Stuart Smith, global design services manager, discusses why structural fatigue in cranes occurs and what can be done to understand and address it.
Understanding structural fatigue in offshore cranes and their overall condition is not only vital in ensuring the integrity of deck operations and meeting safety requirements, it is also proving to be crucial as many cranes and their components have yet to tackle the toughest stage of their lifecycle during decommissioning.
In the past it was common practice to change out structural components on a five-year cycle to ensure their integrity but this maintenance process can take a couple of weeks, hampering or delaying work on deck. The industry has moved away from this view and it is now seen as being more important to understand the condition of safety critical equipment like cranes and how they are deteriorating rather than changing the components on a fixed-term basis.
In around 95% of cases the boom is the section of the structure most vulnerable to fatigue, with its mid-sections particularly sensitive to premature aging. The exception to this can, on occasion, be the A-frame which can also have a shorter than predicted lifespan.
It is not possible to prevent fatigue but understanding its causes enables operators to guard against its effects. Invariably, welds on the structure are where issues begin. Undetectable flaws occurring during the welding process mean that every weld has a potential defect that could present a weakness in the future.
The only way to fully understand the condition of the crane is to conduct a study examining loads and cycles. Typically, the average lifted load offshore does not stray significantly from 3.2Te with a variance of 0.75Te.
Although the weight of the load does play a part in the analysis, it is more pertinent to study the stress range. Calculating the difference between the existing residual strain on the structure, the added stress during an actual lift and the spare lifting capacity provides an accurate picture of the fatigue experienced.
Because of the relatively low average load, the number of lift cycles has a greater effect on the fatigue life of structure than the lifted load itself. This also means there are many large capacity cranes operating at a fraction of their design limits. Lower stress ranges provide longer fatigue lives and occasional lifts at maximum capacity will not significantly affect their fatigue life.
Cranes that predominately operate towards the outer regions of their radius range will result in greater stress ranges and therefore reduce their fatigue life compared to those operating at closer ranges.
All of this can be plotted on an S/N curve with fatigue strength and the number of cycles along each axis with the Miner’s Rule then used to calculate the damage through variable stress ranges.
A critical part of analysing fatigue is determining an accurate life cycle. In the majority of cases gaining reliable information to determine the crane’s life cycle can be a challenge. Many of today’s cranes have a recording load indicator system with a full history of each lift, detailing the weight and the position the crane was in when this took place. This is ideal for fatigue analysis but, a number of ageing cranes will not have this information, or if they do it will only be available for a small proportion of the crane’s life.
It is still possible to establish realistic life cycles for any particular offshore platform crane even without a recording load indicator system. Knowing the type of work, when drilling programmes have taken place and even the deck layout makes it possible to establish the likely lifting frequency, average weights involved and where loads were being moved to and from.
So what happens if fatigue failure occurs? The next step is to take the analysis further and establish a criticality assessment. This involves a stress analysis of the crane that takes into account the potential fatigue failures so it will be possible to understand how this could affect the structure. It may be that if a single weld failure occurs an alternative load path could be available through the structure and the increase in utilisation will not cause yielding or further damage. Alternatively complete structural failure may be the predicted result.