ON THE ISSUE OF CONSTRUCTING OIL AND GAS FIELDS IN SEA AREAS WITH DRIFTING ICE

T. A. Pukhtyakov

(Presented by Professor A. V. Aksarin)

Floating ice is capable of damaging even the strongest pile foundations. In this connection, in sea areas with drifting ice, drilling should be carried out from floating foundations. A large number of types of floating foundations are known [1, 3, 4]; however, all of them experience a certain rolling motion. Below a type of vessel is described whose rolling motion will be minimal. It is called a trimaran and consists of a platform resting on three rocking float-vessels (Figs. 1 and 2).

As is known, under any rolling motion of a vessel, its center of gravity either remains completely motionless (when the length of the vessel exceeds the wavelength), or relatively motionless (when the wavelength exceeds the length of the vessel). In this case the amplitude of motion of the center of gravity of a given vessel will be the greater, the smaller the length of the vessel is in relation to the wavelength. Approximately this quantity may be expressed by the relation

\[ s = h \frac{\lambda - l}{\lambda}, \]

where \(s\) is the amplitude of the rise and fall of the vessel’s center of gravity,
\(h\) is the wave height,
\(\lambda\) is the wavelength, and \(l\) is the length of the vessel.

At the present time, technology makes it possible to build vessels of rather great length. Thus, there is a real possibility of having vessels whose centers of gravity will remain practically motionless under any sea conditions.

The basis of the trimaran consists in the proposal to use three float-vessels, which are arranged in such a way that their centers of gravity form an equilateral triangle. In this case the first of these vessels, 4 (Fig. 2), must be the bow vessel, and the other two, 3, stern vessels. Platform 1 rests on these vessels by means of struts 2. The struts are connected to the load-bearing rods of the float-vessels by hinges. This allows the float-vessels to undergo oscillatory motions relative to the platform in the longitudinal plane. In the transverse plane the float-vessels must not undergo oscillatory motions. This will be prevented by the load-bearing rods, held in a position immovable relative to the platform by the trunnions of the struts.

On the platform, at its geometric center, a drilling derrick 5 is located (Figs. 1, 2).

Fig. 1. Trimaran platform for drilling on three rocking floating vessels (side view). 1—platform proper; 2—platform support columns; 3—stern floating vessels; 4—bow floating vessel; 5—drilling derrick.

Fig. 2. Trimaran platform for drilling on three rocking floating vessels (top view). The designations are the same as in Fig. 1.

It is assumed that on the platform itself there should be located only the equipment that is directly related to the drilling process: winches, mud mixers, etc. Power equipment, reserves of fuel and materials, as well as living quarters for the drillers, should be located on the floating vessels.

But drilling through the structure does not yet mean beginning oil production on it, since floating ice can damage not only the pile foundations for the drilling rigs, but also the platforms with wellhead equipment installed on them. In the USA, in connection with this, wells with underwater wellheads began to be used as early as 1959. At present, the construction of such wells has become fairly widespread [5].

Among the advantages of wells with underwater wellheads, apart from the fact that they are not afraid of floating ice, it should be noted that waves and wind do not have a harmful effect on them, and also that on such wells the possibility of fires is practically eliminated. However, they also have a fairly serious drawback, namely that they are very difficult to service; in particular, produ-

…their repair. In fact, the simplest work under underwater conditions proves to be very difficult, since diving suits greatly impede a person’s movements. In this connection, J. V. Clark [2] recommends using small submarines with manipulators to carry out certain operations on such wells. Of course, the cost of such boats would be very high; nevertheless, some operations would still be impossible to perform with their aid.

Maintenance work on these wells could be carried out with greater efficiency if their wellhead equipment 1 (Fig. 3) were located not on the bottom itself, but on special platforms 2, raised to a certain height above the bottom on piles 3 (Fig. 3). The casing string of such a well 4 must also be raised to this platform. And in order to bring the working conditions at such a well as close as possible to land conditions, the platform should be covered with a metal hood 5 filled with air. For the hood to be stable on the platform, it must be provided with a special projection 6. The presence of this projection makes it possible to install the hood in such a way that its lower edge is below the level of the platform, while the platform itself is above the water level (Fig. 3).

A new question arises: how can a pile foundation be built with a platform located below the water level? Apparently, construction of such platforms should begin no earlier than the completion of the well construction and the receipt from it of a commercial inflow of oil. The proposed work procedure is as follows.

