Mooring and the Mooring Rope - Essay Example

Student’s Name Professor’s Name Business 14th March 2012 Mooring Rope and the Risk of Mooring Introduction The mooring rope serves an important function in the mooring system of any naval vessel. The mooring system consists primarily of lines or ropes, anchors, and to some extent, floats. Beveridge defines the mooring rope as the thick hawsers or lines that are fixed to deck fittings on one end of marine vessels and to fittings on the shore like; cleats, bollards, or rings, on the other end (102). This paper discusses the different types mooring rope, their properties, and the risk of mooring. Mooring and the Mooring Rope Mooring has been described as the act of fastening a vessel; marine or aerial, to a fixed object like a pier, quay, bollard, anchor buoy, or to the sea bed (Beveridge 102). Mooring is usually achieved by using thick mooring ropes, lines or hawsers which are fixed to deck fittings on one end of the naval vessel and to shore fittings like cleats on the other end. Mooring can be by permanent anchor which is realized by the use of a fixed anchor at the bottom of a waterway using a chain, or a cable running to a glide on the surface. These chains, lines, or cables used in permanent mooring are referred to as “a rode” and this permanent type of mooring is often referred to as a rode mooring system (Beveridge 102). Mooring has been practised since the 15th century and vessels at this time were being made to fasten to a variety of shore fixtures like rocks, trees and specially constructed areas like quays and piers. Mooring often requires cooperation between the people on the marine vessel and those on the pier at shore. Large vessels often use heavy mooring ropes which are passed to the people on the shore via weighted jam-packed lines. These ropes are then attached to the bollard and pulled tight. In larger sea vessels, this tightening is accomplished through the help of heavy machinery known as capstans or mooring winches. Heavier cargo ships make use of more than a dozen mooring ropes and sailboats are often used to take five or six of these mooring ropes. There are usually five types of mooring ropes; natural fibre lines, synthetic fibre lines, steel wires, combi-lines and chains (Gaythwaite 5). 1. Natural Fibre Mooring Rope Natural fibre ropes are usually manufactured from manila, sisal, or any other plant fibre. According to Gaythwaite, natural fibres have been used as traditional mooring ropes for quite a long time (11). These ropes are relatively cheap, and easy to handle; especially when they are dry, and have a moderate resistance against abrasion. These fibre ropes however, have a high sensitivity to high temperatures; often caused by sunlight and friction during use. Additionally, these fibre ropes are also extremely sensitive to chemicals and easily absorb water; this makes their handling cumbersome in wet conditions. These ropes from natural fibres have a low load-diameter ratio and a relatively short lifespan. The low load-diameter ratio makes them incapable of absorbing peak loads. 2. Synthetic Fibre Mooring Rope Synthetic fibre ropes are manufactured from nylon, terylene, Polyethylene, HMPE, polyester, dyneema, Kevlar and polypropylene among others. According to Gaythwaite, synthetic mooring ropes are produced from molecules that replicate more or less ad infinitum (13). These ropes are produced as split films, are chemically inert, and posses a low density making them to easily float. Synthetic ropes compared to natural fibres, have a high load-diameter ratio, and are relatively lighter and easier to maintain. They also have a longer lifetime and are relatively cheaper than natural fibres. Additionally, the elongation of synthetic fibre ropes is rather large and their strength characteristics are greatly affected by high temperatures often caused by friction among others. According to Gaythwaite, Nylon is commonly used because it is easy to work with and last for a good number of years (11). The only problem it has is its elasticity which can lead to breakage or snap-back when it is highly-stressed. Kevlar and dyneema mooring ropes on the other hand are safer to use. Their only problem is that they tend to sink rather than float on water and they are costly compared to other types of synthetic fibre mooring ropes. 3. Steel Wires Mooring Rope Steel wire or wire ropes are often hard to maintain and handle. Despite the hard maintenance, this rope type is relatively cheap and can have a long lifetime if properly maintained. Steel wire ropes are rather stiff, have a high load-diameter ratio, and posses a low elongation in proportion to the force of the mooring rope. The stiffness and the weight of steel wires makes them difficult to handle and unsuitable when exposed to dynamic load conditions (Gaythwaite 12). This stiffness in addition causes ship displacements due to the high line forces that take places within the ropes. Due to this, these ropes pose a higher potential of breakage. Steel wire ropes also require good maintenance through lubrication and cleaning, to prevent their corrosion and lengthen their lifetime. These ropes have a higher elasticity and posses a lower weight, a feature which makes them advantageous especially in deep waters using longer lines. 4. Chain Links Mooring Rope Apart from the synthetic and non-synthetic types, mooring ropes can further be made out of steel chain links or chains. Chain mooring ropes forms the most common product applied in mooring lines and they can either be studlink or studless. Studless chains are more suited for permanent moorings. 5. Combi-Lines Mooring Rope Combi-lines on the other hand are a combination of synthetic fibres or tails with steel wires. This combination is particularly used for mooring large sea vessels like tankers. According to Gaythwaite, these ropes posses an excellent shock absorption characteristic given that they have synthetic fibre tails (12). The connection between the synthetic tail and the steel wire is carried out using a proper shackle, which protects the tail eyes with a plastic or leather sheathing. It is a requirement that these tails should not be longer than 10 metres to minimize the need of replacement in case of damage and excessive ship motions. Longer tails are required to obtain the needed flexibility of the mooring system at exposed berths. Other types of combi-lines are the chain at the bottom, chain in between the synthetic fibres, and the chain at the top of the rope combination. In this type of combi-line or rope, the characteristic of each material is used in the best way possible in the rope. The combination of mooring lines made using synthetic line and wire rope mostly results into a hawser. The hawser is usually more elastic and easier to deal with than a wire rope, but less elastic