Turret Design and Construction: Keeping It Off the Critical Path
Authors: Cortijo J.L. , Lamas J.L. 
Published: December 2007

One of the main features that distinguishes an FPSO from a conventional ship, is the turret. The main aspects affecting the turret design are the functional requirements, the mooring loads and the integration of the turret geometry and loads into the ship structure. Once the general arrangement of the turret has been determined; the detailed design of the steelwork, the integration of the mooring lines, the support of the mooring loads, the detailed design of the bearing and hydraulic system and the design of the process pipework and transfer system has to be performed. On the Texaco Captain FPSO, these engineering functions as well as the construction of the turret were performed by the shipyard. This allowed the engineering and construction activities of the turret to be integrated with the overall engineering and construction schedule of the rest of the vessel. This ensured that the control of all aspects of the vessel remained with one entity i.e., the shipyard.

This paper describes the engineering and construction techniques used related to the turret, when these activities were performed relative to other shipyard activities and how in this way, the turret engineering and construction did not impact the project critical path.

Introduction

The turret is generally one of the most critical systems in an FPSO. It represents the "heart" of the unit, which interfaces with the subsea equipment, the mooring and risers systems, the process plant, and the vessel systems.

Due to this broad interaction with the other systems, the design of the turret and its associated systems must be 'frozen' at the very beginning of the project. Procurement of long lead equipment must be undertaken at an early stage to ensure a proper delivery at the construction yard for installation, hook-up and commissioning.

In the following, the experienced gained by ASTANO on how to deal with this matter in order to keep the turret off the critical path will be described. Specifically, the description will be related to the last FPSO unit delivered by ASTANO: The Texaco Captain FPSO.

Background

Preliminary Engineering. An extensive Front End Engineering (FEE) work was carried out on the Texaco Captain FPSO prior to the contract award. After this engineering work was finalized, the design of the turret was completed (subject to final model test results), by using standard validated computer programs for the definition of the size of the mooring system components. The contract for the final design and construction of the FPSO was then awarded by Texaco and the project started on the above mentioned basis.

Model testing and further computational work. Firstly, DMI (Copenhagen, Denmark) was committed to perform a model test programme (wind tunnel and basin tests), in order to confirm the main design parameters (i.e. - basically mooring chain diameter, maximum vessel offsets for the risers system, and turret induced forces and overturning moments). Other information to be used in the design of the unit such as the vessel self-weather vaning ability, the thruster capacity required for heading control and for unwinding of the transfer system, green waters on deck, etc., were also monitored.

Unfortunately, the measured mooring line tensions, vessel offsets and turret induced loads, exceeded by far those predicted using 'the state of the art' computer programs.

Due to the criticality of the impact derived from the results mentioned above, a second model test programme was committed to MARIN (Wageningen, Holland), as well as some analytical work using in-house developed fully dynamic computer programs. P. 509