GB2310832A

GB2310832A - Floating caisson for offshore drilling, workover, production, and/or storage

Info

Publication number: GB2310832A
Application number: GB9703324A
Authority: GB
Inventors: Edward E Horton Iii
Original assignee: Deep Oil Technology Inc
Current assignee: Deep Oil Technology Inc
Priority date: 1996-03-04
Filing date: 1997-02-18
Publication date: 1997-09-10
Also published as: GB9703324D0; AU1498997A; BR9701167A; AU686061B2; NO970796D0; NO970796L

Description

FLOATING CAISSON FOR OFFSHORE DRILLING, WORKOVER. PRODUCTION. AND/OR STORAGE The invention relates to an offshore apparatus for use in drilling, workover, production, and/or storage, and more particularly to a self-buoyant floating caisson for these purposes.

During exploratory and development drilling, it is more economical to minimize the amount of equipment involved until the presence of oil and the potential value of the field can be determined. This is especially true in the offshore drilling industry, where transportation and drilling costs are much higher than land based operations. The offshore drilling industry has approached such concerns through the use of floating vessels or structures that do not require pilings or mooring line anchors to be driven into the sea floor. Such floating structures have typically been semi-submersible jack-up rigs, a vessel that is moored in place by the use of multiple anchors, or dynamically positioned barges that use a number of thrusters to hold the barge in position at the site. More permanent structures are installed for long term drilling and production operations after the field has been determined to be of sufficient value. Dynamically positioned vessels tend to present a large surface to waves and currents, which can result in a substantial amount of power being required to hold the vessel in position.

Therefore, this leaves the need for a dynamically positioned vessel that is less susceptible to waves and currents and uses less power to maintain position.

A first aspect of the invention provides a self-buoyant, floating caisson for use in drilling, workover, production, and/or storage offshore, the caisson comprising: a convex leading edge providing a streamlined shape in cross-section to a trailing edge; a centre well that extends vertically through the caisson; and a thruster attached to each side of said caisson.

A second aspect of the invention provides a self-buoyant, floating caisson for use in drilling, workover, production, and/or storage offshore, the caisson comprising: a convex leading edge that tapers inwardly to a narrower trailing edge to define a symmetrical airfoil shape; a centre well that extends vertically through the caisson; a fore and aft thruster attached to each side of said caisson; and an athwartship thruster on said caisson.

A third aspect of the invention provides a self-buoyant, floating caisson for use in drilling, workover, production, and/or storage offshore, the caisson comprising: a convex leading edge and a convex trailing edge defining a substantially elliptical cross-sectional shape; a centre well that extends vertically through the caisson; and a thruster attached to each side of said caisson.

Embodiments of the invention provide a floating caisson that is particularly useful for offshore exploratory and development drilling, including means for reducing the amount of power required to hold the caisson in position. The caisson is self-buoyant by means of buoyancy tanks. A deck and drilling rig are positioned on top of the caisson. The caisson has a centre well through which drilling and/or production risers may pass. The caisson has a cross-sectional shape that is streamlined to reduce the drag load caused by ocean currents and waves.

For a further understanding of the nature of the present invention, reference should be made to the following description, taken in conjunction with the accompanying drawings in which like parts are given like reference numerals, and wherein: Figure 1 is a side sectional view of one embodiment of the invention; Figure 2 is a perspective view of the embodiment of Figure 1; Figure 3 is a perspective view of an alternative embodiment of the invention; Figure 4 is a side view of another alternative embodiment of the invention; Figures 5A and 5B are views taken along lines 5-5 in Figure 1; and Figures 6A and 6B are views taken along lines 6-6 in Figure 4.

