What should be considered when choosing mooring ropes?
When a ship is equipped with mooring ropes, the tonnage of the ship, the breaking strength of the ropes, the economic status, and the requirements of ship specifications are usually considered.
First of all, the rope should be selected according to the breaking strength and safety factor of the rope. Ships with different natures of operations use different types and specifications of ropes, but no matter what kind of operation they are used for, they must be controlled within their safe strength range.
If it is a new rope, the breaking strength of steel wire ropes of different specifications can be estimated by the manufacturer’s calculation formula, and the breaking strength of new chemical fiber ropes can be estimated according to the formula T=98KD². Among them, T is the breaking force of the chemical fiber rope, the unit is: N; D is the diameter of the rope, the unit: mm; K is the coefficient, the general polypropylene rope is 0.74~0.85, the nylon rope is 1.19~1.33, and the improved polypropylene rope is 1.10 ~1.21, composite rope is 2.0. At the same time, make sure that the instantaneous maximum force of the rope is within its safety strength. We can obtain the safety strength by dividing the breaking strength of the rope by the safety factor. In most cases, 6 is taken as the safety factor, but it varies due to different uses. Choose between 6~10. The strength of the worn rope will be reduced. You can evaluate the remaining strength of the rope by evaluating the ratio of the number of broken rope strands to the total number of strands, so as to determine whether to use it, the inspection process and method for mooring ropes Det Norske Veritas (DNV) has made recommendations.
Secondly, it is necessary to equip and select ropes in combination with the tonnage of the ship, the nature of the operation and the economic strength. The materials and structures of the ropes are different, the characteristics are also different, and the prices are also different.
Among the plant fiber ropes, the white palm rope has high strength and is easy to float on the water surface. The white palm rope is softer, has good elasticity, and is also better in moisture resistance and corrosion resistance. The white palm rope is often used, and the sisal rope and oil hemp rope are less selected. Among the chemical fiber ropes, nylon rope has the highest strength, is not easy to wear, and is not easy to be corroded by acid, alkali, oil, etc., but nylon has a large elongation and elasticity, cannot be exposed to the sun, and its weather resistance is not strong. Disadvantages such as water absorption.
The weight and volume of steel wire ropes are superior to fiber ropes. In addition to high safety strength, they are also resistant to tension and impact, not easy to wear, fatigue and corrosion, soft, safe and reliable to use. Steel wire ropes exhibit various properties due to different production materials and structural structures. Generally, steel wire ropes made of galvanized materials and processed by cold drawing can be used in terms of materials, and their tensile strength exceeds 140kg/mm². In terms of structure, most marine mooring ropes use soft steel wire ropes, and their performance is also related to their composition.
According to the size of the ship, the strength of 500KN can be used as the boundary. If the strength is less than this strength, it is recommended to use 7 fiber cores and 6×24 steel wire ropes. The rope is wound on the winch drum, and the steel wire rope can be used as the core structure rope for ships with larger tonnage, because it can maintain high strength performance as long as it does not exceed the ratio despite bending. By comparison, the strength of the steel wire rope is greater than that of the fiber rope under the same diameter. Large and medium-sized ships generally use more steel wire ropes for the main rope, and fiber ropes for the spare ropes. Small ships usually use fiber ropes as the main rope.
Thirdly, ships are equipped with ropes according to the requirements of classification societies. The current number of outfitting of ships is the basis for the number, length and strength of ropes required by various classification societies in the world. China Classification Society has also made supplementary regulations. and 1.2 as the boundary, when it is greater than 0.9, it is necessary to add 1 to 3 ropes according to the ratio. The ship organization fully considers the requirements of the specification, increases the investment of mooring ropes, and reduces the risk of ropes from the organizational level.