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Volume 15, Number 2March/April 1964

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Sea-Going Drilling Rig

It sails across Persian Gulf waters like a ship but once in place, AMDP-1 is a sturdy, steel island.

Written by The Editors

Oilmen have created some pretty strange-looking shapes in the course of getting petroleum out of the earth, converting it into products, and sending it to market. But nothing in the array of towering refinery units, weird pipe loops, squat spheroid tanks, and wheeled behemoths that trundle through marshland could have challenged for oddity the silhouette of AMDP-1 as it emerged from the hazy Persian Gulf sunrise off the coast of Saudi Arabia on May 10, 1958. And few pieces of oil industry equipment, on land or sea, have matched the hundred-day odyssey of this homely oil field workhorse. Tethered to a stout Dutch tug, it began its ten-thousand-mile journey at Vicksburg, Mississippi, on January 13, 1958, and after cruising at a steady three and a half knots an hour, hove to at the Ras Tanura marine terminal of the Arabian American Oil Company (Aramco) 15 weeks later.

To those who saw it pass, AMDP-1 (the Aramco Mobile Drilling Platform No. 1) must have seemed one of the strangest objects ever to have been borne of its own buoyancy across the ocean depths. A sailor conning it distantly at dawn on the horizon could have been forgiven for assuming that three terrifying sea beasts had fled the pages of ancient marine fables.

Despite its aesthetic shortcomings, AMDP-1 has its own kind of beauty—the elegance of logic. Moving across the water it is a boat; once in place, presto, it is a steel island—it is, in effect, a mobile island. Further, once it had been fitted out with a drilling rig, it borrowed from the towering symmetry of the derrick a handsome air.

Not long after it arrived at Ras Tanura five years ago, it was put to work by Aramco drilling offshore oil wells in the Safaniya Field, where it soon became a familiar sight, one that reflected technical rather than mythic marvels. It has three pylons that drop into the water and become legs. It stands upon the ocean floor and resists the force of heavy seas. It hoists itself out of the water "by its own bootstraps." It carries a full-size drilling rig.

However, its greatest singularity can be seen only in Aramco's ledgers—it saves the company more than $100,000 on every new offshore well. It represents a $1,650,000 investment and is an example of an important victory of technology over the constant increase in oil well drilling costs in offshore fields.

The story of the evolution of AMDP-1 goes a long way back in oil industry history. Almost as soon as they had appeared on the American industrial horizon, the big wooden derricks of the oil fields began their march to the sea. They stopped at the water's edge, but only briefly. By 1894, Summerland, the first offshore oil field to be developed in the United States, had been discovered near Santa Barbara, California. By 1903 a wooden pier that rested on stilt-like piles stretched out into the ocean at Summerland and a number of rigs drilled from it. Soon, men whose chief concern was location of buried pre-historic sea bottoms where oil might be found were coping with marine problems.

As geological data accumulated and geophysical instruments assumed greater precision, the geologists in the oil companies prompted their managements to obtain offshore concessions and risk large investments in wildcatting marine fields. The first great underwater discovery abroad was the Lake Maracaibo Field in Venezuela. Development of the field started in the 1930's. Producing men had to adapt land equipment for overwater drilling. It was a time when drilling rigs were still steam powered, and as one producing man recalled recently, "there were huge boilers on barges all over the place."

The first big break came when land rigs became diesel powered and "the whole system was lightened." The lighter power equipment made overwater drilling easier—the fixed drilling platforms, built upon long piles driven into the bottom, had to carry less weight. But ahead lay the complex problem of keeping costs down, even reducing them, as drilling in lakes, bayous, and coastal waters increased.

The big push into deeper waters came after World War II when world-wide oil consumption grew rapidly and spurred the hunt for new fields. Everything that had been learned about drilling platforms, barges that carried rigs aboard, and huge "sea islands" from which many wells could be drilled, was consolidated in a new technology. However, there was no simple answer, no universal design that would meet the on-site needs of every underwater field.

For instance, the floor of the Gulf of Mexico is soft, silty clay. This mantle is 200 feet deep in some drilling areas. On the other hand, the floor of the Persian Gulf is hard and little penetration is experienced by the feet (spud tanks) of AMDP-1 during drilling. Wave forces vary from place to place around the world, as do tides. The height of a wave is a function of wind direction, velocity, and "fetch" (the distance over which wind blows at a measured velocity). Thus, wave heights differ in offshore oil fields. Weather, of course, varies widely. It is interesting to note that WOW (waiting-on-weather) is the great offshore time waster. Sometimes a desert shamed, a hot, dry summer wind that carries aloft a fog of talc-fine sand particles, can reach 30 to 40 miles into the Persian Gulf and halt drilling.

Engineers have worked out several general types of design solutions for overwater drilling. Extremely costly fixed-pile, self-contained platforms have been built. They were like small communities. Their high initial cost and low salvage value led to smaller platforms that were also mounted on fixed piles but were serviced by tenders (often converted LST's). The next step was to free the drilling platform from a fixed position so that it could drill a hole and then move on to another well site. Thus the mobile platform was evolved. In most cases it requires the service of a tender and has to be moved by a tug. A great deal of ingenuity has gone into the engineering of these various overwater drilling systems. Their capitalization has been quite high, but they have enabled oil men to get further seaward in coastal waters.

