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[FAQ]

For most elements, integration points are used to perform the numerical integration necessary to form the various matrices. For some elements, like most beam and pipe elements, we know the closed form of the integration and the matrices are calculated directly from that. PIPE59 falls into this category and the stiffness and mass matrices are created directly in a manner similar to BEAM4. Some line elements, like BEAM189, require numerical integration to create the stiffness and mass matrices and BEAM189 has 3 integration points along its length. We need the integration points for PIPE59 to calculate the hydrodynamic loads from the WATER Table since the form of the hydrodynamic loads is unknown a priori. Chapter 13 of the Theory Reference describes the integration points for the various elements currently in use; however, PIPE59 is a legacy element, and its documentation is contained in Element Reference > Legacy Elements. For line elements such as the PIPE59 element, the possibility exists for 1, 2 or 3 integration points. PIPE59 has 2 integration points as it relates to the hydrodynamic effects and these are located at X = +/- 0.57735… This is measured in the element’s natural coordinate system that varies from X = -1 to X = +1. So, if the element was 10 units long, the integration points would be located at (1-.57735)*5 = 2.11325 and 1.57735*5 = 7.88675. NOTE: PIPE59 is a legacy element. Although this legacy element is available for use in your analysis, ANSYS, Inc. recommends using a current-technology element such as PIPE288 with KEYOPT(3) = 2, along with the appropriate ocean commands (OCxxxxxx).