Abstract

A diverless guidelineless flowline connection system, the so called direct vertical connection, has been developed, and tested, which sharply decreases connection time, while allowing the layvessel itself to perform both flowline ends connections, thus dispensing with the use of auxiliary tools and making the operation more cost effective and reliable.

This system has been designed to overcome problems associated with conventional pull-in systems, the lay-away method, ROV assisted pull-in systems and 'conventional vertical connections'. The former have already been successfully used in Marlim field, in 25 wells located at water depths ranging from 740 m to 980 m.

The direct vertical connection can now be considered a field proven method. Five import flowline connections have already been successfully performed in Albacora field manifold, installed at 620 m water depth. From now on, based on the excellent results achieved from those installations and previous tests, all deepwater Brazilian subsea trees and manifolds will be designed for direct vertical connection. Sixty six trees and six manifolds incorporating the direct vertical connection system are presently under fabrication in Brazil.

Introduction

A significant aspect which must be considered in developing subsea systems for the production of hydrocarbons in very deepwater is the connection of the flow lines to both subsea trees and manifolds.

Field experience learned from the installation of the first diverless trees offshore Brazil, showed (ref. 1) that conventional pull-in systems were subject to a wide failure mode range and almost always required diver assistance, although designed for diverless operation, Only one out of29 pull-in trees was installed without any diver assistance in Campos Basin, which is, by far, the most prolific Brazilian basin and where all deepwater fields are located.

Fortunately, at that time, subsea wells offshore Brazil were not located at water depths out of divers' reach and the problem was solved simply by using diver assisted trees instead of 'diverless' trees. It is worth mentioning that the former proved to be less diving-time consuming than the latter. However, new discoveries located at water depths beyond 300 m (limit diving depth) have obliged us to face the challenge of designing a reliable diverless flowline connection.

The lay-away system

Instead of attempting to improve existing pull-in methods, an important decision was made: to adopt the lay-away technique (ref. 2), which was devised by Petrobras engineers. The primary objective of the lay-away system is to provide a means of running either tree or production base attached to the flow lines in order to prevent the vulnerable pull-in operations.

So far, 57 lay-away operations, including 36 guidelineless operations, have been successfully conducted in Campos Basin in water depths ranging from 287m to 1027m.

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