OFFSHORE AND SUBSEA LIFTING | APPLICATION REPORT


HEAVY LIFTS, HIGH SEAS


Offshore loads get heavier, and subsea lifts go deeper, all the time. Julian Champkin investigates the hazards and developments.


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ifts in offshore oil and gas are routinely much heavier than on land. Even normal-magnitude marine lifts present hazards.


Complicated factors abound that are unknown on land; and the consequences of a malfunction can be even more extreme, much harder (and much more expensive) to remedy – and much more hazardous to life. One would have thought the deeper


below the surface, the more hazardous the environment. That, says Felix Nyberg, global product manager of Gunnebo, is not necessarily the case. “Corrosion is an obvious hazard; but in


fact, deeper water contains less dissolved oxygen, and so is less corrosive to chains, shackles and other components. Nearer the surface, and especially in the splash zone, is where corrosion is most intense. Not only is there more oxygen, but wave action can scour away any protective layer of corrosion product that has built up and which can act as a barrier to further salt- water action.” Protective paints are one defence. Another is to add a zinc patch or coating as a sacrificial anode; it corrodes away in preference to the steel around it, so preserving the load-bearing component. The price of safety is eternal vigilance, though; both paints and sacrificial anodes need frequent inspection to ensure they are in good condition. Salt-water corrosion is only one of the hazards of subsea lifting. Another is hydrogen embrittlement, also known as hydrogen-induced cracking. Hydrogen diffuses into steel surfaces, especially through micro-cracks and roughness and at points of stress. Inside the steel it penetrates and can cause brittle failure – complete, unexpected, and, in a


30 | May 2021 | www.hoistmagazine.com


hook or lifting chain offshore, extremely dangerous. Vital here, says Nyberg, is ensuring the materials chosen are right for the environment. In particular, you need to check the hardness of your steel - or rather, the lack of it. Hard steels fail in brittle mode - that is, suddenly and without warning. Softer steels on the other hand are ductile: they stretch and deform before failure, so careful and frequent inspection will reveal the necessity for replacement before the accident happens. Check the HRc (Rockwell) hardness value is the indicator here: for offshore conditions, steel with a HRc lower than 39 should be used. Hoisting vessels will move up and down


with the waves; this imparts dynamic loadings that can be many times the actual weight of the load - or which can even turn negative if the vessel suddenly plunges. Those loadings must then take into account when calculating safe working loads. And even the stated SWLs of ropes, hooks and shackles should not be relied upon. Repeated loadings at values close to the SWL bring metal fatigue, which again lead to sudden failure and reduced safe lifetime. Only if frequent loadings are limited to 80% or less of the SWL is the lifetime of the component unaffected. Nyberg’s advice in summary is to choose the right materials and to inspect components often. Hoist-makers William Hackett have similar advice, and indeed as recently as October 2020 issued an industry report on the subject. Its importance is conveyed by its title, which begins, ‘Technical guidance on the effects of Hydrogen Embrittlement for materials used in topside and subsea lifts’ but continues with the words ‘to help minimise risk to human life...’


R The SS-L5 lever patched from William Hackett has quadruple-pawl safety and is DNV-certified for multiple immersions.


The report, peer-reviewed by a number


of organisations and authorities, is intended to help minimise the risk of Hydrogen Embrittlement (HE) and Stress Induced Corrosion Cracking (SICC) in marine and subsea operations. “Hydrogen is famed for causing


notorious structural integrity problems that are difficult to predict, and there is a need for new guidelines and solutions,” says Dr Emilio Martínez-Pañeda, assistant professor at Imperial College London and a world-recognised expert in hydrogen


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