Enhanced Turret Mooring System for Harsh Environment Operation
Authors: Sanna Petter , Askestad Sigmund , Schoemaker Jan , Hansen R. E. 
Published: December 2007

The main design philosophy behind the turret is to achieve simple operation, high reliability and safety level, high production capacity, strength, reduced size and simplicity required for marginal fields and or fields with a short duration production life. This is obtained by choosing a passive mooring system and purpose built inherent flexible structural design and joint connections for equal load distribution on the bearing system. To obtain the required simplicity one objective was to minimize the amount of equipment located on the turret and to locate as much as possible on the ship. The turret is divided into two parts (picture 6). The lower part is the substructure, comprising the buoyancy chamber, columns and bearing arms. The upper part is assembled by Riser Connector Deck (RCD - deck), Emergency Shutdown Deck (ESD - deck), manifold-deck, swivel foundation and their beams and columns which constitute the main structure.

The upper part will be custom designed to a certain extent. It is envisaged that the lower part alone would be installed on a floating storage and offloading (FSO) ship. Development of a floating production, storage and offloading unit (FPSO) would require the upper as well as the lower part.

The turret has an open structure with good natural ventilation and low explosion pressures. Piping within the turret through splash zones is rigid to avoid potential gas leaks and to eliminate wear of the flexible risers in the riser guide pipes, (picture 10)

The structural design of the lower part allows flexibility which, in conjunction with joint connections, results in equal load distribution on the load bearing elements. The standard turret is designed for a mooring system with 10 or 12 mooring lines (102mm chains). A passive system is preferred for simplification of operation and to avoid equipment requiring power, such as individual heavy mooring winches, located on the turret (picture 11).

Free weathervaning is permitted through the special design of the bearing system, and allowing for maintenance and part replacement without operational shutdown.

Relative movements between the ship and the turret are reduced by placing the turret with the bearing system close to the ship’s neutral axis. With the large diameter of the bearing system a moderate bearing surface pressure without lift-off has been achieved. Separate axial and radial bearing systems are included, independent of secondary submerged bearings for extreme conditions, (picture 9 & 12)

The turret can be equipped with a combination of three transfer systems depending on the complexity of the turret and the requirements for continuity of flow from the turret to the ship. A multi path swivel stack is the main transfer device which allows continuous low and high pressure flow between the ship and the turret.

The swivel is passively turned by the overhead gantry. A jumper system allows for continuous flow even during changeover, and reduces the number of flows through the swivel. Hoses are utilized for temporary, non continuous flow. An optional spare swivel replacement system may replace the complete swivel stack, (see picture 15)