OrcaFlex Manual - Orcina

System Modelling: Data and Results, Environment
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3. For analysis of permanent systems (e.g. flexible risers) use expected maximum wave height with the
appropriate return period (commonly 50 or 100 years return period for 5 to 20 year field life) and a range of
associated wave periods. If field specific data are not available, use the period range recommended by Tucker.
6.5.17 Setting up a Random Sea
This section gives information on how to set up a random sea using OrcaFlex's modelling facilities. For a detailed
description of these, see Wave Data.
The most common requirement is to produce a realistic wave train which includes a "design wave" of specified
height Hmax and period Tmax. However alternative requirements are possible and it is sometimes useful to impose
additional conditions for convenience in results presentation, etc.
The height and period of the maximum design wave may be specified by the client, but on occasion we have to
derive the appropriate values ourselves, either from other wave statistics (for example a wave scatter table, giving
significant wave heights Hs and average periods Tz) or from a more general description of weather (such as wind
speed).
Having decided what values of Hmax and Tmax are required, we select an appropriate wave train as follows, using the
facilities available in OrcaFlex.
� Set the significant wave height (Hs) and average period (Tz) for the design storm, and the wave spectrum – ISSC,
JONSWAP, Ochi-Hubble, Torsethaugen and Gaussian Swell options are available.
� Set the number of wave components (typically 100).
� Search through the time history of wave height and looking for a particular wave rise (trough to crest) or fall
(crest to trough) which has the required total height and period. If no wave of the required characteristics can
be found, then adjust Hs and Tz slightly and repeat.
� When the required design wave has been located, you can set the simulation time origin and duration so that
the design wave occurs within the simulation time, with sufficient time before and after to avoid starting
transients and collect all important responses of the system to the design wave. A typical random sea simulation
may represent 5 or 6 average wave periods (say 60-70 seconds for a design storm in the North Sea) plus a build
up period of 10 seconds. If the system is widely dispersed in the wave direction, then the simulation may have
to be longer to allow time for the principal wave group to pass through the whole system. Since short waves
travel more slowly than long ones, this affects simulations of mild sea states more than severe seas.
Setting the Sea State Data
The ISSC spectrum (also known as Bretschneider or modified Pierson-Moskowitz) is appropriate for fully-developed
seas in the open ocean. The JONSWAP spectrum is a variant of the ISSC spectrum in which a "peak enhancement
factor", γ, is applied to give a greater concentration of energy in the mid-band of frequencies. The Ochi-Hubbleand
Torsethaugen spectra enable you to represent sea states that include both a remotely generated swell and a local
wind generated sea.
JONSWAP is commonly specified for the North Sea. Two parameters are sufficient to define an ISSC spectrum – we
use Hs and Tz for convenience. For the JONSWAP spectrum, five parameters are required, Hs, Tz, γ, and two
additional parameters σa and σb (denoted σ1 and σ2 in OrcaFlex), which define the bandwidth over which the peak
enhancement is applied. If you choose JONSWAP then you can either specify γ or let the program calculate it (see
formulae given by Isherwood). The bandwidth parameters are set automatically to standard values). For the North
Sea it is common to set γ = 3.3. If you have to do a systematic series of analyses in a range of wave heights, there are
advantages in keeping γ constant. Note that a JONSWAP spectrum with γ = 1.0 is identical to the ISSC spectrum with
the same Hs and Tz.
Choice of wave spectrum can cause unnecessary pain and suffering to the beginner. For present purposes, the
important point is to get the "design wave" we want embedded in a realistic random train of smaller waves. The
spectrum is a means to this end, and in practice it matters little what formulation is used. The one exception to this
sweeping statement may be 2-peaked spectra (e.g. Ochi-Hubble or Torsethaugen).
Setting the Number of Components
OrcaFlex generates a time history of wave height by dividing the spectrum into a number of component sine waves
of constant amplitude and (pseudo-random) phase. The phases associated with each wave component are pseudorandom.
OrcaFlex uses a random number generator and the seed to assign phases. The sequence is repeatable, so
the same seed will always give the same phases and consequently the same train of waves. The wave components
are added assuming linear superposition to create the wave train. Ship responses and wave kinematics are also