Referring to the drawings, there is shown in Figure 1 a floating caisson 10 according to one embodiment of the invention. Although the basic structure of the floating caisson 10 is known, for example as described in US Patent No. 4702 321, a general description of the structure of the caisson 10 is provided for the sake of clarity. As seen in Figure 1, the caisson 10 is self-buoyant by means of buoyancy tanks 12, and is of uniform cross-section throughout the length of the caisson 10. The caisson 10 may include variable ballast 14, a free flooding area 16, and fixed ballast tanks 18. Risers (not shown) used for drilling or production extend through a centre well 20 in the caisson 10 up to a blow-out-preventer (not shown) at the surface. A deck 22 and a drilling rig 24 may be positioned on top of the caisson 10.

The caisson 10 has a cross-sectional shape that is streamlined to reduce the drag load caused by ocean currents and waves. As seen in the preferred embodiment of Figure 2, the caisson 10 may be constructed with a convex leading edge 26 that tapers inwardly to a narrower trailing edge 32 in the shape of a symmetrical air foil. When in its installed position as seen in Figure 1, the leading edge 26 of the caisson 10 faces into the current.

Fore and aft thrusters 28, on each side of the caisson, and athwartship thrusters 30, are used to maintain position of the caisson 10 without the need for mooring lines. Although only one fore and aft thruster 28 is shown for ease of illustration, it should be understood that at least one such thruster 28 is provided on each side of the caisson 10 such that the thrusters 28 are substantially opposite each other and provide a thrust line that is substantially parallel to the centre line of the caisson 10. As indicated by the arrows, the athwartship thrusters 30 provide a thrust line that is substantially perpendicular to the centre line of the caisson 1 0. The centre line of the caisson 10 extends from the forwardmost portion of the leading edge 26 to the trailing edge 32. Two or more fore and aft thrusters 28 may be provided on each side of the caisson 10 if necessary. The thrusters 28 and 30 are preferably variable pitch thrusters. The use of tunnel thrusters for the thrusters 30 allows them to be safely housed inside the hull and create little or no drag from currents that would otherwise have to be overcome by the fore and aft thrusters 28.

As indicated in Figure 2, the thrusters 28 and 30 are located a sufficient distance below the water line 36 for maximum effectiveness when the caisson 10 is in its normal installed floating position. It is anticipated that the thrusters 28 will be at least approximately thirty metres (approximately one hundred feet) below the water surface.

The symmetrical airfoil shape results in a smoother flow of water across the caisson 10. This results in the need for less power by the thrusters to maintain position of the caisson 10 when compared to a circular caisson that may be about twenty five to thirty metres (eighty to one hundred feet) in diameter. It is estimated that the streamlined caisson 10 of Figure 2 will reduce the drag coefficient by a factor of approximately five when compared to a circular caisson of comparable size. The centre well 20, also illustrated in Figures 1 and 5A, extends vertically through the caisson 10 to accommodate drilling risers or other equipment necessary during exploratory and development drilling operations.

In operation, after the caisson 10 is in position with the deck 22, the drilling rig 24, and all associated equipment installed, the leading edge 26 of the caisson 10 is turned to face the prevailing current. The thrusters 28 and 30 are used to maintain the position of the caisson 10 for drilling operations.

Figure 3 illustrates an alternative embodiment of the caisson, indicated by the numeral 11 0. The caisson 110 is elliptical in shape to reduce the drag load caused by ocean currents and waves. The caisson 110 may be constructed with a convex leading edge 1 26 and a convex trailing edge 1 32 to define an elliptical shape. It should be understood that the side sectional view of Figure 1 is applicable to both the embodiments of Figure 2 and Figure 3. When in its installed position as seen in Figure 1, the leading edge 1 26 of the caisson 110 faces into the current. The fore and aft thrusters 28, on each side of the caisson, and the athwartship thrusters 30, are used to maintain position of the caisson 110 without the need for mooring lines.

The positioning and installation of the thrusters and operation of the caisson 110 are the same as described above with reference to the embodiment of Figure 2. It is estimated that the streamlined caisson 110 of Figure 3 will reduce the drag coefficient by a factor of approximately three to five when compared to a circular caisson of comparable size. The centre well 20, also illustrated in Figures 1 and 5B, extends vertically through the caisson 110 to accommodate drilling risers or other equipment necessary during exploratory and development drilling operations.