compared to the synthetic rope. Another type of combination is the combination of fibres like polypropylene or polyester and polythene or polyethylene which are used in many industrial maritime applications and in mooring smaller crafts. Mooring ropes can further be grouped according to their construction, for example plaited or braided, laid ropes, stranded, or parallel yarn (Gaythwaite 15). The plaited ropes are often described as square braids and can either be double braided or three stranded. These ropes are usually produced on a plaiting machine that has eight reels, with each reel having one strand. Groups of two reels usually interweave as a pair around the other pairs of reels to come up with an eight strands of rope having a square cross-section. These plaited ropes are torque free and come in two categories of eight or twelve strands. Synthetic fibres are usually constructed using double braid or three strand ropes due to their comparatively low cost. Risk of Mooring Studies have shown that synthetic ropes were being damaged in the waters by fish. According to Steele, Thorpee, and Turekian, an analysis of many failed lines has revealed bite patterns and tooth fragments of fish that were used to identify which fish was responsible for damaging of the ropes (97). This fish attack has made the prevention of mooring failure a bit difficult in addition to increase its risks, unless lines that could resist fish bites were used. Apart from mooring failure due to fish attacks there are risks of occurrence of accidents during mooring majorly through; collision, rope and chain failure, abrasion, fatigue, anchor failure storms, and corrosion among others (Steele, Thorpee, and Turekian 100). Fatigue damage mostly occurs at sheaves or connections followed by overloading of the marine vessels. Corrosion on the other hand happens when the wire rope has been in contact with sea water for quite some long time. Higher corrosion rate makes the mooring system incapable of resisting storm loading. Anchor failure is another risk associated with mooring. This risk falls into three categories; anchor wire breaks, insufficient holding, and pennant wire breaks. Rope failure on the other hand, takes place due to the fatigue process and the loading that is related to bending tension experienced in the ropes. Chain failure occurs due to relative motion or rotation between chain links with most of the crucial failure taking place on the leeside lines. Abrasion risks occur when wire ropes are in contact with the sea floor or when each wire moves proportionate to one another and to the bearing surface with the bending of the ropes. Abrasion can also be caused by rocks or debris on the sea bed. Other failure risks are brought about by hardware failures, or failures in the winching equipments among others. Statistics indicate that the risk of storms has the highest impact on the assets in the vessels given that it leads to corrosion. Corrosion consequently has been ranked among the main causes of pollution in the seas. According to Lawson, corrosion failure creates serious pollution risks to the ocean or sea since these wires ropes are in constant contact with the sea water (27). Furthermore, corrosion can lead to crack of chain and wire. Another risk that is evident in the traffic is the risk of collision, which can be caused by neighbouring facilities or insufficient holding. The consequence of the occurrence of this risk is damage to the vessels. The risk presented in mooring winches is the failure of chains and ropes which is caused by the ingress of ropes and chains with sea water for over some time period. The consequence of this is the delay in operation activities. There is also a risk of snap-back especially when the mooring ropes are exposed to stress and break, or if the tension in them is rapidly released. This break can take place in instances where the mooring rope is jammed on a bollard under the ropes of another vessel or is somehow obstructed in any way and abruptly clears under tension. In this clearance under tension, the free ends of the rope tend to ‘oscillate’ violently resulting into a snap-back. In most naval vessels, there are zones known as snap-back zones where mooring crews are usually advised to avoid standing in or close to, since they are danger areas. Mooring crews can incur serious injuries if the snap-back finds them in these snap-back zone areas. According to Lawson a crew member was seriously injured when a roller fairlead detached itself from its pedestal while it was bearing the load of a mooring line that was under tension (27). This crew member happened to be standing in the snap-back zone when the oscillating rope struck him and hurled him into the ship’s foremast causing him severe head injuries. Mooring areas have been additionally observed to be containing many trip and obstruction vulnerabilities. They have also been viewed as hazardous areas majorly because of the great loads that the mooring lines are often subjected to. This means that the mooring ropes are highly likely to part with minimal warning and great force increasing the risk of injury in the mooring areas. Conclusion In conclusion mooring ropes are the thick lines that are usually fixed to deck fittings on one end of marine vessels and to fittings on the sea shore like; cleats, bollards, or rings on the other end. Mooring ropes can be classified into five types; natural fibre lines, synthetic fibre lines, steel wires, combi-lines and chains. Natural fibre ropes are made from manila, sisal, or any other plant fibre. They are easy to handle when dry and have a moderate resistance against abrasion. Synthetic fibres are made from nylon, Polyethylene, HMPE, and polyester among others. They are elastic but are greatly affected by high temperatures. Steel wire ropes on the other hand are stiff and weighty making them difficult to handle and unsuitable under dynamic load conditions. Chain links; which are either studlink or studless, are used mostly for permanent moorings. Combi-lines on the other hand involve a combination of synthetic fibres with steel wires. Risks of mooring are often present in instances like; attack of ropes by fish causing mooring failure; accidents due to collision, rope and chain failure, fatigue, and abrasion among others; storms; and in snap-back situations among others. Works Cited Beveridge, Malcom. Cage Aquaculture. Carlton, VIC: Blackwell Publishing, 2006. Print. Gaythwaite, John. Design of Marine Facilities, for the Berthing, Mooring, and Repair Vessels. Reston, VI: American Society of Civil Engineers, 2005. Print. Lawson, Thomas. Fundamentals of Aquacultural Engineering. Norwell, MA: Kluwer Academics, 2004. Print. Steele, John, Steeve Thorpe, and Karl Turekian. Measurement Techniques, Platforms & Sensors. London: Academic Press, 2009. Print. Read More