Heavy lifts, high seas20 May 2021
Offshore loads get heavier, and subsea lifts go deeper, all the time. Julian Champkin investigates the hazards and developments.
Lifts 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 saltwater 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 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...’
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 embrittlement, who was not directly involved in the report’s findings. He noted, while the scientific community has achieved great progress in using simulation tools to predict the behaviour of components exposed to hydrogen, challenges remain.
“There is a real concern across industry regarding the impact of HE and SICC on chains and links used in hoist and lift products across offshore environments,” says William Hackett director Ben Burgess. “Based on our own experiences of how our products perform offshore, combined with the manufacturing expertise of chain makers McKinnon Chain and outcomes of detailed technical analysis by industry partners, we have identified that as material hardness exceeds 39-40 HRC, the risk of HE and SICC increases as the hardness values rise,” he continues, echoing the points made by Nyberg of Gunnebo. Available hydrogen, the material used and the stress placed on the component are all factors that contribute to hydrogen embrittlement (See Figure 1.).
“The issue of HE is not limited to just one type of offshore activity,” says Burgess. “Examples include the failure of G10 welded chain slings in a container fleet in Norway. In the US, a global oil company had to withdraw a number of lifting appliances and promptly introduced an inspection regime before any future lift work was carried out.
“Meeting the specific International Standards should not be seen as a guarantee that specific equipment is fit for purpose in an offshore environment,” he says. “Specific environmental and performance considerations for equipment which is used offshore needs to be a key part of the material specification and selection process.
“To put this into context, a Grade 8 master link, when correctly heat treated, will provide toughness, tensile strength and resistance to shock absorption in loading, and will do it at hardness levels that enable the steel in the product to withstand extreme conditions when in use in the offshore environment.”
Key areas around HE include causal factors and best practice methods, says the report. Correct materials selection is critical. Operators need to ensure that despite commercial pressures the products used in the offshore environment are fully appropriate for their intended use, and that the environmental conditions, mechanical stresses and material susceptibility have all been assessed rigorously.
“Managing the risks of HE and SICC requires a change of mindset,” says Burgess. “The advancement towards higher grades of steel should be treated with caution. Without proper understanding of the material and its use offshore, the end result is increased risk to operations.”
The company is well-known for its subsea SSL5 lever hoist with its patented Quad Pawl system, which we have covered in these pages before (see Hoist, April 2020). Its design and extensive corrosion protection give it multiple immersion capability, independently verified by DNV. William Hackett has taken further steps to help minimise the risks of HE and extend the lifespan of its master links with the introduction of Zinc-Tough, an innovation that applies a zinc layer to the product which significantly reduces the speed at which corrosion occurs. It extends the product lifespan and also reduces the risk of HE compared to other coating processes such as galvanising and electroplating. William Hackett to date has delivered more than 550,000 master links. “We are immensely proud of our track record in the supply of HA links to Shell, BP, ExxonMobil, and every other major offshore operator,” says Burgess. “We have not had one suspected case of HE to-date, which reinforces the quality approach we take in how our products are manufactured.”
Irizar Forge, a fourth-generation family company based in Lazkao in the Basque country, has developed a speciality in large-size forged lifting components for, but not limited to, the offshore market. Here again, quality of material is everything. The advantages of forged over cast products is in the strength of the finished product, says sales manager Oier Sarasola.
In a cast item the grains of metal are randomly arranged; the molten metal is poured into its mould, cools down, and that’s an end of it. In forging, the red-hot (but solid, not molten) metal is hammered and beaten into shape. “This hammering and beating makes the grains of steel finer and more string-like, and also – importantly - aligns them inside the piece,” says Sarasola. “They follow the flow of the piece, all pointing in the same direction to give a finer, more uniform structure. This is what imparts the added strength.”
Forging is not, though, an easy process to carry out, especially at the huge sizes in which Irizar specialises. It calls for big equipment and highly skilled workers. That affects costings. “But, even if the purchase price is more expensive, the market should take into account that forging is the stronger and safer option, there are fewer maintenance costs and a longer lifespan than with cast or other technologies”, he says. “Hook failure is minimal and is ductile rather than brittle, so plenty of warning is given,” adds Sarasola.
Irizar Forge has always specialised in large size and capacity. “A few years ago, we looked at the largest-capacity cranes, and found they are all offshore,” says Maria Lasa Irizar, director of the company. (Her great-grandfather founded the firm in 1923.) “So, we have especially developed that side of the business; it has become something of a niche for us,” she adds.
“We have standard designs, but most of our business is custom-made to clients’ individual requirements,” says Sarasola. “Shapes are smooth curves so that slings can work safely and efficiently on them over long lifetimes.” Other Irizar products include submersible hooks that are friendly for ROV recovery. “The challenge in subsea lifting is that products must be operated by non-human remotely controlled vehicles; so latches and fittings are designed accordingly, in stainless steel to avoid corrosion. Handles, also in stainless steel, are added for easier handling on deck. Hooks are fully forged in one single piece, so there are no welds which can be points of weakness.”
Irizar recently delivered a 1000t SWL hook block for a heavy lift vessel, and can deliver even higher capacities, with hooks of 1500t SWL as well. These are truly impressive forgings.
