As the global trade in LNG increases across a range of regions and gas tankers grow in size, they need the right lines to aid in manoeuvring and mooring safely, quickly and effectively
LNG carriers operate in all types of climate conditions, from the scorching heat of the Middle East to the bone-chilling cold of the Arctic Circle. But regardless of the climactic conditions, mooring ropes with HMPE are proving increasingly popular. In order to select the right mooring lines for a vessel operation, you need to make sure that they are made from the right HMPE fibre.
The fourth edition of the Oil Companies International Marine Forum’s Mooring Equipment Guidelines (MEG4) came into force in July 2018, and were introduced in the wake of the Zarga incident at the South Hook LNG terminal in Wales in 2015, in which a parted mooring line struck a seafarer, resulting in a serious injury. The guidelines help operators to select and plan the use and maintenance of mooring lines throughout their lifecycle, improving safety and operational performance.
DSM manufactures a HMPE fibre named Dyneema, a strong durable fibre that is 15 times stronger than steel but light enough to float on water. The company was involved in the development of the guidelines and recently published a whitepaper on how to select the right mooring lines, with a focus on safety and reliability.
LNG World Shipping spoke to the paper’s authors, DSM’s segment manager for offshore energy and maritime Jorn Boesten and application manager Jac Spijkers, about the questions operators should be asking concerning the selection on their mooring lines in the light of the MEG4 guidelines.
“The introduction of MEG4 last year set a new standard for hardware and mooring line design, mooring dynamics, inspection and discard criteria, as well as providing recommendations on mooring line strength and most importantly – in our opinion – the questions engineers and buyers should be asking their rope supplier,” says Mr Boesten.
“All of us associated with oil and gas know that the complexity and dynamics the sector is renowned for will only intensify as global gas demand ratchets up [and] In this environment, we all want to ensure that mooring these tankers is done as quickly, safely, and cost-efficiently as possible."
The MEG4 guidelines bring many advantages to operators, and provide a good overview of how to develop, operate and manage mooring equipment.
One of the benefits of MEG4 is that owners are given guidelines on how to compare various materials used in mooring lines and ropes, notes Mr Spijkers.
“You can compare apples to apples,” he says, adding: “That was not possible previously – it allows you to choose the best rope based on the mooring application and vessel type.”
However, selection of rope design, fibre and rope management can remain a challenge, especially for LNG carriers.
“To ensure that the rope has a long service life, you have to make sure it is used properly from the first day,” says Mr Boesten, adding: “It also means you have to choose the right fibre for the right use.”
“Your rope manufacturer can help you with your selection. They can also provide you with the right information on how to get the most from it.”
“One of the problems when selecting a rope is that users often only look at break-strength,” says Mr Boesten. “[they say] it has to be 100-tonnes, which sounds like an enormous amount of break strength.”
“However, there’s so much more to selecting the right rope than that.”
Mr Boesten and Mr Spijkers identified five key factors operators must consider when selecting a HMPE fibre for LNG mooring lines.
Five key factors in selecting the right HMPE fibre for mooring lines
Vessels are always in motion even when docked, meaning that the tension placed on mooring lines is constantly changing. This in turn contributes to material fatigue and reduced service life.
Accurate modelling of such fatigue makes it possible to predict the lifetime of a mooring line, reducing the chances of failure and enhancing mooring safety.
DSM has developed models of mooring lines made with its SK78 material under cyclic loading, factoring in parameters such as line construction, minimum breaking load, duration of use and metocean data.
The high abrasive forces mooring lines are subject to during operations mean durability is a key aspect of performance. However, just because a material performs well in a hex bar or yarn-on-yarn abrasion test, this is no guarantee it will maintain the same performance in fairleads.
The SK78 fibre performs best amongst all HMPE in fairlead abrasion tests and it is possible to increase its performance through use of special coatings that can protect against abrasion. Designed to protect lines against external abrasion from rough surfaces such as fairleads, coatings can deliver significant performance increases for lines in mooring situations, resulting in much longer and more reliable lifetimes. Mr Spijkers also noted that owners can perform training with lines to ensure ropes are handled properly and fairleads used correctly to ensure a long service life.
