*Articles published in CRANE BRASIL
With the evolution of materials science and the need for subsea lifts and installations at ever greater depths, the offshore industry seeks to overcome challenges with technology. Among them is the use of special fiber slings and cables, manufactured with HMPE (High modulus polyethylene) or high molecular density polyethylene, replacing common synthetic cables.
The HMPE gives rise to a very high resistance and light fiber, and when used in the manufacture of cables for slings or direct traction cables, it brings great advantages for use in offshore lifting or installation of equipment on the seabed.
Among the advantages are: resistance equivalent to steel cable weighing seven times less, making it much easier to handle; neutral weight or they float on water, therefore they do not consume crane or winch capacity. For the same case, the own weight of the steel cable can represent up to 40% of the hoisted weight, demanding bigger equipment; high fatigue strength; resistance to ultraviolet rays, chemicals and salt water; high resistance to abrasion and cutting, unlike conventional fibers, which allows for a long service life; low coefficient of friction; easy inspection, maintenance and repair.
Challenges to be overcome
Like any new technology, there are challenges to be overcome, such as: culture and consolidation of wire rope, as today there is technical mastery in its application by engineers and excellent prediction of performance; need to consolidate international technical standards with minimum specifications; need for special equipment to collect and lay the cable with active heave compensation; low coefficient of friction, which can be a disadvantage in some operations; low resistance to high temperatures and possible resonance in deep water installations.
The applications of slings and cables with HMPE fibers are vast, such as the installation of underwater equipment on the seabed, using a crane or winch, where the weight of the cable, which can be up to 3,000 meters long, will practically not contribute for the weight of the lifted load; direct use as crane cable; use as a lifting sling for any application, being lighter than conventional textile strap slings.
Slings are devices that, among other functions, make the connection of the load to be lifted to the lifting equipment, and can be manufactured in different materials, such as steel cable, chain, textiles and special fibers such as HMPE. They can also be formed by combining various materials, with a textile strap in the load region and a steel cable sling in the crane hook region, for example.
There is also a wide variety of capacities, ranging from a few kilograms to thousands of tons. In the latter case, when high lifting capacity is needed, conventional slings become more difficult to use, opening up space for special slings.
One of the most used special slings is the so-called cable laid, which is generally formed by six steel cables arranged (laid) in a helical shape over a central cable. This central cable forms the core of the sling, forming a structure of 7 different cables, with total diameters between 60 and 500 millimeters.
The cables that make up the cable laid must have a steel core, and can be category 1770 and 1960, construction class 6×36. The eyes can be formed mechanically with clips, sockets or with manual braiding, which is the most common form.
The maximum working load of these slings can reach over 3,000 tons for a single leg, and over 10,000 tons can be lifted with a 4-leg arrangement.
Therefore, slings with large capacities are achieved using conventional steel cables. Another great advantage of cable laying is its flexibility compared to a single steel cable of equivalent diameter. The single cable would be much more rigid, making handling impractical, in addition to the economic unfeasibility of manufacturing cables with large diameters.
In addition to the sling with eyes, it is possible to manufacture it in the form of an endless loop, called grommet, with basically the same characteristics as the sling formed by eyes, however, a single continuous cable is used to form the 7 parts.
The design and use of these slings can be done based on the EN 13414 part 3 standard and the IMCA M179 guide, which bring all the criteria for design, manufacture, testing and use of these slings, which expand the options of engineers in heavy lifts (heavy lift ) or when great flexibility of the slings is required.
In previous articles, slings made with high modulus fibers (for example HMPE) and slings made with several layers of steel cables, called cable laid slings, generally used in lifting with heavy loads, were presented.
This article presents the grommet type sling or endless loop, which has a ring as its final geometry, which can be manufactured with steel cable, textile strap or high modulus fiber.
The steel cable grommet was already in full use in lifting abroad, mainly Europe, and was specified for the first time in Brazil by NBR 13541-1 in 2011. Until then, the version of this standard considered only slings formed by legs of a part with eyelets at the ends.
Its ring geometry is obtained from a single cable, so that the two ends overlap in the necessary length for the braiding, performing it by both ends on the main body. This braiding is carried out in accordance with NBR ISO 8794. Obviously, this geometry will generate a seamless end meeting of the cable web, therefore, the strength of the web is always neglected in the calculation, even if it is made of steel.
