Results - Fatigue Analysis & DNV Postprocessing

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Results - Fatigue Analysis & DNV Postprocessing

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The fatigue analysis in this example is carried out for the random sea loading corresponding to Stage 3. The first figure shows a plot of the maximum fatigue damage rate along the pipeline for the random sea loading. The maximum fatigue damage rate occurs at the end of the stinger. A further peak of fatigue damage is seen in the touchdown region of the pipeline also, but is less significant. These observations can be further validated from viewing the animation file associated with the analysis, where it can be observed that large amounts of intermittent contact occurs particularly at these pipeline locations.

Maximum Fatigue Damage Rate

Maximum Fatigue Damage Rate

 

The overall results from the fatigue analysis are summarised in the table below. The total fatigue damage sustained by a weld during the pipe laying operations is estimated at 0.48344E-6. This represents a maximum standby time of 0.12681E+6 hours, roughly 5283.75 days, based on the maximum allowable damage of 0.1. The maximum fatigue damage rate occurring in the pipeline, at the last stinger support, is 0.23221E-9 damage per second.

Table: Normal Lay Fatigue Results

Property

Value

Total Fatigue Damage

0.48344E-6 damage

Maximum Standby Time

0.12681E+6 hours

Maximum Fatigue Damage Rate in the Pipeline

0.23221E-9 damage/s

 

A fatigue histogram plot is also provided in the figure below. This shows the governing stress histogram accumulated by a weld during installation. It can be observed that the predominant stress bin is 0.0-0.5 MPa occurring at almost 5,400 cycles. The number of cycles begins to decrease for each of the stress range bins thereafter, with a minor peak observed at 4-6 MPa.

Governing Weld Stress Histogram 

Governing Weld Stress Histogram 

 

 

DNV Postprocessing Analysis

The results of the DNV Postprocessing for local buckling checks are detailed in the table below. As can be observed, the overbend and sagbend checks passed in both the regular wave and random sea analyses.

Table: Local Buckling Checks for Dynamic Stages

Stage

Max. Overbend Check

Max. Sagbend Check

Status

2

0.3921

0.8478

Pass

3

0.3939

0.8535

Pass

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