Effects of Rogue Waves On Ships
To understand the effects of rogue or freak waves on ships at sea, it is vital to first develop an understanding of rogue waves in itself, the occurrence of such a phenomenon and the reason behind such an occurrence.
The occurrence of sea waves is an uncertain phenomenon. That is, if you consider a certain area of the sea surface, the waves passing through that area would definitely be periodic in nature, but the parameter of each wave (amplitude, time period, frequency, wave height and wavelength) will vary at a rate that cannot be determined to any exact measure.
To understand that with an example, let us suppose that a wave of height 10 meters passes through a given area on the sea surface at this instance. Then one cannot be sure if the next wave passing through the same area on the sea surface will have the same height. This natural uncertainty in the nature of sea surface waves makes it important for us to study sea waves in a probabilistic and statistical approach, rather than a deterministic approach.
Image for representation purpose only
So, we plot the probability of the occurrence of a particular wave height on what is called a histogram. The horizontal axis representing increasing height of wave and the vertical axis representing the probability of occurrence of a wave of the corresponding height.
Figure 1: Sea wave spectrum
In the above histogram, note the following things:
- The probability of a zero wave height is zero (which is evident, since there is no sea without a wave)
- The probability of occurrence of extremely high waves is also tending to zero
- The wave height corresponding to the maximum probability is somewhere higher than zero, in the moderate height region. These are the wave heights we see daily in a normal sea. The height corresponding to the maximum probability is maximum probable height
- The average of highest one-third of waves is the significant wave height (Hs). This is very important, as in, it is considered the reference point of designating a wave as a freak wave
- Any wave that has a height more than that of significant height is called a rogue wave. So, as clearly evident, the probability of occurrence of a freak wave is lower, but due to its height, the energy stored in one freak wave can be high enough to cause damage to ships
To know what effects rogue waves have on ships, we need to know the motion of a ship in a wave. When a ship operates in head seas, it experiences pitching coupled with heaving motions. The more the amplitude of the encountering wave, higher is the pitching and heaving amplitude.
Extreme combined motions of pitching and heaving in ships result in the forward part of the ship plunging into the sea surface after it encounters a wave. So, in some cases, when the sea state is high, there is a probability of increased height of waves that a ship may encounter. Sometimes, these waves may be higher than that corresponding to significant wave height, which is called freak waves or rogue waves.
The problem with such waves, is that they cannot be predicted, owing to the natural uncertainty of sea waves.
The following are the effects on ship that occurs because of encountering rogue waves at sea:
When a ship encounters high waves (especially in head seas), high amplitude pitching and heaving combined, produces an effect that sends the bow out of the water. As the wave passes aftward, the bow falls onto the surface (or slams the surface), with high acceleration, resulting in tremendous slamming forces in the forward structure of the ship.
Image for representation purpose only
Formation and Propagation of Cracks:
Due to high slamming and pounding forces in the forward structure, the hull at the bow section is often prone to cracks that can propagate over the entire depth of the bow section.
Buckling of plates:
The shell plates at the bow and the bottom plating upto 25 percent of the ship’s length aft of the forward perpendicular is subject to effects of slamming which result in buckling of these plates. Especially the bottom plating in the forward region, because in most lading cases, the ship is in hogging condition, which maintains the bottom shell in a state of compression. Major augment of stresses in the bottom plating therefore result in exceeding the buckling stress of the material, which may be much lower than the ultimate tensile stress.
When forward structures have been subjected to large number of cycles of freak waves or slamming forces over a longer period of time, the structure undergoes fatigue. If scantling and structural surveys are not carried out regularly, then ultimate failure, leading to complete rupture of bow sections is not an impossibility when encountered with freak waves.
So designers have over the years, developed methods to combat freak waves by incorporating various factors of safety in structural design. Broadly, we will discuss them under the following distinctions:
Inclusion in Structural Formulae:
When the scantlings of a ship are calculated in preliminary design phase, designers use empirical formulae suggested and tested by classification societies. These formulae have been developed over extensive observation and analyses of statistical data of stressed that ships are subjected to at sea, and accordingly, factor of safety are considered in determing the scantlings, so as to prevent failure due to waves that are above the significant height.
Additional Strengthening against Slamming:
The hull girder is additionally strengthened at the bow. Some of the additional structures that are included are discussed below:
- Panting stringers that run longitudinally, are welded to the sideshell forward of the collision bulkhead. The height between subsequent stringer is usually 2 to 2.5 meters
- Panting Beams run transversely inside the deck shell, joining the panting stringer to the centreline wash bulkhead
- Angled pillars are used to support the panting beams at the centreline
- Panting web sections or perforated flats are used after between every one or two panting stringers. The side frames are end connected to these and the panting stringers via tripping brackets to ensure smooth stress flow
- Plate floors are used at every frame space ahead of the collision bulkhead
Figure 2: Additional strengthening of forward structure
Navigational Measures Taken On-Board:
Navigational measures are also taken on board a ship, to combat rogue waves, especially in high sea states. Remember the initial paragraphs of this article where we discussed about pitching and heaving motions induced onto a ship? The idea of navigational measures is just to reduce the pitching amplitudes.
The pitching amplitudes depend on the encountering angle and the encounter frequency. The encounter frequency is changed by changing the speed of the ship, and the encounter angle is altered by changing the heading of the ship. Though the latter may not always be feasible on fixed routes, the former is mostly used to reduce unwanted motions on the higher side.