Lines made with synthetic fibres will elongate proportionally with time as they are sensitive to long-term loads, in a process known as creep in which as long molecular chains slide along each other, the lifetime of the line is ultimately reduced.
“All materials creep over time,” says Mr Spijkers. “Tension on your line will cause creep. The rope will elongate and won’t go back to its original length.”
The degree of creep experienced by a line is affected by factors including fibre type, ambient temperature and applied load, with very high loads or high temperatures accelerating the process. This is particularly significant in high molecular-weight polyethylene (HMPE) and increases when using mooring lines in high temperatures such as those seen in the Middle East.
“If you have a long mooring line — say over 50 m – creep is bound to occur,” says Mr Boesten, adding: “This is not something you will notice as a seaman.”
It is also worth noting that there are significant differences in creep resistance and characteristics across different types of HMPE fibre.
If creep is inevitable but difficult for mariners to detect in everyday use, vessel operators must find a way to accurately predict the levels of creep their lines will experience over their lifetime.
“It’s tough for the seaman to monitor the condition of the rope - that’s why we take that risk out of the picture,” says Mr Boesten.
Following a multi-year research programme, DSM has developed a Creep Performance Tool that factors in parameters including HMPE type, temperature, time and tension to predict creep rate and elongations and estimate the creep lifetime of applications made using SK78 fibres.
High and low temperature performance
Ships moor in all types of conditions and LNG carriers are no different, with LNG terminal conditions running the gamut from Arctic levels of coldness to equatorial heat. Novatek, developer of the Yamal LNG project in Siberia, has just announced a second Arctic LNG project known as Arctic LNG 2 will be built in Siberia. Meanwhile, in the Middle East new projects are in the works such as four new mega-trains at the Ras Laffan facility, with would increase its nameplate capacity by over 40%. Given these differing climactic conditions, vessel operators need to know their mooring lines will function reliably regardless of the ambient temperature. And in addition to climactic conditions, movements and tension in mooring lines can generate internal friction, adding heat and increasing the core temperature of a line.
While HMPE fibres can withstand low temperatures, many have a relatively low maximum operating temperature – another key factor when considering line composition.
Feedstock that goes into manufacturing of HMPE fibres can be tailored to meet specific environmental conditions. Mr Boesten explained: “We engineer the fibre so that it meets all of the requirements of mooring LNG carriers anywhere, including the Middle East where temperatures can reach above 40 degrees centigrade.
“We accomplish that through the formulation of the feedstock for the Dyneema fibre,” he added.
Again, modelling and testing has a role to play here, with some manufacturers having developed models to accurately predict the temperature performance behaviour of ropes across a range of environmental conditions. All to increase the safety levels when mooring in warm environments.
Low environmental footprint
Now more than ever, businesses must be aware of the environmental footprint of their operations, and mooring lines must be taken into consideration here too. Selecting a line with a longer life expectancy means less material will be used over the lifetime of a vessel and enable faster mooring times with a reduced need for employment of tugs – with the additional benefit of lowering associated costs.
In addition to this, the material a line is made out of itself has its own carbon footprint, with substances such as steel tending to have higher carbon footprints per unit of strength. Use of alternate materials and carbon footprint savings generated during production can all play their part in reducing environmental impact.
By bearing in mind the above five factors when selecting a material for lines, and combining this with the right line design and a strong maintenance and service package, owners can give themselves an accurate picture of how their lines will perform over time,
One of the big changes in mooring operations in recent years has been increasing adoption of ropes made from synthetic material such as high molecular-weight polyethylene (HMPE), rather than steel wire, as was previously the case. One of the earlier vessels to use a full set of ropes with Dyneema SK78– COSCO Shipping Energy’s VLCC Cospearl Lake – recently marked 10 years of service with the same set of 22 synthetic lines.
Synthetic materials such as HMPE are significantly lighter than steel wire ropes of the same strength and diameter and are safer for crews to handle with reduced likelihood of injuries and less risk of steel wire rope-related backlash.
Mr Boesten sees manufacturers as having a key role to play in ensuring a culture of safety around mooring lines, saying: “In the end, it’s all about being safer. It’s about educating the market.”