19-12-2012, 02:36 PM
Fatigue Assessment
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Introduction
Fatigue is a failure mode that will occur under severe cyclic conditions, e.g. a pipeline subjected to waves. If a large enough number of load fluctuations are allowed then fracture will eventually occur. The load itself may not be large enough to cause immediate failure, but it is the number of load fluctuations that causes failure.
Fatigue failure will occur at the weakest part of a piping system were the stress level is highest. Welds are normally weaker than smooth pipe since there will always be small imperfections or fractures in any weld. The size of these imperfections relates to the weld quality. If the cyclic stress range goes above the lower threshold for fatigue these small imperfections or cracks will start to propagate/grow and may reach a critical size ending in fatigue failure.
Damage may appear for every load cycle, and the damage accumulates. When the accumulated damage has reached a critical level (critical crack size) failure will occur.
The most important component under fluctuating stress and strain is called the stress range (or amplitude). A stress range is the algebraic sum between a peak stress and its subsequent valley.
Fatigue Curve
The m and a constants depict which fatigue curve to be used. They are dependant on which weld detail is under consideration as well as how the weld is tested. There are two welds details that are applicable for the Valhall Flank gas lift lines.
- According to EN 13445-3 table 18-4 they are detail no. 7.1: Full penetration butt welded neck flange and detail no. 1.5: Full penetration butt welds. The fatigue class is decided based on what kind of testing is performed. If testing group 1 or 2 is applicable then fatigue class 63 may be used as long as full penetration is assured, if testing group 3 is applicable then fatigue class 40 is to be used. The lower fatigue class number used results in the shorter calculated fatigue life.
- According to PD 5500 fatigue class E is applicable for the two weld details as long as full penetration can be assured as well as the weld is proved free from significant defects by NDT testing.
Imposed Displacements
All loads with a cyclic nature are to be considered for the gas lift lines. The piping will be subject to pressure cycles, thermal cycles and forced displacement cycles which stem from ocean waves. The waves deflect the riser conductor giving horizontal movements of the wellhead (x-mas tree).
The major contributor for the fatigue life for gas lift lines are the wellhead horizontal displacements. The effect of the waves on the riser pipe in relation to horizontal wellhead movements depend on the supporting of the conductor. Conductor centralizers are normally installed in order to limit the shacking of the wellheads. These should be tight in order to limit the horizontal movements as much as possible. Any reduction of wellhead horizontal displacement will reduce the cyclic stress range and subsequently affect the fatigue life significantly.
Conductor analysis is normally performed where the horizontal displacements of the wellhead may be found.
Number of Cycles
Another major contributor to the fatigue life is the number of cycles for each stress range. The number of horizontal displacements for the wellhead and subsequently the gas lift lines is related to the number of waves acting on the riser conductor. Wave occurrence diagrams are used for finding the number of cycles anticipated to act on the riser conductors. These are usually found in environmental or oceanographic design data reports for the oil field in question. The Valhall field is in the vicinity of the Greater Ekofisk area as explained in report no.: S/EPT/073/04 July 2007 Valhall Field Oceanographic and Meteorological Design Data Summary. The Ekofisk environmental data chapter 3.3 Fatigue Waves shows all wave occurrences over a 30 year period from 8 approach directions in table 3.3-1. See section Fatigue Waves below.
An extreme conservative approach to fatigue life calculations would be to consider all waves, regardless of wave height, to deflect the wellhead the full 10mm horizontal movement. This is not implemented in the Valhall Flank gas lift fatigue calculations.
A conductor analysis would show which wave heights are to be taken into consideration. A conductor analysis for the gas lift wellheads are not currently found so an analysis for an Ekofisk 2/4M riser is used, ref. page 13 and 39 of doc.: Riser Analysis, ConocoPhillips Ekofisk II Development Project, 2/4M. H042729, Rev NC.
Piping Restraints
The supporting of the gas lift lines closest to the wellhead depicts the stress range. The more restraints there are the higher the stress range will be as the flexibility of the piping is reduced. The restraints also affect the natural frequency of piping systems. A natural frequency above 4 Hz is an indication that the piping is adequately supported. For piping connected to wellheads this goal is often not obtained since it directly affects the fatigue lifetime.