Dossier Océan et énergie - Énergie Thermique des Mers
Sommaire IOA News Letters
DEMONSTRATION OF SLANT DRILLING TECHNIQUES
Justus Muller and George
Hawaii Institute of Geophysics
Demonstration of the technology of drilling slanted shafts under the shoreline will occur at the Natural Energy Laboratory of Hawaii (NELH), located in Kona, Hawaii. Researchers from the Hawaii Institute of Geophysics will drill the first shaft to house cable conduit for the Deep Undersea Muon and Neutrino Detector (DUMAND) Project. The second shafe will be for ocean thremal energy conversion (OTEC); five additional pilot shafts will be drilled to further support OTEC and marine aquaculture.
The state of Hawaii established NELH in 1974 to facilitate ocean-related R&D of energy and aquaculture projects. The major asset of the NELH site is the steep ocean bottom gradient that allows intake of deep cold ocean water from depthis in excess of 2,000 feet using short pipeline lengths (6,000 to 8,000 feet in range). Total water pumping capacity of the numerous pipelines is more than 20,000 gallons per minute.
Several of these pipelines are exposed to damage from storm waves while others are buried in a trench blasted through the surf zone. However, these methods of installation are unacceptable for future piplines. Slant drilling can replace these methods and this project will determine whether or not the technique is economucally, technically and environomentally feasible for installing pipeline shore crossings in Hawaii.
Slant Drilling Technique
Also known as horizontal directional drilling, "slant" drilling is not the same "horizontal" drilling used in oil fields. A slant-drilled shaft begins at the surface at an angle normally about 12 degrees below the horizontal plane. The drill rig's tilt and azimuth are adjusted to aim the drill bit and the drilling action is obtained by pumping fluid down the pipe and through a positive-dis-placement turbine unit located immediately behind the bit.
Pilot shafts in lengths varying from 500 to 1000 feet will pass under the shoreline roughly parallel to the seabed. Offshore, the shafts will begin to curve upward and penetrate the seabed at diver depth, about 60 feet below the surface.
The drill bit navigation system will be calibrated by aligning it with the local horizontal plane and with the direction to the drill breakout point. A microcomputer will be used to help select the three-dimen-sional route between the ends of each drilled shaft.
During drilling, the drill bit aezimuth and pitch angles will be monitored by a navigation sensor. Data will be telemetered to a computer and steering adjustments will be made to control the X-Y-Z position of the seafloor breakout. Precision and accuracy of no worse than 0.5 percent of the total shaft length-3 feet over a 600-foot run-is expected.
The conventional approach in drilling for oil and gas uses a very expensive driller's mud. The mud powers, lubricates and cools the drill motor. In addition, the mud ifits the cuttings to the surface and regulates the pressure at the bottom of the hole to prevent a blowout. At NELH, an inexhaustible supply of seawater will be used to operate the drill.
Two factors make drilling at NELH easy. First, the lava in the area in open-pored and porous to depths of at least 2000 feet. Secondly, ages of ocean submergence have filled these pores with seawater.
Reaming and Casting
After the pilot shaft is drilled, it can be reamed to a much larger diameter. Rotation and thrust will be provided by the slant drill rig. Reamed shafts will be lined with a thick-walled high-den-sity polyethlene (HDPE) plastic casing. Casings will be pulled in from the shore end via a "turnaround" sheave anchored at the ocean end of the shaft. As the specific gravity of HDPE is about eight percent less than that of seawater, it means that the water-filled shaft liner can easily be made neutrally buoyant during insertion and the contribution of weight to to pull-in friction should be negligible.
Other uses in Hawaii for shfat-drilled pipelines include upside-down wells to tap perched water in the mountain, sewage disposal lines,waste disposal, and pilot tunels. In the Pacific area, atolls could use slant-drilled pipelines for OTEC plants that produce electricity and fresh water. A drill barge could be brought into the lagoon and a pipeline drilled from the lagoon to the open ocean. The cold, nutrient-rich seawater can be released into lagoons that are almost devoid of nutrients.
(Above material abstracted from: Muller, Justus and Ggorge Wilkins, Demonstration of slant drilling techniques to drill and case hard-rock conduits for cable crossings under stormy shorelines, Oceans 91 Proceedings, October 1-3, 1991, Honolulu, Hawaii USA, Vol. 1, pp. 286-289)