In this example I’ve used a point 70m in front of the stern just for illustrational purposes. Export the model such that the origin is horizontally more-or-less in the center. You will need an external program for this. Next create a panel model to be used for the diffraction analysis. Choose a single heel, draft and trim to model.For heel and trim it is recommended to use 0 if possible. It is recommended to select a draft that corresponds to the draft of the vessel during the most critical part of the operation. However we can only provide hydrodynamic data for a single situation as every vessel can have only a single active set of hydrodynamic properties. Instead of following the rest of this guide you can also do: Preparationsĭuring the operation the draft, heel and trim of the vessel may change. This is exactly how DAVE exports vessels to Orcaflex. Instead it will be included by adding 6D buoys or other items with inertia to the vessel Again, we will not include this in the vessel-type. This is the part that orcawave calculates (or any other diffraction package such as wamit, capytaine, etc). This includes forces of waves on the vessel but also the damping of the vessel due to its own motions in still water. These are the fluid forces acting on the vessel due to the motions of the vessel relative to the fluid or the fluid relative to the vessel. They will not interfere with any of the data entered in the vessel-type. The good news: all these items will be modeled outside of the vessel and vessel type. This includes ballast tanks, deck cargo, items in a crane, pipes running overboard, mooring. This is the gravity acting on the vessel and everything on and/or attached to it. In orcaflex this is going to provide the data for vessel-type→stiffness, added mass, damping. In DAVE this is beautifully implemented using the “buoyancy-shape” node. This depends only on the shape of the hull of the vessel. This is simply the water that the vessel displaces when there are no waves. The simple solution is to keep fluid and structure separate, which is exactly what we will do. But when a vessel is composed of multiple parts this combination is very troublesome. Here G is the gravity acting on the vessel and M is the metacenter, a point derived from linearizing the horizontal displacement of the center of buoyancy due to vessel roll or pitch.Ĭombining fluid and structure is okay for simple vessels. Typically these two are combined, resulting in the familiar GM value. The static component due to the weight of the vessel as well as the static component due to the displaced fluid.
Hydrostatics is the most tricky part as it comprises two components. Static fluid effects (Static pressure distribution) All actions are marked as:īut we will start with some theory to outline where we are going.įor a typical first order time-domain simulation involving floating bodies the following information is needed: But also for a simple vessel I recommend to use this method. In those situations the standard formulation of hydrostatic stiffness becomes extremely complicated. Especially if the vessel is composed of multiple elements such as cranes, cargo is placed on it or lifted from it or multiple vessels elements are interconnected. This may sound difficult but it actually makes many things a whole lot easier. It also introduces an alternative way of modeling the hydrostatics by removing the gravity effects from the stiffness matrix. This guide presents a step-by-step guide for setting up a vessel with first order diffraction data from orcawave. Setting up the vessels for such a model using orcaflex is easy but not trivial, but in the end it is just book-keeping. Many offshore operations can be modeled using first order time domain analysis.
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