Ship Structural Details
The structural failure of the NEW CARISSA was likely the result of a combination of sea floor scouring effects, bottom pounding, and transverse bending from waves. The eventual break-up of the vessel into two pieces can be viewed as a direct result of a combination of these effects.
Vessels that strand broadside to incoming seas and surf on a sandy bottom are subject to significant scouring of the sea floor, which can quickly lead to excessive hull stresses. As the incoming waves strike the vessel's side, they are deflected fore and aft until they are able to sweep around the ends of the vessel at high velocity. The high velocity water flow at the ends scours away the sand beneath the bow and stern, and can deposit the sand on the seaward and leeward sides of the vessel near amidships. This scouring and depositing can quickly develop significant hogging stresses on the vessel as a pinnacle is effectively created under the vessel near amidships. (In a hogging condition, the deck of the vessel is placed in tension, and the bottom structure in compression.) The scouring can also place excessive localized compression forces on the bottom structure as the weight of the vessel is transferred to the sand pinnacle, resulting in crushing of the bottom structure. Thus, it is good practice to try to prevent a stranded vessel from broaching by whatever means possible, including ground tackle, tug assistance, and ballasting. If the ship has broached in such conditions, an extraordinary effort should be made to turn the vessel end on into the seas .
Vessels that strand in heavy seas will likely suffer structural damage due to pounding impact between the sea floor and the vessel bottom. When a vessel initially grounds in large seas, the ground reaction will not normally be high enough to prevent the vessel from lifting off the bottom as large swells pass. As a result, the vessel will drop back on the sea floor with significant impact as each wave crest passes. This impact can quickly lead to structural damage at the point of ground contact. One simple method to eliminate liveliness and bottom impact is to ballast a stranded vessel down as hard as possible. This can increase the bottom contact, spread the load, and reduce localized bottom stresses .
The damage incurred on the bottom of the vessel that resulted in breaching of the fuel tanks and engine room was most likely due to excessive localized compression from scouring and progressive damage from pounding. The vertical fracture on the side shell of the vessel was reported by on-scene personnel to be associated with a vertical buckle that was seen to be working in and out in response to wave impact. This buckling was most probably the result of compressive hogging stresses (from bottom scouring) in combination with transverse bending stresses created by large waves striking the port side of the vessel and flexing it laterally about the stranding pinnacle. The transverse bending due to wave action would act to cyclically superimpose tensile stresses on the port side, and compressive stresses on the starboard side with each wave impact.
Design and Construction Factors Relative to the Failure
An important consideration when reviewing this failure is that the NEW CARISSA was constructed of high strength steel (exact ultimate strength unknown). It is generally understood and accepted by designers and engineers that high strength carbon steel structures have less fatigue resistance (strength under cyclic loading) than similar structures constructed of mild steel. There have been many examples in the marine industry of premature fatigue failures of high strength steel structures. This characteristic of high strength steel was likely a contributing factor to the rapid rate of crack growth on the stranded NEW CARISSA as it was exposed to excessive cyclic loading from the heavy seas and surf.
Operational Factors Relative to the Failure
It is also an accepted principle of materials engineering that high strength carbon steels are very sensitive to high temperatures while in service. Sustained elevated temperatures can quickly anneal the steel by removing strengthening precipitates from the matrix of the material, resulting in the loss of high strength properties. Additionally, rapid cooling (quenching) of hot steel is known to cause embrittlement, which further degrades the fracture resistance of the structure. The burn subjected the vessel to both effects; the extremely high and sustained temperatures had an annealing effect, and areas of the structure subject to wave contact were repeatedly subjected to heating and quenching. Figure 2 depicts the heat-affected areas of the vessel where the coatings were burned away leaving bare steel and rust. It shows severe heat warping and coating damage in the failure region and clearly indicates that the highest temperatures occurred in that location.
End Result
The stranding and structural failure of the NEW CARISSA highlights the impressive forces that can quickly be imposed on a vessel broached in heavy surf on a sandy beach. The forces on such a vessel have been discussed as being a combination of 2 mode global bending of the hull girder (hogging and transverse bending), localized vertical compression from the formation of a pinnacle below the vessel, and impact forces from bottom pounding. These bending and impact loads all stem directly from or are amplified by the effects of bottom scouring.
When considering the likely response of the NEW CARISSA's hull steel to high temperatures, and reviewing the pictorial evidence of the heat damage, it can be concluded that the burn attempt significantly hastened the break up of the vessel. The fact that there was limited visible damage to the hull structure before the burn (just 3 hours before catastrophic failure of the structure) also supports this conclusion. The multi-member structure of a large modern ship is complex and redundant and will normally suffer extensive visible progressive damage before catastrophically failing.
In his report on the incident, Captain Hall stated that, "It is very unlikely, and there is no evidence, that the explosions or fire contributed to the vessel breaking in half" . For the reasons discussed above, the author does not reach this same conclusion. It is the author's position that incident responders contemplating the use of in-situ burning as a spill prevention or response strategy must be fully cognizant of the predictable structural ramifications of such action. In particular, the effect that these ramifications will have on subsequent salvage and response efforts should be of highest concern.
As previously stated, it is unlikely that the NEW CARISSA structural failure will herald changes to future structural designs or standards. However, a review of the case provides insight to engineers and incident responders on the expected structural behavior of such severely stranded vessels, and highlights possible implications of the in-situ oil burning technique.
References:
[1] Hall, Captain M. J., United States Coast Guard, "Crisis on the Coast" Federal On Scene Coordinator's Report - Grounding of the NEW CARISSA, June 1999
[2] Milwee, William I. Jr., Modern Marine Salvage, The Society of Naval Architects and Marine Engineers, New Jersey City, NJ, 1996
Comments