In 1949, Aramco began its offshore exploration in the waters of Saudi Arabia. In 1950 the first overwater fixed drilling platform was erected not far from shore near Safaniya. The platform was served by a barge. The following year the Safaniya Field was discovered and a program of delineation drilling was begun. A special barge built to serve drilling platforms was purchased in Venezuela from the Creole Petroleum Company and towed to Saudi Arabia by tug. It had been known as the Queen Mary and the nickname remained.

During the early development of the Safaniya Field, the world's largest offshore reservoir, Aramco continued to use the fixed platform system of drilling. Heavy steel piles had to be driven into the sea bottom to support each platform. A drilling rig and derricks had to be mounted on each, and then dismounted when the well was completed—a time-consuming and costly procedure.

Aramco engineers began to survey the mobile drilling platforms in use in the industry. In the period 1955-1957 the Air Force placed four massive three-legged platforms—called "Texas Towers"—in early-warning radar service off the east coast of the United States. The triangle design had also found favor in many drilling companies, and Aramco's engineers, working with the design engineers of the R. G. Le Tourneau company, created a triangular mobile platform based upon Persian Gulf requirements. The platform was finished in 1957 and towed to Saudi Arabia early in 1958. It provided the means for a cost break-through in Aramco's offshore drilling.

The Aramco platform, without drilling rig, is a model of simplicity in engineering design. Seen from above it is an isosceles triangle with a pylon (also triangular) rising from each point. The deck is 94 feet on two sides and 104 feet on the third. The flat bottom of the platform is identical—the two surfaces are ten feet apart and are, of course, enclosed by steel sides. The enclosed interior is divided by water-tight compartments. In the water this steel tank becomes a hull and is buoyant.

The three equilateral pylons, or legs (depending on whether they are up in the air or down in the water), are each 125 feet long and individually interlaced with tubular steel bracing. When viewed from the side, as one approaches on the water, their mode of elevation and retraction becomes clear. Three points of each leg are made up of gear teeth over its length from top to footing (a spud tank forms the foot). Three big gear boxes are mounted around each leg in the hull/platform. The gears for each leg let the leg down into the water until the foot is firmly set upon the sea floor. The gears continue to turn and the hull climbs up the legs and becomes a platform.

AMDP-1 operates in conjunction with the Aramco Drilling Tender No. 1 (ADT-1) which went into service in March, 1961, about four months after the barge Queen Mary was disabled by a storm. Before AMDP-1 is towed to a new drilling site, a fixed production platform is prepared. This platform is smaller and can be constructed from lighter materials than those formerly used because with the advent of AMDP-1 the platforms no longer had to support a drilling rig and the tons of drill pipe that hang from the derrick in a deep hole during drilling.

When it is towed to a new site, AMDPT's apex is its prow. Its stern, or base side, has a large slot which fits around and over the fixed production platform at the new well site. Once AMDP-1 is positioned around the production platform, the drilling rig and derricks are skidded from the center of the triangular deck over the slot. The rig is now in position to drill. The drill stem will pass through an opening in the fixed production platform.

When the hole is completed, the tender is towed off and then AMDP-1 is towed away. The fixed production platform remains with its big Christmas-tree valve complex marking a completed well. And AMDP-1 proceeds to its next assignment where the fixed production platform is already waiting.

The foregoing simplified procedure by-passed one crucial step—pre-loading. As mentioned earlier, an Aramco offshore well requires thousands of feet of drill pipe which may weigh as much as 100 tons. This weight, plus other operational loads must be taken into account when AMDP-1 is being footed on the sea bottom. It would not do to have the huge structure continue to settle during drilling as the drill stem lengthens and becomes heavier. Therefore, when the hull has climbed up out of the water about a foot and a half, sea-water is pumped into compartments in the hull until the weight of the added water equals the anticipated weight of the drilling load. The pre-load is then discharged, and the platform elevates itself to a position where its bottom is 24 feet above the water at low tide, based upon accepted marine standards. This elevation places the platform about 14 feet above the surface at high tide. This height accommodates ten-foot wave crests with an added four-foot surge clearance in case of severe storm conditions.

The tender carries all the auxiliary needs of the drilling rig—work water, drinking water, drilling mud, drilling cement, power generators, drill pipe, well casing, and so on, plus living quarters for the drilling crews.

Everything about AMDP-1 is proportioned on a massive scale. It can drill in low-tide depths of 77 feet of water. Its total displacement with drilling equipment in place is 1,001 tons. It is a mobile drilling island of impressive stability—blunt, solid, hefty. However, the level of its platform can be controlled with unusual precision and delicacy. Should an inclination develop of as little as three-tenths of a degree in any direction, an immediate adjustment is made. A pair of opposing levels, called inclinometers, tells the operator the direction and degree of tilt.

AMDP-l's platform climbs at the rate of one foot per minute—slow but secure.

Well by well, AMDP-1 has earned back for Aramco the original cost of $1,650,000—but Aramco's engineers are ever pioneering new ways to further reduce drilling costs. Since it arrived in Saudi Arabia the hybrid hull/platform has undergone modifications which make it more stable afloat and permit it to stand in depths of water greater than it was originally designed to do.

AMDP-1 would surely not excite the eye of an admiral accustomed to the sleek lines of giant sea queens. It has, however, more than earned the Navy tribute, "Well done."

This article appeared on pages 3-7 of the March/April 1964 print edition of Saudi Aramco World.


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