After assembling the grommet, paint the region of the braid and the region where the web meets, in order to avoid bending at these points.
Dimensioning is done by NBR 13541-1 and its inspection and use are specified by NBR 13541-2. In the project, special attention must be given to the calculation of the reduction of resistance of the sling due to bending in the support region, which can be a shackle, hook or trunnion. The smaller the bending diameter, the greater the capacity loss. The loss calculation can be performed according to ISO 19901-6.
The main advantage of the grommet in relation to conventional slings with eyes is that it can be shorter in length, which can be useful when there are geometric limitations. For example, a 51mm diameter grommet can be less than one meter long, while a conventional 2-eye sling must be at least 3.2 meters long.
A very advantageous combination is to use grommets manufactured using the cable-laid technique (see previous article), leading to slings with very high load capacities combined with high flexibility. For example, a grommet with an equivalent diameter of 342 millimeters and a breaking load greater than 8 thousand tons can be built. The calculation of this type of grommet is not contemplated in Brazilian standards, but can be done by the EN 13414-3 standard.
Publication of the NBR ISO 18264 standard
In the first part of this series of articles on special slings, slings made with HMPE fibers (high modulus polyethylene) were presented, which have high tensile strength and low weight, bringing several benefits to some types of offshore lifting.
Although steel cable slings dominate the offshore scenario in Brazil, there is an increasing technical basis for the application of high modulus fiber cables in lifting activities. Proof of this is the publication in March 2020 of the NBR ISO 18264 standard – Textile slings – Fiber cable slings for general purpose lifting operations – High modulus polyethylene (HMPE).
This Brazilian standard, developed by CB-50 (offshore equipment and structures) contains the same text as the international ISO standard, published in 2016, and specifies the requirements regarding manufacturing, safety, workload determination tests, use , inspection, among others. It includes slings with one to four legs, with eyelets or endless loop (grommet), made with braided cables with 8 or 12 legs, as well as cables with cover, made only with HMPE fibers.
The standard allows the combination of cables with lifting accessories such as shoes, shackles, pins, load rings, among others.
Unlike standards for steel cable slings and conventional textile straps, NBR ISO 18264 establishes equations for reducing the Maximum Working Load (CMT) of the sling as a function of the ratio of the bending diameter of the cable and the diameter of the cable itself, and this ratio cannot be less than 1 under any circumstances. When this ratio is less than 3, a reduction is necessary, which can reach 50%. With a diameter ratio greater than or equal to 3, reduction is not necessary, which is an advantage over steel cable slings, which in this situation lead to a 17% loss of strength.
A curious point of the Brazilian version of NBR ISO 18264 is that it still does not establish a safety factor for calculating the CMT of the sling, clarifying that a specific standard for this is under development. As a reference, the European Union uses a safety coefficient of 7, Japan 6 and the United States 5.
With the evolution of international standards and learning from the successful experiences of lifting with HMPE slings around the world, today we have one more option to use in Brazilian projects, bringing more efficiency to offshore lifting.
Extended Body Shackles
Shackles are one of the most used accessories in lifting and moving loads, as they are versatile, practical, standardized and facilitate the various connections necessary for lifting.
Among the various types of shackles, the wide body shackles, wide body shackles or sling shackles stand out. They are characterized, as the name says, by the larger diameter in the body part (curved region) in relation to conventional shackles, precisely where the support of the slings occurs.
It is a fact that a sling when bent on a pin loses tensile strength in relation to the sling without bending, and this loss of strength is greater the smaller the bending diameter is. For example, this loss can reach 50% when a wire rope sling is bent onto a pin with a diameter equal to its own diameter.
2 to 2.5 times larger bending diameters
To reduce bending loss, extended body shackles are very effective as they have a bending diameter about 2 to 2.5 times larger than conventional shackles with the same workload. This allows you to use the slings more efficiently, better matching the strength of the shackle with the strength of the sling.
For example, a 76 mm diameter steel cable sling, grade 1960, construction class 6×36, steel core, with the body bent over a conventional 120 ton shackle, results in a sling with a working load of 87 tonnes . If, for the same sling, a 125 tonne flared body shackle is used, the sling will have a capacity of 106 tonnes, an increase of 22%.
Strength losses in conventional alloys
In the case of conventional tubular textile strap slings, there is also a loss of strength due to bending, but unlike steel cable slings, there is no formulation that estimates the loss of strength as a function of the bending diameter. What is done in this case is to determine the minimum bending diameter that allows the strap to reach its maximum breaking load without the bending influence.