1. The well is filled with clay mud, and the required number of piles is driven next to it.

2. From the drilling vessel, a caisson is installed over the piles and lowered to a certain depth (five to six meters). After this, a considerable part of the piles will be under above-water conditions.

Fig. 3. Layout of the wellhead equipment of wells in an underwater field. 1 — Christmas-tree fittings; 2 — platform for placing the wellhead equipment of the well; 3 — piles for installing the platform; 4 — well casing string projecting above the bottom level; 5 — hood; 6 — projection for installing the hood; 7 — transition gallery; 8 — piles for installing the transition gallery; 9 — rest chamber for maintenance personnel; 10 — water-sealing elbows; 11 — pump for pumping water out of the water-sealing elbows; 12 — pipeline for pumping out water; 13 — branch pipe for discharging, by the pump, water into the sea; 14 — sump (recess in the water-sealing elbow to improve pumping of water from it); 15 — air line for supplying air to the connecting gallery; 16 — oil pipelines (well flowlines).

1. The piles are cut off at the specified depth, and a platform for placing the wellhead equipment is welded to them or fastened by some other method.

2. At the specified depth, the casing string of the well is cut off, and either a Christmas tree or other equipment is installed on it. At the same time, discharge lines are led away from it. After this, the well is again stimulated, and the stream of fluid is directed into the discharge line (16) (Fig. 3).

3. The caisson is raised, and the cap described above is installed on the well.

In order that the field workers, while in underwater conditions, may move freely from the mouth of one well to the mouth of another, it is recommended that the platforms be connected by passage galleries 7 (Fig. 3). The passage galleries are to be installed, like the platforms near the wells, on piles 8. At certain points in such galleries it is necessary to place rest chambers 9, and at the ends, water-sealing bends 10.

Figure 3 shows the operating scheme of such a bend. It amounts to the following. Suppose that the cap has been removed from one of the wells. The platform of such a well will immediately be flooded with water, which will also fill the water-sealing bend. However, the water will not pass farther than the bend into the gallery. It will be prevented by the air pressure inside the gallery (see Fig. 3). After a new cap has been installed over the given well, air is intensively pumped into the gallery through air line 15; it breaks through under the cap and gradually fills it. The liquid level under the cap then gradually drops, and its lowering continues until it reaches the lower edge of the cap. After this, pump 11 is switched on; by means of pipeline 12 it pumps water out of the given bend and, through nozzle 13, discharges it into the sea. In order that removal of water from the water-sealing bend may be more complete, a recess, or sump, 14 is made in it, directly from which pipeline 12 begins.

By means of galleries similar to the one described, it is possible to connect not only individual wells but also the entire field with the shore, if the field is located comparatively close to the coast. If, however, the field is located far from the coast, then it will be more advantageous to transport the service personnel to the field by other means, for example, in small underwater boats.

Of course, servicing underwater fields will not be possible without diving work, but it will be many times less in volume than such work at a field where the well mouths are located on the very bottom of the sea. The situation is similar when comparing such fields with onshore ones. Labor costs at them will exceed labor costs in comparison with ordinary fields. However, the need for motor fuel requires development of as large a number of oil fields as possible, including those located in sea areas with periodically appearing ice.

A significant reduction in the cost of servicing such fields may be provided by automation, which at present is being applied ever more widely at onshore fields. Here, incidentally, it should be noted that modern automatic devices, without special alteration, can be transferred to conditions of somewhat elevated pressures, which will exist under the caps in underwater conditions. Most of these automatic devices will require fundamental redesign if they are to be adapted for work on the seabed without being covered by caps filled with air.

References

1. E. Jermen. Oil developments of England in the North Sea.—In: Oil and the Sea. Vol. I. “Nedra,” Moscow, 1967.

2. J. V. Clark. Remote control of operations on the ocean floor.—In: Oil and the Sea. Vol. II. “Nedra,” Moscow, 1968.

3. G. Lill. The Mohol Project.—In: Oil and the Sea. Vol. I. “Nedra,” Moscow, 1967.

4. L. A. Lukyantsev. On the question of drilling in offshore oil-bearing areas. Izvestiya TPI, Vol. 217, 1971.

5. K. Horn. Methods of completion and operation of offshore wells.—In: Oil and the Sea. Vol. II. “Nedra,” Moscow, 1968.