GB2339730A - Floating caisson with lower section of reduced cross-sectional diameter - Google Patents

Description

2339730 FLOATING CA-LSQONS FOR OFFSHORE DRILLING/PRODUCING The invention

relates to floating caissons for offshore drilling/producing, which may be used for drilling 5 and/or producing hydrocarbons offshore.

In the offshore oil industry, floating structures are used in areas where deep water results in the cost of a jacket fixed to the sea floor being too expensive to realize a sufficient economic return, even for large oil reserves. Such f loating structures have been semi-submersibles, a column stabilized vessel that is moored in place by the use of multiple anchors, single column spar type structures that are moored in place by multiple anchors, dynamically positioned vessels that use a number of thrusters to hold the vessel in position at the site, and tension leg platforms (TLP's).

Each structure has advantages and disadvantages. For example, while dynamically positioned vessels eliminate the need for anchors and mooring lines, they present a large surface area to waves and currents, which can result in a substantial amount of power required to hold the vessel in position. The large surface area also results in the vessel being subject to heave, pitch, and roll motions in response to wave action. The semisubmersibles present less surface area to waves and so are less susceptible to pitch and roll motions but are still subject to heave motions and are not designed to store large quantities of oil.

minimizing environmentally induced motions is desirable not only from a safety and comfort standpoint, but also from an operational standpoint since drilling and producing risers which are connected from the vessel to the sea floor wellhead must be designed to accommodate the motions of the structure. The cost of designing and building risers is directly related to the amount of heave, pitch, and roll of the structure, as well as the wave, current, and gravity forces acting on the risers themselves.

The TLP is relatively successful at minimizing heave, pitch, and roll. However, the TLP is a relatively shallow draft structure that is expensive and limited to moderate water depths.

Further, it is virtually immobile' once it has been installed.

As the deck loads to be supported by the spar type structures (a single column hull such as that described in U.S.

Patent No. 4,702,321) have increased (20,000 tons to as much as 40,000 tons), the diameter of the spar hull has necessarily become larger. As the diameter of these spar hulls becomes larger, fabrication becomes more difficult. Transportation of large spar type structure to the installation site, whether on heavy lift vessel or by floating the completed hull, may also present difficulties. Prior spar structures have been designed and built with uniform cross sections as shown in U.S. Patent No.

4,702,321 and also with enlarged lower hull cross sections such as in the Brent Spar.

C The present invention provides a self buoyant, floating deep draft caisson for use in drilling and/or production of wells offshore. The cross sectional diameter of the lower portion of the caisson is less than the upper portion of the caisson. The reduced cross sectional diameter of the lower portion increases the natural period of the structure in heave and also increases the hydrodynamic damping of the structure in heave.

The caisson is self buoyant, preferably by means of a buoyant upper section and is held in position by mooring lines. The caisson may have a center well through which drilling and/or production risers pass. The reduced cross section portion is generally below the hard buoyancy tanks. Water entrapment plates and/or helical strakes may also be utilized in conjunction with the caisson.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:

Fig. I is a side view of a caisson embodying the invention; Fig. 2 is a side sectional view of the caisson of Fig.

1; Fig. 3 is a side view of an alternative embodiment of the invention; Fig. 4 is a view taken along lines 4-4 in Fig. 3; Fig. 5 is a side view of another alternative embodiment of the invention; Fig. 6 is a side view of a further alternative embodiment of the invention; Figs. 7A and 7B illustrate one method of transportation and loading/unloading of the alternative embodiment of Fig. 3; Fig. 8 illustrates, from the alternative embodiment of Fig. 3, the principle of entrapment plates adding hydrodynamic mass and increasing the natural period of the structure; and Figs. 9 and 10 compare a conventional spar type structure with that of an embodiment of the invention, and illustrate the principle of increasing the righting moment by increasing the distance between the center of buoyancy and the center of gravity of the fixed ballast.

Referring to the drawings, Fig. 1 shows a side view of a caisson 10 according to an embodiment of the invention.

Although the basic structure of a typical floating caisson 10 is generally known, for example as described in U.S. Patent No. 4,702,321, a general description of the structure of caisson 10 is provided for the sake of clarity. As seen in Fig. 2, caisson 10 is self buoyant by means of buoyancy tanks 12. Caisson 10 may include variable ballast tanks 14, oil storage compartments 16, trim tanks 18, and fixed ballast 20. As seen in Figs. 1 and 2, caisson 10 may be held in position at the offshore site by mooring lines 22. Mooring lines 22 are attached to one end to fairleads 23 on the caisson and at the other end to anchors or pilings, not shown, that are driven into the sea floor. Risers, not shown, may be used for drilling or production operations and extend through a center well 24 in the caisson. A deck 26 is supported on top of the caisson 10. The deck 26 provides space for operating equipment and working and living quarters for personnel.A drilling rig 28 may be positioned on the deck 26 for drilling operations.

Whereas a typical floating caisson 11, seen in Fig. 9, will have a uniform cross section throughout its length, caisson 10 has a cross section taken along its lower portion 13 that is of a lesser diameter than a cross section taken along its upper portion 15. The reduced diameter of the lower portion 13 preferably begins below buoyancy tanks 12 and 14. Cone shaped section 17 serves as a transition from the larger diameter upper portion 15 to the smaller diameter lower portion 13.

