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Fatigue - Hybrid Staged Operations - User Inputs

Introduction

The Hybrid Staged Operations fatigue option is similar in behaviour to the Staged Operations approach. For example, it is also used to assess the fatigue damage associated with scenarios where the structure under consideration changes considerably over time.

However this hybrid option has some common traits with the Normal Lay fatigue approach as well. In particular, for each stage included in the analysis the specified welds are automatically moved along the pipe string using Pull Length and Cycle Time inputs. The reasoning behind this process can be explained when one considers the key limitation underlining the Staged Operations option.

The Staged Operations fatigue capability assumes that the tracked pipeline welds remain in the same place for the entire duration of each stage. This assumption works fine if a detailed number of stages are used to model the entire installation process, for example a new stage is created after every few pipe pulls. However if one wishes to use only a handful of stages then this assumption may result in an uneven distribution of weld damage, meaning certain welds will have an over-conservative fatigue damage value while others will have an under-conservative value.

By moving the welds internally during each stage, in accordance with the input Pull Length and Cycle Time, the Hybrid Staged Operations fatigue option overcomes the limitation discussed above. This in turn results in a more uniform distribution of tracked weld damage along the pipeline, even for analyses with a small number of stages.

Please note that by addressing the limitation of the traditional staged approach, the hybrid method inadvertently adopts some of the restrictions associated with the Normal Lay capability. In particular, for each stage of the analysis the hybrid option assumes that the calculated dynamic response of the pipeline is applicable to the entire duration of the stage, or in other words, the response is assumed to be representative of all pipe lengths encountered during the stage.

Taking the above into account, the Hybrid Staged Operations option works best when the user identifies and selects the crucial stages of the installation process, where there are important changes in the dynamic behaviour of the pipeline. By analysing these stages one should effectively capture the majority of the fatigue damage incurred by the pipeline during the installation procedure.

 

User Inputs

You identify your fatigue analysis as a Hybrid Staged Operations analysis in the Analysis Type drop-down list. You choose an Analysis Option from the related drop-down list as before. You nominate a Pipeline and Vessel Reference Point in the same way as for a Normal Lay run. In the Suspended Length drop-down list you specify whether the length of suspended pipe length is Increasing or Decreasing during the analysis. This indicates what direction the tracked welds are to be moved during each pull or cycle of the individual stages.

You define the Tracked Weld Reference Point – Length along Pipeline as you would for Staged Operations fatigue.

Vessel Weld Locations is used in the same way as previously to define the locations of welds on the vessel. The Tracked Weld Locations and Weld Properties button behave in similar fashion to the Staged Operations capability in that you use them to define the locations of the various welds to be tracked during the analysis, as well as their associated properties.

The Allowable Fatigue Damage dialog is the same here as for the Normal Lay case, but the format of the Cycle Time dialog is different, as the figure below shows. You use it to define the Actual Duration and the Maximum Single Cycle Time for each analysis stage as you would for Staged Operations. However you also use it to specify the average Pull Length and Cycle Time inputs for each stage; the significance of these inputs will become apparent in the ‘Fatigue - Hybrid Staged Operations - Operation’ article.