On the Question of Drilling in Offshore Oil-Bearing Areas

L. A. Pukhlyakov

(Presented by Professor S. S. Sulakshin)

The question of extracting oil from areas located within the sea was raised as early as 1896 by engineer Zglenitsky, who asked that two sections of the seabed be allotted to him for oil exploration. Zglenitsky proposed that drilling be carried out from a piled foundation. In 1900 Zglenitsky’s proposal was considered by a special commission and rejected, but the commission recognized the possibility of using for oil production about 300 desyatinas of the seabed of Bibi-Eibat Bay after it had been filled in. In 1906 an international competition was announced for the best project for filling in this bay, and in October 1907, out of 16 projects submitted to the competition, the project of engineer Pototsky was recognized as the best; it provided for enclosing the marine area with a breakwater and subsequently filling it in. Construction of the breakwater began in 1909, and in 1911 the filling of the bay began. The filling work was directed by the author of the project, engineer Pototsky. By 1918, out of 300 desyatinas of enclosed space, 193 desyatinas had been filled in. At that point the work was halted and resumed only after the end of the Civil War [5, pp. 115—118]. In 1922, oil production began on the filled-in territory, and in 1927/28 it reached 0.82 million tons [1, p. 272].

Subsequently, oil workers abandoned the filling of sections of the seabed and, for the construction of drilling rigs at sea, began to erect piled foundations. At present, in addition to the Caspian Sea, oil from piled foundations is produced in the Persian Gulf, where in 1963 its production exceeded 50 million tons, in the Gulf of Mexico, and in other places. In 1963, world oil production from the seabed amounted to approximately 100 million tons [6]. Major oil fields have been discovered in the coastal waters of Brazil, Argentina, Tunisia, Gabon, Lebanon, Indonesia, and other places. The largest gas field, Slochteren, has been discovered off the coast of Holland. In the USSR, oil production from the seabed in 1955 amounted to approximately 4 million tons [4], and in 1967 to about 11 million tons.

Here, however, it should be noted that the described method of drilling marine oil-bearing areas entails several serious shortcomings, the chief of which is the impossibility of developing great depths. At present (1968), areas of the sea floor with depths down to 33 m have been developed, and in prospect there stands the problem of developing areas with water depths down to 100 m. There is at present not even any question of constructing pile foundations in areas with great depths, while the oil-bearing capacity of many such areas is quite probable. There is one more shortcoming of the method under consideration: the extraordinarily high cost of constructing the pile foundations themselves. In this connection, the question of drilling from floating foundations has now (1968) arisen.

Simultaneously with the problem of prospecting for oil and gas, the problem arose of drilling in the open sea for the purpose of studying rocks exposed on its bottom. In fact, on the surface of our planet land occupies only about 29.4%, and only this part of it has been sufficiently studied. Of the remaining surface of the Earth, man has only the most general conception, and in order to eliminate this gap it is necessary to carry out drilling at various points on the ocean floor. Since 1959 this question has been discussed in the press [8], and in 1961 the Americans carried out drilling in the ocean at a depth of about 3.5 km. In doing so, the problem of mooring the vessel to the given point was successfully solved, but the problem of rolling remained unresolved. The vessel from which drilling was carried out experienced inclinations at times of up to 17°, and this greatly hindered the drilling process. At present in the USA a new vessel is being built for this purpose, in which anti-rolling devices are being provided [2].

Many authors proposed getting rid of rolling by various methods, but all these methods were in general reduced to one thing: to submerge the drilling vessel, or the most massive part of it, to a certain depth, into a zone where the influence of waves is significantly reduced.

Thus, as early as 1905 I. S. Zakovenko proposed a caisson-pontoon [5, p. 116], that is, a bell immersed below the water level, from which the water is displaced by air. It should be noted, however, that the pressure in this bell must be at least five times higher than normal, and therefore it would be very difficult for a person to work under such conditions. Subsequently, other authors also came forward with similar proposals, but they were never implemented.

In 1945 R. Dawson [7] proposed for this purpose a submersible barge, the drilling platform of which was to be located above the water. The stability of the vessel was to be achieved by the fact that the greater part of it was to be located at a certain depth, in a zone where the influence of waves practically does not penetrate. It is precisely this principle that will be used in the new American vessel from which drilling in the ocean will be carried out at a depth of more than 5 km.

In the USSR, for engineering-geological investigations on the sea floor within small depths, a floating installation has come into use, which at the moment of operation is lowered to the bottom [3]. The maximum water depth at which this installation is used is 10 m.

Below is described a semi-submersible cross-shaped vessel which differs substantially from the one just described (Figs. 1 and 2). An application for this vessel to the Committee for Inventions of the USSR was made by the author of the present article in 1963 (application No. 855441/27-71), but a certificate of authorship was refused on the grounds of usefulness in 1965. In the USA, an application for the same vessel was made in 1966 by P. Gibson et al., to whom in 1969 patent No. 3486343 was issued (application 15.06.66, published 30.12.69).

In the working state the hull of this vessel must be located below the surface of the water and therefore retain stability; however, the drilling

Fig. 1. A semi-submersible cruciform vessel in the semi-submerged state (longitudinal section): 1—longitudinal (machinery) hull; 2—drilling tower; 3—comb floats; 4—conductor (central pipe for lowering the drilling tool to the seabed); 5—opening for raising equipment to the drilling platform; 6—hoisting system; 7—ladder; 8—main propeller

equipment from it to the seabed must be lowered from a platform which, in this case, will be located above the water level, in the upper part of the drilling tower. To lighten the tower, it is proposed to place as many mechanisms as possible in the hull.

