Critical Period During Dry Docking

The "critical period" in dry docking refers to the specific time when the ship transitions from being fully supported by buoyancy (floating in water) to being supported by the blocks in the dry dock. During this time, the ship's stability is at its most vulnerable.

Key Points About the Critical Period:

1. Loss of Stability (GM Reduction):

   • As the water level decreases during dry docking, the ship's center of buoyancy (B) moves downward and eventually disappears when the ship is fully supported by the blocks.
   • The ship's metacentric height (GM), which measures stability, decreases as the buoyancy reduces and may even approach zero temporarily, causing instability.

2. Importance of the Critical Period:

   • If stability is not carefully monitored during this phase, the ship could develop a dangerous list (tilt) or even capsize.
   • Proper positioning on the blocks and monitoring the ship’s center of gravity (G) are crucial to ensuring safety.

3. Critical Factors to Consider:

   • The ship’s longitudinal and transverse centers of gravity.
   • The distribution of weight onboard the ship.
   • The rate of dewatering in the dry dock.

Effect of Buoyancy During Dry Docking

Buoyancy plays a crucial role during the dry docking process and becomes a potential hazard during the critical period.

Key Points About Buoyancy:

1. Buoyancy Forces:

   • Buoyancy is the upward force exerted by water that counteracts the weight of the ship.
   • As water is drained from the dry dock, the buoyancy forces gradually decrease, and the ship transfers its weight to the blocks.

2. Danger of Uneven Distribution:

   • If the ship is not aligned properly, uneven loss of buoyancy can cause the ship to tilt or shift suddenly.
   • This misalignment can result in damage to the hull or failure of the supporting blocks.

3. Structural Stress:

   • During the transition, parts of the ship may experience increased stress as the weight shifts from buoyancy to the blocks. If not evenly supported, this can cause structural damage.

Why Buoyancy is Dangerous in the Critical Period:

1. Dynamic Instability:

   • As the water level lowers, buoyancy disappears unevenly. This creates a moment of instability, especially if the ship’s center of gravity is too high or not centered properly.

2. Impact on the Hull:

   • The hull may experience uneven stresses, leading to potential deformation or damage if the ship is not perfectly aligned on the keel blocks.

3. Critical Timing:

   • The transition from buoyancy to block support happens quickly, making it essential to carefully control the process to avoid accidents.

Mitigating Risks During the Critical Period

1. Precise Block Placement:

   • Ensuring the blocks are correctly positioned and can evenly support the ship’s weight is essential.

2. Slow Dewatering:

   • Reducing the water level gradually allows for careful monitoring of the ship’s position and stability.

3. Weight Distribution:

   • Before dry docking, ballast and cargo should be managed to ensure an even weight distribution and a low center of gravity.

4. Continuous Monitoring:

   • Stability parameters (such as GM) and structural stresses should be monitored throughout the process.

In summary, the critical period is the phase where the ship transitions from floating to being fully supported by the dry dock blocks, making it a period of potential instability. Proper planning, precise execution, and continuous monitoring are essential to mitigate risks associated with the loss of buoyancy and stability during this time.