AU686061B2

AU686061B2 - Floating caisson for offshore drilling

Info

Publication number: AU686061B2
Application number: AU14989/97A
Authority: AU
Inventors: Edward E Horton III
Original assignee: Deep Oil Technology Inc
Current assignee: Deep Oil Technology Inc
Priority date: 1996-03-04
Filing date: 1997-02-27
Publication date: 1998-01-29
Anticipated expiration: 2017-02-27
Also published as: BR9701167A; NO970796L; GB9703324D0; NO970796D0; GB2310832A; AU1498997A

Description

p -1- P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT .r *r a..

Invention Title: FLOATING CAISSON FOR OFFSHORE DRILLING The following statement is a full description of this invention, including the best method of performing it known to us: GH REF: P24704-C:RPW:RK CASE 5749 FLOATING CAISSON FOR OFFSHORE DRILLING BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is generally related to an offshore apparatus for use in drilling and production of offshore wells and more particularly to a dynamically positioned deep draft floating caisson.

10Q. 2. General Background '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 S 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 CASE 5749 -2vessel that is less susceptible to waves and currents and uses less power to maintain position.

SUMMARY OF THE INVENTION The invention addresses the above need. What is provided is a floating caisson that is particularly useful for offshore exploratory and development drilling that includes 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 0 caisson. The caisson has a center well through which drilling and/or production risers pass. The caisson has a cross section r o shape that is streamlined to reduce the drag load caused by ocean currents and waves.

BRIEF DESCRIPTION OF THE DRAWINGS For further understanding of the present invention reference should be had to the following description, taken in conjunction with the accompanying drawing in which like parts are given like reference numerals, and wherein: e Fig. 1 is a side sectional view of an embodiment of the invention.

Fig. 2 is a perspective view of an embodiment of the invention.

Fig. 3 is a perspective view of an alternate embodiment of the invention.

Fig. 4 is a side view of an alternate embodiment of the invention.

Fig. 5A and 5B are views taken along lines 5-5 in Fig. i.

Fig. 6A and 6B are views taken along lines 6-6 in Fig. 4.

CASE 5749 -3- DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, it is seen in Fig. 1 that the invention is generally indicated by the numeral 10. Although the basic structure of floating caisson 10 is known as that 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. i, caisson 10 is self buoyant by means of buoyancy tanks 12, and is of uniform cross section throughout the length ~of the caisson 10. Caisson 10 may include variable ballast 14, free flooding area 16, and fixed ballast tanks 18. Risers used S for drilling or production extend through a center well 20 in caisson 10 up to a blow-out-preventer at the surface. A deck 22 and drilling rig 24 may be positioned on top of caisson Caisson 10 has a cross section shape that is streamlined to reduce the drag load caused by ocean currents and waves. As seen in the preferred embodiment of Fig. 2, 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 Fig. 1, the leading edge 26 of 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 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 CASE 5749 -4is substantially parallel to the centerline of the caisson As indicated by the arrows, athwartship thrusters 30 provide a thrust line that is substantially perpendicular to the centerline of the caisson 10. The centerline of the caisson 10 extends from the forward most portion of the leading edge 26 to the trailing edge 32. Two or more of fore and aft thrusters 28 may be provided on each side of the caisson 10 if necessary. Thrusters 28 and 30 are preferably variable pitch thrusters. The use of e* S tunnel thrusters for thrusters 30 allows them to be safely housed 19 inside the hull and create little or no drag from currents that Swould have to be overcome by fore and aft thrusters 28.

As indicated in Fig. 2, the thrusters 28 and 30 are located

S

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 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 when compared to a circular caisson that may be eighty to one hundred feet in diameter. It is estimated that the streamlined caisson 10 of Fig. 2 will reduce the drag coefficient by a factor of approximately five when compared to a circular caisson.

Center well 20, also illustrated in Fig. 1 and 5A, extends vertically through the caisson 10 to accommodate drilling risers or other equipment necessary during exploratory and development CASE 5749 drilling operations.

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

Fig. 3 illustrates an alternate embodiment of the caisson, indicated by numeral 110. Caisson 110 is elliptical in shape to reduce the drag load caused by ocean currents and waves. Caisson lQ 110 may be constructed with a convex leading edge 126 and a convex trailing edge 132 to define an elliptical shape. It should be understood that the side sectional view of Fig. 1 is o..

applicable to the embodiment of Fig. 2 and Fig. 3. When in its installed position as seen in Fig. 1, the leading edge 126 of caisson 110 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 110 without the need for mooring lines. The positioning and installation of the thrusters and operation of the caisson are the same as described above in reference to the embodiment of Fig. 2. It is estimated that the streamlined caisson 110 of Fig. 3 will reduce the drag coefficient by a factor of approximately three to five when compared to a circular caisson. Center well 20, also illustrated in Fig. 1 and 5B, extends vertically through the caisson 110 to accommodate drilling risers or other equipment necessary during exploratory and development drilling operations.

Fig. 4, 6A, and 6B illustrate an alternate embodiment of the CASE 5749 invention wherein the caisson 10 or 110 is provided with a plurality of plates 34 along the length of the caisson 10/110.