The preferred ratio between the diameter of the lower portion 13 and the upper portion lies in the range of 0.9 to 0.5.

Reducing the diameter of the lower portion 13 of the caisson, relative to the upper portion 15, provides several benefits, some of which seem contrary to what would be expected.

The structural steel per unit length of cylinder is significantly reduced. This means that the overall length of the lower portion 13 can be increased for the same weight of structural steel required for a shorter caisson of uniform cross section throughout its length.

The increased length, in turn, increases the effectiveness of fixed ballast 20 at the keel, which in turn decreases the pitch of the caisson in response to environmental forces. This is illustrated in Fig. 9 and 10 where the length between the center of buoyancy and fixed ballast, represented by the letter is compared between a typical caisson 11 of uniform cross section to the caisson 10 of the invention. Fig. 9 and 10 also illustrate the principle of increasing the righting moment and thereby creating a more effective use of fixed ballast utilizing the same amount of structural steel in the lower portion 13 of the caisson 10.

Current loading on the lower portion 13 of the caisson 10 is reduced as a result of the reduced cross section area.

Fig. 3 and 4 illustrate an alternate embodiment of the invention wherein one or more plates 30 are rigidly attached to the lower portion 13 and extend radially outward therefrom. The plates 30 are preferably at the depth at which the water is relatively unaffected by wave motion when the caisson is in its operative installed position. It is preferable that the plates 30 have a width such that the combined diameter of the lower portion and the plates 30 be equal to or approximately equal to the diameter of the upper portion 15. Therefore, it can be seen that the ratio between the diameter of the lower 13 and the width of the plates 30 will be dependent upon the difference in diameter between the lower portion 13 and upper portion 15. For example: If the outer diameter of the upper portion 15 is one hundred feet and the outer diameter of the lower portion is eighty feet, then the width of the plates 30 would be approximately twenty-five percent of the diameter of the lower portion 13. However, if the outer diameter of the upper portion is one hundred feet and the outer diameter is ninety feet, then the width of the plates 30 will be approximately only eleven percent of the diameter of the lower portion 13.

The plates 30 provide the following benefits. The plates entrap water, thereby increasing the effective mass of the caisson, which in turn gives it a longer natural period, thus shifting it away from the period of maximum wave energy and, therefore making it less responsive in heave. The plates 30 also provide additional hydrodynamic damping in the vertical direction to the caisson because of the turbulent flow around the plates.

This, in turn, limits the amplitude of heave resulting from any excitation forces at resonance with the natural period of the structure. The benefits and advantages of plates 30 are more fully discussed in U.S. Patent No. 5,722,797.

Fig. 7A and B illustrate the loading/offloading and transportation of the alternate embodiment of the caisson in Fig.

3 on a heavy lift vessel 32. The vessel 32 is ballasted down to load or unload the caisson and deballasted for transportation.

A benefit of the plates 30 and lower portion 13 having a combined diameter that is approximately the same as the upper portion 15 is realized during these operations, since the need for special supports to maintain the caisson in a level position is eliminated.

Fig. 5 illustrates an alternate embodiment of the invention wherein one or more helical strakes 34 are rigidly attached to the lower portion 13 and extend around and along the length of the lower portion 13. Strakes 34 serve to reduce vortex induced vibrations that may result from water current flowing around the caisson. The strakes break up the vortices that tend to form around a bare cylinder. Providing strakes 34 on the lower portion 13 can be particularly important when the caisson is operating in regions of deep, high velocity currents because the strakes limit the current induced motions and reduce the drag force on the caisson.

Fig. 6 illustrates an alternate embodiment of the invention where both the plates 30 and strakes 34 are included on the lower portion 13.

Because many varying and differing embodiments may be made within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.

Claims (4)

1 A self buoyant, floating deep draft caisson for use in drilling and/or production of wells offshore, the caisson having upper and lower portions, with the lower portion having a cross sectional diameter that is smaller than the cross sectional diameter of the upper portion of the caisson.

2. A caisson according to claim 1, comprising one or more plates that extend radially outwards from the lower portion of the caisson at a depth at which the water is relatively unaffected by wave motion when the caisson is in its operative installed position.

3. A caisson according to claim 1 or claim 2, comprising a plurality of helical strakes that extend around and along the length of the lower portion of the caisson.

4. A self buoyant, floating deep draft caisson for use in drilling and/or production of wells offshore, wherein:

the caisson has upper and lower portions, with the lower portion having a cross sectional diameter that is smaller than the cross sectional diameter of the upper 25 portion of the caisson; one or more plates extend radially outwards from the lower portion of the caisson at a depth at which the water is relatively unaf f ected by wave motion when the caisson is in its operative installed position; and a plurality of helical strakes extend around and along the length of the lower portion of the caisson.

S. A self buoyant, floating deep draft caisson for use in drilling and/or production of wells offshore, the caisson being substantially as herein described with reference to and as illustrated in Figs. 1 to 8 and Fig. 10 of the accompanying drawings.