Aqwa allows for the modeling of installed tethers, such as those used on Tension-Leg Platforms (TLPs), and drilling risers. As those features share some basic properties, they are combined into a single Tether/Riser object.

To add a Tether/Riser:

Tethers/Risers may only be joined between a fixed point and a structure, therefore the Connectivity field in the Details panel is not editable for this type of connection. The Start Fixed Point drop-down list of defined fixed points allows you to select a fixed point for the tether/riser, while the End Connection Point can be selected from a drop-down list of existing Connection Points defined on the structures.

The Tether/Riser Local Axes (TLA) are defined with their origin at the position of the Start Fixed Point. The TLA Z-axis points from this origin toward the End Connection Point. The TLA X-axis is parallel to the Fixed Reference Axes (FRA) XY plane, and at right-angles to the TLA Z-axis. The TLA Y-axis follows the right-hand rule. When the tether/riser is vertical in the FRA, the TLA will be parallel to the FRA. For more information, see Mass and Stiffness Matrices in the Aqwa Theory Manual.

Tethers/Risers can consist of up to 24 Tether/Riser Sections, which can be defined under the Connection Data object and accessed as required to build up a number of Tether/Riser connections. For each tether/riser, the Section Type field allows you to select the Tether/Riser Section via a drop-down list. Once chosen, the unstretched Length of that section can be entered. The whole tether/riser make-up is summarized in the Tether/Riser Definition Data table.

In the Graphical Window, while the tether/riser object is selected in the Outline panel, the tether/riser is drawn in its unstretched condition. Any length from the Start Fixed Point to the End Connection Point which is not covered by the Tether/Riser Section Selection is shown as a yellow dashed line. When the tether/riser object is not selected in the Outline panel, the tether/riser is drawn in its stretched condition.

Boundary conditions at each end of the tether/riser must be defined using the details under Tether/Riser Boundary Conditions at Fixed Point and on Structure. For each attachment point you must define an Axial Stiffness (in the direction of the Local Tether/Riser X-axis), and a Rotational Constraint which may be either Define Stiffnesses or Encastre Condition. Setting Rotational Constraint to Define Stiffnesses allows you to set rotational stiffnesses about the Local Tether/Riser Y- and Z-axes, while Encastre Condition prevents any rotation about the Local Tether/Riser Y- and Z-axes.

For a tether/riser which consists of more than one section, it is also possible to define Intermediate Constraints along its length. The Additional Constraint field contains a drop-down list of the positions between tether/riser sections. Selecting one of these positions allows you to set the Constraint Type which must be one of the following:

Finally, there are a number of Tether/Riser Specific Options that may be optionally defined. Although some of these options are more relevant to the connection when it is considered as a tether or a riser, there is no limitation on the combination of options that may be used.

The Number of Elements field allows you to specify the number of finite elements which are used to model the tether/riser, up to a maximum of 250 elements.

The Tether Group Multiplier option specifies that a single modeled tether should be considered as a group of tethers, which has the effect of multiplying the tether forces acting on the connected structure by the specified integer factor.

The Anchor and Structure Cap Areas options can be used to set the areas of the tether/riser that are not subject to external water pressure at the fixed point and connection point, respectively. These values are used in the calculation of the tether/riser effective and wall tensions.

The Internal Fluid Pressure and Density options can be used to define the properties of the fluid carried by the riser, which are also used in the calculation of the tether/riser effective and wall tensions. If you define a non-zero pressure, you must also define a non-zero density (and vice-versa).

The Axial Stress Impact Factor and Impact Half Life options can be used to define the tether axial stress peak and decay during an impact. The initial axial stress at impact is determined from the product of the impact velocity and the stress impact factor, while the exponential decay of the axial stress with time is determined from the impact half life as:

If you define a non-zero stress impact factor, you must also define a non-zero impact half life (and vice versa).

The Lower Stop Position defines the distance of the tether lower stop below the fixed point position. Note that if the lower stop distance is input as zero, the tether can never be free-hanging.