MoorPy Usage

Advanced Features

(A list of key functions to be added here)

Variable Rope Stiffness Behavior

MoorPy supports separate static and dynamic stiffness coefficients, to approximate the variable stiffness characterisics of synthetic fiber ropes. For using this capability, the mooring line type information must include a static stiffness value (EAs), and a dynamic stiffness value (EAd). An additional factor on EAd that scales with mean tension (EAd_Lm) can also be included. When using a mooring line properties library (i.e., a yaml file), these values should be specified by the EA_MBL, EAd_MBL, and EAd_MBL_Lm keywords, respectively. See the (moorprops library section to be added) for more information.

Two System methods control switching between static and dynamic stiffness properties:

activateDynamicStiffness switches the mooring system model to dynamic line stiffness values. It also adjusts the unstretched line lengths to maintain the same tension at the current system state. If EAd has not been set, it will not change anything (call it with display > 0 to display a warning when this occurs).

system.activateDynamicStiffness()

revertToStaticStiffness resets the mooring lines to use the static stiffness values and return to their original unstretched lenths.

Additional Parameters in MoorPy

Some of MoorPy’s objects have additional parameters beyond those specified in the input file, which can be used to add more features to the simulation.

  • Body.Awp: This specifies a waterplane area for the body, which is used to provide a vertical hydrostatic stiffness.

  • Point.zSpan: The zSpan parameter lists the lower and upper extents of the point’s volume, relative to the point coordinate, r. The Point’s volume is assumed evenly distributed between zSpan[0] and zSpan[1], and this affects hydrostatic calculations when the Point crosses the free surface (not used any other time).

Advice and Frequent Problems

General unexpected behavior

Whenever the system is setup or modified (e.g., adding objects), the System.initialize method must be called to register all connected objects in the system.

Errors when running from an imported MoorDyn file

When initializin a MoorPy System from a MoorDyn-style input file, there are several common sources of error:

  • The section headers (e.g., “——— Lines ———-”) may not have the keywords MoorPy is expecting. Refer to the sample above for the correct format. It has changed since 2021.

  • The type keywords or number of expected entries in a line may be based on earlier MoorDyn version and not match what MoorPy expects.

  • The input file may not contain all the body information needed by MoorPy. Does the body type need to be specified as coupled? If the body is floating, its hydrostatic properties will need to be added in MoorPy manually because they are not contained in a MoorDyn input file.

Errors in finding system equilibrium

Solving system equilibrium can be the most difficult part of a MoorDyn analysis. If the system equilibrium solve is unsuccessful, some of the possible causes are

  • The system equilibrium sovle includes a floating body, and that body does not have adequate hydrostatic properties.

  • The mooring system results in a numerically-challenging stiffness matrix - for example if some lines are much shorter than others, or if there are taut lines with very high stiffnesses.

Confusion with calculating system stiffness matrices

Stiffness matrices can be calculated for multiple mooring objects using multiple different methods in MoorPy. The two main methods of calculating stiffnesses are through a finite difference method and an analytical method. The finite difference method slightly perturbs each DOF of the MoorPy object and calculates the change in force on the object before and after the perturbation. This change in force divided by the change in displacement provides the stiffness value. The analytical stiffness method derives the stiffness of each mooring line using the Catenary equations and translates that stiffness to whichever point (or body) the mooring line is attached to.

  • Finite difference

    • The 3x3 stiffness matrix of a Point object at its given location can be found by running point.getStiffness()

    • The 6x6 stiffness matrix of a Body object at its given location can be found by running body.getStiffness()

    • The “nDOFtype” x “nDOFtype” stiffness matrix of a System can be found by running system.getSystemStiffness()

    • The nCpldDOF x nCpldDOF stiffness matrix of a System can be found by running system.getCoupledStiffness()

  • Analytical

    • The 2x2 or 3x3 analytical stiffness matrix of a Line object is calculated internally when solving for the Line’s end forces in line.staticSolve()

    • The 3x3 analytical stiffness matrix of a Point object at its given location can be found by running point.getStiffnessA()

    • The 6x6 analytical stiffness matrix of a Body object at its given location can be found by running body.getStiffnessA()

    • The “nDOFtype” x “nDOFtype” analytical stiffness matrix of a System can be found by running system.getSystemStiffnessA()

The overall mooring system stiffness matrix is usually of interest to most users. This can be found by running one of the three System stiffness matrix methods. The one best to use depends on the types of other objects in the MoorPy System.

  • The getSystemStiffness() method calculates the combined stiffness matrix of all “DOFtype” objects in the mooring system. It has a default value of “free” to the “DOFtype” input, meaning that it will calculate the combined stiffness matrix of all “free” objects (e.g., points, bodies) in the system.

    • For example, a three-line mooring system with two line types in each mooring line, where each connecting point between the two line types is a “free” floating Point object, and a free floating Body object on the surface, will result in a 15x15 stiffness matrix (a 6x6 matrix for the body and 3 3x3 matrices for the connecting points)

  • The getCoupledStiffness() method calculates the system stiffness matrix for all “coupled” DOFs specifically, while equilibrating the free, uncoupled DOFs. This would be similar to running getSystemStiffness(DOFtype=”coupled”), except this method solves for equilibrium in the free floating points while calculating the stiffness of the coupled DOFs, rather than only solving for the stiffness matrix of all “coupled” DOFs.

    • For example, using the same setup described above, except that the body is a “coupled” DOFtype, the result of running getCoupledStiffness() will be a 6x6 matrix, since the body is the only “coupled” object in the mooring system.

  • The getSystemStiffnessA() method calculates the combined analytical stiffness matrix of all “DOFtype” objects in the mooring system. It calls the analytical stiffness calculation methods of other objects in the mooring system and combines their analytical stiffness matrices into one global system stiffness matrix. It has a default value of “free” to the “DOFtype” input.

Other errors

There are too many variables at play to provide decisive general guidance, but we aim to expand the advice in this section as more user feedback is received.