Line results: Fluid loads

Note: If the line experiences sea state disturbance, this will be accounted for in the fluid load results.

Sea surface Z

Available at nodes. The global $Z$ coordinate of the sea surface directly above the instantaneous position of the node.

Depth

Available at nodes. The depth of the node beneath the sea surface $(= \text{surface }Z - \text{node }Z)$.

Sea surface clearance

Available at nodes. The vertical clearance from node centre to the instantaneous sea surface. A negative value indicates that the node is submerged.

Proportion wet

Available at nodes. The proportion of the part of the line that the node represents, that is submerged in the sea. The value is in the range 0 to 1, a value of 0 meaning no submersion and 1 meaning completely submerged.

Sea velocity, sea X velocity, sea Y velocity, sea Z velocity,
Sea acceleration, sea X acceleration, sea Y acceleration, sea Z acceleration

The magnitude and global $X$, $Y$ and $Z$ components of the water particle velocity (due to current and waves) and acceleration (due to waves) at the node position. If the node is above the water surface then the value at the water surface is reported.

Relative velocity, normal relative velocity, x relative velocity, y relative velocity, z relative velocity

Available at nodes. Relative velocity is that of the fluid relative to the node, i.e. $\vec{v}_\textrm{fluid}-\vec{v}_\textrm{node}$. The results reported are the magnitude of the relative velocity and its components relative to node axes.

The fluid velocity used is that of the principal fluid affecting the node. The principal fluid is defined as follows:

  1. If proportion dry $\gt$ 0.5 and include wind loads on lines is true, then the principal fluid is the air.
  2. Otherwise the principal fluid is the sea.
Note: If the principal fluid is the sea and in the case of a node that is above the water surface, the relative velocity is based on the fluid velocity at the surface point vertically below the node.
Warnings: The relative velocity results are derived using the node velocity results, so the velocity results accuracy warning applies here.
The relative velocity results are reported without taking into account possible containment scaling arising from participation in line contact.

Strouhal frequency

Available at nodes. The Strouhal frequency is defined to be $St\,v_\mathrm{n}/d_\mathrm{n}$ where $St=0.2$ is Strouhal number, $v_\mathrm{n}$ is the normal component of relative velocity and $d_\mathrm{n}$ is the normal drag diameter.

Warning: Strouhal frequency is reported without taking into account possible containment scaling arising from participation in line contact.

Reynolds number

Available at nodes. The Reynolds number is a measure of the flow regime. OrcaFlex offers a number of different options for the calculation of Reynolds number.

Warnings: Reynolds number is derived using the node velocity results, so the velocity results accuracy warning applies here.
Reynolds number is reported without taking into account possible containment scaling arising from participation in line contact.

x drag coefficient, y drag coefficient, z drag coefficient, lift coefficient

Available at nodes. These are the drag and lift coefficients used in the corresponding drag and lift calculations.

For constant coefficients then these results report the values given by the user's data (except for a node at the junction between two sections with different coefficients, where an effective average value is used). For coefficients which vary with Reynolds number or with height above seabed, these results report the computed value which was used.

Warning: For coefficients which vary with Reynolds number, the drag coefficient is derived using the node velocity results, so the velocity results accuracy warning applies here.

If the line uses a wake oscillator VIV model with inline drag amplification then the amplification factor is included in these results. The inline drag amplification factor is itself available as a separate result.

Drag force, normal drag force, x drag force, y drag force, z drag force, lift force

Available at nodes. These are the drag and lift forces used in the corresponding drag and lift calculations. These forces are reported per unit length. Drag force components are relative to node axes.

These results are not reported if:

Fluid inertia force, x fluid inertia force, y fluid inertia force, z fluid inertia force

Available at nodes. The fluid inertia force is the inertia term of Morison's equation. These forces are reported per unit length. Components are relative to node axes.

The fluid inertia force results are not reported if the node is owned by the inner line of a line contact relationship with containment enabled.

Morison force, normal Morison force, x Morison force, y Morison force, z Morison force

Available at nodes. The Morison force is the sum of the drag force and the fluid inertia force, that is the total force defined by Morison's equation. These forces are reported per unit length. Components are relative to node axes.

These results are not reported if:

Wake velocity reduction factor, wake Cd, wake Cl

Available only at nodes, for lines which have sections which react to wake effects.

Wake velocity reduction factor is applied to the velocity at the node as a result of upstream wake effects. Wake Cd and wake Cl are the drag and lift coefficients respectively, used to calculate the hydrodynamic forces at the node as a result of any upstream wake effects.

Note: Wake cl is positive when the lift force is applied in the $y$-direction of the upstream wake's frame of reference and negative when the lift force is applied in the $-y$-direction.

Slam force, slam GX force, slam GY force, slam GZ force

Available at nodes, for lines which have any sections with non-zero slam data. Slam force is the magnitude of the total slam force per unit length, due to the line half-segments on either side of the node entering the water or approaching the surface from below. Slam GX force, slam GY force and slam GZ force are the components of that total slam force per unit length in the global $X$, $Y$, and $Z$ axis directions.