Mooring lines
A mooring
line connects an anchor system to an offshore unit. There are basically three
main mooring line types. A mooring line system can consist of chain mooring
lines, wire mooring lines or synthetic fiber ropes or a combination of the
three.
Chain mooring line
In shallower water (up to 100 m) the chain mooring line is the most common type of mooring line. The strong catenary stiffness effect, low elasticity and the highbreaking strength make this mooring line suitable for long-term mooring. The chain line is not applicable in deep water because of the high vertical load. Instead off all chain mooring lines often chain segments are used near fairlead and bottom. The chain mooring line is relatively heavy.
There are two types of chain links namely studless and studded chain links. The studded chain link will better prevent knot formation but will be more sensitive for fatigue failure than the studless chain link.
In shallower water (up to 100 m) the chain mooring line is the most common type of mooring line. The strong catenary stiffness effect, low elasticity and the highbreaking strength make this mooring line suitable for long-term mooring. The chain line is not applicable in deep water because of the high vertical load. Instead off all chain mooring lines often chain segments are used near fairlead and bottom. The chain mooring line is relatively heavy.
There are two types of chain links namely studless and studded chain links. The studded chain link will better prevent knot formation but will be more sensitive for fatigue failure than the studless chain link.
Wire mooring line
The wire mooring line is lighter and has a higher elasticity than a chain mooring line. These applications make that the steel wire mooring system is used for higher water depths. The catenary effect is a lot lower than with a chain mooring line. Typical wire rope constructions are spiral strand, six strand rope and the multiple strand rope. The costs of a wire mooring line are relatively low.
The wire mooring line is lighter and has a higher elasticity than a chain mooring line. These applications make that the steel wire mooring system is used for higher water depths. The catenary effect is a lot lower than with a chain mooring line. Typical wire rope constructions are spiral strand, six strand rope and the multiple strand rope. The costs of a wire mooring line are relatively low.
Synthetic fibre rope
A synthetic fibre rope has a light weight (almost neutrally buoyant). The rope is highly extensible and is useful for very deep water depths. The costs of a synthetic fibre rope are relatively high.
Conventional fibres
The most common materials used for fibre mooring lines are polyester, polyamide, polypropylene and polyethylene. Some ropes are made of combinations of these materials.
Polyester
Polyester is the most durable of the common materials with high strength. Polyester has a low co-efficient of friction and a relatively high melting point. It has good resistance against external abrasion and does not lose strength rapidly due to cyclic loading.
Polyamide (Nylon)
Polyamide rope loses around the ten percent of its strength when wet. It has the highest elasticity of regularly used materials with good temperature and abrasion resistance.
Polypropylene
Polypropylene rope has approximately the same elasticity as polyester rope. Polypropylene has limited temperature resistance and has poor cyclic loading characteristics.
High modules fibres
These fibres are much stronger than conventional synthetic fibres such as polyamide, polyester and polypropylene.
The strength is similar to the wire but has a significantly lower weight.
Aramid fibres
Aramid fibres typically have high strength and low stretch. It does not creep and does not melt but chars at high temperatures. It is susceptible to axial compression but has very good fatigue properties (tension-tension fatigue life)
Liquid Crystal Polymer (LCP)
Liquid Crystal Polymer fibres have high strength and low stretch and excellent resistance to creep and flex fatigue. The fibre has a temperature resistance between HMPE and Aramid (melting point of 300°C). LCP fibres have excellent long-term durability to fatigue, cutting and abrasion.
High modulus polyethylene (HMPE)
High Modulus Polyethylene is a fibre with a high strength per weight ratio and low stretch characteristics. HMPE fibres have very good fatigue and abrasion properties but limited temperature resistance, having a melting point of 147°C and a maximum continuous working temperature of 65°C. HMPE has good resistance to axial compression, a low coefficient of friction and good abrasion resistance.
A synthetic fibre rope has a light weight (almost neutrally buoyant). The rope is highly extensible and is useful for very deep water depths. The costs of a synthetic fibre rope are relatively high.
Conventional fibres
The most common materials used for fibre mooring lines are polyester, polyamide, polypropylene and polyethylene. Some ropes are made of combinations of these materials.
Polyester
Polyester is the most durable of the common materials with high strength. Polyester has a low co-efficient of friction and a relatively high melting point. It has good resistance against external abrasion and does not lose strength rapidly due to cyclic loading.
Polyamide (Nylon)
Polyamide rope loses around the ten percent of its strength when wet. It has the highest elasticity of regularly used materials with good temperature and abrasion resistance.
Polypropylene
Polypropylene rope has approximately the same elasticity as polyester rope. Polypropylene has limited temperature resistance and has poor cyclic loading characteristics.
High modules fibres
These fibres are much stronger than conventional synthetic fibres such as polyamide, polyester and polypropylene.
The strength is similar to the wire but has a significantly lower weight.
Aramid fibres
Aramid fibres typically have high strength and low stretch. It does not creep and does not melt but chars at high temperatures. It is susceptible to axial compression but has very good fatigue properties (tension-tension fatigue life)
Liquid Crystal Polymer (LCP)
Liquid Crystal Polymer fibres have high strength and low stretch and excellent resistance to creep and flex fatigue. The fibre has a temperature resistance between HMPE and Aramid (melting point of 300°C). LCP fibres have excellent long-term durability to fatigue, cutting and abrasion.
High modulus polyethylene (HMPE)
High Modulus Polyethylene is a fibre with a high strength per weight ratio and low stretch characteristics. HMPE fibres have very good fatigue and abrasion properties but limited temperature resistance, having a melting point of 147°C and a maximum continuous working temperature of 65°C. HMPE has good resistance to axial compression, a low coefficient of friction and good abrasion resistance.