Fatigue - Normal Lay - User Inputs

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Fatigue - Normal Lay - User Inputs

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You identify your fatigue analysis as a Normal Lay analysis in the Analysis Type drop-down list.

You must then select an appropriate option from the Analysis Option drop-down list. Three options are provided – Full, Previous and Restart. The first indicates that you want to perform a random sea dynamic analysis as a precursor to the fatigue analysis; the second indicates that you want to base the fatigue calculations on the results from a dynamic analysis you’ve already run; and the third indicates that you have already performed a fatigue analysis and now wish to re-run it using different fatigue parameters, such as S-N curve or SCF. How this Restart option is different from Previous will be explained in the 'Fatigue - Normal Lay - Operation' article.

You must designate a particular section of the model as representing the main pipeline on which you are going to define weld locations, and this is done by selecting a component from the Pipeline drop-down list. The list you are presented with here will be a list of all of the Pipe Section and Line components in your model. Note that the main pipeline you nominate should run all the way from the seabed up to the vessel (or vice versa, depending on how the line/pipe section was added to the model in the Model component), for reasons which the 'Fatigue - Normal Lay - Operation' article will make clear. Also, in the case of lines you should only select ones that are fully comprised of pipe section components, as the presence of other components, such as cables, will provide unrealistic fatigue predictions.

The Vessel Weld Reference Point, which is used subsequently in the designation of weld locations on the vessel, is assumed to lie at the vessel end of the main pipeline. You must designate which end of the pipeline is the vessel end by selecting an appropriate Vessel Connection Point from the Vessel End drop-down list, which displays a list of all the vessel connection points defined in the Model component.

The Vessel Weld Locations dialog, shown in the figure below, allows you to define the locations of welds on the vessel. The locations are specified in terms of a Weld Distance (length) along the main pipeline from the Vessel End of the pipeline.

Vessel Weld Locations – Normal Lay

Vessel Weld Locations – Normal Lay

You can designate weld locations individually, or identify equally-spaced welds, using the Number and Spacing inputs. For example, the specification in the figure below means that you want the first of 10 welds to be 20m from the vessel, followed by 9 welds 10m from the first.

Equally Spaced Welds

Equally Spaced Welds

The Weld Properties dialog, shown in the figure below, is used to associated properties with the welds. You use it to select an S-N Curve component from a list of those defined in the project, and specify appropriate Bending SCF and Axial SCF values. For normal lay analyses, the weld properties you specify are assumed to apply to all welds in the model, so you are only required to specify single inputs for S-N Curve, Bending SCF and Axial SCF.

Weld Properties – Normal Lay

Weld Properties – Normal Lay

The use of the dialogs in the Damage section is straight-forward. There is only one self-explanatory entry in Allowable Damage; the value you specify here is used to calculate the standby time. There are two inputs on the Cycle Time dialog, Pull Length and Cycle Time itself. Pull Length defines the length of pipeline paid out per pull and Cycle Time is the length of time between pulls. The significance of these inputs becomes apparent in the 'Fatigue - Normal Lay - Operation' article.

The Generate Database Output, Delete Stress History Timetraces and Generate Histogram Output drop-down lists provide you with control over the level of generated output. If you set Generate Histogram Output to Yes, then the Histogram Bins dialog becomes available. This dialog allows you input the upper bound of each stress bin you wish to define. The significance of these inputs will also become apparent in the 'Fatigue - Normal Lay - Operation' article.