Hysteretic Bending |
 
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Hysteretic Bending |
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Flexcom provides an option to specify non-linear bending behaviour with hysteresis. This is an important effect observed in flexible risers during normal operation. Internal pressure in the riser causes interlayer friction to build up between the tensile armour layers leading to a hysteretic stick-slip bending response.
Flexcom employs a sophisticated algorithm that computes bending hysteresis for all types of loading. The input data is a backbone curve comprising a series of piecewise-linear moment/curvature values, which is representative of the operating conditions. The input format is similar to the specification of a non-linear elastic bending stiffness relationship as described above. Flexcom modifies the hysteresis curve as required to initiate the step change in non-linear stiffness when the bending reverses.
The hysteresis option is by default included in both static and dynamic analyses. An option is available to suppress hysteresis in a static analysis, since bending hysteresis effects occur after a pipe becomes pressurised. When this option is invoked, a linear elastic bending stiffness is used for static analyses that precede the first dynamic analysis. The linear bending stiffness is assigned as the last slope of the moment/curvature backbone curve. The hysteresis bending stiffness is Symmetric by the definition above.
When conducting a global analysis of a flexible riser, a cautious approach is often adopted whereby only the relatively small elastic component of the pipe bending stiffness is included in the analysis. This comparatively small bending stiffness produces conservative over-estimates of the pipe bending curvature, which is normally acceptable provided the extreme and fatigue design criteria (limit states) are not exceeded. However, in recent years this simplified design approach has been shown to exceed the design criteria at the touchdown of deepwater catenary risers and in fatigue analysis involving wet armour wires.
When fully pressurised, the interlayer friction between the tensile armour layers causes the dynamic bending response of flexible risers to become highly damped. During dynamic excitation, the pressurised pipe bending stiffness is characterised by a stick-slip (high-low) effect as adjacent armour layers slide over one another. This stick-slip effect produces hysteresis loops during cyclic loading, unloading and load reversal.
Further information on this topic is contained in the following sections:
•*GEOMETRIC SETS is used to assign geometric properties to element sets.
•*BENDING HYSTERESIS is used to define hysteresis moment-curvature backbone curves for non-linear materials.
•*NO HYSTERESIS is used to suppress bending hysteresis effects.