THE EVOLUTION DEVELOPMENTS OF OFFSHORE PIPELAYING AND CURRENT

Abstract

In 1952, the installation of the first 8 miles of 8-inch pipeline in shallow water from Grand Isle near New Orleans, using a pipelaying barge similar in many respects to that used to lay existing pipelines in Bass Strait, marked the beginning of a new era in offshore pipelaying.

Prior to 1952, "offshore pipelaying" consisted of river crossings and swamp work, and for this work land spread equipment was simply transplanted onto floating structures or barges.

As the successful search for oil progressed into deeper waters, it became apparent that the old onshore pipelaying techniques could no longer be applied offshore. Such terms as tension-shoe, stinger, weight-coat and riser formed the new jargon and diving equipment and techniques associated with pipelining were also seen to undergo major revision and development.

Surprisingly, the old river-crossing technique for water depths less than 100 feet, using controllable buoyancy tanks, is still used for installations in water-depths too deep for the more conventional pipelaying barge with tension shoe and stinger.

One of the deepest permanent installations by this updated "old" method is a 9⅝" diameter pipeline reaching depths in excess of 1,000 feet whereas the current depth limit for lay-barge installation is about 600 feet. It is worth noting that experiments carried out in the early 1960's demonstrated the laying and recovery of two miles of pipe from a depth of 8,500 feet, using this method, known as "the S-Curve method".

More recently, a patent has been registered for a vertical pipelaying method for depths of 1,000 feet or more, using a lay-barge rigged with pipelaying "towers", for making additions to the pipe string whilst it is held at the surface in a vertical position.

The assembled pipeline will describe a single shallow curve from vertical at the surface, to horizontal on the sea-bed, thus eliminating the characteristic S-curve of a stinger-launched pipeline.

Another interesting development has been the appearance of a pipelaying reel-barge capable of a laying rate in excess of 8,000 feet per hour, and capable of laying up to 12-inch diameter pipelines. The pipeline is welded and radiographed onshore, and spooled onto a giant reel. The barge is then towed along the right-of-way as the pipe unwinds from the reel. The reel also doubles as a huge pipe-tensioning device, with the result that pipelaying in water depths of 1,000 feet for 8-iuch diameter and 500 feet for 12-inch diameter pipelines is possible.

The dominant external influences on any known pipelaying method are water depth, pipe diameter, weather conditions and sea-currents.

The product of water depth and pipe diameter determines the weight of pipe-string to be supported by the floating equipment, and when variables of wind, waves, swell and current are added to this otherwise static load, the resultant combination provides the basis for selection or design of the floating construction equipment to be used on any given job. Without looking too far into the future, it would seem reasonable to envisage the conventional pipelaying barge being replaced by a sea-bed based pipelaying habitat, crawler mounted, simultaneously laying pipe and cutting trench; the weight of pipe-string would be reduced to only a few percent of that associated with a floating pipelayer, and the influence of surface weather conditions completely eliminated.