Test of double bottom tanks how to proceed

Answer

Testing double bottom (DB) tanks is required following major structural repairs, drydocking surveys, or as part of statutory classification survey cycles (Special Surveys) to confirm structural strength and watertight integrity.

                                               DOUBLE BOTTOM TANK TESTING

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Hydrostatic (Head of Water) Test                                                                                 Pneumatic / Air Test

• Statutory strength & tightness test                                                         • Welded seam leak detection

• Water filled to top of air pipe / 2.45m head                                            • Air pressure ~0.2 bar with

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Types of Tests

1. Hydrostatic (Head of Water) Test: The primary statutory method to test both structural strength and watertightness.
2. Pneumatic (Air) Leak Test: Used primarily during construction or after plate renewals to test weld seam tightness using soapy water before hydrostatic testing.

Step-by-Step Hydrostatic Testing Procedure

Phase 1: Pre-Testing Preparation & Safety

• Risk Assessment & Permits: Issue an Enclosed Space Entry Permit if personnel must enter adjacent spaces (e.g., dry cofferdams, adjacent tanks, or bilges) during the test.
• Isolate Systems: Isolate fuel oil/ballast filling lines, cross-overs, and stripping lines connected to other tanks using blank flanges or double valve isolation (Lock-Out/Tag-Out).
• Ventilation & Air Vent Check: Verify that the tank air vent pipe (gooseneck) is completely clear, unobstructed, and free of rust or stuck float balls to prevent over-pressurization.
• Internal & External Visual Check: Inspect internal floor plates, intercostals, framing, and soundings. Ensure all manhole covers are fitted with new gaskets and bolts are cross-torqued evenly.

Phase 2: Filling & Pressurization

• Controlled Filling: Slowly fill the tank with ballast water using a ballast/general service pump or gravity head. Slow filling prevents air locks and sudden pressure spikes.
• Achieving Statutory Head: Continue filling until water overflows through the air vent pipe on the open deck.
  • Class Requirement: The head of water must reach the top of the air pipe, or a minimum height of 2.45 m above the highest point of the tank top (whichever is greater).

Phase 3: Inspection & Holding Period

• Holding Pressure: Maintain the static water head for a minimum of 15 to 30 minutes to allow the structure to settle under load.
• Inspection Zones: Walk around all boundaries of the pressurized tank and check:
  • Tank top plating inside engine room or cargo holds.
  • Boundaries with adjacent DB tanks, cofferdams, and duct keels.
  • Bilge wells and margin plates.
  • Outer bottom shell plating (if vessel is in drydock).
• Pass Criteria: No permanent structural deformation, no weeping or sweating at welded seams, and no drop in the water level at the air vent pipe.

Phase 4: Draining & Documentation

• Safe Draining: Open drain valves or pump out ballast water slowly to avoid creating a vacuum inside the tank. Ensure air vents remain open during de-ballasting.
• Record Keeping: Log the test details (date, tank tested, head maintained, duration, and Class Surveyor signature) in the Ship’s Official Log Book and Engine Room Log Book.