Mooring System: From Initial Design to Offshore Installation
Authors: Portella Ricardo B. 
Published: October 2008

The Petrobras 40 is the result of the conversion of the former McDermond derrick barge DB100 into a semi-submersible production unit of 150.000 bpd oil production. The naval design of the unit has being carried out by PROJEMAR S.A. and the process plant design by IESA Engineering. FPSO Construction Inc is holding the conversion work at Jurong Shipyard in Singapore and the whole project managed by MarÍtima.

Designed to operate in South-Marlin field - Campos Basin, offshore Brazil during 20 years, the Petrobras 40 will be moored in 1080 m of water depth by a passive 16 lines chain-polyester-chain taut-leg mooring system. The Unit was planned to be installed in February 2000 and is now scheduled to be in the field by the middle of the year.

This paper describes the work performed for the complete design of the mooring system. The initial design concept, the model tests performed at MARINTEK model basin, the optimization of the system based on model tests results and offshore installation considerations, the final mooring analysis and the offshore installation procedures are presented.

The modifications in the initial concept due to the design evolution are discussed and the offshore experience used to evaluate the design basis.

Introduction

The intent of this paper is to describe the mooring system design from the initial design concept to the offshore installation that has been carried out for the semi-submersible unit Petrobras 40.

The platform was designed to operate at South Marlim Field - Campos Basin - Offshore Brazil with a peak production of 150,000 barrel of oil per day. The average water depth at the location is 1080 m.

The mooring system is a passive taut-leg system composed of 16 chain-polyester-chain lines designed for 20-years of life in the location.

The system was designed according to API RP 2SK as a permanent mooring system. Site environmental conditions were specified for the design based on Metocean Data obtained by Petrobras.

Offshore operational procedures for the mooring legs installation and platform hook-up have been developed to be carried out by an installation contractor.

Mooring System Design Strategy

The mooring system design strategy adopted for the P40 project was based in a methodology that divided the design in two distinct phases: Phase One, or initial design, carried out in order to provide data for the tank model tests and Phase Two, or final design, carried out after the tests. The two phases are divided by the tank model tests, and can be summarized on the following :

Phase One: Pre-dimensioning:

The objective of this phase was the dimensioning of the mooring system to be tested in the tank tests. Additionally it provided the initial data to be used on the technical specifications of the on-vessel and off-vessel mooring system components, that initialized the procurement process of these equipment. Phase one had the following steps:General Arrangement DefinitionWind Tunnel Model TestsMotion AnalysisStatic Riser AnalysisInitial Mooring System DesignComputer Simulations of Tank Test ConditionsTank Model Test