Trondheim Tekniske Fagskole The “Design Wave Philosophy’’ Calculation of the design wave Wave forces on semi-submersible platforms Wave forces and bending moments in FPSO-ships Platform movements in large waves Examples of heavy weather damage *****What is a Rogue Wave ?***** Why, where and when ? Shall we design against Rogue and Freak Waves ? What can a platform master do against Rogue and Freak Waves ? Remote-sensing of sea conditions Search And Rescue and emergency operations Decision making in an emergency What is a Rogue Wave ?

Trondheim Tekniske Fagskole What is a rogue wave ?

Trondheim Tekniske Fagskole What is a rogue wave ? The various definitions What are the problems for design ? What are researchers working on ?

Trondheim Tekniske Fagskole What is a rogue wave ? Let’s start with “What is a Wave ?”

Trondheim Tekniske Fagskole What is a rogue wave ? In practice, you know it was a rogue wave once you saw it.

Trondheim Tekniske Fagskole What is a rogue wave ? A good excuse when crew failed to install port hole storm covers ?

Trondheim Tekniske Fagskole What is a rogue wave ? A 15° roll of the camera angle ?

Trondheim Tekniske Fagskole What is a rogue wave ? Just something that happens ?

Trondheim Tekniske Fagskole What is a rogue wave ? In physics and/or statistics, there are several definitions:

Trondheim Tekniske Fagskole What is a rogue wave ? Some say a wave that is more than twice the significant wave height, but that may not be a reliable definition.

Trondheim Tekniske Fagskole What is a rogue wave ? If significant wave height was constant and equal to 10 meters, one would encounter a wave of: 18.5 m every 3 hours 21.5 m every day 25.0 m every month 27.5 m every year 30.0 m every 20 years 31.5 m every 100 years 33.1 m every 1000 years 34.8 m every years The maximum in one day is also the maximum in the 20 minutes or 3 hours surrounding it, so twice the significant wave height is by no means abnormal: it happens every day !

Trondheim Tekniske Fagskole What is a rogue wave ? One of those waves in a [large] set of a given height that make the set more numerous than it should be from conventional theory.

Trondheim Tekniske Fagskole What is a rogue wave ? From the figures above, theory states that there should be 500 waves above 2Hs in a year. If one measures 1000 of them, they are rogue waves: about 500 freak and 500 normal extremes and of course we can’t distinguish easily which are which.

Trondheim Tekniske Fagskole What is a rogue wave ? A wave of unexpected severity given the prevailing sea conditions at the time it occurs

Trondheim Tekniske Fagskole What is a rogue wave ? A wave of unexpected severity given the prevailing sea conditions at the time it occurs. Unexpected: A good deal of damage can be avoided if those exposed are fully aware there and then. Severity: Steepness, Front steepness and other characteristics may be more damaging than mere height.

Trondheim Tekniske Fagskole What is a rogue wave ? Reconstructed water surface elevations over a 1000 m span, from T-30s (blue) to T (red) for the New Year Wave. The famous “Draupner wave”

Trondheim Tekniske Fagskole What is a rogue wave ? The various definitions What are the problems for design ? (calling again on Sverre Haver) What are researchers working on ?

Trondheim Tekniske Fagskole Air gap If waves do not reach the deck, wave forces are rather mild since waves go through a rather transparent structure, but if they hit the deck… How high above sea-level should the deck sit ?

Trondheim Tekniske Fagskole Are rogue waves a problem for structural design? Could be – but that depends very much what is meant by rogues waves. At least two options: a) ”Classical” extreme waves i.e. rare members of a population of waves defined by modelling the surface process as a piecewise stationary and homogenous slightly non-Gaussian process. This type of extreme waves are presently accounted properly for by the offshore industry provided some accidental wave load scenario is implemented. Traditional shipping may have some room for improvements. However, waves that are likely to cause structural failure do exist within this population, but their annual probability of occurrence should be lower than say

Trondheim Tekniske Fagskole Are rogue waves a problem for structural design? b) Freak (extreme) waves; defined as typical members of a population being in agreement with physical mechanisms well beyond those under a). This type of extreme waves are outside our design events. If such a population exists, it may challenge present design recipes if it effects the – annual probability of exceedance level at a given site.

Trondheim Tekniske Fagskole Suggested definition of freak waves A freak wave event is an event that is governed by mechanisms and/or phenomena well beyond those underlying the piecewise stationary and homogenous second order model of the sea surface. A second order model is selected as a reference population for non-freak events because that is the most sophisticated model available for routine design. A factor definition is not very useful since it may – with low probability – be exceeded even under Gaussian assumption for the sea surface elevation.

Trondheim Tekniske Fagskole Can freak waves represent ”an ugly” mechanism? YES – IF THEY MAKE THE UPPER TAIL (10 -3 – ) FOR THE ANNUAL EXTREME VALUE DISTRIBUTION AT A GIVEN SITE SIGNIFICANTLY FATTER

Trondheim Tekniske Fagskole The Challenge! KNOWN THREAT Major threat of safety – but can be accounted for. Wave-parallel: ”Classical” extreme waves UNKNOWN THREAT Difficult to approach in a rational way – but can be crucial regarding safety. Wave-parallel: Freak extreme waves Does a separate freak wave population exist????

Trondheim Tekniske Fagskole What is a rogue wave ? The various definitions What are the problems for design ? What are researchers working on ?

Trondheim Tekniske Fagskole Scientists work Two main approaches: Look for theoretical models that enable to construct mathematical rogue waves that are as similar as possible to measured ones. Analyse occurrences to find characteristics of the wave/sea state/storm that distinguish them from innocuous ones.