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TitleOCIMF (Snap Back Danger)
TagsChemistry Materials Rope Chemicals Wear
File Size725.2 KB
Total Pages11
Table of Contents
                            6.3.3 Bend Radius
		6.3.4 Identification
		6.3.5 Handling, Maintenance and Inspection
		6.3.6 Standard Specifications
                        
Document Text Contents
Page 1

The three-strand rope is the most common form of twisted rope. It is adequate for some tasks, but
is prone to hockling (see figure 6.7). This significantly reduces strength, making it a bad choice for
use as a mooring rope.

The six-strand with core structure, used in Jetkore and Atlas ropes, is a twisted rope similar to
conventional wire rope. It is not as prone to hockling as three-strand rope and is sometimes used
for mooring lines.

The eight-strand rope, sometimes called square braid or plaited, is constructed of braided pairs of
strands. It has essentially the same strength as a three-strand rope of the same size. It does not
hockle and is more durable than twisted rope. It is a good rope structure for mooring lines and for
most other purposes on ships.

Double braid rope, sometimes called braid-on-braid, is constructed of a core braided of many small
strands and surrounded by a cover which is also braided of many small strands. It is generally
stronger than other ropes of the same diameter because the structure is denser. It is commonly used
for mooring hawsers at single point moorings (SPMs) and for tails on wire ropes.

6.3.3 Bend Radius

The strength loss due to bending is apparently not as critical for conventional fibre lines as for wire
given their ability to allow migration of load. Newer types of synthetic fibres such as the aramid
types are more bend radius sensitive. As for all fibres the strength loss and durability factors may
depend upon the specific material and construction. The rope manufacturers guidelines should be
consulted for each specific application.

6.3.4 Identification

If the composition of a mooring rope is not known, certain characteristics of appearance, burn
properties and density may help to identify the material.

Fibre appearance
Polypropylene fibres are usually coarse and thicker than a human hair and are normally
flat instead of round. Polypropylene is often dyed black to provide resistance to sunlight.
Sometimes other dye colours, such as yellow or orange, are used.

Thus a coloured or black rope with coarse flat fibres is probably polypropylene. The only
exceptions to this are polyethylene and Karat, which can have a similar appearance.

Since polyethylene ropes may not be quite as strong as polypropylene, a burn test may be
necessary to distinguish between the two for critical applications where there is a possibility
that polyethylene ropes have been furnished.

Karat rope fibres are similar in appearance to polypropylene. They are straw coloured, as
opposed to the distinctive yellow colour of some dyed polypropylene fibres.

Nylon and polyester fibres are almost always very thin and fine, distinguishing them from
polypropylene. However, it is virtually impossible to distinguish between nylon and polyester
by appearance. New nylon fibres have a bluish tinge, but even experts cannot always tell them
apart, especially if the rope has been used. The only reliable way is by a burn test.

Aramid ropes are normally covered by a braid strand or extruded plastic jacket. The aramid
fibres are very fine and straw coloured.

Density
Polypropylene and polyethylene are lighter than water and can readily be distinguished from
polyester and nylon by their density. Place several fibres or yarns in a glass or dish of water
and stir, if necessary, to remove any air bubbles adhering to the sample.

If the sample floats, it is polypropylene or polyethylene. If it sinks, it is polyester or nylon.

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Synthetic lines are not very resistant to cuts and abrasion, and should not be exposed to
conditions which might damage them. If they are used in fairleads previously used with wires,
make certain the fairleads have not become grooved or roughened by the wires. It may be
necessary to grind the fairleads smooth.

Care should be taken when dragging synthetic lines along a deck. Avoid sharp edges and
rough surfaces. Small lines should be carried instead of dragged when possible.

When dirt, grit or rust particles are allowed to cling to and penetrate into synthetic ropes,
internal abrasion will result. The rope should be brushed or cleaned before storing.

Twisted ropes can be harmed by kinking, which may form into hockles if not properly re-
moved. When a kink forms, the load must be removed and the kink gently worked out.

Twisted rope must be coiled in the proper direction. Most lines are right-hand lay and should
be coiled clockwise. When removing new rope from a coil, suspend the coil on a shaft and
rotate it.

Winch-mounted synthetic lines should be end-for-ended after about two years to distribute
wear, unless inspection dictates a shorter schedule.

Storage
Synthetic lines should be stored in clean, cool, dry surroundings. Excessive heat can damage
synthetic fibres, especially polypropylene and polyethylene. Do not store synthetic ropes near
steam pipes or against bulkheads which may reach high temperatures.

Ultraviolet rays from sunshine can damage fibres. Polypropylene and polyethylene are especially
vulnerable. The potential degree of damage increases as rope size decreases. Never store
small polypropylene or polyethylene ropes in direct sunlight.

