Costa Concordia - Damage and Stability

Costa Concordia was delivered in 2006 and would have been designed to satisfy the SOLAS 90 deterministic damage regulations rather than the current SOLAS 2009 probabilistic damage stability regulations. SOLAS 90 specified the number of flooded contiguous watertight compartments that the vessel should be capable of withstanding based on the factor of subdivision (F);

• for F between 0.33 ­ 0.5 two compartments.
• for F equal to 0.33 or less the three compartments

Costa Concordia was apparently designed with a factor of subdivision equal to 0.386 and hence required the intact stability to be adequate to withstand the flooding of any two contiguous watertight compartments. Hull damage length of 53 metres (from frame 52 to 125) and variable width up to 7.3 metres. 

This damage resulted in the progressive flooding of five watertight compartments WTC, 4, 5, 6, 7 and 8. Watertight compartment No. 5 completely flooded in minutes, with rapid flooding occurring in WTC No. 6 and progressive flooding of WTC No. 4, 7 and 8. The flooding of these five compartments dramatically increased the ship's draught so that Deck 0 (bulkhead deck) started to be submerged and water also started to enter WTC No. 3 through a stairway enclosure connecting the deck to Deck C. The progressive flooding of these five compartments prior to their complete flooding (occurred in about 40 minutes) would also have resulted in a substantial free water surface effect, which would have had a resulted in a significant detrimental effect on the vessels stability, causing the first significant heeling to starboard, which increased the progressive flooding of WTC No.3. Forty-five minutes after the collision, the heeling to starboard reached 10°, and just before it grounded just over an hour after the impact, it was almost 20°. Then, 15 minutes after it grounded, the heeling was more than 30°. Below is a preliminary time table of the events and incidents and the development of the angle of list by the Italian Marine Casualty Investigation Central Board:

This damage resulted in the progressive flooding of five watertight compartments WTC, 4, 5, 6, 7 and 8. Watertight compartment No. 5 completely flooded in minutes, with rapid flooding occurring in WTC No. 6 and progressive flooding of WTC No.  4, 7 and 8. The flooding of these five compartments dramatically increased the ship's draught so that Deck 0 (bulkhead deck) started to be submerged and water also started to enter WTC No. 3 through a stairway enclosure connecting the deck to Deck C. The progressive flooding of these five compartments prior to their complete flooding (occurred in about 40 minutes) would also have resulted in a substantial free water surface effect, which would have had a resulted in a significant detrimental effect on the vessels stability, causing the first significant heeling to starboard, which increased the progressive flooding of WTC No.3. Forty-five minutes after the collision, the heeling to starboard reached 10°, and just before it grounded just over an hour after the impact, it was almost 20°. Then, 15 minutes after it grounded, the heeling was more than 30°. Below is a preliminary time table of the events and incidents and the development of the angle of list by the Italian Marine Casualty Investigation Central Board:

The flooded compartments contained a number of critical systems such as main diesel generators, ballast and bilge pumps, electrical propulsion motors. The flooding of these compartments resulted in black out of main electrical network, loss of propulsion and various high capacity sea-water service pumps.

The combination of factors effectively meant that Costa Concordia was damaged beyond any condition that it had been designed for and hence the irreversible flooding was beyond any manageable level.

Watertight Doors

The watertight doors of the flooded compartments were closed at the time of the collision. Initially there had been some debate about the vessel sailing with some watertight doors open and there certainly seems to be a bridge recording of the captain ordering the closing of both the watertight doors in the bow and the engine room just after the collision and then again sometime later another order to “Shut the door’s. Shut all the watertight doors immediately.” SOLAS regulations II-1/22 (paragraph 4), previous SOLAS regulation II-1/15 (paragraph 9.3), does permit a ship’s flag administration to allow under some circumstances certain watertight doors to remain open during navigation. Such doors must be clearly indicated in the ship’s stability information. An initial assessment also considered the possibility of water leaking out through the watertight door No.24 on Deck A into WTC No.4.

Computer Simulation of Costa Concordia Flooding

It should be noted that initial computer simulations attempts to model the flooding following the collision correlate well with the flood water heights in various spaces provided in the statements from crew members. However, the simulations showing the heel angle following the roll to starboard until the final grounding do not seem to match the heel angles from the on board inclinometer. The explanations offered for this non-correlation was flooding of additional spaces not considered in the simulation model and possible entrapped air in flooded spaces. However, the simulation also did not seem to take into account the wind heeling moment.

If only two WTC had been breached the ship would have still have had a significant positive GM. Since the damage was on the port side you would have expected an initial heel to port, however, unless there was significant asymmetrical flooding the ship would have likely slowly righted itself as the compartments fill with water. With five compartments damaged there would have initially been a reduced but still positive GM values. Under these conditions the wind heeling moment would have a significant effect and this may explain the slow transition from port to starboard heel angle as the ship came to a rest then turned beam on to the wind and drift back to shore. With further progressive flooding the resulting increasing free surface, loss of buoyancy and waterplane area, and the wind heeling moment would eventually resulted in a negative GM, which means the vessel take up an angle of loll to counter this. Once the vessels aft draft exceeds the bulkhead deck (Deck 0) this leads to future progressive flooding, loss of stability and rapid increase in the angle of loll.