WO1999057011A1

WO1999057011A1 - Dynamically positioned semi-submersible drilling vessel

Info

Publication number: WO1999057011A1
Application number: PCT/US1999/009325
Authority: WO
Inventors: Eugene E. Begnaud; Calvin V. Norton; Bruce G. Malcolm
Original assignee: Friede and Goldman Ltd
Current assignee: Friede and Goldman Ltd
Priority date: 1998-05-01
Filing date: 1999-04-29
Publication date: 1999-11-11
Anticipated expiration: 2000-11-01
Also published as: AU3872099A

Abstract

The invention relates to an offshore structure (10), and more particularly to a semi-submersible vessel designed to operate in harsh environment, such as North Sea, Newfoundland and the like. The vessel has a twin-hull pontoon structure (12, 14) that supports four corner caissons (20, 22, 24, 26) extending vertically from the pontoons above an operational draft of the vessel. A pair of horizontal braces (38, 40) connect opposing pairs of caissons, the braces extending in a general plane perpendicular to longitudinal axes of the pontoons. Each brace has a 'redundancy' feature, being divided into two independent watertight compartments. If one compartment wall is damaged, the other half allows to retain operational stability of the vessel in at least a survival draft. Station keeping is accomplished with a full dynamically positioned system consisting of a plurality of thruster assemblies mounted on the starboard and port sides of the pontoon hull. Each pontoon (12, 14) has a passageway for allowing access to the thruster assembly for installation and repair, if necessary.

Description

DYNAMICALLY POSITIONED SEMI-SUBMERSIBLE DRILLING VESSEL

BACKGROUND OF THE INVENTION

The invention relates to an offshore drilling unit, and more particularly to a semi-submersible vessel for conducting offshore operations in moderate or severe environments, such as the Gulf of Mexico, the North Sea, offshore

Newfoundland, and the like.

In recent years, the drilling operations have been conducted at increasingly greater distance from the shoreline, placing the offshore production or drilling facilities in often severe weather conditions. In such environments it is particularly important to have a stable floating facility for supporting the mineral exploration and production operations, as well as providing living accommodations to the crew and storage for the necessary equipment. In deep waters, over 7500 feet, it becomes particularly advantageous to deploy floating semi-submersible vessels, as opposed to fixed bottom anchored structures. Designs of semi-submersible vessels utilize buoyant pontoons, or lower hulls which support a plurality of vertically extending columns, the upper portions of which carry a working platform. Some of the semi-submersible vessels can have a single caisson, or column, usually denoted as a buoy while others utilize three or more columns extended upwardly from buoyant pontoons.

In many such structures, vertical braces are used between the columns, the braces contributing to the water plane area of the vessel. The braces are usually constructed with smaller diameters than that of the columns and are therefore more vulnerable to the environmental and mechanical damage. If the connecting braces are damaged, the entire structure becomes jeopardized.

Many known semi-submersible vessels are made with independent propulsion system, often in the form of thrusters that are attached to the hulls, or pontoons and facilitate moving of a vessel from one location to another. The thruster system also helps to retain the semi-submersible vessel at a predetermined location and reduce the loads on risers, tendons and other lines extending to the ocean floor. Another consideration that is taken into account when designing semi- submersible vessels is resistance of the vessels to heave and roll motion induced by waves. The vessels must have sufficient stability to withstand wave motions to allow the mineral exploration and production operations to be carried out in safety. This consideration becomes particularly important in harsh environments where strong winds and waves are prevalent throughout the year. The present invention contemplates provision of a semi-submersible vessel, particularly adapted for use in hostile environment, a vessel with improved safety features and high maneuverability.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a semi- submersible vessel with improved safety features.

It is another object of the invention to provide a semi-submersible vessel with dynamically positioned station keeping system that operates in the absence of anchors. These and other objects of the invention are achieved through a provision of a semi-submersible vessel for use in particularly harsh environment. The vessel comprises a pair of parallel buoyant pontoons having a generally rectangular configuration and divided into a plurality of independent compartments. Four column, or caissons are mounted on the pontoons and extend vertically to support the upper deck of an offshore platform suitable for conducting mineral exploration or production operations. The vessel is moored at a location using dynamically positioned thruster assemblies mounted on the pontoons. The thruster unit extends below the pontoon, while a motor which independently drives a respective thruster unit, is mounted inside a thruster room within the pontoon. A passageway between the thruster rooms in the pontoons allows access to the thruster assemblies, with the passageway communicating with the platform at a level above an operating draft of the vessel.