Synthetic fibres are also subject to chemical damage. Their susceptibility depends on the
chemical and the fibre. Nylon is attacked by acids and bleaching agents. Polyester is attacked
by some alkalis. Industrial solvents, including paint thinners, will damage most synthetic lines
if they are stored in paint lockers or near paints and paint fumes.

Oil and petroleum products will not normally damage synthetic fibres. Nonetheless, care
should be taken to avoid contact with them. If a rope becomes oily, it is more difficult to
handle. Dirt and grit will adhere to the oil and cause internal abrasion of the rope. If the
line becomes oily or greasy, it should be scrubbed with fresh water and a paste-like mixture
of granulated soap. For heavy accumulations of oil and grease scrub the line with a solvent
such as mineral spirits; then rinse it with a solution of soap and fresh' water.

Inspection and replacement
Synthetic lines should be examined frequently while in service. They should be checked for
obvious signs of deterioration before each use and undergo a thorough inspection at least once
each year.

Some signs of damage such as hockling, cuts, surface abrasion and fusion are readily visible.
Others are not as evident. While it is not possible to prescribe definitive retirement criteria,
the following sections discuss the types of damage and wear experienced by ropes and provide
general guidelines.

Cuts
The degree of damage caused by a cut depends on the depth and extent of the cut and on the
rope construction. Each strand of a thrcc-strand, six-strand or eight-strand rope carries a
substantial portion of the load. If any one strand is significantly weakened by a cut, then the
strength of the entire rope is significantly decreased. In general, any cut which penetrates
through 25% of the area of one or more strands critically weakens the rope. The rope should
be cut and spliced or retired.

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Double braid ropes have many more strands. In conventional synthetic fibre double braid
ropes the cover and the core each carry about 50% of the load. Thus, one or several cut strands
in the cover normally do not significantly reduce the strength. If more than about 10% of the
entire cover strands are cut, then the double braid rope should be retired.

In the case of the newer types of synthetic line such as the aramid fibres, almost the entire
load is carried by the inner core. Therefore, should the external sheath be damaged the internal
load bearing fibres may rapidly degrade through exposure to ultraviolet rays or through
mechanical wear. It is consequently advisable to inspect these lines on a regular basis with a
view to pre-emptive repair as necessary.

External abrasion and fusion
A moderate amount of external abrasion is normal and can be tolerated in most synthetic
ropes. The abrasion is evident as a general fuzzy appearance. If abrasion reduces the solid
diameter by more than about 5%, then the rope should be retired. If the abrasion is localised
and the remainder of the rope is in good condition, then the rope may be respliced.

Severe localised abrasion may be of concern. Severe abrasion of even one strand in three-
strand, six-strand or eight-strand rope can significantly reduce the strength of the strand and
upset the rope structure. The abrasion affects a number of yarns as it extends along the strand,
so the degree of damage is not necessarily proportional to the depth of abrasion. If the
abrasion on any one strand penetrates more than about 15% of the strand area, the rope
should be cut and spliced.

Internal abrasion
Internal abrasion is caused by the strands and yarns rubbing against each other as the rope
undergoes cyclic loading. It is a form of fatigue entirely different from the type of fatigue
experienced in metals.

The rope should be examined for signs of inter-strand abrasion. Carefully open the structure
of three-strand, six-strand or eight-strand rope to examine the surfaces of the strands at points
where they contact each other. A general fuzzy appearance at the points where strands rub
against each other is an indication of moderate internal abrasion. If the abrasion has pro-
gressed to the extent that some yarns are worn through, the rope should be retired.

Internal abrasion in double braid rope is harder to detect because it may appear to be
normal external abrasion. Closely examine the broken yarns which appear on the strands at the
surface. If they have broken in the valleys between the strands, then it is internal abrasion.
This internal abrasion probably extends throughout the entire rope structure. If it is severe,
it has significantly decreased the rope strength and the rope should be retired.

Hockling
Hockling normally occurs only in twisted ropes. A hockle resembles a knot in the rope, as
shown in Fig. 6.7. Hockles greatly reduce the strength of the rope. When a hockle appears in
a rope which is otherwise in good condition, it should be cut out and the rope spliced.

Hockles occasionally occur in the individual strands of three-strand, six-strand and eight-strand
ropes. Such hockles upset the balance of load carried by the strands. The rope should be cut
and spliced.

Broken Core
The core of a double braid rope may break under high load without resulting in immediate
rope failure. Under load, the rope will have a smaller diameter at the point of core break.
Under no load, the rope may bend more freely at this point. If the core is broken, the double
braid rope should be retired.

Ultraviolet Damage
Ultraviolet rays from the sun destroy the strength of polypropylene and polyethylene fibres.
The weakened fibres can easily be rubbed off the surface of the rope. The significance of the

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