The S-N high cycle fatigue assessment is conducted to determine the pipeline fatigue life under the high cyclic temperature and pressure fluctuations. A fatigue assessment of response that is associated with number of cycles more than 10000 is denoted high cycle fatigue, where the assessment is performed based on calculation of elastic stresses.
The S-N fatigue assessment is based on Palmgren-Miner rule, which does not take into account of mean stress, frequency and loading sequence. The fatigue assessment is carried out for the following locations:
• Girth weld cap;
• Girth weld root;
• Longitudinal seam weld (except seamless pipe).
• axial stress;
• hoop stress;
• Bending stress.
• Stress concentration factor (SCF) at the girth weld including the effect of misalignment;
• Type of S-N curve i.e. D, E, F, F1 or F3;
• Condition of weld i.e. seawater cathodic protection (CP), in air or free corrosion.
The longitudinal stress range at each single location along the pipeline route can be obtained from Finite element analysis (FEA) model or calculated in accordance with DNV-OS-F101 Section 4.1.1 conservatively assuming the pipeline is fully restrained.
During pipelay, tension is applied to the pipeline by tensioner located at the firing line on the lay vessel. The tension applied will determine the curvature of the pipeline between the stinger and the touchdown point. Please note that the higher the tension applied, the touchdown point of the pipeline will be further away and vice versa.
Prior to pipeline installation operation, the installation analysis will be performed. The purpose of the installation analysis is to determine the tension required during the pipelay to ensure the strain/stress of the pipeline overbend and sagbend are within the allowable limit in accordance to the codes and standards.
The tension discussed above is also known as the top tension. Due to the tension applied, there will be residual lay tension acting on the pipeline upon completion of pipeline installation. The residual lay tension is the stress that remains in the pipeline after the tension applied to the pipeline has been removed.
There are several ways of calculating the residual lay tension which depends on the stage of engineering design:
1) Assume as 10% of the lay tension. (This is a common assumption used in calculating the fully restrained pipeline effective axial force and some other analyses if no sufficient information is available. Note that this assumption may not be suitable for some analyses as the residual lay tension may be more than 50% of the lay tension in certain scenarios)
2)Simple residual lay tension - the residual lay tension calculation will be added in the future, if there is any request!
3) Detailed FE analysis - Model the progressive pipe lay using the FEA software (i.e. Abaqus, Ansys, Orcaflex etc.) and check for the effective axial force of the pipeline upon completion of the pipe laying.
The question I often received is whether to land the line on
the seabed during J-tube pull-in?
I have carried out the sensitivity study by performing the
static analysis for J-tube pull-in. The results show that:
If the pipeline is rigid and the clearance between the bell
mouth and seabed is small, the most favorable way of J-tube pull-in is to
firstly land the rigid riser onto the seabed and followed by J-tube pulling;
However, if the pipeline is flexible and the clearance between the bell
mouth is bigger (e.g. 2 m to 3 m), the most favorable way of J-tube pull-in is
to pull the flexible riser directly into the J-tube without touching the
Please note that the answer given above is only based on the sensitivity study I have carried out and may not be applicable to all cases.