Hooks and fastening are not the only components of undersea lifting; the wire, chain or rope needs attention also. A trend in onshore lifting is replacement of wire rope by synthetic. The same trend is visible offshore. Synthetic rope offers a number of advantages over steel: it does not corrode, has similar strength size as steel wire and is extremely light and easy to handle on deck during the rigging process. The density of the rope is close to the density of water, which means that irrespective of the depth of operations the fibre rope has no impact on the overall payload weight, whereas with wire or chain, the deeper the operations it is important to consider the self-weight of the wire to the weight of the payload at depth. “That means that on a deep or very deep subsea lift, the load carried by the lifting machinery remains constant using fibre,” says Nyberg. “With a wire rope, at the deepest level the hoist is carrying the huge weight of wire rope as well as the load itself.” The difference is considerable: 4,000 metres of 88mm fibre rope weighs approximately 20 tonnes, compared to over 200 tonnes for the equivalent length of steel rope. A 150t SWL crane with fibre rope crane can lift loads at depths of 3,000 m that would require a 250t SWL wire-rope crane to lift the same payload.
There is, though, a drawback: synthetic is more elastic than steel. A synthetic rope stretches under a load, so when in a lifting operation it is wound, under tension, onto a drum winch, it is in a stretched state. Storing a stretched rope at high tension is inadvisable; the contained energy can be dangerous, the winding is uneven, and frictional heat is created as the rope relaxes.
Scottish company Parkburn Precision Handling Systems, based in Hamilton, has developed a solution specifically to address this problem. Its patented Deep Water Capstan de-tensions the synthetic rope as it winds it in. It consists of two separate drums that intersect: each drum has 16 “fingers” which interleave with those of the other drum to provide the rope-bearing surface. The drums are offset to each other around the rotational centreline to create a natural helix and a slightly elliptical cable path through the machine. This results in a unique tensioning/de-tensioning profile that spreads the work evenly through the rope as it passes through the machine. After it emerges from the Deep Water Capstan the rope can be stored at low tension. Each drum has its own motor and gearbox, supplied by Dana SAC. The device can be used on cranes also: a 150t variant of Parkburn’s Deep Water Capstan lies at the heart of MacGregor’s FibreTrac 1500 marine crane.
Another offshore trend is the increasing size and weight of lifts. Topsides for rigs are standardly now constructed as everlarger modules to be transported and lifted into place rather than being constructed offshore. An essential component of such lifts is the spreader beam, and specialists Modulift have just constructed their largestever beam. It can lift a staggering 2,000t and has a 33m span. The feat of below-thehook engineering was delivered for offshore work to the Dutch company Safe Lifting Europe. It is not the first record-breaking beam it has supplied for Safe Lifting. An earlier beam for the same client, of 1,500t at 20m span, was at the time also a record-breaker for Modulift. “We are working in a market that is showing unlimited potential in terms of capacity,” says Jacques Vroegop, technical director, Safe Lifting Europe.
“We could be talking about much heavier lifts becoming commonplace. At the moment we are working with cranes offering up to 1,000t capacity but we are in a very dynamic sector.” “Satisfying though it has been to process these recent orders, we are not surprised,” says Sarah Spivey, managing director, Modulift. “We have been aware of the potential at the super-heavy end of the market for a long time—the middle remains quiet. I don’t expect this to be our highest capacity beam for the long term. We have the engineering capability to go to 5,000t and the boundaries will continue to be pushed.” Modulift is already looking at upgrading to 3000t for an upcoming project with a client.
Increasing size is affecting lifting vessels also. Fred. Olsen Windjammer installs wind farms, and its Brave Tern and Bold Tern are purpose-built heavy-lift specialist lifting vessels designed to jack themselves off the seabed while they crane the towers and rotors into place. They also carry both towers and rotors on their decks from the loading ports to the site of installation. The Bold Tern is currently working on the world’s largest offshore windfarm, the Hornsea One off the Yorkshire coast. The project is the first to have more than 1GW of capacity; it will power more than a million homes. Its sister ship is currently in Taiwan working on the new Yunlin windfarm but is shortly to be upgraded to include in her superstructure the highest crane on the market. When the work is completed in February 2022, she will be one of the few vessels capable of installing foundations and towers of next generation wind turbines. Such turbines will have tower heights of 138m to the nacelle, and rotors reaching 220m above sea level. The Brave Tern, when raised on her four legs above the surface, will be able to reach 238m with the main hook and 256m with fly-jib.
Her new and unique crane will be a 1600t LEC 65500 leg-encircling crane from Huisman. It will be fitted around the aft port leg of the Brave Tern; the lambda-shaped boom is very stiff, giving reduced motions at the tip; the compact size of the crane in combination with its low own weight and high, 1600mt, lifting capacity make it unique and suitable for both the installation of wind turbines – which requires long boom and light weight crane design - and the installation of foundations - which need short boom and strong crane construction. The crane is fully electrically driven, giving reduced power consumption, higher reliability and a more environmentally-friendly machine. “This unique crane has been enhanced with key updates and an even more extreme boom,” says Alexandra Koefoed, CEO, Fred. Olsen Windcarrier. “The increased outreach capacity of the new crane allows us to stow the wind turbine components in a more flexible way despite the increased crane weight, thus maintaining or exceeding the payload we carry for our clients. This is a considerable lifetime extension for the vessel, as the weight and dimensions of wind turbine components continue to increase.”
For lifting engineers a life on the ocean waves will clearly continue to offer ever increasing opportunities.