Mooring Lines
An efficient mooring system should provide maximum stabilization to a platform whilst powerful forces (wind, tide, current, and waves) act against it. Providing sufficient resistance against these forces can be essential to the safety of the ship.
Mooring lines play a large role in the overall mooring system. Below we will discuss a few factors to consider when determining the proper construction and material of your mooring line.
Line Construction
Rope is normally described as number of strands X number of wires per strand
(For example, 6 X 41)
Number of Strands
- 3-Strand: This is the most common construction of rope but its tendency to hockle (kink) reduces its strength, making it a poor choice for mooring.
- 6-Strand: This rope has a core structure (multiple strands surrounding a single strand) and is less prone to hockling. This construction is sometimes used for mooring, such as with Aramid rope.
- 8-Strand: This rope is also commonly referred to as square braid or plaited and is a good structure for mooring lines. It is not prone to hockling, is more resilient than regularly twisted rope, and is torque free.
- Double braid rope: This rope has very good abrasion resistance, is relatively flexible, has a good strength to weight ratio, and has a good structure for mooring lines.
Number of Wires Per Strand
- More wires will give the line greater flexibility and fatigue resistance but less abrasion resistance.
- Fewer wires will give the line less flexibility and fatigue resistance but greater abrasion resistance.
Rope Terminations - Since most of the handling and abrasion occurs on the ends of a rope, termination methods play an important role on the life of the rope.
Line Material
There are two distinct materials to choose from when designing a mooring system: steel wire rope and synthetic fiber rope. The table below compares steel with two commonly used High Modulus Synthetic Fibers (HSMFs).
Material |
Estimated Life |
Strength (Compared
to Steel) |
UV
Resistance |
Approx. Weight (per 100') |
Steel |
1.5 years |
1 |
Very Good |
185 lbs. |
Kevlar |
5 years |
5 |
Poor |
36 lbs. |
Dyneema© |
5 years |
10 |
Good |
26 lbs. |
- Steel Wire Rope - Steel is a commonly used material for mooring lines because it is strong and can be bought at low cost. There are, however, disadvantages of using steel wire lines
such as:
- Steel requires a large amount of space for storage
- The lubrication process necessary for steel lines is inconvenient because of required disposal methods due to pollution
- Steel lines are vulnerable to corrosion and have high maintenance costs
- Recoil from stored energy in steel lines is extremely dangerous for the crew
- Synthetic Fiber Rope - There are many types of synthetic rope used with mooring lines, some more advantageous than others. Synthetic lines have torque-free construction, are easy to check and repair in field, do not corrode (eliminating a lubrication process), and are lighter and more flexible than steel. The main disadvantage of synthetic line, in general, is its weak resistance to abrasion. This, however, can be remedied by protective jackets or coatings on the line. Below we will look at the characteristics of some common synthetic fiber lines.
- Nylon: This fiber has excellent resistance to continued loading, high chemical resistance, high elasticity (making it useful in Ship-to-Ship operations), and doesn't float. However, nylon's wet strength is 80% that of its dry strength, has a lower fatigue life, and can be damaged by acids. Note: Nylon is often quoted by its dry strength when being compared to other synthetics, therefore a decrease in strength should be taken into account when considering nylon.
- Polyester: This fiber has exceptional abrasion resistance, a relatively high melting point (making it resistant to fusion), high chemical resistance, high dry and wet strength, does not float, and is the most durable of common materials. Polyester lasts approximately 190 times longer than nylon and 570 times longer than polypropylene. However, polyester can be damaged by alkalis.
- Polypropylene - This fiber is commonly used for messenger lines, but is not recommended for mooring lines. Although it is lightweight and low cost, polypropylene is weaker than polyester, has a low melting point, can be damaged by UV rays (by actinic degradation), has reduced cyclic load characteristics, and can fuse under high friction.
- Aramid - This fiber is classified as a High Modulus Synthetic Fiber (HMSF) and it is considerably stronger than most common synthetic fibers (like the ones listed above). Aramid has high strength and low stretch, excellent cut resistance, is light weight, does not float, good chemical and thermal stability (chars, not melts, at high temperatures), and superb tension-tension strength. However, aramid fiber has very poor UV resistance, poor abrasion resistance, and is one of the more costly synthetic fibers. If aramid fiber rope is fitted with a protective jacket, it greatly increases its strength translation. Aramid is often referred to as "Kevlar."
- High Modulus Polyethelene (HMPE): This fiber is another HMSF and is very effective in mooring lines. Its high strength per weight ratio makes it fifty times stronger than steel and forty percent stronger than Aramid. This fiber has very good fatigue and abrasion resistance, low stretch, and good UV resistance. However, HMPE may require fiber rope tails to provide extra elasticity, depending on the type of operation. Common HMPE fibers are Dyneema and Spectra.
Below is a chart comparing abrasion and cost of common synthetic fibers
Line Material |
Abrasion Resistance (Wet/Dry) |
Cost |
Dyneema© |
Excellent/Good |
High |
Aramid |
None/Poor |
High |
Nylon |
Poor/Excellent |
Moderate |
Polyester |
Good/Good |
Moderate |
Factors such as abrasion and UV damage can sometimes shorten the lifespan of a synthetic rope. To prevent this, protective jackets or protective coatings should be used. With the correct termination methods and protection, synthetic fiber should show a 100% strength translation.
PLEASE NOTE: Each mooring system is subjected to different environmental conditions. Therefore, a complete mooring analysis is recommended to determine the best material and proper rope construction for your mooring lines. Also, please be aware that synthetic technology is still growing and information should be checked to verify its consistency with current technology.
References
- OCIMF, Effective Mooring, Witherby & Co., London, 2005.
- OCIMF, Mooring Equipment Guidelines, Witherby & Co., London, 1997.
- Advanced Design Consulting, Inc., Advanced Fibers, Anti-Friction Materials and Jackets for Navy Rope. www.adc9001.com.
