Moored Vessel |
 
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Moored Vessel |
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Similar in some respects to the CALM buoy modelling facility, Flexcom can perform static and dynamic analyses of a model which explicitly includes a moored vessel. Again you model the vessel with an assemblage of standard beam-column elements, and locate a node at the vessel CoG. In this case, the additional “vessel” elements connect the vessel CoG to all of those riser nodes whose motions are defined by the motions of the moored vessel. These nodes are referred to as “attached” nodes in the following for convenience. There should be one (and only one) vessel element for each attached node, connecting it to the CoG node. These vessel elements should again be rigid or stiff, and should have zero mass and zero drag and buoyancy diameters.
The objective of a Flexcom dynamic analysis of a model which includes a moored vessel is to calculate the response of the vessel to i) current; ii) wind; iii) thrusters; and iv) drift forces calculated from a wave spectrum and user-input Quadratic Transfer Functions (QTFs). A key output from this type of analysis is the second order drift motion of the vessel (CoG node), which is in fact routed to an ASCII file as the analysis proceeds. These motions can then be subsequently added to first order (high frequency) vessel motions in a dynamic analysis of a detailed model of one or more of the mooring lines or risers.
In the normal way, the model will comprise of nodes and elements representing the mooring lines and risers (if present) which are connected to the vessel. The vessel itself will be modelled with an assemblage of standard beam-column elements, with a node located at the vessel CoG. These “vessel” elements will connect the vessel CoG to all of those mooring line and riser nodes whose motions are defined by the motions of the moored vessel. There should be one (and only one) vessel element for each attached node, connecting it to the CoG node. The vessel elements should be rigid or stiff, and should have zero mass and zero drag and buoyancy diameters. In order to keep the model size to reasonable proportions, and because long simulations of the low frequency vessel response may be required, the risers and mooring lines should be modelled with fewer elements than would typically be used if the wave frequency response of these components were the primary analysis objective.
After the various mooring analysis stages are run, the vessel low frequency or drift response is available as a function of time. Subsequently, more detailed models of one or more of the mooring lines and/or the risers would be developed, to determine the dynamic response of this component or components to wave frequency loading in the normal way - the model(s) in this case would not include the vessel explicitly. Rather the dynamic excitation would now consist of a combination of first order (high frequency) motions determined from RAOs, plus the second order motion time history produced with the earlier model. Flexcom includes an option to specify a vessel motion combination of this nature.
The rationale for this solution procedure is that, in accordance with mooring codes of practice, vessel low and high frequency motions are typically considered independently. In addition, the high frequency vessel response is typically not influenced by the restraint applied by mooring lines and any risers present.
Further information on this topic is contained in the following sections:
•Calculation of Vessel Forces explains in detail how the forces on the moored vessel are calculated.
•Analysis Procedure presents some general guidelines on the steps involved in performing a full dynamic analysis with a moored vessel.
•Input Formats describes the relevant input formats for the definition of current coefficients, wind coefficients and QTFs.
•Vessel Heading discusses vessel heading in the context of moored vessel analysis.
•*MOORED VESSEL is used to define a moored vessel and its associated properties.
•*ANALYSIS TYPE is used to specify the analysis type. Specifically, TYPE=MOORING is used in conjunction with *MOORED VESSEL.
•*WIND is used to specify wind loading.
•*WIND COEFF is used to specify wind coefficients used to determine the wind loading on a floating body or moored vessel.
•*CURRENT COEFF is used to specify current coefficients used to determine the current loading on a floating body or moored vessel.
•*QTF is used to specify Quadratic Transfer Functions (QTFs) that allow the slow drift loads on a floating body or moored vessel to be determined.
•*MOMENTS is used to specify the value of Molin’s yaw coefficient for a moored vessel, and also to specify the fractions of Molin’s Moment and Munk’s Moment that are applied to the moored vessel.
•*THRUSTER is used to specify thruster loads on a moored vessel.
If you would like to see an example of how these keywords are used in practice, refer to D01 - Moored Vessel.