A pair of horizontal braces are secured between opposing pairs of columns, the braces extending generally perpendicularly to longitudinal axes of the elongated pontoons. The braces are provided with "redundancy" features - they contain two independent watertight compartments. A bulkhead divides an oblong brace into two independent sections, so as to retain stability of the vessel even if the structural integrity of a compartment wall is compromised by wave motions or physical objects striking the brace. In that manner, even if one section of the wall is damaged, the second independent buoyant compartment will help to retain structural capability at severe environmental occurrences.

To minimize drag forces induced by wave motions, the columns and hulls of the pontoons are provided with outwardly convex, rounded corners.

This design allows for less drag force to be transmitted when mooring or moving the vessel. The pontoon hulls have compartments for storing ballasts, drill water and other necessary supplies. The columns, or caissons house reserve mud tanks, ballasts tanks, force air ventilation supply and other necessary machinery and equipment. It is an integrated box deck structure that houses drilling and ship service equipment, power generation, storage for liquid mud and other miscellaneous objects. Living quarters and service equipment is mounted on the platform, as well.

BRIEF DESCRIPTION OF THE DRAWINGS Reference will now be made to the drawings, wherein like parts are designated by like numerals, and wherein Figure 1 is a perspective view of the semi- submersible vessel in accordance with the present invention.

Figure 2 is an outboard profile illustration of the vessel in accordance with the present invention. Figure 3 is a cross-sectional of the brace for use in the vessel of the present invention.

Figure 4 is an elevation of bow of the semi-submersible vessel of the present invention.

Figure 5 is an elevational view of a stern of the vessel of the present invention.

Figure 6 is a longitudinal sectional view of starboard outboard portion of the vessel.

Figure 7 is an elevational view of columns forming part of the vessel of the present invention. Figure 8 is a schematic view illustrating a general arrangement of a pontoon hull.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings in more detail, numeral 10 designates the semi-submersible vessel of the present invention. The vessel comprises a pair of floating buoyant hulls, or pontoons 12, 14 divided into a plurality of watertight compartments for accommodating ballasts, as well as allowing access to thruster assemblies, as will be described in more detail hereinafter. Secured to the upper portions 16 and 18 of the pontoons 12 and 14, respectively, are columns, or caissons 20, 22, 24 and 26 that extend in a tranverse relationship to the vertical axes of the pontoons 12 and 14.

The upper portions of the columns 20, 22, 24, and 26 carry a platform 30 adapted for supporting mineral exploration and production operations. As shown in Figure 2, parts of the columns 20-26, as well as pontoons 12 and 14 are submerged below the water surface to an operational draft 32, while the upper portions of the columns 20-26 and platform 30 are elevated to a level above maximum expected wave for a particular location. The ballast chambers in the pontoons 12 and 14 provide the necessary room for introducing a ballast, for example, sea water to partially submerge the structure. The operational draft 32 is generally higher than a survival draft 34

(Figure 2) by 15 to 25 feet. When the vessel 10 encounters particularly harsh conditions, wave and hydrostatic forces act on the pontoons and on the columns, causing the vessel 10 to move vertically and angularly, subjecting the vessel to heave and pitch motions of the wave. Heave and pitch or roll motion is due to the wave actions on the pontoons and columns. These forces which cause vertical and angular motions of the vessel result in the vessel shifting in relation to a vertical to axis 36 of the vessel 10. The angular motion of the vessel 10 is reacted by inertia and hydrostatic forces.

The horizontal braces 38 and 40 extend between columns 20, 24, and 22-26, as can be seen in Figures 1, 4 and 5. The horizontal braces 38 and 40 connect the opposing pairs of columns at a vertical level above the pontoons 12 and 14 and below the survival draft 34 of the vessel 10.

Each brace 38 and 40 is provided with a "redundancy" feature. As shown in Figure 3, each brace 38 and 40 has a generally oval, or oblong cross section and is provided with an internal bulk head 42 which divides the brace 38, or 40 into two equal longitudinal sections. The bulk head 42 defines two independent compartments 44 and 46 in each of the slender braces 38 and 40. The compartments are watertight and if the wall of one of the compartments is damaged or penetrated, the second compartment will survive, continuing to connect the columns and contribute to the stability of the vessel 10 on the water.

