SHEAR7 Interface |
 
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SHEAR7 Interface |
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In fluid dynamics, vortex-induced vibrations (VIV) are motions induced by the periodic irregularities in the external flow around a cylinder due to separation of the boundary layer due to excessive curvature. Low pressure eddies, or vortices, are formed changing the pressure distribution along the surface. These vortices, when not symmetric around the body with respect to mid-plane lead to differences in lift forces on each side of the body. This imbalance in forces leads to a motion transverse to the direction of the flow. When the shedding frequency of the vortices is close to the natural period of the riser, lock-in can occur, causing periodic oscillations of the structure.
Because of these periodic oscillations, fatigue damage can be incurred by the structure. VIV can be an important consideration for an engineer in the design and analysis of offshores structures; especially in the design of long slender structures that we are often concerned about in Flexcom.
Most users will be familiar with the SHEAR7 program which estimates fatigue damage caused by vortex induced vibrations (VIV). SHEAR7 is a program developed by a team at MIT lead by Professor J. Kim Vandiver and is recognized as the primary software in the prediction of vortex-induced vibrations (VIV) for risers. The program allows users to estimate structural response, drag force amplification factors, and predict fatigue life. SHEAR7 evaluates which modes are likely to be excited by vortex shedding and estimates VIV response. In calculating riser VIV response, one of the SHEAR7 requirements is a knowledge of the riser natural frequencies, mode shapes, and modal curvature distributions. SHEAR7 does itself include an analytical module which can calculate this data for a large class of mainly simplified structural models. However if your riser does not correspond to one of these structural models, then you must supply the eigensolution data to SHEAR7 as part of your VIV analysis input data. Modes includes a facility to produce this data as part of the eigensolution output, and in a format ready for direct input to SHEAR7.The Flexcom-SHEAR7 interface has the capacity to aid the end user in the calculation of VIV fatigue life through the conversion of Flexcom model and mode shapes into SHEAR7 input and running SHEAR7.
This section describes the capability of Flexcom to interface with SHEAR7, a program which estimates fatigue damage caused by vortex induced vibrations. SHEAR7 must be licensed separately through AMOG.
Further information on this topic is contained in the following sections:
•Additional Output and Repeat Runs
•*RISER TYPE ($MODES section) is used to specify the riser type and other parameters relating to the Modes facility for generating output for subsequent input to SHEAR7.
•Numerous keywords are contained in the $SHEAR7 section, a relatively recent addition to Flexcom. This input section provides a streamlined interface with SHEAR7, allowing you to define all of the required SHEAR7 inputs directly within Flexcom itself, resulting in a seamless integration of the modal and VIV fatigue analysis stages.
For full details of how Modes interfaces with SHEAR7, refer to SHEAR7 Interfacing Operation.