Inside the drilling tower (2) there is a conductor (4)—a pipe for lowering the guide string and the drilling tool to the seabed. The upper end of the conductor must always rise above the sea surface; this will ensure that water cannot enter the internal cavity of the vessel.

To prevent the sudden sinking of the vessel if it suffers some kind of damage, it is provided with two comb floats (3). The comb floats are a series of hollow tanks fastened to trusses. During wave motion they themselves must undergo certain inclinations, but their centers of gravity will remain immobile or almost immobile; this means that the rolling they produce will be very slight.

Fig. 2. Semi-submersible cruciform vessel in the surface state before installation of the comb floats. The dotted line indicates the outline of the underwater part of the vessel

The amount of submergence of the vessel must be regulated by the degree of filling of the ballast tanks located in the bow and stern parts of the vessel. To prevent the vessel from capsizing at the moment it surfaces, it must be cruciform (Fig. 2). In this arrangement, one of the hulls (the longitudinal one) must be the main hull and contain most of the mechanisms, ballast tanks, and comb floats, while the second (transverse) hull must be auxiliary and serve as accommodation for service personnel, stores, and the like. The presence of the transverse hull, of course, must lead to the vessel being unable to develop high speeds when moving from one point to another. However, however small these speeds may be, they will far exceed the speeds at which drilling rigs are moved on land.

The length of each of the hulls should be on the order of 200 m, and the height of the drilling tower 85 m, of which about 40 m should be above the waterline. With a drill-pipe stand length of 25 m, this will make it possible to raise the drilling platform as much as 10 m above the water level. If it becomes necessary—

...the possibility of using derricks 37 m long, the drilling tower can be made higher.

It will obviously be impossible to assemble such a vessel in a dock. In this connection, the separate parts of the vessel—the right and left parts of the transverse hull and the longitudinal hull (without the tower)—should be manufactured as independent vessels, and their assembly into a single whole should be carried out on the water. Finally, after assembly, the drilling tower can be installed on them.

In this connection, the conductor (the central pipe for lowering equipment to the seabed) should also be manufactured in sections.

At the moment when the longitudinal hull is launched, only the base of this pipe should be secured in it—its lower part, with a length corresponding to the height of the longitudinal hull. The rest of it must be assembled after the drilling tower has been installed in place. The comb floats must be installed simultaneously with the drilling tower.

After drilling has been completed at a certain point, a production casing string must be lowered from the vessel into the well; it must rise above the water. Therefore, in order to move from this point to a new one, a trough up to 5 m deep (high) from the plane of the bottom must run along the bow part of the longitudinal hull. When the vessel floats up, the upper part of the trough must rise above the water level and make it possible for the vessel to leave beneath it the end of the production casing string protruding from the water.

To provide for small movements of the vessel in the lateral direction, and also for turning it about its axis without shifting the center of gravity, it must be equipped not only with the main propeller (8), but also with two auxiliary propellers, perpendicular to the main one, located in the bow and stern parts of this hull.

When drilling at sites with depths greater than 400 m, the vessel’s position at the given point must be maintained by the operation of the propellers. At depths from 400 to 50 m the vessel can be anchored. The use of anchors in the form of large boxes with opening bottoms will be especially rational. When filled with sand, these boxes will hold the vessel in the semi-submerged condition necessary for the work, provided there is an excess of buoyant force (and this will give it exceptionally high stability). If it is necessary to move to a new point, the bottoms of the boxes are opened, the sand is discharged onto the seabed, and the empty boxes are raised to the surface. At depths of less than 50 m, the semi-submersible cruciform vessel should simply lie on the bottom and remain in that state until the work at the given point is completed. If the bottom proves uneven, the vessel must rest on it only at two points—the bow part of the longitudinal hull and one of the ends of the transverse hull. Its other two ends must be kept afloat, using the buoyant force of the rear comb float.

For the operation of wells constructed with the aid of such a vessel, vessels of a similar type, but of much smaller dimensions, may be used. In addition, pile foundations can be built next to them (if the depths of the seabed in the area of such wells do not prevent this). However, these foundations now need not be as massive and costly as when drilling towers are built on them; and, most importantly, they will not have to be built where no oil is found. Thus, the use of the vessel considered here will yield substantial savings in the exploration of offshore oil fields.

References

1. S. M. Lisichkin. Essays on the Development of the Oil-Producing Industry of the USSR. Izd. AN SSSR, 1958.

2. G. M. Maksimovich. The second stage of drilling a well into the Mohorovičić layer. “Neftyanoe khozyaistvo,” 1965, No. 1.

3. A. S. Marchenko. A floating installation for engineering-geological investigations at sea. “Razvedka i okhrana nedr,” 1963, No. 11.

4. On the results of fulfillment of the fifth five-year plan for the development of the USSR and the Union republics in 1951–1955. Gospolitizdat, 1956.

5. N. Ya. Udynsky. Drilling operations. In: The Oil Industry of the USSR. Gostoptekhizdat, 1958.

6. “The Black Gold” of the World Ocean. “Novoe vremya,” No. 22, 1965.

7. R. Dawson. Deepwater submersible barge, patent 2653452. United States Patent Office, 1953.

8. G. Lill and W. Bascom. A bore-hole to the Earth’s Mantle. Nature, No. 4681, vol. 184, 1959.

NOTED MISPRINTS