To improve the drag characteristics of the vessel, that is to minimize drag, the bow corners of the pontoons are formed with rounded vertical side walls 50, 52, 54 and 56 (Figure 1, bow portion). Angular vertical side walls 51,

53, 55, and 57 are provided in the aft sections of the pontoons 12 and 14, as can be seen in Figures 1 and 8.

As can be better seen in Figure 7, each column, or caisson 20, 22, 24, and 26 has rounded corners 60 of a discrete radius. This radius can be, for example 1/4 or greater of the depth or width of the generally square caisson.

Such rounded corners allow to minimize drag forces, while still providing the necessary strength to the corner columns. Additionally, the constructability of the caissons is greatly improved, as flat panels can be mostly used.

The vessel 10 is provided with a full dynamically positioned system of station keeping. The system consists of eight 360° fixed pitch variable speed azimuthing thruster assemblies secured to the bottom of the pontoon 12, and in a preferred embodiment, a pair of thruster assemblies is located at starboard and a pair at port side of each pontoon 12 and 14 (See Figure 8). It is believed that this system is sufficient for water depths up to 10,000 feet. Mounted on the main deck 64 of the platform 30 are a pair of winches

66, one at each forward column 20 and 24 (Figure 4). The winches 66 are designed to hold a length of wire rope for harbor mooring. If desired, the number of winches can be increased up to eight, for an 8-point pre-set mooring configuration. Turning now to Figure 6 of the drawings, the thruster units 62 are seen connected to a drive motor means 68 positioned in a specially provided thruster room 70. The motor means 68 can be AC drive, or other similar means. To facilitate maintenance, installation and removal of thrusters 62, a passageway 72 is formed in the hull body 12 and 14. The passageway 72 extends between thruster rooms 70, the passageway 72 communicating with platform 30 at a level above the operational draft 32. The thruster rooms 68 can be accessed directly from the upper deck 64, or cranes can be used to get access to the thruster rooms 70.

The vessel 10 uses ballasted pontoons and caissons for operational stability. The substructure loads are distributed through columns 20-26.

The caissons 20-26 carry a box-shaped buoyant attachments 74 secured at a general area of survival draft 34. By carefully selecting the position of the members 74, it is possible to maximize the survival conditions of the vessel. An additional benefit of the buoyant members 74 is that there is no reduction in load capability on the vessel between operating draft and survival draft.

The members 74 give variable load capability and water plane displacement. The member 74 do not substantially affect water plane area at operational draft as their position is below the draft level 32. It is important that the members 74 do not extend past the flat portion of the caissons 20-26, but rather extend in vertical alignment with the outer wall of the lower hull pontoon. Since the outer limits of the attachments 74 do not pass the vertical line of the pontoon outer wall, ease of construction of the vessel 10 at dry dock is facilitated. The outer limits of the width of the outer walls of the members 74 terminate before the outwardly convex corners 60 of the columns 20-26 begin to curve.

The vessel 10 provides a facility for conducting mineral exploration in relatively deep waters of up to 10,000 feet. The vessel 10 is equipped with a pair of cranes 80, 82 mounted on crane pedestals 84. Drilling and production operations are conducted through the platform 30 in a manner well known to those skilled in the art. Riser sections 86 (Figure 5) are stored on the main deck, and can be delivered to the drill floor 88 in an efficient, cost effective manner.

The compartments formed in the pontoon 12 and 14, in addition to holding salt water ballasts are equipped for housing pumps, storing drill water, diesel oil and other necessary materials. The caissons 20-24 are also compartmentalized to hold reserve mud tanks, brine tanks, base oil tanks, ballast tanks, vents, ventilation supplies and return pipes for lower hull and column machinery compartments.

Other equipment necessary for offshore operations, such as, sewerage treatment unit, portable water tanks, storage tanks can be positioned in other 7 compartments within the columns 20-26. The upper hull, or platform 30 structure uses an integrated box deck structure to house drilling and ship service equipment, power generation and storage for liquid mud, sacks, and other variable materials and equipment. Two gantry cranes are provided. One of the gantry cranes 90 can be used to run a riser, while another gantry crane 92 can be provided over the blowout preventer for splitting the storage area stack. This storage area can also be used to run sub-sea trees. The living quarters on the platform 10 can accommodate the necessary number of crew members for conducting the mineral exploration and production operations. The rounded corners of the pontoons and columns require less effort to maintain position of the vessel at a selected location above the drilling area. As a result, less power is required, less fuel is used, and the mineral exploration and production operations run more efficiently. The vessel 10 uses no anchors for remaining in place but rather a dynamically positioned system utilizing azimuthing thrusters for increased efficiency of the vessel.

