The VLT: A Single-Point Mooring for 100+ Risers
Authors: Garnero Charles , Heyl Caspar N. , Boatman L. Terry , McCollum Jerry 
Published: October 2008

Based on existing installed systems, it is generally perceived that the number of risers for a turret moored FPSO is limited to approximately 50. Riser congestion, available turret space as well as bearing size and load carrying capacity all tend to limit the number of risers for existing turret moored systems. Until now, spread moored FPSOs have generally been considered the only viable option for field developments requiring 50-100 risers. This paper describes the design and analysis of the Very Large Turret (VLT) and how previously limiting parameters are overcome for a cost effective single point mooring system with the capacity for 100+ risers.

The VLT application lies in deep to ultra deep water. The VLT is an internal turret with the ability to support both flexible and steel pipe risers. The ability to support a multitude of risers make it well suited for field developments with minimal or no subsea manifolding where each wellhead receives a dedicated riser bundle. The VLT is particularly well suited for field development concepts currently being considered for offshore Brazil and West Africa.

The results presented herein illustrate that the large diameter of the turret, typically considered a design constraint, actually contributes to the design efficiency and its ability to function in a dynamic environment. The VLT's inherent flexibility results in an improved bearing load distribution and the turret's unique arrangement requires a turret steel weight that is lower than other existing turret systems.

Today more than ever, large field developments (100,000 - 250,000 bopd) are being considered. Ship-shaped production platforms with the storage capacity of a VLCC or larger are being specified. Passive fully weathervaning mooring systems that allow tandem offloading are preferred over spread moorings due to higher operation up time and lower risk of tanker collision. The VLT satisfies these needs.

Introduction

The maximum production capacity of Floating Production, Storage and Offloading units (FPSOs) has steadily increased over the recent years. Production rates for these large fields are now reaching as high as 250,000 bopd. The increased flow capacity has driven a corresponding increase in the quantity and size of risers between the subsea wells and the floating vessel. As shown in Table 1, the selected mooring system for these vessels has almost invariably been a spread mooring.

Table 1: Large Field Development FPSOs (Available in full paper)

All risers must pass through the inside diameter of the turret bearing on a single point moored FPSO. Until development of the VLT (PATENT PENDING), the bearing opening has been perceived as the governing factor limiting the number of risers that can physically fit within a turret. To achieve the production flow capacity, two methods have been used. One method has included the use of a spread moored vessel with riser porches on either side of the vessel beam. The other method includes the use of increased subsea manifolding requiring fewer (but larger size) risers. A third method is now available using the VLT.