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Evaluation of Wellhead Fatigue With Structural-Monitoring Data

Topics: Offshore

There is an increasing need to understand the strength and durability of subsea wellheads and conductors. Recently, in-situ monitoring of drilling-riser and wellhead response has been used for projects in the Gulf of Mexico and the North Sea. Wellhead-fatigue-life predictions, for which minimal in-service validation has traditionally been available, have been found to be conservative by factors of 10 or greater on the basis of the field-measurement data collected.

Fatigue of Subsea Equipment

Fatigue is a mechanism of progressive and localized structural damage and occurs only when a material is subjected to cyclical loading. Fatigue damage can occur even when a structure experiences loads that generate stresses significantly below the elastic yield strength of the material. Because subsea wellheads are exposed to constantly varying loads of high magnitude, their fatigue response is of particular concern.

The fatigue life of a component is greatly affected by the likelihood that flaws exist within it and by the complexity of its geometry. If welds are included in the manufacturing process of a component, or are used to join components, crack-like flaws will be introduced. Alternatively, the presence of sharp corners, notches, holes, or threads leads to concentrations of stress in certain areas of a component. The ratio of the maximum stress in such an area to the average stress in the component is termed the stress-concentration factor (SCF) or the stress-amplification factor (SAF). The higher the SCF or SAF, the shorter the fatigue life under a given regime of load cycling.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 23981, “Evaluation of Wellhead Fatigue Using In-Service Structural-Monitoring Data,” by Phil Ward, Alex Rimmer, and Hugh Howells, 2H Offshore, prepared for the 2013 Offshore Technology Conference, Houston, 6–9 May. The paper has not been peer reviewed. Copyright 2013 Offshore Technology Conference. Reproduced by permission.
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Evaluation of Wellhead Fatigue With Structural-Monitoring Data

01 April 2014

Volume: 66 | Issue: 4

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