Wind Data

The wind parameters recorded by the buoy during the corresponding time window are given in Table 7.5.

Figure 7.9 Limb 3 tension (upper) and buoy excursion to the west (lower) time series plots for the 240 second data.

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Table 7.5 Wind parameters recorded by the SWMTF buoy during the time window under consideration.

Parameter Value

Mean wind speed 12.4 m/s

Maximum wind speed 19.5 m/s

Minimum wind speed 6.5 m/s

Mean direction (emanating from) 089°

7.3.4 Current Data

Current profile data is plotted from the 10 minute ensemble commencing 09.30 09/10/2010 and is given in Figure 7.10.

The mean water level depth at this time is 31 metres. It is clear that the tidal current at the surface is running in a southerly direction at approximately 0.5 m/s with a velocity component to the east of approximately 0.05 m/s. This resolves to a tidal current at the surface of 0.5 m/s (1 dp) flowing towards a bearing of 174°.

At the surface there is considerable current attributable to the wind and wave action that is superimposed on the tidal current. The ADCP bins within the wave elevation zone (31m +/- wave amplitude) return a maximum mean current of 0.32 m/s to the west and 0.55 m/s to the south. The resultant of these components is a surface current of 0.64 m/s towards a bearing of 210°.

Figure 7.10 Current profiles eastward (LH) and northward (RH) during the peak event.

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7.4 Orcaflex

Orcaflex is a commercial marine dynamics software package used to conduct static and dynamic analysis of many different types of offshore systems. It is a 3D, non-linear, finite element program operating in the time domain and being capable of dealing with large magnitude deflections (Orcina, 2014). Orcaflex can model the coupled behaviour of a surface vessel and its mooring system. In what is often referred to as a discretised cable model, Orcaflex employs an idealised system of mass components (nodes) and visco-elastic elements (segments) to represent cables and mooring lines (Masciola et al., 2011).

Some of the important options selected within Orcaflex for this work are described in the following sections.

7.4.1 6D Buoys

Floating bodies fall into two main categories in Orcaflex; vessels and buoys. A 6D buoy represents the fullest modelling available in terms of imparted loads and kinematics. They have mass, moments of inertia, added mass, damping and drag. 6D buoys are subject to wave slam forces, connection loads, fluid flow effects, applied loads and contact forces (Orcina, 2014).

Three subsets exist for 6D buoys; these are lumped buoys, spar buoys and towed fish. Spar buoys are intended for modelling axi-symetric buoys having a vertical axis. Hydrodynamic loads are calculated according to Morison’s equation implying that the buoy in question is small in relation to the wavelength (Orcina, 2014).

7.4.2 Waves

Orcaflex allows one or more wave trains to be defined. Orcina (2014) suggest that a single wave train is normally sufficient in all but complex cases such as a crossing sea.

Each wave train can be specified by a regular wave theory such as Airy, a particular type of spectrum for random waves such as JONSWAP, or by a time history input file (Orcina, 2014).

When a time history input file is used to define the wave environment, Orcaflex performs a FFT (fast Fourier transform) on the time series for surface elevation.

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The programme then assigns a single Airy wave to each of the frequency components that result from the transform. These Airy waves are then used in combination to recreate the waveform described by the input file (Orcina, 2014). Importantly with this method, the input time series must be appropriate for the FFT. “The FFT requires the number of samples it uses from the time history file, N say, to be a power of 2, and it produces N/2 components. Because of this, the time history file must contain a sequence of N samples that covers the period of the simulation, where N is a power of 2 that is at least twice the specified minimum number of components” (Orcina, 2014). To achieve this, a time series that is greater in duration than the simulation is input to Orcaflex and the wave origin and duration is set to define the time window of the simulation. Orcaflex will then use a longer duration of the time series for the FFT.

7.4.3 Lines

Orcaflex provides for three main types of line:

• ‘Homogenous pipe’ which is used to represent pipes where the properties can be defined by material properties such as Young’s modulus.

• ‘Equivalent line’ which represents multiple pipes either arranged concentrically or adjacently.

• ‘General’ which is used in all other cases. In this category the functional properties of the line such as axial stiffness, bending stiffness and linear density are input directly. This category of line is therefore appropriate for ropes, chains, umbilicals etc.

Geometry and mass can be defined by the user from which the programme will derive buoyancy. Bending stiffness and torsional stiffness can be defined or deemed to be negligible. Axial stiffness can be defined as a profile of tension (kN) against extension (%). Where appropriate generic values for these parameters can be used via Orcaflex’s line wizard (Orcina, 2014).

7.4.4 Integration Methods

Orcaflex provides two integration methods, explicit and implicit. The explicit integration is described as robust and reliably accurate but computation time can be much higher than implicit integration (Orcina, 2014). By contrast the

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implicit integration is much quicker but the accuracy of results can be sensitive to the time step selected for use. Orcina recommend that results from implicit integration simulations are compared to results from explicit simulations if possible (Orcina, 2014).