Many changes and modifications can be made in the design of the present invention without departing from the spirit thereof. I, therefore, pray that my rights to the present invention be limited only by the scope of the appended claims.

Claims

8We Claim:

1. A semi-submersible vessel, comprising: a pair of buoyant pontoons; a plurality of vertically extending columns supported by said pontoons, upper portions of said pontoons carrying a platform adapted for conducting offshore operations therefrom; and at least a pair of horizontal braces extending between said columns, each of said braces being divided into a pair of independent watertight compartments by a longitudinal separation wall so as to retain operational stability of the vessel when structural integrity of a compartment wall is compromised.

2. The vessel of claim 1 , wherein each of said braces has a generally oval cross-section to minimize drag force on each of said braces induced by wave motions.

3. The vessel of Claim 1 , further comprising a dynamically positioned station keeping means connected to said pontoons for retaining said vessel in a desired offshore location.

4. The vessel of Claim 3, wherein said station keeping means comprises a plurality of independently controlled thruster assemblies.

5. The vessel of Claim 4, wherein each of said thruster assemblies comprises a thruster unit mounted below a pontoon and a driving motor positioned inside a pontoon and operationally connected to said thruster unit.

6. The vessel of Claim 4, wherein a passageway is formed in each of said pontoons, said passageway allowing access to said thruster assemblies, said passageway communicating with said platform at a level above an operational draft of the vessel.

7. The vessel of Claim 3, wherein said mooring means comprises a plurality of independently operating thruster assemblies, said thruster assemblies being mounted in pairs adjacent a starboard and port side of each of said pontoons.

8. The vessel of Claim 1, wherein each of said columns has an integral vertical wall, said wall being provided with outwardly convex corners to facilitate ease of station keeping and retention of said vessel in a designated offshore location by minimizing hydrodynamic drag forces.

9. . The vessel of Claim 8, wherein a radius of each of said convex corners is at least one fourth of the width of the column.

10. The vessel of Claim 1 , wherein each of said pontoons comprises a hull body having a bow portion, said bow portion having curved corners to minimize drag forces induced by current forces on the vessel and transit speed.

11. A semi-submersible vessel, comprising: a pair of parallel buoyant pontoons; a plurality of vertically extending buoyant caissons supported by said pontoons and arranged in pairs on each of said pontoons, upper portions of said caissons supporting a platform for conducting offshore operations; at least a pair of horizontal braces extending between said caissons in a transverse relationship to longitudinal axes of said pontoons; and a station keeping means for retaining said vessel in a desired offshore location, said station keeping means comprising a plurality of independently operating thruster assemblies mounted on said pontoons, said thruster assemblies being accessible through a passageway formed in each of said pontoons, said passageway communicating with said platform from a level above an operational draft of said vessel.

12. The vessel of Claim 11 , wherein said thruster assemblies are mounted in pairs along a starboard and port side of each of said pontoons.

13. The vessel of Claim 11 , wherein each of said braces is divided into a pair of watertight compartments divided by a longitudinal centerline bulkhead so as to retain operational stability of the vessel when structural integrity of a compartment wall is compromised.

14. The vessel of Claim 11 , wherein each of said braces has a generally oblong cross-section so as to minimize hydrodynamic drag forces.

15. The vessel of Claim 11 , wherein each of said caissons has an integral exterior vertical wall, said wall being provided with outwardly convex corners to decrease drag forces acting on said vessel at an offshore location.

16. The vessel of Claim 15, wherein a radius of said convex corner is at least one fourth the width of the caisson.

17. The vessel of Claim 11 , wherein each of said pontoons has a generally rectangular cross section with outwardly convex corners to decrease drag forces acting on said vessel at an offshore location.

18. The vessel of Claim 17, wherein a radius of said convex corner is at least one fourth the depth of each of said pontoon for minimizing lateral or horizontal drag forces from current velocities.