Cover types

Cover type data

Cover types are used to model buried lines. Each cover type object encodes the properties of a material which can be used to cover part of a line. An in depth discussion of the following data can be found in the associated theory topic.

Resistance scale (per unit length)

The characteristic scale of the force that resists the upward motion of the line through this cover type. It is usually set equal to the peak resistance (per unit length), which is the maximum resistance to uplift that the cover can apply to a unit length of line. A safety factor is sometimes included (although this could also be incorporated into the normalised resistance). The resistance scale is often a function of the cover height above the line. The resistance scale can be set to a constant value, or a variable data function of cover height, or can be provided by a cover load model.

Normalised resistance

This is a non-dimensional factor that dictates what proportion of the resistance scale the cover will apply to the line. The uplift resistance acting on a buried line is computed by multiplying the normalised resistance by the resistance scale.

The normalised resistance can be a constant value (e.g. a safety factor), but will more commonly be a variable data function of upward pipe displacement (non-dimensionalised by the mobilisation scale), measured relative to the initial position of the line, just after it was buried.

If you are using a variable data normalised resistance curve, then you must also define an unloading stiffness, $\lambda$. This determines what happens to the uplift resistance when the line descends back into the cover after uplift. The unloading stiffness can be set to ~, N/A, or any non-negative number. A value of zero means that there is no unloading (so that the full weight of the soil will always continue to act on the line once it has been mobilised); a value of ~ specifies that the initial slope of the profile, taken at zero displacement, will be used; a value of N/A specifies that the resistance unloads elastically upon reversal of motion (i.e. the resistance tracks back down the original profile whenever the displacement decreases).

Mobilisation scale

Available only when the normalised resistance is a variable data function of non-dimensionalised upward pipe displacement. The mobilisation scale is the non-dimensionalising factor for the x-axis of the variable data curve. The non-dimensionalising factor of the y-axis is the resistance scale per unit length. The mobilisation scale will commonly be set equal to the peak mobilisation distance, which is the displacement needed to mobilise the peak uplift resistance. The mobilisation scale can be either a constant value, a variable data function of cover height, or it can be provided by a cover load model.

Downward stiffness

This data determines the downward resistance generated by downward motion through the cover. To specify a linear stiffness, enter a single stiffness value that is the reaction force applied per unit depth of penetration into the cover (beneath the infill level) per unit area of contact. For nonlinear stiffness, use variable data to specify a table of reaction force per unit area of contact against depth of cover penetration. It can also be set to ~, which means to use the seabed normal stiffness at the seabed surface (i.e. at zero seabed penetration) beneath the node. The downward stiffness is always elastic.

Ratcheting length

Available only when the downward stiffness is non-zero. This corresponds to the length scale over which the downward stiffness data takes effect upon reversal of motion, which is determined by how much of the covering material falls into the void beneath the pipe as it rises through the cover. A value of infinity means that the downward stiffness only takes effect once the pipe has dropped back down to its original as-buried position; a value of zero means that the downward stiffness takes effect immediately once the pipe begins to sink back down into the cover after uplift; and a positive value means that the downward stiffness only applies when the pipe descends by this distance from the maximum extent of its uplift (or it descends back to its original as-buried position). The level below which the downward stiffness takes effect is known as the infill level. The ratcheting length can be either a constant value, a variable data function of pipe diameter, or it can be provided by a cover load model.

Layers

Available only when a cover load model is in use. This is an advanced feature for modelling lines buried under multiple layers of different covering materials. More details can be found in the theory topic.

Pen

Defines the colour, line style and thickness of the pen used for drawing this cover type. For each line that uses this cover type, there is a choice, on the line data form, of whether to draw its covered sections or not.

Cover load models

A cover load model allows you to specify many of the properties of a cover type in terms of exact mathematical formulae. It is a Python variable data source with the following outputs and inputs:

Outputs

• R_c: the resistance scale per unit length.
• d_m: the mobilisation scale.
• d_r: the ratcheting length.

Inputs

• D: the covered line diameter of the line.
• H: the initial cover height, measured from the uppermost point of the line's cross-section (at each node) when it was in its as-buried position. If there is only a single layer of cover, then H will be a scalar quantity; if there are multiple layers of cover, then H will be a Python list.
• z: the upwards displacement of the line from its initial, as-buried position; this is also known as mobilisation.

Some illustrative examples of cover load models can be found in the buried line examples.