Evaluation of Wellhead Fatigue With Structural-Monitoring Data
Authors: Carpenter Chris 
Published: February 2015

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 The paper has not been peer reviewed.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.

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.

There are two primary mechanisms by which environmental loads cause fatigue of offshore structures. These are vortex-induced-vibration (VIV) fatigue, driven by current loads, and wave-induced fatigue, caused by wave loading. VIV fatigue occurs if the frequency with which vortices are shed from the leeward side of the riser, a natural effect for a cylindrical body placed in flow, matches one or more of the natural frequencies of vibration of the riser and conductor system. These rapid oscillations generate significant stress fluctuations and can cause significant fatigue damage very quickly. By contrast, wave-induced fatigue is a constant effect, but generally results in lower fatigue-damage rates at the wellhead. This is because wave loading will excite the upper portion of the riser, creating loads and motions that are transmitted down to the wellhead. Hydrodynamic damping and inertia associated with the lower riser portion will act to reduce these motions. For specifications of both VIV and wave-induced-fatigue monitoring systems, please see the complete paper.

Besides the wellhead and connector, areas of concern include the welds between the wellhead and the adjacent pipe and the seam weld in the conductor pipe. A typical SCF in one of these welds is approximately 1.3, generated primarily from the geometric misalignment and ovality of the components on either side of the weld. However, because of flaws and residual stresses induced by the welding process, the minimum fatigue life may occur in the weld despite the lower SCF. Subsea wellheads are positioned below the blowout preventer (BOP), the primary control mechanism for the well, and therefore are critical in ensuring that well control can be maintained. Furthermore, inspection of these components for flaws after installation is not feasible. These factors lead to the application of a safety factor that can cause analytical fatigue-life predictions to fall short of the target life.