Encyclopedia of Ocean Engineering

Living Edition
| Editors: Weicheng Cui, Shixiao Fu, Zhiqiang Hu

Catenary Anchor Leg Mooring

  • Liping SunEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-981-10-6963-5_141-1



A typical CALM system consists of the mooring buoy, chain and anchor system, mooring assemblies, and floating and submarine hose strings. The buoy body provides the necessary buoyancy to keep the system afloat. A rotating deck or turntable is mounted on top of the buoy body to transmit mooring force from the mooring assemblies through the buoy to the anchor chains. The CALM anchoring system consists of 4–8 anchor chains extending from the buoy body radially out and terminating at firmly embedded anchors or anchor piles (Luo et al. 2015).

Scientific Fundamentals

System Composition

The CALM system consists of a large buoy, which supports a number of catenary chain legs anchored to the sea floor (Fig. 1). Riser systems or flow lines that emerge from the sea floor are attached to the underside of the CALM buoy. Some of the systems use a hawser, typically a synthetic rope, between the vessel and the buoy. Since the response of the CALM buoy is totally different than that of the vessel under the influence of waves, this system is limited in its ability to withstand environmental conditions. When sea states attain a certain magnitude, it is necessary to cast the vessel off. In order to overcome this limitation, rigid structural yokes with articulations are used in some designs to tie the vessel to the top of the buoy (Vryhof 2005).
Fig. 1

CALM (www.sbmoffshore.com, 2012)

CALM system can be used both in shallow water and deepwater area and are connected to a shore storage facility (tank farm) or to offshore production platforms by means of a submarine pipeline. For this application, the CALM is used as an offloading system for a deepwater Floating Production Storage and Offloading unit (FPSO).

CALM is also a complex nonlinear system for oil tanker or FPSO. In addition to the three degrees of freedom in the horizontal plane (surge, sway and yaw), the entire external transmission system has 12 degrees of freedom (buoys 6, tanker or FPSO 6) of the motion response. If the simultaneous CALM system-FPSO-the coupling motion analysis of the tanker, it will reach 18 degrees of freedom. Because the incident wave period is generally greater than 8 s, the longitudinal oscillation is the main motion form of the buoy. Therefore, it is necessary to take into account the pitching motion characteristics of single point mooring system in the design of pipeline (SBM 2012).

The CALM system allows the moored tanker to weathervane. With this principle, the tanker offers to the environment (waves, current, and wind), a direction of least resistance, thus the system can operate in much higher conditions than the other systems. The CALM system reacts against the force created by the environment on the moored tanker by a set of two spring effects: (a) The nylon mooring hawser, (b) The buoy mooring legs, which are installed in a Catenary configuration (GL 2014).

CALM-catenary anchor leg mooring can be capable of handling very large crude carriers. This configuration uses six or eight heavy anchor chains placed radially around the buoy, of a tonnage to suit the designed load, and attached to an anchor or pile to provide the required holding power. The anchor chains are pretensioned to ensure that the buoy is held in position above the PLEM. As the load from the tanker is applied, the heavy chains on the far side straighten and lift off the seabed to apply the balancing load. Under full design load, there is still some meters of chain lying on the bottom. The flexible hose riser may be in one of three basic configurations, all designed to accommodate tidal depth variation and lateral displacement due to mooring loads. In all cases, the hose curvature changes to accommodate lateral and vertical movement of the buoy, and the hoses are supported at near neutral buoyancy by floats along the length (Chakrabarti 2005). These are
  • Chinese lantern, in which two to four mirror symmetrical hoses connect the PLEM with the buoy, with the convexity of the curve facing radially outwards.

  • Lazy-S, in which the riser hose leaves the PLEM at a steep angle, then flattens out before gradually curving upwards to meet the buoy approximately vertically, in a flattened S-curve.

  • Steep-S, in which the hose first rises roughly vertically to a submerged float, before making a sharp bend downwards followed by a slow curve through horizontal to a vertical attachment to the buoy (API RP 2SK 1993).

Historical Development

The history of mooring system started from the development and use of the single point mooring device of catenary buoy by the US navy during World War II, and then developed rapidly. Numerous different mooring systems have been developed over the years (Kent et al. 2007).

The world’s earliest single point mooring system is CALM. CALM mooring system is the most widely used single mooring. There are over 560 single point mooring system over the world. And there are about 500 CALM systems by 2008. In 1997, a polyester rope + anchor chain catenary mooring system is first used for a FPSO in Brazil. Afterwards, more and more fiber mooring systems appeared in the Gulf of Mexico (Winkler and Mckenna 1994). Since early 2000, the CALM design has been used and adapted to deepwater conditions, greater than 1,000 m (ABS 2014). Installed CALM in deepwater are listed as following:Agbami (Nigeria, 1435 m), Kizomba A&B (Angola, 1200 m, 1000 m), Dalia (Angola, 1341 m), Erha (Nigeria, 1190 m), Akpo (Nigeria, 1285 m), Bonga (Nigeria, 1000 m), Girassol (Angola, 1320 m), Greater Plutonio (Angola, 1310 m).

Key Technology in the Development of CALM

Since the 1970s, the research on the CALM system can be divided into the following two aspects: (1) engineering design – the deploy/recovery of mooring system, the composition of mooring line, the arrangement of mooring system, and the development of anchor. (2) theoretical research – environmental load, static analysis of anchor mooring line, dynamic response between mooring system and mooring floating body.

