I. INTRODUCTION

Back in 1975 a number of major oil companies had decided to initiate exploration drilling programms in very deep water : below 300 meters and often up to a 1000 meters of water.

Those deep water drilling programms initiated the development of a number of new technologies in many different fields related to drilling. Among those, one can list the following :

  • Dynamic positioning or drill ships, new riser technology, new B.O.P. technology.

A number of specialized drilling vessels, incorporating those new technologies, were ordered by offshore drilling contractors.

At the same time a question was asked to the diving contractors: what kind of services will they be able to offer in water depths exceeding those normally within the reach of commercial saturation diving?

Various types of answers were possible, based on different concepts :

  • the one atmosphere diving suit,

  • the remote controlled vehicles (R.C.V.),

  • the conventional oil field submarine,

  • the one atmosphere bell.

At this point in time, the decision to develop more particularly one solution rather than the others was obviously influenced by the past experience of the contractor involved.

Our company had been, at this time, already involved with a variety of one atmosphere vehicles. Some of those were not at all related to drilling operation and were mainly concerned with transfer, of personnel from surface to on bottom one atmosphere enclosure. However, some experience had already been gained with one atmosphere assistance to drilling:

On board the first dynamic positioning drillship, the PELICAN, a conventional diving bell was modified along the following lines:

6 thrusters were added, a large view port was fitted instead of a lateral hub door, some navigational equipment was added, as well as a simplified manipulator arm. This bell was used for all the routine work and when a major intervention on the subsea drilling equipment was required, a conventional dive was run.

As much as 150 one atmosphere dives were logged with this bell and considerable experience was gained on what should be the functional specification of a bell specially designed for this purpose.

Also, when the occasion came to design a one atmosphere bell specifically for drilling assistance, the following major functional specifications were decided :

Visibility:

It was considered as a point of major importance since most of the tasks to be performed on subsea drilling equipment require firstly a thorough inspection of the equipment to be worked on. Also good visibility downwards was required both during descent for safety reasons and also for search on bottom of lost equipment.

Pressure rating of the hull:

It was initially decided that a thousand meters was to be considered as the minimum operating depth with a possibility to extend the working depth to 1500 meters.

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