On the Issue 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 the 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 pile-supported foundation. In 1900 Zglenitsky’s proposal was considered by a special commission and rejected, but the commission recognized it as possible to turn over 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, of the 16 projects submitted for this competition, the project of engineer Pototsky was recognized as the best; it provided for enclosing the marine section with a breakwater and subsequently filling it in. In 1909 construction of the breakwater was begun, and in 1911 the filling of the bay. The filling work was directed by the author of the project, engineer Pototsky. By 1918, of the 300 desyatinas of enclosed space, 193 desyatinas had been filled in. At this 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 in of sections of the seabed, and for the construction of drilling rigs at sea they began to build pile-supported foundations. At present, in addition to the Caspian Sea, oil is produced from pile-supported foundations 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]. The largest 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 has several serious shortcomings, the principal one being the impossibility of developing great depths. At present (1968), sections of the seabed with depths of up to 33 m have been developed, and in the future there remains the problem of developing sections with water depths of up to 100 m. There is not even any question at present of constructing pile foundations in sections with great depths; meanwhile, the oil-bearing potential of many such sections is quite probable. There is another shortcoming of the method under consideration: the extraordinarily high cost of constructing the pile foundations themselves. In this connection, at present (1968) the question of drilling from floating bases has arisen.

Simultaneously with the problem of searching for oil and gas, there arose the problem of drilling in the open sea for the purpose of studying the 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 notion, and in order to eliminate this gap it is necessary to drill at various points of the ocean floor. Since 1959 this question began to be 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 tying the vessel to the given point was successfully solved, but the problem of rolling remained unresolved. The vessel from which the drilling was carried out at times experienced inclinations of up to 17°, and this greatly interfered with the drilling process. At present, in the United States a new vessel is being created for this purpose, in which anti-rolling devices are provided [2].

Many authors proposed various ways of eliminating rolling, but all these methods, in general, came down to one thing: to submerge the drilling vessel, or its most massive part, to some depth, into a zone where the influence of waves is considerably reduced.

Thus, as early as 1905 I. S. Zakovenko proposed a caisson-pontoon [5, p. 116], that is, a bell lowered below the water level from which the water has been displaced by air. It should, however, be noted that the pressure in this bell would have to 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 put forward similar proposals, but they were never realized.

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 having the greater part of it located at a certain depth, in a zone where the influence of waves practically does not penetrate. This very principle will be used in the new American vessel from which drilling in the ocean at a depth of more than 5 km will be carried out.

In the USSR, for engineering-geological surveys on the seabed within small depths, use has been made of a floating installation which, at the time 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 cruciform vessel which differs substantially from that 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 grounds of utility in 1965. In the United States, an application for the same type of vessel was filed in 1966 by R. Gibson et al., to whom patent No. 3486343 was issued in 1969 (filed 15.06.66, published 30.12.69).

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

Fig. 1. Semi-submersible cruciform vessel in the semi-submerged state (longitudinal section): 1—longitudinal (machinery) hull; 2—drilling tower; 3—float-fins; 4—conductor (central pipe for lowering the drilling tool to the seabed); 5—opening for lifting equipment onto 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 half of the drilling tower. In order to unload 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 prevent water from entering the internal cavity of the vessel.

To prevent the vessel from suddenly submerging if it suffers some damage, it is fitted with two float-fins (3). The float-fins consist of a number of hollow tanks secured on trusses. During wave action they themselves should undergo certain inclinations, but their centers of gravity will remain stationary, or almost stationary, and this means that the rolling produced by them will be very slight.

Fig. 2. Semi-submersible cruciform vessel in the surface state before installation of the float-fins. 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 ballast tanks located in the bow and stern parts of the vessel. To prevent capsizing of the vessel at the moment of its surfacing, it should be cruciform (Fig. 2). In this case one of the hulls (the longitudinal one) should be the main hull and should contain most of the mechanisms, the ballast tanks, and the float-fins, while the second (transverse) hull should be auxiliary and serve for housing the operating personnel, for storage facilities, and the like. The presence of the transverse hull, of course, will mean that the vessel will not be able to develop high speeds when moving from one point to another. However, no matter how low these speeds may be, they will greatly 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; the height of the drilling tower—85 m, of which about 40 m should be above the waterline. With a stand length of 25 m, this will make it possible to raise the drilling platform above the water level by up to 10 m. If the need arises—

the possibility of using stands 37 m long, the drilling derrick 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 derrick)—should be built as independent vessels, and their assembly into a single unit should be carried out on the water. Finally, after assembly, the drilling derrick can be installed on them.

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

At the moment 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 should be assembled after the drilling derrick has been installed in place. The floats—hydrofoils—should be installed simultaneously with the drilling derrick.

After drilling has been completed at a given point, a production casing string must be lowered from the vessel into the well, and 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), measured from the plane of the bottom plating, must run along the bow part of the longitudinal hull. When the vessel is afloat, the upper part of the trough must rise above the water level and allow the vessel to leave beneath it the end of the production string protruding from the water.

To provide for small lateral movements of the vessel, and also for turning it about its axis without shifting the center of gravity, it should 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 in areas with depths of more than 400 m, the vessel’s position at the given point should be maintained by the operation of the propellers. At depths from 400 to 50 m, the vessel may anchor. It will be especially rational to use anchors in the form of large boxes with opening bottoms. When filled with sand, these boxes will hold the vessel in the semi-submerged state required for operation, while an excess of buoyant force is present (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 pours out onto the seabed, and the empty boxes are raised to the surface. At depths of less than 50 m, the semi-submerged cruciform vessel should simply lie on the bottom and remain in this state until work at the given point is completed. If the bottom proves uneven, the vessel should rest on it at only two points—the bow part of the longitudinal hull and one of the ends of the transverse hull. Its other two ends should be kept afloat by using the buoyant force of the rear float—hydrofoil.

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 may be built alongside them (if this is not prevented by the depths of the seabed in the area of such wells). However, these foundations now need not be as massive and expensive as in the construction of drilling towers on them; and, most importantly, they need not be built where no oil is found. Thus, the use of the vessel considered here will provide significant 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 borehole into the Mohorovičić layer. Neftyanoe khozyaistvo, 1965, No. 1.

3. A. S. Marchenko. A floating installation for engineering-geological surveys 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. Udynskii. 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.

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