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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).
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| 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.
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| 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.
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| 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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