- 115 Downloads
It gets its name from the shape of the free hanging line, one end of the mooring line is connected to the floating structure, and the other end is fixed at the anchor point of the seabed; the part that is suspended in the seawater will take a catenary shape, which is similar to the rope that is fixed at both ends and freely placed under the uniform force. The catenary forms can be seen everywhere in nature; when you wake up in the morning to see spider webs covered with water drops, when you see the ropes on the suspension bridge, and when you see the wires between the two poles, you know what kind of shape are they? The answer is the catenary. As early as 1690, Jacobi Bernoulli proposed the famous “catenary problem,” but it was not until Newton and Leibniz invented the calculus to get the correct answer. The catenary is a magical curve that has been used extensively in buildings, bridges, ship, and ocean platform moorings (Wren et al. 1989).
In marine engineering, spread catenary mooring systems and single-point catenary mooring systems are two common arrangements (Skop 1988). Mono-hull ship and semisubmersible platforms have traditionally been moored using a spread catenary mooring system, and the points are connected to different positions on the platform so that the platform cannot rotate freely, and the orientation of the platform remains basically unchanged. In some cases, excessive displacement caused by environmental forces may cause large loads on the mooring system. In order to overcome this drawback, a single-point mooring system has been developed which is characterized in that a plurality of mooring lines are connected at a point on the longitudinal centerline of the platform. The platform will have a wind vane effect to reduce the environmental loads caused by wind, current, and waves. Single-point moorings are used primarily for ship-shaped vessels. They allow the vessel to weather vane. This is necessary to minimize environmental loads on the ship-shaped vessel by heading into the prevailing weather (API 2005).
Theoretical Analysis Methods
d: the water depth plus the distance between sea level and the fairlead in [m]
F: the force applied to the mooring line at the fairlead in [t]
S: the unit weight of the mooring line in water in [t/m]
The abovementioned mooring line can be composed of a single component material or a combination of multiple components on the basis of different needs. The most common product used for mooring lines is chain which is available in different diameters and grades. Although the steel chain is resistant to abrasion and is not easily damaged, the quality is high and the cost is high. Since the deep water mooring system has strict weight restrictions, the full chain system is generally not used. When compared to chain, wire rope has a lower weight than chain, for the same breaking load and a higher elasticity. The most common combination method is three components: top chain + steel cable + bottom chain. The use of multicomponent mooring lines needs to focus on the length and diameter of each component and the choice of buoys or weights. In this way, the top and bottom chains prevent damage to the top mooring line from long-term friction of the fairlead and subsea portion as well as undulating collisions (Smith and Macfarlane 2001).
Selection of Anchors
Development and Challenges
In general, the traditional catenary mooring positioning system occupies an important position in the mooring positioning system of the mobile platform. However, the influencing factors to be considered in deep-water mooring systems are more complicated than those in shallow water, and their research needs more attention and consideration. On the one hand, the traditional steel anchor chain and wire rope system, with the increase of water depth, its self-weight increases, and horizontal stiffness decreases sharply, while the weight increase leads to the increase of the cost, and it will also cause the multifaceted impact about deployment and recovery of the mooring system. On the other hand, the new concept and new materials for the deep-water mooring system are continuously developed. For example, polyester ropes with light weight and high strength and weight ratio have replaced steel cables in some cases. The application of the new material can not only reduce the weight of the mooring system, but also its mechanical properties are improved to suit deep water. The lightweight catenary mooring system developed in recent years overcomes many shortcomings of the traditional catenary mooring system in deep sea mooring, but neither the lightweight catenary line nor the traditional can’t overcome the excessive deviation of the horizontal level under sea conditions; the greater the water depth, the more obvious this shortcoming. In addition, the use of such mooring systems in deep water requires a large number of mooring lines of larger diameter, which can take up a lot of space on the installation vessel. There are many problems that need to be solved, and the effectiveness and economy of the multiple component systems also need further study.
- American Petroleum Institute (API) (2005) Design and analysis of station keeping systems for floating structures. API recommended practice 2SK, 3rd edn. API Publishing Services, Washington, DCGoogle Scholar
- Banfield SJ, Flory JF (2009) Improved mooring line technology for tankers and gas carriers at exposed berths [C], OCEANS 2009 MTS/IEEE, Biloxi, Mississippi, USA. pp 1–10Google Scholar
- Kato S (1982) 24. A program for two-dimensional dynamic analysis of forces on mooring line by use of lumped mass method (an outline of programs compiled in Ship Research Institute-No. 5). Nutrients 21(4):247–249Google Scholar
- Ma G, Sun LP, Shang-Mao AI et al (2015) A nonlinear mechanical model for extensible slender rods. Appl Math Mech 36(4):371–377Google Scholar
- Ruinen R, Degenkamp G (2001) Anchor selection and installation for shallow and deepwater mooring systems. In: The eleventh international offshore and polar engineering conference. International Society of Offshore and Polar Engineers, Stavanger, p 7Google Scholar
- Samad FIA (2009) Performance of catenary mooring system. TJ Mechanical Engineering & MachineryGoogle Scholar
- Vryhof anchors b.v., krimpen a/d yssel, the Netherlands, 2005Google Scholar
- Webster WC, Lambrakos K, Kim J, Jing X (2012) Rod dynamics with large stretch. In: Proceedings of the ASME 2012 31st international conference on Ocean, Offshore and Arctic Engineering, Rio de JaneiroGoogle Scholar