Haixiao Liu et al. (2014) used a specially designed experimental system to investigate the nonlinear mechanical behavior of synthetic fiber cables, including how stiffness changes, how the main factors affect the hydrodynamic stiffness changes, and the nonlinear tension-elongation relationship. The similarity criterion of the hydrodynamic stiffness of fiber cables derives from scale analysis and experiments verification. The accuracy of empirical expressions for hydrodynamic stiffness currently used is verified by measured data. The results show that the uniform load is an important factor affecting the hydrodynamic stiffness, and the effect of the amplitude of strain change on the stiffness cannot be ignored. The cyclic load is also an important factor affecting the hydrodynamic stiffness. Based on the measured data, an empirical expression considering both uniform load, strain amplitude, and the number of cyclic loads is presented. It is the only expression that can evaluate the change of hydrodynamic stiffness under long-term cyclic loading.

Minzhe Li (2013) designed a mooring system for semi-platform by combining the semi-physical simulation and mooring system control method, to control the mooring line retracting and deploying to realize platform positioning and to detect the positioning effect under different sea conditions.

Amir G. Salem et al. (2012) studied the method of estimating pitch motion of CALM system in frequency domain and verified it by experiment. The hydrodynamics of CALM system is directly related to the fatigue life of the mooring system under the coupling interaction of the mooring line and the oil pipeline. It is very important to accurately predict the hydrodynamic response of the buoy.

H. S. Da Costa Mattos and Chimisso (2011) studied the image model of creep test for low-density and high-strength HMPE fibers. In the macroscopic method, besides the traditional variables (pressure, total strain), scalar variables related to damage induced by creep process are introduced, and the evolution law of the damage variable is proposed. The life and elongation of HMPE specimens were predicted by creep tests at different loading levels and room temperature, which were in good agreement with the experimental results.

Davies P et al. (2011) studies the effects of uniform load, load range, and stiffness loading frequency on synthetic fiber cable including polyester cable, aramid cable, and HMPE in dry environment experimentally. In subsequent experiments, the bending stiffness of aramid cable and HMPE cable and the operation in deep water were discussed.

Meng Yuan (2010) used finite element time-domain method to construct the numerical calculation model of mooring cable for single-point and multi-point mooring system. Given a sinusoidal movement at the top of SPM system, the dynamic response of the whole mooring line was calculated. Taking the multi-point mooring system of Spar platform as an example, the time-history variation of the tension at the end of the mooring line was calculated. The theoretical calculation results are consistent with the model test.

Francois and Davies (2008) carried out a sampling polyester cable test of cable with breaking strength of 70 tons and a full-scale test with the breaking strength of 800 tons. Viscoelastic response of the cable is considered for the quasi-static stiffness to reduce the change of uniformly distributed load under changing environmental conditions. The upper and lower boundaries of stiffness only considered the effect of uniformly distributed load.

Jing He (2007) used single-objective and multi-objective optimization methods to optimize the length, material price, and fracture strength of the two-component catenary mooring line of CALM system.

Y. T. Chai et al. (2002) proposed a semi-analytical quasi-stationary method based on catenary equation to deal with the interaction between the three-dimensional multi-mooring line partial subsection state and all subsection suspension state and the seabed, which can efficiently analyze the parameters of multi-point mooring system and flexible riser under different states.

Casey and Banfield (2002) investigates polyester cables with hydrodynamic axial stiffness ranging from 600 to 1000 tons, and points out that the strain amplitude is indeed a variable affecting the hydrodynamic stiffness.

Fernandes et al. (1999) carried out a full-scale polyester cable test with a diameter of 0.127 m. It is found that the dependence of hydrodynamic stiffness on frequency is weak.

Key Applications

The CALM is the most popular and widely used type of offshore loading terminal. CALMs have been deployed worldwide for a variety of applications, water depths, and vessel sizes ranging from small product carriers to Very Large Crude Carriers (VLCC). Because of safe and easy berthing and un-berthing operations, the CALM is equally the preferred offshore terminal of Mooring Masters and Tanker Captains (Luo et al. 2015).

CALM type single point is divided into the earliest Wheel CALM type, Turntable CALM type, and the newly developed Turret CALM type. At present, the most commonly used type is Turntable CALM. Compared with Wheel CALM, it uses large diameter rolling bearing at the center of the buoy to replace Bogey Wheel and transfer mooring force. In this way, the liquid rotary joint is completely independent of the turntable and bearing structure and no longer acts as a bearing member. The single point in this form consists of the following parts: buoy body, rotary turntable device, pipeline system and liquid rotary joint, anchor system, pipe manifold baseplate, underwater hose and floating hose, mooring rope device, and auxiliary equipment. The schematic diagram of the three types of buoy structure is shown in Fig. 2 (BV 2006).
Fig. 2

Schematic diagram of the three types of buoy structure (www.sbmoffshore.com, 2012)

The CALM system has no restrictions on the size of the tanker, is easy to maneuver, works around the clock, requires the least staff, and has strong adaptability to the environment. The advantages of the CALM system can be summarized as follows:
  1. 1.

    There is no need for a natural and excellent deep-water port, and there is no need for fixed dock facilities to provide loading and unloading operations.

  2. 2.

    The system is of lower cost.

  3. 3.

    It can be installed in open sea areas that do not require protection.

  4. 4.

    Can work normally all the year round.

  5. 5.

    Easy to install.

  6. 6.

    The mooring system is simple.

  7. 7.

    It can transmit a variety of oil and gas products at the same time.

  8. 8.

    The shuttle tanker has its own mooring propulsion system.

  9. 9.

    The terminal does not require a special tugboat for berthing and offshore, only a smaller vessel is required to serve (Luo et al. 2015).




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Copyright information

© Springer Nature Singapore Pte Ltd. 2019

Authors and Affiliations

  1. 1.College of Shipbuilding EngineeringHarbin Engineering UniversityHarbinChina

Section editors and affiliations

  • Liping Sun
    • 1
  1. 1.Harbin Engineering UniversityHarbinChina