DICAS Mooring System: Practical Design Experience to Dismystify the Concept
Authors: Portella Ricardo B. , Mendes Cristiane 
Published: October 2008

An overview of the DICAS (Differentiated Complacent Anchoring System) mooring system and its practical utilization is presented based on some of the projects where PROJEMAR S.A. took part and its adoption was required or considered as the mooring system best solution. A description of the mooring system concept, the evaluation of the methods and computational tools normally used on the mooring system calculation, design criteria, model tests and the practical design solutions adopted on 3 (three) different projects designs are discussed to show why and when this solution can be considered a mooring alternative for FPSOs and FSOs operating in mild environmental conditions.

Introduction

The DICAS (Differentiated Complacent Anchoring System) was initially presented as an alternative solution for deep water mooring systems of FPSOs and FSOs intended to operate in mild environmental conditions by PETROBRAS during the 1997 Offshore Technology Conference [3].

During 5 (five) years of continuous development, several mooring systems designs have been conducted considering this new semi-weathervane concept, including some new platforms that are under construction/conversion and will be operating quite soon on the offshore field.

The paper intends to present a resume of the learning obtained conducting some of these mooring systems designs. 3 (three) different project designs developed for 3 (three) different units operating in 3 (three) different offshore oil fields give the basis for the conclusions achieved, and besides the fact that not all of them came true, the experience acquired on their development gave an excellent back ground to deep discuss the DICAS mooring system concept and its practical application.

First project considers the use of a small FSO with one flexible riser to operate offshore Brazil in 150 m of water depth, the second project considers a medium size FPSO with 37 (thirty seven) flexible risers to operate offshore Brazil in 700 m of water depth, and the third project is a large FPSO derived from the conversion of a VLCC with 107 (one hundred and seven) flexible risers in a free hang configuration to operate offshore Brazil in 800 m of water depth.

Each of the projects analyzed is related to a different size of unit operating in a different water depth with a different risers system configuration, what gives a great overall view of the practical possibilities for the adoption of this brandy new mooring system concept, showing some of its advantages and limitations and trying to dismystify its utilization.

The DICAS Mooring System Concept

Developed by PETROBRAS [3] to be an alternative for permanent mooring systems on FPSOs and FSOs intended to operate offshore Brazil, the DICAS (Differentiated Complacent Anchoring System) concept is in fact a sophistication of the very well known "spread" mooring system concept.

What differs a DICAS mooring system from a conventional spread mooring system is that different stiffness are applied on the mooring lines located at the bow and at the stern of the vessel, allowing the system to be partially weathervane.