Advanced Technology

Smooth featureless surfaces inside pipelines and devices that contain high speed flows are critically important to maximizing oil and gas throughput, aren't they? Maybe not. At one time, this was considered true, because the idea was to reduce the interior surface area to the minimum needed for maximum laminar flow speed. The ideal flow equation is moderately complex for variable flow speeds and multiple phases, but the rationale was always the same.

Smooth interior surfaces are key to maximum pipeline flow ... right?
Perhaps not.

Can pipeline flows be increased with interior features?

Smooth featureless surfaces inside pipelines and devices that contain high speed flows are critically important to maximizing oil and gas throughput, aren't they? Maybe not. At one time, this was considered true, because the idea was to reduce the interior surface area to the minimum needed for maximum laminar flow speed. The ideal flow equation is moderately complex for variable flow speeds and multiple phases, but the rationale was always the same.

Now, some researchers believe the use of specialized features on interior pipe surfaces may actually improve flow performance. The point is not keeping the interior flows as laminar as possible, which means accepting a lower speed flow, but controlling the turbulence that results with higher throughput volumes by using these surfaces.

Fluid dynamicists are tackling flows with higher Reynolds numbers now, such as in air-fuselage interaction in high multiples of sonic speed, but their findings apply to viscous flows in pipelines. Tiny regular features called riblets are being experimented with in air flows over airplane wings to reduce drag.

In the marine environment, the external surfaces of marine organisms are designed to minimize drag. They are not smooth either. Fish (scales), sharks (denticles), and dolphins all have featured surfaces. Features apparently prevent turbulent flow products - eddies, vortices - from penetrating the boundary layer and reaching fixed surfaces, where they induce drag. Surface features break down large eddies into smaller ones, allowing the fluid's viscosity to recapture them quicker.

Will we see dimpled or ribbed pipeline interiors soon? No, even if they are found to be more flow efficient. There is still a great deal of research left on other implications of featuring. What about the impact on pigging, inspection, and measurement? Also, featured interiors could offer more points of attachment or nucleation for paraffins, waxes, and hydrates.

At the same time, interior features may offer an unexpected benefit in that higher turbulence rates induced by greater flows in the pipeline may actually decrease the rate of deposition of paraffins and hydrates. What if the features were introduced periodically in key sections to create a cleaning turbulence or during slower phase flows. Keeping paraffins and hydrates in emulsion and unattached is one of the goals of research, and induced turbulence might meet that need.

Flow loops to scientifically examine hydrate and paraffin development and attachment have been set up recently under contract to the joint-industry organization, Deepstar, and research is getting underway. Examination of featured pipeline interiors and induced turbulence may not be far behind.

Pipeline blockage economics altered by CT, tractor units

Long subsea tiebacks to platforms and long exposure to near-freezing temperatures in deep water pose a significant blockage problem for production design. Producers have only a few years' experience with long tiebacks and are unable to test flow production streams adequately under such conditions. Even after accounting for such conditions, the well flow chemistry can change during the life of the well, increasing the chances for blockage. A number of pipelines and pipeline sections have been abandoned when such unplanned events occur.

Chemical inhibitor injection lines and flow pigging loops are the natural protection for long offset flowlines, but the extra costs of such equipment and the loss of production, especially in the case of separate wells, frequently overwhelm profitability. Texaco and others are examining the possibility of using coiled tubing to remove paraffin, wax, and hydrate blockages in pipelines and are proposing trials of various methods in tests loops to develop a standard practice.

There is a question as to whether the technique, using either chemicals or mechanical means, can solve a blockage problem, and further, how long it would require to cut hard deposits. Some wax blockages resemble concrete. In such cases, coiled tubing units could employ well tractor units that use traction wheels against the line's interior surface to power the cleaning assembly into the blockage. Without weight, coiled tubing has no other means of penetration.

In any case, solving the problem after the blockage occurs can greatly reduce production costs by allowing producers the option of remediating the problem when it becomes truly troublesome instead of installing injection and pigging loop equipment just in case. (For more information on this subject, see lead article on page 28 of this issue and page 30 - Drilling & Production column - in the October, 1996 issue.)

Stress measured in tiny temperature changes

Under stress, compression heats a metallic material. Tensioning cools it. The difference from one extreme to the other is measured in thousandths of a degree. Being able to graphically display the temperature variations on a color screen provides a large amount of information during or after a stress generating process.

Stress Photonics of Madison, Wisconsin (US) has developed an infrared camera - processor package that does just that. The package helps spot poorly designed materials, weak areas or links, fastened or welded materials with excessive or insufficient compression or tension.

Supercritical CO2 bath strengthens cement bond quickly

A supercritical carbon dioxide bath strengthens and reduces the weight of cement mixtures significantly. Los Alamos National Laboratory (US) and Materials Technology of Reno, Nevada are experimenting with various uses for the super-strong product.

Supercritical carbon dioxide acts more like a fluid than a gas, but is able to penetrate the smallest cavities within the cement mixture. There, the fluid dissolves all water molecules, resulting in curing of the cement in minutes. Incinerator ash and other environmentally undesirable byproducts can be incorporated into the cement.

Methane hydrates release may be part of global warming

The argument over whether current climate aberrations and warming could be caused by mankind, by earth's natural warming and cooling cycles, or induced by solar activity may not be easily resolved, but a fourth possibility now is receiving more investigation in climate research symposia.

The past release of sufficiently large accumulations of methane hydrates from the seafloor, in the permafrost regions, and under retreating ice glaciers could have resulted in massive extinctions of life on earth. The reason for the attention is that global methane hydrates contain nearly 30 times the amount of carbon dioxide in the atmosphere. When the hydrates emerge into the sunlight, they quickly oxidize into carbon dioxide.

Packaging incineration, generation, distillation for offshore MODUs

The use of mobile jackup and semisubmersible drilling rigs for combined offshore incineration, power generation, and water distillation provides an attractive package for under-developed countries with a need for all three. The addition of natural gas from nearby depleted reservoirs provides an even stronger commercial combination, since onshore garbage incineration is a less onerous environmental problem in under-developed countries.

This proposal was developed by Woods Hole Oceanographic Institute (US) and other groups as a series of alternatives (Ocean Enterprise Concept) to onshore problems. Increasingly, the regulatory and environmental considerations associated with land location of the three processes favors an offshore site.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.

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