Upstream oil industry bias has always favored use of steel pipe. Despite some disadvantages, it can be manufactured to meet almost any pressure requirement at almost any size. And, more importantly, steel is considerably less expensive than its composite material rival known as flexible pipe. As a result, particularly on land installations, flexible pipe until recently has usually used only been used in low-pressure or temporary installations.
But in the 1980s, the oil industry began to look at doing business in extreme water depths and at using floating production platforms to process and store oil in areas where there was no pipeline infrastructure. Within this new environment, flexible pipe became a high demand item as floating production, storage and offloading (FPSO) ships and semisubmersibles quickly became the deepwater industry production installation of choice, first in Brazil and Southeast Asia and later off the coast of West Africa.
Flexible pipe holds two very strong attractions as risers for floaters. First it is considerably lighter than rigid pipe and is neutrally buoyant. That means the vessel's limited space and buoyancy capabilities can be used for storage capacity or topsides processing facilities rather than to support thousands of feet of heavy steel pipe.
But even more attractive for floating vessels subject to considerable vertical movement, or heave, is flexible pipe's bending capabilities. Without that defining characteristic of the pipe the vessel would be unable to move more than minimally and few if any days in a given year would be sufficiently calm to allow production.
Flexible pipe also became a deepwater staple because since it is installed from a reel on the back of a ship with connections made in advance on the deck, the need for divers or remotely operated vehicles to perform that task is eliminated. Consequently it can be installed in substantially less time than traditional rigid pipe and in deep water, time is big money. According to the literature of one leading vendor, installation rates for flexible pipelines often exceed 1500-ft per hour.
As a consequence of these technical advantages, by the end of the 1980s, flexible pipe was being qualified and installed in more than 3,000-ft waters, mostly as FPSO risers. Today, manufacturers of flexible pipe, essentially a composite of two or more different materials working in concert to provide specific characteristics, are preparing to install their wares in water depths of greater than 6,000 ft.
And as with nearly all technologies, the ultra deepwater presents challenges to flexible pipe that are yet to be met, among them pipe lightening and flow assurance.
Lightening The Pipe
For the most part, the industry has engaged in two approaches to reducing the weight of flexible pipe. The first is to simply optimize pipe design for the service and water depth environment in which it will work. Simply put that means installing the lightest pipe possible within carefully calculated safety margins. But that is, at most, a short-term approach.
The second, more general method is to replace the traditional steel exterior known as "tensile armor" whose function is to supply collapse resistance, with one made of much lighter composite material in the upper sections of the pipe (where it will be subject to less water-induced collapse pressure).
"At the moment we are seeking industry participation in that effort," said one flexible pipe expert whose company is currently testing operator interest in such a joint interest project. "Right now the cost of composites is much higher than steel but as the cost of composite manufacturing goes down it will probably become competitive."
But, he says, even when composites become competitively priced, due to their inability to handle compression, they will not likely be used near the seabed where wave action would subject them to considerable compressive loads.
Flow Assurance
In many areas of the world, increasing depth means increasing pipeline distance from seafloor wellhead to production riser. This means produced oil, gas and water must travel considerable distances through pipes run along the ocean floor bathed in near-freezing water.
As the production flows, it cools and at some point crosses that pressure-temperature point known as the "cloud point" where asphaltines, or wax, forms on the pipe walls. Likewise when water and gas are present in the flow stream, hydrates can form. In either case, the ensuing blockage can and often does stop fluid flow altogether.
In shallower waters this problem, known in the industry as "flow assurance", is fairly well under control through the use of chemicals injected into the flow line at the well head and through the use of insulated pipe. But such measures are not likely to work over the many miles of pipeline running from wellheads located in up to 12,000-ft waters as are now being contemplated.
Flexible pipe manufacturers are now working on flow assurance problems via several avenues, including re-circulating fluid to heat the pipe and by introducing electric current to the pipe in a process called direct resistance heating. Each, still in the development stage, has drawbacks and advantages.
The key advantage to sending currents of electricity down the flexible pipe is efficiency. Since it is comprised of composite and steel fibers, it will act as a resistor and convert electrical energy to heat.
"I think [direct resistance heating] is more desirable for a couple of reasons," said Halliburton's Mark Kalman, whose company, Wellstream, came to Halliburton via the Dresser merger. "When you use electricity the heating is uniform. That is how resistors work.
"A definite disadvantage when you are circulating fluid is as you give up heat you give up temperature and so the temperature difference goes down and so your heat input is going to be reduced."
But there are considerable problems with providing a constant electrical charge to long sections of pipe. "One of the drawbacks to electrical heating is you are talking about a substantial consumption of power," Kalman said. "You would have to have facilities to provide that. You can't just plug in the wall when you are offshore."
One other avenue of research, receiving less attention from flexible pipe manufacturers than from other researchers engaged in general composite materials development, is to develop better-performing insulation materials. But though they hold promise, say the experts, to date those materials that might meet the requirement of such a harsh environment are still prohibitively expensive.
Underbalanced Drilling
Flexible pipe today may hold the key to overcoming one of offshore drilling's greatest ultra deepwater hurdles caused by the close proximity of fracture gradient and pore pressure in deepwater. Beyond about 7,500-ft waters, this phenomenon makes traditional well control all but impossible because, simply put, mud sufficiently heavy to contain formation pore pressure is also very nearly sufficiently heavy to fracture it and one fluctuation will indeed do that.
One solution to the problem may lie in underbalanced drilling, a practice not yet possible from floating drilling units because the slip joint on the riser that allows for rig heave cannot handle any pressure at all. And when drilling underbalanced pressure is always present on the riser annulus.
A joint interest project being field tested now in Brazil is using flexible pipe to run from a rotating control head placed on top of the drilling riser to the rig. That way the flexible pipe performs the same duties as the top joint of an annulus return line while its flexibility allows the rig to move vertically without a slip joint.
"One of the great things about this system is it can be put in place and removed within a matter of hours," said Weatherford underbalanced drilling expert, Don Hannegan, whose company is part of the JIP along with Petrobras and others. "That means when you are drilling a section that doesn't have that small frac gradient-pore pressure window, you can just return to traditional drilling."
While flexible pipe usage has spread in the oil industry during recent years, upstream petroleum engineers generally view it as a tool that in some few instances can save time or money. With recent advances in composite materials and the extreme requirements of ultra deepwater, however, flexible pipe may soon be recognized as an enabling technology rather than a convenient innovation.
by John Murphy, Houston
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