Plates 34 begin at or near the lower end of caisson 10/110 and are spaced along the length of caisson 10/110. Plates 34 are rigidly attached to the caisson 10/110 to extend radially outward from the caisson 10/110 so as to be horizontal relative to the water surface 36 when caisson 10/110 is in its installed position at sea. The plates 34 act to effectively trap water between the ~plates during heave, pitch, and roll motions induced by waves and Q currents. The effectively trapped water gives additional mass, which increases the natural period of caisson 10/110 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 requires less strengthening for towing and upending, which means that less steel is required to build the caisson and thus reduces 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 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. Although the plates 34 are illustrated as rectangular, they may be of any shape and size in order to obtain the desired effect upon the natural period of the caisson :.0/110.

Because many varying and differing embodiments may be made CASE 5749 -7within 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.

0*V, 4 oo**

S

S.

ee* *o,

Claims

1. n-~Aa self-buoyant, deep draft floating caisson for use in drilling, workover, production, and/or storage offshore, the improvement comprising: the self-buoyant, deep draft floating caisson having a convex leading edge that tapers inwardly to a narrower trailing edge; the self-buoyant, deep draft floating caisson having a centre well that extends vertically through the caisison; first thrusters attached to each side of the self-buoyant, deep draft floating caisson, each of the first thrusters having a thrust line substantially parallel to the leading edge to trailing edge centre line of the self-buoyant, deep draft floating caisson; and second thrusters positioned adjacent the trailing edge of the self-buoyant, deep draft floating caisson and having a thrust line that is substantially perpendicular to the leading edge to trailing edge centre line of the self-buoyant, deep draft floating caisson, the first thrusters and second thrusters eliminating the need for mooring lines to hold the self-buoyant, deep draft floating caisson in position.

2. The self-buoyant, deep draft floating caisson of 25 claim i, further comprising a at least one plate that extends radially outward from the self-buoyant, deep 0 draft floating caisson below the water level when the O self-buoyant, deep draft floating caisson is in its operative installed position, the at least one plate S 30 acting to effectively entrap water during heave motions whereby the additional mass of the entrapped water increases the natural period of the self-buoyant, deep draft floating caisson in heave beyond the periods of maximum wave energy.

3. l Aa self-buoyant, deep drafting floating caisson for use in drilling, workover, production, and/or r: storage offshore, the improvement comprising: i the self-buoyant, deep draft floating caisson S:24704C/703 9 having a convex leading edge that tapers inwardly to a narrower trailing edge to define a symmetrical airfoil shape; the self-buoyant, deep draft floating caisson having a centre well that extends vertically through the self-buoyant, deep draft floating caisson; fore and aft thrusters attached to each side of the self-buoyant, deep draft floating caisson; and athwartship thrusters on the self-buoyant, deep draft floating caisson, the fore and aft thrusters and athwartship thrusters eliminating the need for mooring lines to hold the self-buoyant, deep draft floating caisson in position.

4. The self-buoyant, deep draft floating caisson of claim 3, wherein the fore and aft thrusters and athwartship thrusters are variable pitch.

The self-buoyant, deep draft floating caisson of claim 3, wherein the fore and aft thrusters have a thrust line substantially parallel to the leading edge to trailing edge centre line of the self-buoyant, deep draft floating caisson.

6. The self-buoyant, deep draft floating caisson of claim 3, wherein the athwartship thrusters have a thrust line that is substantially perpendicular to the leading 25 edge to trailing edge centre line of the self-buoyant, deep draft floating caisson. A

7. -Ina self-buoyant, deep draft floating caisson for use in drilling, workover, production; and/or storage offshore, the improvement comprising: 30 a self-buoyant, deep draft floating caisson having a convex leading edge and a convex trailing edge to define a substantially elliptical shape; a self-buoyant, deep draft floating caisson having a centre well that extends vertically through the self-buoyant, deep draft floating caisson; first thrusters attached to each side of the f C C self-buoyant, deep draft floating caisson, each of the c V thrusters having a thrust line substantially parallel to S:24704C/703 10 the leadig edge to trailing edge centre line of the self-buoyant, deep draft floating caisson; and second thrusters positioned adjacent to the trailing edge of the self-buoyant, deep draft floating caisson and having a thrust line that is substantially perpendicular to the leading edge to trailing edge centre line of the self-buoyant, deep draft floating caisson, the first thrusters and second thrusters eliminating the need for mooring lines to hold the caisson in position. fo s ?UO c P cLU-ot c-FtCs-hi, CCiiSQ 0s cJoIV\di I-

8. Thc caaacon of claim 7, frther comprising at least one plate that extends radially outward from the self-buoyant, deep draft floating caisson below the water level when the caisson is in its operative installed position, the at least one plate acting to effectively entrap water during heave motions whereby the additional mass of the entrapped water increase the natural period of the self-buoyant, deep draft floating caisson in heave beyond the periods of maximum wave energy.

9. A self-buoyant, deep draft floating caisson substantially as herein described with reference to the accompanying drawings. e Dated this 7th day of August 1997 DEEP OIL TECHNOLOGY, INCORPORATED SBy their Patent Attorney 25 GRIFFITH HACK S:24704C/703 CASE 5749 ABSTRACT OF THE DISCLOSURE A dynamically positioned floating caisson particularly useful for offshore exploratory and development drilling that includes means for Lreducing 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 vertical center well through which drilling and/or production risers pass. The caisson has a cross section shape that is streamlined to reduce the drag load caused by ocean currents and waves. o c 0* os