KUP 13 Have thorough knowledge of the Principles to be observed in keeping a navigational watch

Basic Introduction

Intact buoyancy refers to the buoyancy provided by the undamaged, watertight compartments of a ship when it has experienced damage or flooding. It is the upward force exerted by the water on the remaining intact volume of the hull below the waterline. In simpler terms, imagine a ship with several compartments. If one compartment is flooded due to damage, the ship loses the buoyancy that compartment provided. However, the other undamaged compartments still contribute buoyancy, helping to keep the ship afloat. This remaining buoyancy is the intact buoyancy.

Intact buoyancy is crucial for a ship’s survival in a damaged condition. It helps to counteract the weight of flooded compartments: The upward force of intact buoyancy opposes the downward force of the flooded sections, preventing the ship from sinking further. It ensures that a ship remains afloat and upright under normal conditions, which is fundamental to protecting the lives of those on board and the cargo being transported. Maintaining intact buoyancy allows a ship to navigate through various marine environments safely, preventing sinking or capsizing due to water ingress or improper weight distribution. Additionally, a vessel with optimal buoyancy achieves better fuel efficiency and handling characteristics, which are important for economic and environmental reasons. Compliance with international maritime safety regulations, such as those enforced by the International Maritime Organization (IMO), also necessitates maintaining intact buoyancy to avoid penalties and ensure the vessel’s continued operational approval. In essence, intact buoyancy is a critical factor in ensuring that maritime operations are conducted smoothly, safely, and by global standards. Naval architects carefully consider intact buoyancy when designing ships, aiming to create vessels that can withstand damage and remain afloat with sufficient stability. They use calculations and simulations to estimate the intact buoyancy in various damage scenarios, ensuring the ship’s structural integrity and safety.

Damaged stability

Imagine a ship with a forward compartment breached and exposed to seawater. The ship loses buoyancy in that area and sinks lower until the remaining intact hull sections provide enough lift to regain equilibrium. As the ship’s center of buoyancy (LCB) shifts, the ship must also tilt (trim) to align the center of gravity (G) and LCB vertically. This means the ship, originally floating at waterline W0L0, now floats at a new waterline W1L1.

If this new waterline W1L1 rises above the bulkhead deck (the deck where the damaged compartment ends), it’s usually assumed the ship would be lost. This is because the water pressure in the flooded compartment could force open hatches, leading to uncontrolled flooding. However, in reality, the ship might still stay afloat for a significant period.

We’ve mainly focused on a ship’s stability in undamaged conditions. Internal damage, like an explosion, might compromise the structure but rarely causes severe instability. Such incidents are unlikely to sink a ship, though evacuation might be necessary. A ship sinks only when enough water enters the hull, either overwhelming its buoyancy or drastically reducing stability, leading to overturning. Water can enter through collisions, grounding, or leaks. It’s crucial to assess the remaining transverse and longitudinal stability, as both can be significantly reduced.

Three scenarios can unfold:

1. Foundering: Insufficient buoyancy causes the ship to settle and sink.
2. Capsizing: Inadequate transverse stability causes the ship to roll over.
3. Plunging: Insufficient longitudinal stability causes the ship to sink by either the bow or stern.

Effective ship subdivision can limit flooding and its impact. The level of subdivision varies with ship size and function, with passenger ships having the most. Openings in lower decks and bulkheads are minimized, and watertight doors/hatches are typically closed at sea. Remote or local closure is usually possible. While smaller, numerous compartments maximize stability, they hinder operations, necessitating a compromise. Warships have more extensive subdivision to better withstand combat damage. The ability of compartments to be filled with water, also known as permeability, needs to be factored into considerations.

Permeability

Most ship compartments are not vacant. They contain equipment, fittings, and sometimes liquids in tanks. Even empty spaces have structural elements like stiffeners and brackets. So, if a compartment is breached, the water can not fill it completely. The ratio of the water volume that can enter to the total compartment volume is known as permeability.

Damage stability assessment methods:

Calculating the new waterline after damage requires a step-by-step approach because the usual assumptions of minor changes do not hold true. There are two ways to do this: the lost buoyancy method and the added weight method. These methods result in different metacentric height (GM) values, but they yield the same righting moment. Ultimately, both methods should lead to the same final drafts.

Lost buoyancy method

Initially, the volume of the damaged compartment up to the original waterline and the lost waterplane area are calculated, considering the compartment’s permeability. Assuming the original waterplane area is A and the lost area is μa (where μ is permeability), with the lost volume of buoyancy being μv, we can then estimate the initial parallel sinkage the ship experiences.

A second estimate is usually needed due to changes in waterplane area with draft. This involves using the characteristics of a waterplane halfway between the original and flooded levels. The longitudinal center of flotation and moment to change trim (MCT) are calculated for this intermediate waterplane, again accounting for permeability. With subscript m representing intermediate values:

where x̄ is the distance of the lost volume’s centroid from the center of flotation.

New drafts can be found from sinkage and trim. Further refinement is possible if either value is large, or results can be verified using Bonjean curves, which consider flooding and permeability.In the lost buoyancy method, the center of gravity (G) remains unchanged unless severe damage removes ship structure or equipment. This method is typically used for damage stability calculations.

Added Weight Method

In this alternative approach, the water flooding the damaged compartment is treated as additional weight. Permeability is factored in when calculating this weight, and the free surface effect of the incoming water is also considered. However, all hydrostatic data used are those of the undamaged ship. Initially, the calculation proceeds like any other weight addition, but once the new waterline is determined, the extra water entering the ship up to that level must be accounted for. This may require further iterations of the calculation.

Both methods described assume the breached compartment extends above both the original and final waterlines. If not, the actual floodable volumes need to be used, and the assumed waterplane characteristics adjusted accordingly. It’s clearly advantageous for a ship to have ample potential buoyancy above the intact waterline, known as the reserve of buoyancy.

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