Other approaches

a slack moored system will become considerable, thus introducing an undesirable error in the measurements of velocity fields and other ocean variables.

Subsurface buoy systems are also classified as single leg and multi-leg subsurface buoy systems. The great majority of oceanographic subsurface buoy systems have a single anchoring point. Cost efficiency and ease of deployment result from their simple configuration. As the buoy would be much below water surface, the wave force and the motion of the buoy due to waves would be less.

Two part buoy systems are used when the need arises to provide motion stability for underwater sensors and at the same time to provide a surface expression for telemetry of data or for relocation of the main buoy system. The motion stability is provided by using a subsurface supported buoy system.

A typical deep water buoy system may involve multiple lines with lengths varying from a few tens of metres to thousands of metres. When these are considered in conjunction with the equipment deployed from or associated with the moored buoys, the losses resulting from a mooring failure can be significant. Consequently there is much interest in design and analysis methods applicable to the buoy mooring systems.

In general, based on the use of the buoy mooring system it can be grouped as (i) buoys used for monitoring or measuring the parameters of importance to oceanographers and naval scientists and (ii) buoys used for engineering purposes such as mooring oil tankers.

6.7.2 Oceanographic Buoy Systems

An oceanographic buoy system can be defined as a floating structure deployed in the ocean for the purpose of measuring environmental data (Berteaux, 1976). Because of their inherent capacity of efficiently providing long term series measurements of meteorological and oceanographic parameters, a relatively large number of buoy systems are deployed each year in world’s oceans.

Buoy systems are used in the ocean environment for monitoring weather, oceanographic and defence related data acquisition, and also as vehicles for electronic navigational systems. An array of hydrophones used in conjunction with a subsurface moored buoy and a telemetry system can be used as a passive sonar to detect and transmit sounds in the sea either due to surface or subsurface ships. Standard navigation systems can be installed aboard floats, provided with power and moored offshore to extend the range of precision navigation. In the Ocean Acoustic Tomographic System (OATS), the use of buoy cable system would help to get very valuable oceanographic and scientific data.

6.7.3 Offshore Floating Storage Systems

Since many large oil fields are in remote places where harbours are non-existent, a need is felt to have artificial berths to moor the tankers during their loading operation. Many configurations of offshore tanker terminals are attempted. The single point mooring system (SPM) has emerged as the most rapidly deployed, economical and safest to operate. SPM enables economic transport of crude oil where use of pipelines is not technically or economically feasible because of rough seabed, topography or long distances from shore.

Single point mooring terminals are, as the name implies, facilities of small horizontal dimensions, to which large vessels are moored by means of a bow hawser or by any other means which allows the vessel to rotate 360° around the mooring point. Generally, single point mooring terminal can have two functions. Primarily, it affords a safe mooring to the vessels. Secondly, it can form a link in the transport of oil.

The single point mooring terminal can assume many forms. Of the more than 300 SPMs now in use around the world (Maari, 1985) approximately 80 percent are of the type -single buoy catenary anchor leg mooring (CALM). CALMs have been employed as loading terminals since 1961. The CALM (Figure 6.35) basically consists of a cylindrical buoy type float anchored to the seabed by a number of radial catenary chain legs (up to eight chains) while the vessel is moored to the buoy by one or more elastic synthetic (usually nylon) lines. This system employs the properties of the catenary to supply the

R. Sundaravadivelu 1

Fig. 6.36 Catenary Anchor Leg Mooring (CALM) Terminal

elasticity required when holding large tankers in open seas. The buoy is cylindrical and can have an outside diameter between six and twenty metres and a height between four and eight metres.

Single buoy, multi-leg mooring systems are the most commonly used offshore loading facility which has grown in significance in the recent years through the use of single point mooring systems for the exploitation of marginal fields and in the development of moorings for deep water production facilities. One of the principle tasks of the designer of catenary moorings is to ensure that the system characteristics are such that the movement of the floating unit under extreme environmental conditions remains within acceptable limits. Scrutiny of the calculation of catenaries show that chains with their high weight per unit length often have high energy absorption capacity. Whilst chain has this very desirable property of being a good energy absorbing catenary, it unfortunately suffers from that well known failing character, being only as strong as its weakest link. This constitutes the second major design criteria i.e., the tensile loads under extreme wave conditions should be less than the proof loads.

The typical examples of different types of offshore loading systems are given below:

(a) Rotating manifold CALM system installed at Buchan, United Kingdom (Figure 6.36a ) (b) Soft yoke CALM system installed at Palanca, Angola (Figure 6.36b.)

(c) Rigid yoke CALM system installed at Cadlao, Philippines (Figure 6.36c)

Water depth can vary to a practical maximum of about 130 metres. Normal operating sea states with the tanker moored are the significant wave heights in the range of about four metres. High waves at a CALM terminal can generate prohibitive forces in the anchoring chains. This is especially the case where the ratio of maximum wave height to water depth is very high. CALMs have been installed in hostile areas such as North Sea with a maximum survival wave height up to 28 metres (Montrose Field) and the Enchora Field in Brazil with a maximum wave of 21 metres. Current velocity can be a limiting factor for the submarine hose system but CALM terminals have been installed and are operating successfully in currents of up to 2 m/s (four knots). Wind is not a significant factor because it only affects the SPM indirectly through forces applied on the tanker. Normal operating (with ship moored to the CALM) wind velocities are about 40 knots and design wind speeds are up to 70 knots.

R. Sundaravadivelu 1