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Modelling introduction |
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Modelling introduction |
To analyse a system using OrcaFlex, you first build a mathematical model of the real-world system, using the various modelling facilities provided by OrcaFlex. The model consists of the marine environment to which the system is subjected, plus a variable number of model objects chosen by the user, placed in the environment and connected together as required. The objects represent the structures being analysed and the environment determines the current, wave excitation, etc. to which the objects are subjected.
The objects available are summarised below; detailed information on each is given later.
are used to model ships, floating platforms, barges etc. They are rigid bodies whose motions are prescribed by the user. The motion can be specified in a number of ways: directly by a time history motion data file or specifying response amplitude operators (RAOs) for each of 6 degrees of freedom (surge, sway, heave, roll, pitch and yaw), indirectly by giving first order wave load RAOs and/or second order wave drift QTFs, or a combination of the two. They can also be driven around the sea surface, at user-specified velocities and headings, during the course of the simulation.
are simple point bodies with just three degrees of freedom – the translational degrees of freedom (X,Y and Z). Unlike a vessel, whose response to waves can be defined by its data, the motion of a buoy is calculated by OrcaFlex. 3D buoys are not allowed to rotate and are intended only for modelling objects that are small enough for rotations to be unimportant.
are much more sophisticated than 3D buoys – they are rigid bodies with all six degrees of freedom, and OrcaFlex calculates their translational and rotational motion. Several different types of 6D buoy are available, for modelling different sorts of marine object.
| Note: | Although called buoys, 3D and 6D buoys do not need to be buoyant and so can readily be used to model any rigid body whose motion you want OrcaFlex to calculate. |
are catenary elements used to represent pipes, flexible hoses, cables, mooring lines, etc. Line properties may vary along the length, for example to allow a buoyant section to be represented. Line ends may be fixed or free, or connected to other objects such as vessels or buoys, and ends can be disconnected in the course of a simulation.
Each line can also have a number of attachments. These are elements attached to lines at user-specified locations, and provide a convenient way of modelling items such as floats, clump weights, or drag chains.
are massless connections linking two other objects in the model. Two types are available. Tethers are simple linear elastic ties that can take tension but not compression. Spring / dampers can take both compression and tension and both spring and damper components can be nonlinear.
are also massless connections between two (or more) objects in the model. The connection is by a winch wire, which is fed from and controlled by a winch drive mounted on the first object. The winch drive can be operated in either length-controlled mode, in which it pays out or hauls in the winch wire at a user-specified rate, or in tension-controlled mode, in which it applies a user-specified tension to the winch wire.
are geometric shapes and three types are available. Elastic solids can be used to act as physical barriers to restrict the movement of the other objects in the system; they are made of an elastic material and so apply a reaction force to any object that penetrates them. Trapped water shapes can be used to model parts of the sea, such as moonpools, that are shielded from the waves. Drawing shapes are purely for visualisation; they have no influence on or interaction with the rest of the model.
Several different elementary shapes (cuboids, planes and cylinders) are available, and they may be placed together to build more complex compound shapes. They may be fixed or connected to other objects such as vessels or buoys.
are massless objects intended to provide general-purpose connections between objects. Constraint objects comprise two frames of reference that can translate and rotate independently. One frame may be connected to a parent object, and the other frame can have one or more child objects connected to it. Constraint objects allow degrees of freedom to be introduced, to be fixed, or to have imposed motions.
are composite objects which model horizontal axis wind turbines. An OrcaFlex turbine object comprises dedicated models for the generator, gearbox, hub and blades. Blades can be rigid or flexible to capture aeroelastic coupling effects; they share many of the structural characteristics of lines. A blade element momentum (BEM) method, generalised to 3D, is applied to calculate the aerodynamic load.
Of these various object types, the lines, links, winches and constraints all have the property that they may be used to connect together other objects. To assemble a model, therefore, involves creating objects and using the lines, links, winches and constraints, as appropriate, to connect the objects together as required.
The number of objects in the model is only limited by the memory and other resources available on the computer being used. Similarly, there are no built-in limits to the number of connections between objects. As a result very complex systems can be modelled, though of course the more complex the model the longer the analysis takes. A comprehensive collection of example models of varying complexity is provided for OrcaFlex.
Computer programs cannot exactly represent every aspect of a real-world system – the data and computation required would be too great. So when building the model you must decide which are the important features of the system being analysed, and then set up a model that includes those features. The first model of a system might be quite simple, only including the most important aspects, so that early results and understanding can be gained quickly. Later, the model can be extended to include more features of the system, thereby giving more accurate predictions of its behaviour, though at the cost of increased analysis time.
Once the model has been built, OrcaFlex offers a variety of analyses: