FatFree

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Alfa out of range error when pressing Calculate

An alfa out of range value error indicates that either the current or wave induced flow velocity is negative, which is an input error. In the ‘Current Template’ it will show that the considered input data produces negative mean values on the fitted Weibull distribution.

FatFree calculates one possible Weibull distribution based on RPV values, but as just a numerical fitting it can lead to such problems. Please refer also to the User Manual. It is recommended to review the current input data and/or calculate independently a Weibull distribution fit on existing data and use the Weibull distribution option for current modeling.

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Can FatFree be used for riser fatigue analysis?

Fatfree was developed based on DNV-RP-F105 that meant to be applicable to pipelines.

There are differences between a riser and a pipeline, for example current and wave induced flow over the pipeline is constant for pipelines, whereas in the riser it changes with water depth/length. The user shall be responsible to assess the impact of this assumption/simplification on their proposed case. The application of the DNV-RP-F105 for risers should be done at designer’s discretion. Reference is made to DNV-RP-F105 Appendix C.

The user may also note that “user defined” option is available for response data.

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How to use mode shape and stress amplitude input in the MultiMode sheet?

In case of mode shape input, the user must provide location coordinates and the associated mode shape deflections, which must be normalized to the maximum deflection (i.e. the maximum displacement value is 1 or -1). The location coordinates must be equally spaced along the input given. For each location FatFree will then calculate the associated unit diameter stress amplitude. This calculation is done in accordance with DNV-RP-F105 Section 6.7.4 / DNVGL-RP-F105 Section 6.6.2, i.e. it is assumed the stress amplitude is purely due to bending. The local curvature required for stress amplitude is calculated from the given mode shape by second order finite difference. Because of this, FatFree calculations will not be performed in first two and last two locations.

In case of direct stress amplitude input, the user must provide location coordinates and the associated stresses amplitudes (in MPa), which must be scaled to one (total/outer/hydrodynamic) diameter deflection (i.e. the maximum displacement value is D or -D). FatFree will consider each stress amplitude given for its calculations only modifying it to account for the two hot spots evaluated (weld root and weld cap).

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How to use span areas in the Fatfree MultiMode sheet?

The number of span areas is related to continuous lengths of the pipe where the spans interact. Different span areas do not interact (modes from different areas do not compete and the fatigue damage from multiple areas are not added even if the location belongs to multiple areas), they are different calculations. The user is required to provide the start and end points of each span area, which shall correspond to values provided in the Location column.

The span areas may overlap and not all locations must belong to a mode, but each mode must contain a minimum of five (5) locations. Each inserted mode shape must then be identified with the span area that it is related to, only the data in the locations between the beginning and end of that span area will be used. A calculation according to DNV-RP-F105 Appendix A is then created for each span area.”

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'overflow error' when 'wave modelling' is enabled in deep water condition

The overflow-error is caused when calculating the wave number for deep water. In the current implementation a first guess is made assuming deep-water. Then this solution is improved by a Newton iteration – although not necessarily in a deep water case. Since VBA does not have a tanh-function by default, exponential functions are used instead, which cause an overflow.

In deep water there should be no contribution of waves to the fatigue lifetime.

Hence a quick solution to the problem would be to set the “Wave Modelling” option to “No Wave”. 

This is a bug and will be fixed in a future release.

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Range class failed

Error message:

Question: Error prompted when performing calculation in FatFree

Answer:

This error most probably related to wrong input in the histogram. To solve this in general, check:

  1. Make sure the directions are specified in numerical format (degree):

  1. Check number of discrete directions and number of discrete Hs value to reflect the same input in the Hs Histogram.

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Water depth and wave theory issues

The water depth where FatFree is applicable is the same water depth as where DNV-RP-F105 is applicable. In general there is no limitation on water depth for the response models (VIV) as long as the wave induced flow velocity is calculated with a sufficient accuracy. Note that both the estimate of the wave induced flow velocity and also assessment of FLS/ULS aspects related to direct wave action (Morrison’s force) as described in DNV-RP-F105 are based on the linear Airy’s wave theory. Whether this is accurate enough depends on the water depth and wave parameters like wave height and period. Therefore there is no specific limit on the water depth and it must be considered from case to case.

Another aspect is the fact that when the nonlinear wave effect becomes significant due to shallow water, the pipeline normally needs to be buried for stability and protection reasons anyway, so the case of assessing free spans in real shallow waters has not been very essential.

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What does the warning message “Premises for Cross-Flow RM violated - check low KC!” mean?

The warning message “Premises for Cross-Flow RM violated - check low KC!” is given by FatFree when the premises stated in DNV-RP-F105 paragraph 4.4.10 are violated.

For low KC numbers the vortices are shed off in a gate 45degrees to the flow direction and creates a vertical load frequency twice the wave frequency. This means that you may see cross-flow vibrations at lower reduced velocity than the typical onset values around 3. The cross-flow response model given in DNV-RP-F105 and FatFree does not capture this effect and the results may be non-conservative. 

Please see the note in paragraph 4.4.10 for guidance.

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What is the number of location points in the MultiMode sheet?

The number of evaluation points would be the discretization of the available mode shape data (for example, the number of nodes if a FEA was done). The number of evaluation points can be 1 (single location analysis, and Mode Shape input is not supported) or greater than or equal to 5 (multi-location analysis). 

The number of evaluation points is common for both input options.  Input is given per mode and not per span, so input may include multiple spans and (in the case of multi-location analysis with multiple span areas) even non-interacting free spans.

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