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How high and dangerous can a monster wave be and what can we do to protect vessels and offshore structures from the raw force of the sea? The Ocean Engineering Section of the Technical University, Berlin, is developing new methods for the generation of tailored wave sequences for model tests.

Rough waves – simulation and impact
on marine structures

By Dipl.-Ing. Christian Schmittner,
Dipl.-Ing. Sascha Kosleck and
Prof. Dr.-Ing. Guenther Clauss,
Institut of Land and Sea Transport Systems, Ocean Engineering Section,
Technische Universität Berlin,
schmittner@naoe.tu-berlin.de,
kosleck@naoe.tu-berlin.de,
clauss@naoe.tu-berlin.de

Since time immemorial, seafarers have been heard telling unbelievable stories of gigantic waves or white walls of water washing men and cargo overboard, smashing hulls and capsizing and drowning even the strongest built ships. These accounts of survivors of such rogue waves, were often contributed to a vivid fantasy or to an oversized portion of rum. Times have changed, ships have become bigger and stronger, but still vessel or offshore structures are lost and damaged in severe weather.

In the last years research projects have focused on extreme waves, their formation and impact on marine structures in order to throw light on this mystery. Fortunately, the chances to encounter a rogue wave are rare – unfortunately, observations, photos or measurements also.

But even though their probability is very low, they are physically possible. With the improvement of wave measuring techniques over the last years a number of rogue waves have been reported from all over the oceans. Nickerson shows photos of two exceptional and frightening events (see Figure 1).

Figure 1. Rogue wave observations. The Bay of Biscay (left) and the Atlantic Ocean, south of Newfoundland (right).

Rogue waves, also known as freak waves or monster waves, are commonly defined by a ratio of Hmax/Hs > 2.0, where Hmax is the maximum wave height and Hs the significant wave height – the wave height an experienced mariner would guess – in a given sea state.

One of the best documented rogue waves is the so called New Year Wave (see Figure 2). This giant wave hit the Draupner jacket platform in the Norwegian sector of the North Sea on the 1st of January 1995 with a maximum wave height Hmax of 25.63 m and a wave crest height _c of 18.5 m and caused sever damage on deck. Beside the wave height itself, this wave is of special interests as the significant wave height Hs during this time was “only” 11.92 m, resulting in a Hmax to Hs ratio of 2.15. Note, that for the design of marine structures often a traditional value of 1.86 is used.

Figure 2. New Year Wave, North Sea, Draupner jacket platform (Hs = 11.92 m, Hmax = 25.63 m = 2.15 _ Hs , _c = 18.5 m, water depth d = 70 m).

Exceptional waves have also been reported from different places all over the world, many of them having a height of 2.3 to 2.5_Hs, a few even 2.9_Hs. It is a challenging question which maximum wave and crest heights can develop in a certain sea state. The most common causes for their appearance or whether they vary from place to place is still hard to say.

In addition to the global parameters Hs (significant wave height) and Tp (zero-up-crossing period) of a given sea state, the occurrence of a rogue wave and its destructiveness depend on various factors, such as the individual wave height and shape, the superposition and interaction of single wave components and/or the existence of strong currents, running against the direction of wave propagation.

Simulation
How dangerous is a specific rogue wave? Even though numerical simulation tools are straight forward, these questions can often only be answered by experiments. The Ocean Engineering Section of the Technical University Berlin, led by Prof. Guenther Clauss, develops new methods for the generation of tailored wave sequences for model tests. Extreme waves are generated by using so-called wave packages, i.e. the wave generator first generates short waves followed by longer waves. Due to the dispersive characteristic of water waves – longer waves propagate faster than shorter ones – the long waves catch up with the slower waves and superimpose at a specified position in the tank creating a rogue wave.
Figure 3 presents the genesis of a 3.2 m rogue wave generated in the large wave channel of the Coastal Research Centre in Hannover/Germany, breaking on a wave breaker constructed for the protection of offshore structures and coastlines. Using this wave packet technology the impact, e.g. pressure, forces and extreme loads, can be investigated at model scale.

Figure 3. 3.2 m rogue wave smashing on a wave breaker in the large wave channel of the Coastal Research Centre in Hannover, Germany.

Beside the largest wave a combination of successive waves or local wave characteristics (e.g. wave steepness) can also lead to extreme structure responses. As an example for a dangerous wave group, the previous mentioned New Year Wave has been generated in the wave tank (see Figure 4 for a comparison of the recorded real wave and the wave tank simulation).


Figure 4. New Year Wave, comparison of original recording (blue) and wave tank simulation (red); Ocean Engineering Section, Technische Universität Berlin, Germany.

Using this real scenario the forces and motions of a semisubmersible for the exploration of oil fields and a FPSO:s (Floating Production Storage and Offloading ships) have been investigated. Even though the New Year Wave is a rogue wave it is – with a height of “only” 25.63 m – still not the highest wave expected during the life time of an offshore structure or ship. Nevertheless, the bending moments of the investigated FPSO:s where just within its calculated design loads. Figure 5 presents the impact of the simulated New Year Wave on the bow section of the FPSO.


Figure 5. Impact of the New Year Wave on the bow section of a FPSO (Model Scale 1:81); Ocean Engineering Section, Technische Universität Berlin, Germany.

Depending on the position of the maximum wave related to the ships position the characteristic of the bending moment as well as its maximum and minimum vary significantly. A rogue wave developing right at the FPSO:s midship section does not automatically cause the highest bending moments.

Figure 6 shows the characteristic of the bending moment at midship for several different positions x of the maximum wave height. x= initial denotes the rogue wave occurring right at midship position whereas x= –3/2 lpp describes a rogue wave occurring 1.5 ship length upstream of the midship section and x=3/2 lpp the same wave occurring 1.5 ship length downstream. The figure shows, that the maximum sagging moment (MS= 5.61_106 KNm) develops with the maximum wave peak occurring 1/4 ship length upstream of the midship section while the maximum hogging moment (MH= 4.39_106 KNm) develops with the maximum wave peak 1/4 downstream. Note, that the design moment for the investigated FPSO is 5.64_106 KNm.

Figure 6. Bending moments at FPSOs midship section depending on the position of the maximum wave height.

The information collected from these and related experiments with waves having a ratio of Hmax/Hs > 2.15 and _c/Hmax > 0.6 (crest-draught asymmetry), proves that rogue waves are serious events which should necessarily be considered in the design processes of ships and offshore structures.

Latest update 18-10-2006 8:49

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