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Model setup

The model includes a single Generic Line connected to a vessel and the seabed.

Defining the vessel

The vessel is added to the model by clicking the floater button in the tool bar of the GUI :

Once created, all properties of the support vessel can be defined by editing the object which can be done by double clicking the vessel either from the 3D View Window or the Model Browser. The window used to define these properties includes several tabs:

The "Drawing" tab allows you to define the main geometric properties used for display in the 3D View window. The coordinates of the geometric reference point can be set within the edit boxes located above this tab. The coordinates of the center of motion (point to which motion RAOs or other displacements are applied) can be also set from this panel and must be defined with respect to the geometric reference point.

Note

The center of motion is not necessarily the same point than the geometric reference point. The geometric center point is used for display in the 3D View window and serves as a reference to define the position of fairlead and hang-off points. Usual practice is to set this point at the keel level, mid-ship on the main axis of the vessel. The center of motion is then defined with respect to the geometric reference point.

Vessel motion properties are input through the RAOs in the "motion" tab.

The "Fairlead/hang off point" tab allows you to define the coordinates of the connection points between the floater and the line. Connection with the vessel is made at a specific hang-off point defined through its coordinates relative to the geometric centre point of the vessel and departure angle. The departure angle is defined through the azimuth and elevation parameters, as detailed in the picture presented in the Fair-lead/Hang-off tab of the vessel definition pane.

In this example, the departure angle is set to 30 degrees and the elevation is set to 187 degrees. The riser departure angle forms therefore an angle of 30 degrees with the vessel axis on the starboard side. The line runs from End_1 to End_2 with an elevation angle of 187 degrees so that the line forms a vertical angle of 3 degrees in the starting plane of the line.

The departure angle will however have no impact on the riser motions during both static and dynamic simulations in this case since the riser is just pinned at the hangoff. The departure angle will serve as a reference built-in angle from which angular deflection could be measured at post-processing stage prior to designing the bending stiffener.

Setting seabed properties

Seabed properties are defined using the Sea & ground tab in the Model browser. Several type of seabed profile could be defined: flat, with slope and user- defined. One or several connection points for the line could be defined on the seabed with specific azimuth/vertical departure angles.

Pipe/Soil interaction

Pipe/soil interaction is handled through a specific Line/surface contact property that must be defined prior to the analyses in the Contact Types model browser folder. Contact is modelled with a normal stiffness of 1000kN/m2 and Coulomb friction in this case. Axial and lateral friction coefficients are defined with a friction mobilization distance of 1E-03m that represents the slippage necessary to mobilize the full friction force. User should not forget to complete the Contacts tab in the calculation parameters windows when defining the analysis.

Defining mechanical properties of a line

The segment property can be opened from the Line Type directory in the Model Browser.

The input pane type used to define the pipe properties is "Flexible Pipe" which means that 6 DOF beam elements are used and defined through the mass per unit length, buoyancy and drag diameters, hydrodynamic characteristics, and principal stiffness terms.

The finite element model selected is a bi-nodal beam that means that the shape function is linear. Tri-nodal beams corresponding to quadratic shape function are rarely used and it is recommended to use bi-nodal in most cases.

The mass per unit length refers the weight in air and is also used to determine the inertia of the pipe in air when subject to motions. The submerged weight of pipe per unit length is automatically derived by the GUI from the mass in air per unit length and the buoyancy diameter input by the user. User may also impose the submerged weight in water with the "Apparent weight option". In this case, the calculation of the submerged mass using lineic mass in air and the buoyancy diameter is not used and instead the apparent weight entered is used directly in the calculations.

Two options are available to define an internal content,

  • Add the weight directly due to the internal content into the lineic weight and/or apparent weight of the line. The internal cross section of the pipe is set to 0.

  • Set the internal cross section area of the line correctly and define the internal fluid density along the line from the Internal fluid tab in the line definition window. This allows you to easily model lines with several internal contents without changing the mechanical properties of this line or adjusting the submerged weight.

Defining the line

The riser is added in the model by clicking on the line button , and then selecting the "Generic line" option for most cases. A new line appears in the Model directory and could be further defined through the "Structure" and other tabs used to set all the properties of the lines components.

This new line is made up of a single Section between the two end nodes. Sections can be split into several sections to create new intermediate connection points when needed. Each section may also be given different initial shapes, such as straight, catenary, upper and lower arc, or a user- defined shape. Any section is composed of one or several Segments. Segments are defined through their length, associated pipe mechanical properties, and segmentation (the number of finite elements along the segment).

In this model, the riser is modelled as a single Section between two connection nodes.

The first end node is pinned to the hang-off point defined on the vessel while the second end node is connected to the Ground Connection node defined within the Sea & Ground component using a user-defined connection. The user-defined connection used for node End_2 leaves vertical displacement free to allow the riser to lie exactly on the seabed (instead of having the centre line of the pipe at the seabed level as when using a pinned connection).

There is no need for any other connection point along the line and therefore the line does not need to include more than a single section. This section is however made of several Segments which are given specific lengths, pipe properties and element segmentation. Though the pipe property remains unchanged all along the riser, two sections are used in this case to allow for different element lengths to be used along the upper catenary and in the touch-down point area. The first segment is meshed with 10m long elements whereas the segmentation is refined in the second segment with 5m long elements to improve the accuracy of the results.

Current

The Current profile is defined by specifying the current velocity and heading for several water depth levels. Both current velocity and direction are interpolated linearly between the specified levels.

The following options are available to activate the current during the static analysis:

  • The full current velocity is applied from the beginning of the static analysis (no ramping),

  • The current velocity is increased linearly during the static steps (linear ramped),

  • The current velocity is increased over the static steps following a user-defined loading table.

During the dynamic analysis, users could apply either the constant current for all time-steps, or define a new current profile at specific times to set-up varying current direction and speed.

Waves

Both regular and irregular wave components are defined in this model. The regular wave is based on the Airy wave model. The two irregular waves simulating swell and wind seas are modelled as JONSWAP spectra for which the required maximum height over significant height is set to 1.86. Whatever the total simulation time, this ratio ensures that the maximum wave height that will be encountered during the dynamic simulation period will be at least 1.86 times the significant height. It is therefore not required to run the simulation over 3 hours to catch a maximum wave height representative of actual conditions. Each wave spectrum is then split into 200 single wave components between user-specified periods of 3s and 25s.

The wave heading and wave periods must lie in the range of relative headings and periods for which vessel motions RAOs was defined.

Offset

The offset component is a static displacement applied to the vessel in several static steps. The complete offset amplitude will be reached for the last static load step if the linear quasi-static evolution was selected.