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Lifecycle stage

In the context of engineering, structural fatigue is the process whereby fluctuating loads cause damage in two phases – crack generation and crack propagation. The magnitude of load to cause this will typically be much less than the single load application needed to cause damage. While some types of structure can tolerate extensive cracking without compromising their load-carrying capacity, others may be more susceptible to abrupt collapse or excessive deflection if a crack progresses to a critical size.

Fatigue cracks may commence depending on the number of load cycles, but they may occur in rapid succession, or can also develop with substantial time lapses between them. Fatigue performance is articulated as ‘Life’ (in other words the period before expected failure under the predicted loading regime).


Static (constant amplitude) and Random (variable amplitude):

The definitions used to describe individual fatigue loading cycles are shown in Fig.1 below. The definition of stress range is important in the fatigue analysis of welded structures because of the relatively uncontrolled nature of the metal formation and therefore the discontinuities and defects that may be present. Another parameter used to describe fatigue loading cycles is the Stress Ratio, which is defined commonly as the lower limit stress/ upper limit stress. (Or, Min Stress Ratio = Max Stress Figure).

Fig 1. Definition of parameters describing Static (constant) amplitude fatigue loading cycles

Source: Platform, Pipeline and Subsea Technology – Fatigue Design

Fig 1. Definition of parameters describing Static (constant) amplitude fatigue loading cycles

Fig 2.  Shows some general Random (variable amplitude) loading examples for different scenarios

Source: Platform, Pipeline and Subsea Technology – Fatigue Design

Fig 2. Shows some general Random (variable amplitude) loading examples for different scenarios

In the main, engineering structures are always subjected to some type of variable amplitude or random loading.

Fatigue damage tends to take the form of cracks particularly at welds, for instance, at the end of structural members as they frame into joints. Originally, early jacket designs focussed on experiences in Mexico, however in the UKCS, such designs are unable to withstand the more severe wave environment. This has contributed to having numerous structures, or some components of them, requiring fatigue damage repairs or needing to be strengthened to prevent any anticipated damage.

In the case of corrosion, this can result from under-designed or failed cathodic protection. This is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded “sacrificial metal” to act as the anode. The sacrificial metal then corrodes instead of the protected metal.

For structures such as long pipelines, where passive galvanic cathodic protection is inadequate, but localised corrosion can occur due to galvanic corrosion as is the case when carbon steel caissons house stainless steel pump or strainer components. In either scenario thinning or perforations of the carbon steel can occur.

There are many reasons why an asset may need intervention whether strengthening, modification or repair. Extending the life of existing offshore wells is one of the biggest challenges facing many oil and gas operators around the world today. One of the concerns is whether there is enough margin against fatigue failure to carry out future operations, including workover or plug and abandonment (P&A) activities.

There are several factors that make managing the risk of fatigue failure for workover or P&A complicated:

1. A lack of ‘as-built’ data about the well and previous drilling operations
2. Less fatigue resistance than in modern wells
3. The use of larger, heavier modern blowout preventers (BOPs) on obsolete equipment.

Nevertheless, findings from routine inspections can continue to uncover even more damage or fabrication defects.

The need for additional conductors, or life extension solutions to enable a field to continue production after its intended design life could also result in a requirement for further strengthening or modification.


In 2018 Claxton’s sister company, 2H Offshore launched the measurement-based Wellhead Fatigue Joint Industry Project (JIP). With this project 2H works alongside operators and clients to improve riser, wellhead and conductor fatigue estimates and make drilling operations more reliable and efficient.

The project aims to provide a measurement-based foundation for drilling riser system modelling to ensure that riser, wellhead and conductor fatigue damage can be more accurately assessed. Currently, eight major oil and gas operators have joined the JIP.

“Riser, wellhead and conductor fatigue life is one of the most important aspects of drilling system integrity assurance. However, analytical parameters related to riser and BOP hydrodynamics, vortex-induced vibrations (VIV) and conductor-soil interaction are still not fully understood.” Explains Bulent Mercan, 2H Offshore Project Manager.

“To obtain acceptable fatigue life, subsea engineers are having to take costly measures such as the installation of strakes around the riser joints, improvements of fatigue details at welds and connectors, vessel upgrades and more. A reliable set of analytical parameters leveraging diverse field measurements will lead to improved accuracy and more efficient drilling operations. Industry collaboration through sharing resources will play an important role to achieve this goal.”

Other sources:

Asset life extension case study pack
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Asset Life Extension Case Study Pack

Download the Claxton structural asset life extension case study pack for details on slot recovery techniques, centralizers, replacement platform guides and other bespoke solutions.