Abstract

This paper describes a development program being conducted by Petrobras to develop reliable and through life cost effective moorings for depths to 3,000 m. Four alternative solutions are considered: spread catenary, taut leg, taut with submerged buoys and differentiated compliance (DICAS) mooring systems. For each system, four main aspects are being investigated: materials, design procedure and criteria, installation and maintenance. This program has started in 1996. Results already obtained show that: its is economically feasible to moor exploration and exploitation floaters in water depths to 3,000 m. The paper also discusses the most cost effective options for these moorings and key parameters to select, design, install and operate them.

Introduction

Since 1986 Petrobras has foreseen that fiber ropes would play a key role in extending the depth capability of drilling and production vessels. Initial studies concentrated in substitutes for steel wire rope in catenary systems with grounding chain and drag embedment anchors. Kevlar aramid ropes were qualified for this application by developing handling procedures and conducting two sea trials1.

However, the introduction of fiber ropes in spread moorings called for a systems approach to design and materials selection. A pilot study showed that, to meet standard requirements for maximum offset for flexible and/or drilling riser, polyester (PET) fiber ropes would be the most cost-effective option2.

Detailed studies characterized the relevant properties of these ropes, appraised their use in catenary and taut leg moored semisubmersibles in water depths to 1,500 m, and developed basic design procedures and criteria3,4.

From 1992 to 1996 basic installation procedures were developed and tested. Also medium to long term trials in catenary system were set up. Initial Results from field samples were promising.

By this time Petrobras was faced with the challenge of developing Marlim Field, using several floaters. Congested seafloor layout and provision for workover demanded short radius mooring systems for these floaters. An appraisal of the state of the art and internal experience with PET fiber ropes led to the adoption of taut leg systems based on these ropes for three semi-submersibles for this field5.

Also the challenge of installing FPSO-II in 1,420 m of water in Marlim South Field, with the deepest anchor set in more than 1,600 m, led to the choice of a catenary system with long PET fiber rope inserts for the mooring lines.

Ultradeep Water R&D Program

With the ongoing exploration and exploitation projects in mind and foreseeing the need to moor drilling, production, storage and offloading facilities in water depths to 3,000 m, a research and development program was put together by Petrobras.

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