Mooring Line MBL
marineusedgood offers mooring ropes. Ship mooring remains an essential function of the shipping industry. There are a variety of standards, guidelines and recommendations for mooring systems, ranging from mooring equipment and arrangements to mooring practices. However, incidents of injuries to ships and dock personnel still occurred during mooring. OCIMF has made major revisions to the mooring guidelines in its fourth edition, with a focus on the safety of ships and terminal personnel. It addresses four important areas of focus:
● Lessons learned from the accident, most notably from the HMSF mooring line accident.
● Human-centred mooring design and human factors in mooring operations.
● International Maritime Organization (IMO) new and developing regulations and guidance on mooring safety.
• Alternative mooring techniques and how to safely incorporate them into the design of ship and dock mooring systems.
Mooring line
1. Introduction
Mooring lines are an integral part of any mooring system. This section of guidance will help interested parties understand the lifespan of ropes and build a stronger framework for how to select, maintain and replace ropes. The complete life cycle of each rope should be recorded on the ship operator's rope management plan (LMP), which should be forwarded to future ship operators.
1.1 Life cycle of mooring lines
The life cycle of a mooring line consists of four stages
During the selection phase, the user works with the manufacturer and/or supplier to gain a better understanding of the cable performance and maintenance requirements, and finally purchase the cable. A common framework for how ship operators and manufacturers can share this information is detailed in Appendix B: Guidelines for Procurement and Testing of Mooring Lines and Stern Lines.
During the operational phase, the LMP manages the mooring lines during use in conjunction with the vessel's SMS and PMS. The aim of the LMP is to record the history of the purchase and use of mooring lines so that ship operators can make them obsolete before they fail.
Finally, there is a learning phase. It is about understanding mooring line performance and mooring line management and improvements in technology. It involves communication between the manufacturer and the user, i.e. feedback from the manufacturer/test facility to the user on cable test results and development, and user feedback on the performance of the cable during use. This feedback is used to predict the rope phase-out plan and to improve the design and performance of the mooring ropes.
1.2 Design of mooring system and selection of mooring ropes
1.2.1 Strength criteria and terminology
The Ship Design Minimum Breaking Load (Machine Design MBL) is the minimum breaking load for new, dry mooring lines, in compliance with the binding requirements of Standard Environmental Conditions. The ship design MBL is the core parameter to which all other components of the ship's mooring system are dimensioned with specified tolerances.
The Line Design Breaking Force (LDBF) is the minimum breaking force of a new, dry, spliced mooring line when tested in accordance with Appendix B. This applies to all mooring line and tail line materials, but those made of nylon are wet and splice tested. This value is declared by the manufacturer on each mooring rope's certificate (see Appendix B) and is noted on the manufacturer's rope data sheet, as described in Appendix B. When choosing a rope, the LDBF of the rope should be 100%-105% of the ship's design MBL.
The LDBF of nylon (polyamide fibers also known as polyamide) mooring ropes should be specified for the wet break test because nylon changes its strength properties once exposed to water and usually does not dry completely to its original structural state.
The Working Load Limit (WLL) is the maximum load that the mooring line should be subjected to in operational use, calculated according to standard environmental conditions. WLL is expressed as a percentage of the ship's design MBL and should be used as a limit value in both ship design and mooring operations analysis. WLL should not be exceeded during rope operation.
Just as SWL is the limit for fixed equipment, the WLL value will also be used as the limit for standard ambient conditions and mooring layouts when designing the mooring system. The WLL for steel wire ropes is 55% of the ship's design MBL, while the WLL for all other ropes (synthetic fibers) is 50% of the ship's design MBL.
Although it is technically more accurate to associate the WLL with a specific mooring LDBF, the differences between the MBL and LDBF of ship designs from different manufacturers are negligible. Using the Ship Design MBL allows analysis and comparison using a single value.
Design Basis Load (DBL) is the design load of the fitting, calculated by multiplying the ship's design MBL by the geometric factor (GF).
1.2: Comparison of working range of mooring lines
The usual operating load should be lower than WLL. Working loads that meet and exceed standard environmental limits will increase the rate of cable fatigue and damage, and may result in accidental or accelerated cable damage.
1.2.2 Stiffness of mooring lines
Typical load elongation characteristics of steel cable, conventional fiber and HMSF cable. Traditional fiber cables such as nylon, polyester and polypropylene have lower stiffness under the same load compared to steel cables and high modulus synthetic fiber (HMSF) cables.
Typical load/tensile stiffness properties of steel and synthetic fiber cables up to 50% LDBF for synthetic fiber cables and up to 55% LDBF for steel cables.
The stiffness properties of fiber ropes and tail ropes can vary widely depending on various factors, such as: number of load cycles, range of cyclic loads, relaxation time, speed of load change, whether the rope is wet or dry, etc. For example, in an open position and with high rope loads, the mooring ropes are subject to constant periodic loads due to the motion of the waves. This increased load and cyclic loading increases the stiffness of the cable, resulting in a steeper curve than shown above. This is especially true at higher average loads. The ship operator should discuss the ship's environmental conditions with the rope manufacturer to address this phenomenon in the system design.
1.2.3 Influence of D/d and deck equipment on rope performance
The D/d ratio is the bending diameter of the mooring equipment divided by the diameter of the mooring line.
Any bending of the rope will reduce its breaking strength, and repeated bending will shorten the life of the mooring rope. In order to maximize the strength and service life of the mooring line, the D/d ratio should be as large as possible.
It is recommended that designers of mooring arrangements aim for a D/d of at least 15 for mooring fittings. This ensures that performance degradation due to bending is kept to a minimum. D/d resulting from deck equipment design is only one of the factors affecting the condition and service life of mooring lines, but it is a fundamental consideration during the assessment of rope strength and expected service life.
