What is Critical Period in dry docking and why it is called Critical?

Ever wondered what is critical period in ship’s dry docking and why it is called critical? Docking and Undocking are an integral part of any ship’s life at sea. Docking can be scheduled , or unscheduled (in case of any major accident) or can be done if a ship is undergoing any conversion.

This topic will cover some basic aspects with respect to Docking of vessel. 

Whenever a vessel proceeds to Dry Dock, every Dry Dock facility (whether floating or fixed) have certain requirements which normally comes through the Docking Master. Professionals expert in Docking and Undocking of vessels are called Docking Master. These requirements can be with respect to certain draft, trim, amount of ballast, Air draft or any other requirement or a combination of requirements. They will also demand for the Docking Plan of the vessel. I will discuss the Docking plan and other plans in detail in my upcoming topics.

As a Master of the vessel, it is very important to ensure that Docking condition of the vessel is calculated in advance and ship’s meet the stability criteria during such operation. 

Docking condition normally forms part of the vessel’s Trim and Stability Booklet and shall be carefully studied. 

An critical period in dry docking condition plan for a vessel has been given below:

Above plan shows the arrangement of the vessel. While below table shows the location of various compartments and their corresponding frame locations.

Master shall pay attention to the value of GM in the Docking Condition. It is very important that the vessel has sufficient positive GM and required trim before she docks. The blocks (normally made of reinforced concrete having wood padding on top ) are arranged throughout the dock prior arrival of the vessel to the dock. The position of blocks is determined keeping in mind the length / breadth of the vessel and also the position of Bottom plugs and Doppler/EM Log and Echo sounder sensor.

Usually, the dock is full with water when the vessel enters it and once inside, the pumping out is commenced. 

Continuous monitoring is done by the Docking Master with respect to vessel’s position as the water level in the dock falls. The vessel usually needs to be in the centerline of the dry dock so that complete keel rests firmly on the blocks. To ensure this, the rate of pumping is reduced when the stern is about to touch the blocks. Forward and aft mooring lines are passed from the vessel to the shore. To keep the vessel aligned with the centerline of the dock normally a Leica theodolite is used. Theodolite is a surveying equipment which is widely used to measure the horizontal and vertical angles. You must have seen the instrument especially during road / building / bridge construction by civil engineer.

Normally, the stern touches the blocks first. When the ship’s stern touches the keel blocks, this contact creates a Normal Reaction or Upthrust. The value of this Normal Reaction force or upthrust increases as the water level further reduces in the dock. This results is sudden reduction in the value of GM which is the measure of the vessel’s stability. 

And if, at this time, the vessel doesn’t have sufficient positive GM, the sudden reduction in the GM may result in negative GM which may result in the vessel to capsize. 

Over here it is important to define what is Critical Period.

What is Critical Period and why it is called ‘critical’ ?

A Critical Period is the interval of time from, when the stern of the vessel touches the blocks to the time when the entire weight of the vessel is borne by the blocks (i.e. the vessel sits completely on blocks). This period is very crucial and continuous monitoring is required.

The word ‘critical’ means something which has the potential to cause a disaster. So, it is logical as to why the critical period is ‘critical.

Also, vessel shall keep minimum required trim so that the critical period is reduced to minimum. 

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Fair Seas & Bonn Voyage!

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