Fatigue - Staged Operations - Operation
A Staged Operations fatigue analysis begins in the normal or default operation with a series of random sea dynamic analyses of the individual stages. For each analysis stage, the actual weld locations are determined by the Tracked Weld Locations and Common Reference Point inputs. Note that it is possible (even likely) that not all models will be able to accommodate the total number of tracked welds. Consider for example a fatigue analysis of a start-up via sheave operation. You might specify 50 tracked welds at 5m intervals from the pipeline free end; but the model for the first dynamic analysis might include only 100m of pipe, in which case only 20 tracked welds are accommodated. Naturally the other 30 tracked welds, when they appear in subsequent phases, have zero damage from phases in which they were not present. Note also that the tracked welds do not have to located at regular intervals from the reference point.
Once each dynamic simulation is complete, time histories of combined stress are generated for each of 8 points around the cross-section circumference at each tracked weld location. The locations of the tracked welds will not in general coincide with nodal locations, so cubic spline interpolation is required. Rainflow cycle counting is then used to transform each time history into a value of rate of fatigue damage in the same way as for Normal Lay fatigue, except that in this case the option is provided to specify different S-N curves for different weld locations. In this fatigue analysis type also the stress ranges are output to a binary storage file for possible reuse in a repeat fatigue analysis, possibly with different S-N curves or SCF values. Note however that repeat fatigue analyses with different SCFs will not be meaningful if different SCFs for bending and axial stresses are used.
Given that the damage rates for the tracked welds are representative of multiple locations within each stage, the damage rates for each weld are then multiplied by the specified Actual Duration for the stage, to give the fatigue damage occurring at each weld in each stage. The total fatigue damage at each weld is obtained by summing the fatigue damage occurring in each stage. This defines a plot of total fatigue damage as a function of distance from the Common Reference Point, which is the main fatigue analysis output.
If you have set Generate Stress Histogram to yes, then nodal stress histograms are produced for every stage as per the Normal Lay fatigue operation. The nodal stress histogram data for each stage is summed together as the stages progress. So the final stage contains the total accumulated nodal stress histogram data for the Staged Operation. Tracked weld histograms are also estimated. As per the Normal Lay fatigue operation a profile of stage ‘bin rates’ is established along the pipeline for each bin at each of the circumference location. Once this profile is defined the ‘bin rates’ are multiplied by the Actual Duration for the stage in order to obtain a tracked weld histogram for each tracked weld. The tracked weld histogram data is summed together as the stages progress so the final stage contains the total accumulated tracked weld stress histogram data for the Staged Operation.
Once the total fatigue damage is known, the duration that the suspended pipeline can be held stationary in each stage without exceeding the specified Allowable Fatigue Damage may be determined. The duration is calculated by subtracting the total fatigue damage from the allowable fatigue damage. This difference is then divided by the maximum damage rate for each stage (that is, the highest damage rate from all the tracked welds in that stage), to give a maximum allowable standby time for that stage.
Fatigue rates at the vessel weld and touchdown locations may also be significant factors in governing standby time, so other important output parameters are maximum fatigue damage rates at these locations in each stage. The critical locations in each stage are the specified Vessel Weld Locations and the touchdown point (the touchdown location at each stage is identified in a similar manner to the Normal Lay fatigue approach). Given that the fatigue rate of the vessel and touchdown locations may not be representative of multiple locations within each stage, the fatigue damage rate is multiplied by the Maximum Single Cycle Time to give the maximum fatigue damage occurring in a single pull. Since the vessel welds or the touchdown point are not followed (hence the term vessel welds rather than tracked welds), then the total fatigue damage may not be calculated. For these locations, the fatigue damage rate and the maximum fatigue damage occurring in a single pull are output.
An illustrative flowchart for the Staged Operations fatigue approach is provided in the figure below.
Flowchart for Staged Operations Fatigue