLATE • Heavy steel doubler welded below • Absorbs brass bob hammer blows • Prevents tank bottom puncture Doubling Protection 3. SUCTION PROXIMITY • Placed close to suction strum • Reflects true suction level • Eliminates dry pump suction True Suction Datum
Figure 4: Safety Features. A thick steel striking plate protects the tank bottom from the sounding rod, a self-closing parallel plug cap guards against a cap left open, and the pipe is located close to the suction pipe to give a correct reading for pump suction.