Latest Breakthrough in Turret Moorings for FPSO Systems: The ForgivingTan kerrrurret l nterface
Authors: Pollack J. , Pabers R.F. , Lunde P.A. 
Published: October 2008

Ship-shaped vessels have seen a tremendous growth in their use as Floating Production and/or Storage vessels. Along with this growth has come a need to moor in ever deeper water depths and with more flow lines. The most efficient means of dealing with the mooring of these vessels has been the incorporation of a turret. This turret allows the vessel to weathervane while providing a geostationary hull area to bring in flow and mooring lines. It is desirable to have these turret systems weathervane passively, which requires an efficient bearing system, particularly as many turrets are retrofitted into converted vessels. Interfacing these bearing systems with the turret and vessel structure requires extensive design and analysis to assure their mutual deformations will not affect the longevity of the bearing system.

A self-aligning bearing interface, incorporating the use of elastomeric supports, has been developed which eases the design of this bearing interface and reduces bearing loads. The elastomeric support system prevents the transmission of vessel deformations to the bearings thus assuring a low torque, long life, high external load capacity, bearing system.

Introduction

Excluding the fluid swivel stack, the most complex, expensive, and design controlling component in a turret is the bearing system. This system is composed of the bearing itself and the support system. A turret bearing typically consists of a high capacity roller bearing having a certain radial, axial and moment capacity. The rated capacity of these bearings can, however, only be achieved if they are properly supported in housings that do not deform beyond certain limits. Applied moment interacts with and reduces the available axial capacity of the bearing.

The design presented in this paper uses an elastomeric support system that assures vessel deformation will not be passed to the bearing housing. In addition, the bearing support system can be hydraulically aligned in situ, thus avoiding expensive machining of foundations on the vessel. The elastomeric components are arranged in a manner to create a spherical support, greatly reducing bearing bending moments that result from bearing misalignment and column run out tolerances. This bearing system has been installed on an FPSO operating in the Gulf of Thailand and will also be installed on two large deepwater turrets offshore Brazil later this year.

Background

The use of Single Point Moored turret systems has increased as these systems can permanently hold any vessel on location in any body of water. To minimize mooring loads, all turrets weathervane, thus a bearing system is used to attach these turrets to the moored vessels. Most turrets are built to weathervane passively, requiring low bearing turning resistance to avoid winding up chains and flowlines. Bearings with rolling elements are the only proven passive, large diameter, high load capacity, low friction bearings.

Present turret state-of-the-art has relied on one piece roller bearings that have a maximum manufacturing limit of 8 meters diameter. Segmented roller bearings can be used up to 14 meters, but to-date there is no operating experience with these bearings on FSO or FPSO turret systems.