Economical and political circumstances have been continuously pushing exploration and production of mineral resources However, for much of the world, the best potential for major new oil and gas reserves lies beneath 450msw Eventual welding repair operations at such depths can only be carried out by fully automatic systems, part of a diverless underwater working station Such station should feature flexibility inside the working cell, self-sufficiency and reduced distances for data transfer and proximity between control units and their executive counterparts To achieve these objectives a repair station consisting of a submersible, alignment frames and a purpose built habitat has been proposed This study presents a conceptual description of this repair station, its main elements, working procedures and development requirements to realize it

1. Introduction

Since the number and age of offshore installations raises every year, there IS an increasing need for repair & maintenance systems and procedures Although underwater pipeline repair technology has been of concern to companies for a long time, the actual depth range in which repair work could be performed is skill more or less behind the depths in which pipelines are laid

Taking the Mediterranean Sea as an example, more than 80% of its area lies on depths exceeding 200m and more than 50% at l000m In addition to that an increasing number of new fields are being discovered in depths beyond 500m in the Mediterranean [Tassini, 1987] and at the Campos Basin in offshore Brazil Freire, 19891 Operators hesitate to exploit these resources without contingency procedures for pipeline repair and maintenance since an eventual pipeline failure in deep waters could have dramatic consequences to the environment and production

Although successful hyperbaric manual welding trials have been executed in 600m simulated water depth [Jansen et al, 1987], 180m appears to be the limit for offshore manned intervention, at least in the Norwegian Sector of the North Sea Pelauze, 19891

It seems evident that pipeline repair concepts for water depths greater than 300m–400m must focus on robotics and remote sensing if repair intervention is going to catch up the capabilities that already exist in the drilling, production and transportation phases of the offshore petroleum industry

The two different diverless underwater repair methods generally considered for deep water repairs are mechanical connectors and hyperbaric welding Various repair systems based on mechanical connectors have been designed, such as the TITUS system [Lenque and Tangeland, 1989] Mechanical connectors are believed to offer slightly better pre-conditions for automation than hyperbaric welding and as a matter of fact, some of the available systems have already undergone offshore trials However, they also present some disadvantages such as the requirement for accurate metrology of the contact surfaces, the probable need of dummy nodes and the lack of mechanical strength Moreover, depending on the system, the effort for pre and post repair activities offers no additional time benefit [Alexander and Quin, 1984]

Currently, hyperbaric welding is still the best known repair method either as a manual operation (carried out by welder-divers) or as diver assisted mechanized welding

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