On-Site Welded Repairs To Offshore Structures Using Dry Underwater Habitats

ABSTRACT Three offshore structures fabricated of tubular steel sections recently were repaired by welding wherein the welding was accomplished in underwater dry habitats. The equipment and procedures utilized for these repairs were similar to that developed for performing hyperbaric welded pipeline connections in underwater dry habitats. Regarding that process, working depth requirements for the repair of platforms is well within the demonstrated capability of the process for pipeline connections in underwater dry habitats based on recent demonstrated performance in the field. The platform damage was of an unplanned nature and structural reassessment was carried out to determine the repair requirements prior to repair of the damage. The repair then was carried out in accordance with the results of that reassessment. The developed state of the art of hyperbaric welding and the ability to produce a satisfactory weld is a most essential ability of this process. There are, however, a large number of key abilities and skills other than the welding that are required for over-all success in carrying out a successful program of underwater repairs of structures. The required skills and abilities are reviewed. INTRODUCTION The numbers and relative ages of offshore structures, utilizing tubular steel-construction techniques are increasing at locations around the globe. This is a result of the continued expansion of offshore oil and gas production. During their period of useful life, these structures normally have maintenance programs carried out during which regular inspections, cleaning, and other functions are performed as a normal part of the operation of the platform. It is to be expected, however, that in addition to this, during installation or use some of these structures will sustain damage that requires maintenance in the form of repairs of an unplanned nature. Such has been the case for a number of these structures, three of which recently have been repaired by my company by welding in underwater dry habitats to effect the repairs. The procedures and techniques used in making those repairs are reported herein along with the skills required in the process. The repairs discussed were on platforms in the British and Norwegian sections of the North Sea. However, the procedures used in these repairs are typical and the principles used are the same for similar repairs at any geographic location. The structures requiring repair typically have been located in unprotected areas of the sea and have oil-producing operations carried out on and around them. This combination of location and activity has afforded a number of circumstances that have resulted in structural damage. For example, damage has resulted from natural causes after failure of the corrosion prevention system, unplanned events such as impacts with a supply vessel or falling load. For structures of this type, after structural damage is sustained, structural reassessment is used to determine whether the repair must be made. This is necessary because of the considerable redundancy in the placement of structural members, resulting in more than one possible path for reacting a given structural load.