For the mooring of tankers at exposed offshore locations the oil industry is showing great interest in Turret mooring systems, because of their mechanical simplicity and cost effectiveness. The first turret mooring system, the Jabiru disconnectable Riser Turret Mooring, offshore Australia was installed in 1986. It was followed by a mooring system of much simpler concept: the stern mounted Turret mooring of the storage tanker in the Rospo Mare field, offshore Italy. Both systems are described in detail in the paper.
More turret systems of the relatively simple stern or bow-mounted concept have been installed or are planned.
Meanwhile, on the drawing board, the turret mooring concept has been developed into designs which comply with a wide range of criteria. Large diameter Internal Turrets have been designed, to provide space for multiple well, deep water developments. For the use In restricted water depths, in harsh North Sea environmental conditions, a compact Internal Turret has been developed, which can be incorporated In either a purpose designed, new built vessel as well as in an existing tanker.
A novel disconnectable turret arrangement has also been designed to deal with disconnection In high sea states and to allow reconnection in significant environmental conditions. The paper discusses these latest developments In detail and demonstrates how far the turret concept has advanced, to make it applicable for the most extreme design criteria.
Single Point Mooring (SPM) systems are commonly used to moor tankers at exposed offshore locations.
A large variety of SPM designs developed in the past 20 years, "custom-made" solutions to various mooring requirements.
Many of the mooring systems have been designed to use chains to provide the restoring forces against the environmental loads which offset the vessel.
The most elementary SPM system is the Catenary Anchor Leg Mooring (CALM) buoy (see Figure 1). This system consists of a floating buoy, moored to the seabed by a number of chains. The tanker moors to the buoy by means of a hawser arrangement. On the buoy the hawser is attached to a turntable, which allows the ship to weathervane about the buoy.
The CALM buoy is generally utilized as a loading/offloading terminal, to which tankers are temporarily moored for only the duration of the fluid transfer operation.
For the permanent mooring of tankers a buoy mooring system was developed with a triangular shaped rigid arm connection between the vessel and the buoy. Its apex is fixed to the turntable on top of the buoy, while its base is attached to the ship by means of hinges, which allow relative pitch angles between the tanker and the arm. This mooring concept, the so-called Single Buoy Storage (SBS) system, Is utilized for permanently moored storage tankers as well as for tanker based Floating Production Storage and Offloading (FPSO) systems.