Figures 4, 6A and 6B illustrate another alternative embodiment of the invention wherein the caisson 10 or 110 is provided with a plurality of plates 34 along the length of the caisson 10/110. The plates 34 begin at or near the lower end of the caisson 10/110 and are spaced along the length of the caisson 10/110. The plates 34 are rigidly attached to the caisson 10/110 to extend radially outwards from the caisson 10/110 so as to be horizontal relative to the water surface 36 when the caisson 10/110 is in its installed position at sea. The plates 34 act effectively to trap water between the plates during heave, pitch, and roll motions induced by waves and currents.

The effectively-trapped water gives additional mass, which increases the natural period of the caisson 1 0/11 0 and shifts the natural period beyond the periods of maximum wave energy. This results in the ability to design the caisson 10/110 to have a shallower draft than would be required without the plates. Another advantage is that the shorter length of the caisson requires less strengthening for towing and upending, which means that less steel is required to build the caisson, thus reducing the cost. Also, the caisson can be used in shallower water with the plates 34. The upper portion of the caisson 10/110 extends above the water approximately fifteen metres (approximately fifty feet) and supports the drilling rig 24 and deck 22.

Although the plates 34 are illustrated as being evenly spaced apart, the spacing between the plates may vary depending upon the desired effect upon the natural period of the caisson 10/110. Also, although the plates 34 are illustrated as being rectangular, they may be of any shape and size in order to obtain the desired effect upon the natural period of the caisson 10/110.

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 embodiments herein detailed, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.

Claims

1. A self-buoyant, floating caisson for use in drilling, workover, production, and/or storage offshore, the caisson comprising: a convex leading edge providing a streamlined shape in cross-section to a trailing edge; a centre well that extends vertically through the caisson; and a thruster attached to each side of said caisson.

2. A caisson according to claim 1, wherein said thrusters have a thrust line substantially parallel to the leading edge to trailing edge centre line of said caisson.

3. A caisson according to claim 2, comprising a second thruster having a thrust line that is substantially perpendicular to the leading edge to trailing edge centre line of said caisson.

4. A caisson according to claim 1, claim 2 or claim 3, comprising one or more plates that extend radially outwards from the caisson below the water level when the caisson is in its operative installed position.

5. A self-buoyant, floating caisson for use in drilling, workover, production, and/or storage offshore, the caisson comprising: a convex leading edge that tapers inwardly to a narrower trailing edge to define a symmetrical airfoil shape; a centre well that extends vertically through the caisson; a fore and aft thruster attached to each side of said caisson; and an athwartship thruster on said caisson.

6. A caisson according to claim 5, wherein said thrusters are variable pitch.

7. A caisson according to claim 5 or claim 6, wherein said fore and aft thrusters have a thrust line substantially parallel to the leading edge to trailing edge centre line of said caisson.

8. A caisson according to claim 5, claim 6 or claim 7, wherein said athwartship thruster has a thrust line that is substantially perpendicular to the leading edge to trailing edge centre line of said caisson.

9. A self-buoyant, floating caisson for use in drilling, workover, production, and/or storage offshore, the caisson comprising: a convex leading edge and a convex trailing edge defining a substantially elliptical cross-sectional shape; a centre well that extends vertically through the caisson; and a thruster attached to each side of said caisson.

1 0. A caisson according to claim 9, wherein said thrusters have a thrust line substantially parallel to the leading edge to trailing edge centre line of said caisson.

11. A caisson according to claim 9 or claim 10, comprising a second thruster having a thrust line that is substantially perpendicular to the leading edge to trailing edge centre line of said caisson.

1 2. A caisson according to claim 9, claim 10 or claim 11, comprising one or more plates that extend radially outwards from the caisson below the water level when the caisson is in its operative installed position.

1 3. A self-buoyant, floating caisson for use in drilling, workover, production, and/or storage offshore, the caisson being substantially as herein described with reference to and as illustrated in the accompanying drawings.