For example, a red tubular textile strap with a working load of 5 tons has a nominal diameter of 30 mm. If it is connected to a shackle by simple contact (see NBR 15637-2), a shackle with a body diameter greater than or equal to 30 mm will be required, thus, at least a shackle with a working load of 12 tons must be used. Therefore, to use a 5 ton strap, it is necessary to use a 12 ton or larger shackle. If a flared body shackle is used, a shackle with a working load of 7 tons, more compatible with the capacity of the sling, can be used.
Flared body shackles not only increase the strength of the slings but also increase durability, preserving investment in accessories and increasing the safety of lifting operations.
In previous articles, several types of slings for offshore lifting were shown, such as composite slings, called cable-laid and HMPE fiber straps and cables, each type having advantages and disadvantages in the application, leaving it up to the lifting engineer to choose the best one solution for the project.
Increasing the options for lifting, we have grade 8 chain slings, already widely used in Brazil, and slings with higher grades, 10 and 12, increasing the possible lifting solutions.
Not all chains are suitable for lifting
Chain slings, for some time in the past, were considered unsafe for use in offshore lifting, however, this false impression was due to the use of chains that were not suitable for lifting, with low grades and without proper heat treatments.
With the increase of the grade from 8 to 10, or recently, to 12, the chain becomes lighter for the same capacity, or it is possible to increase the capacity, maintaining the weight of the sling.
For example, a grade 8 chain sling has a maximum working load (WML) of 8 t; if it is grade 10, the CMT rises to 10 t and if it is grade 12, the CMT increases to 12.5 t, a gain of more than 50% in capacity, increasing the weight of the sling by 23%, comparing grades 8 and 12.
Advantages and disadvantages of chain slings
In addition to the use of chain slings in conventional lifting, in the offshore area, there are advantages over steel cables and straps, such as the ease of shortening the length, very high resistance to cutting and abrasion, as well as great flexibility, which can be used in unfavorable geometries and decommissioning of structures and equipment. In addition, they are very useful for creating lifting points on structures, allowing the installation of winches, hoists, blocks, etc.
As disadvantages, we can mention weight, need to import grade 12 slings and capacity limitation per sling leg, usually reaching 40 tons.
Grade: designation of the minimum breaking strength of the steel from which the chain links are manufactured, in hundreds of megapascals (MPa). For example, grade 8 chain steel must break with a minimum stress of 800 MPa. Grade 10 chain steel must break with a minimum stress of 1000 MPa.
textile chain slings
Steel chains form very versatile lifting and lashing slings that are durable, flexible and easy to shorten. However, it has the disadvantage of being much heavier than other slings of the same strength, made of steel cable or textile straps.
To achieve an optimal product, incorporate all the advantages of chains and eliminate the disadvantage of greater weight, the textile chain (soft chain) was created, which is up to eight times lighter than a steel chain of the same resistance, made possible by the use of HMPE fibers, High Molecular Weight Polyethylene, in Portuguese (see part 4).
For example, a grade 8 chain, diameter 13 mm and Maximum Working Load (CMT) of 5.3 tons, with a length of 10 meters, weighs 36 kgf. The textile chain with the same length and 5 ton CMT weighs 5.8 kgf.
This weight reduction allows the rigger to handle much less weight in a working day, with less fatigue and greater productivity.
Summarizing the advantages of textile chain slings, both for lifting and for lashing for transport, we have:
- Up to eight times lighter than steel chain;
- Much more resistant to cutting and abrasion than textile strap slings;
- Safer to handle and more ergonomic than steel chain, avoiding personal accidents;
- Greater resistance to chemical attacks, grease, dirt and salt water, compared to common textile straps;
- Manufacturing with less impact on the environment than steel chain;
- Quieter in use than steel chain;
- Does not absorb water;
- Floats on water;
- Preserves the cargo surface;
- It does not conduct electric current;
- Corrosion free;
As a disadvantage, we can mention the lower availability of the material in the Brazilian market, as it is a product of recent development. Today, there is only one manufacturer in Brazil. Abroad, a large and traditional manufacturer of steel lifting accessories was the pioneer in the manufacture of textile chain.
With this sling option for lifting and mooring, which incorporates high technology materials, the lifting engineer can optimize his projects, bringing more safety to operations and more comfort to operators.