Abstract

Recently, Aker Solutions delivered a movable chain jack system to Petrobras's P58/P62 FPSOs and a movable windlass system to ENI's Goliat FPSO. This paper highlights the main differences between the two systems and when it is beneficial to employ movable systems. There are many parameters to consider in determining which system to use - also a traditional system involving a single winch or chain jack per mooring line may in many cases be preferred. The movable chain jack concept is designed to operate multiple mooring lines within the same cluster. A single chain jack is lifted by a skidding gantry and moved to the next mooring line and so forth. Installation and messenger chains are moved using a large sliding chain locker allowing for later offloading of the surplus chain. The movable windlass system is also designed to operate multiple mooring lines within the same cluster. However, in this case the winch is rotary and can operate via electric or hydraulic power. One of the main considerations is to move the windlass and keep the mooring lines intact without cutting them.

Introduction

The primary purpose of an offshore mooring system is to maintain a floating unit on station within specified tolerances. These tolerances are often based on offset limits determined from the configuration of the riser system. The mooring system provides restoring forces that act against environmental forces (wind, waves and currents) that are trying to move the unit off station. Most mooring systems for offshore application are spread moored or single point moored (including turret systems). Spread mooring is the conventional mooring approach and is widely used for both drilling and production units. Spread mooring keeps the unit in position at an optimal heading but does not allow weathervaning. Single point mooring (internal/external turrets) is used primarily for ship-shaped vessels allowing the vessel to weathervane. However, a spread moored system can also be applied to ship-shaped vessels and we see many of this type off the coast of West Africa and Brazil.

Traditionally, on-vessel mooring equipment has consisted of individual winches, fairleads and chain stoppers located at each mooring line. The mooring lines can be chain, wire rope, fiber rope or a combination. This paper discusses movable chain jacks and winches recently supplied for spread mooring systems on both ship-shaped and cylindrical- shaped FPSOs.

1. Traditional Spread-Moored Platform Designs

Position moored production and drilling units come in various shapes and sizes. Traditionally, we have seen semisubmersible, spar and ship-shaped units. All these units have applied winch or chain jack systems. Figure 1 shows typical designs that have been in use for several decades.

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