US3653349A

US3653349A - Self-propelled semi-submersible drilling rig

Info

Publication number: US3653349A
Application number: US38218A
Authority: US
Inventors: Elliot E Brown
Original assignee: Ocean Drilling and Exploration Co
Current assignee: Ocean Drilling and Exploration Co
Priority date: 1970-05-18
Filing date: 1970-05-18
Publication date: 1972-04-04
Anticipated expiration: 1989-04-04
Also published as: FR2088411B1; NL7100096A; GB1302227A; FR2088411A1; DE2062057A1; JPS5218471B1

Abstract

A self-propelled drilling rig of the semi-submersible type for drilling wells at marine locations generally from an afloat position and having hull members provided with self-contained propulsion units for propelling the rig between marine locations.

Description

PATENTEDAPR 4 I972 SHEET 1 [IF 3 Elliot E Brown IN VE N TOR A TTORNEY PATENTEU APR 4 I972 SHEET 2 BF 3 Elliot 5 Brown IN VENTOR ATTORNEY SELF-PROPELLED SEMI-SUBMERSIBLE DRILLING RIG As offshore drilling has progressed into ever deepening waters, bottom-supported drilling units reached their economic depth limit, and the industry has had to develop floating structures for such service. Two general types of drilling structures have been developed and have enjoyed considerable success and acceptance by the oil industry. One type was an adaptation of a conventional ship, usually selfpropelled, by adding moorings to hold it in a drilling position and special machinery for the drilling operation, the drilling operation usually being conducted through a hole cut through the center of the vessel.

The second successful type is the semi-submersible barge, usually of large dimensions, consisting of a lower base assembly or hull, tubular supporting columns, and an upper working deck or platform. The hull in some cases was a substantially continuous hollow, box-like, structure, and in others consisted of a plurality of hollow tubular members connected to fonn a base. The hull was provided with tubular supporting columns on which was mounted an upper working deck or platform. Such a semi-submersible barge is designed to float on its lower hull and normally is moved by tugs or other draft vessels from location to location in this afloat position. When arriving at the drilling location, a number of anchors and chains are streamed, usually with the aid of attending vessels, and the hulls are then flooded in order to partially submerge the unit, hence the designation semi-submersible. In this partially submerged operating position the lower hull is well below the water line, and the upper deck remains substantially above the water line, leaving sufficient clearance under the deck for maximum expected wave height. The columns joining the upper deck and lower hulls provide nautical stability and structural support, and are arranged in such fashion as to achieve these properties and still retain minimum obstruction or maximum transparency to wave action.

It has been well established that the semi-submersible type of platform offers many advantages in operation over the socalled ship-type, the principal one being that of greater stability, meaning that the semi-submersible responds in a much lesser degree to the forces of wind and waves, and its roll, pitch, heave and other movements are considerably less. This minimal movement is of course very desirable for the efficient drilling of wells, as excessive motion makes satisfactory drilling virtually impossible, thus making the semi-submersible particularly advantageous in heavy weather areas. Also, the forces on the moorings are substantially reduced.

These advantages of the semi-submersible are achieved by virtue of its usually large overall dimensions, together with the dispersed and highly transparent configuration of the vertical members and the constant inertia and drag of the large hulllike members at a depth well below the water line, where in addition, they are less affected by waves than they would be at the surface.

The principal disadvantage of the existing large semi-submersible units when compared to the ship-type units lies in its greater difficulty in being moved from place to place, and particularly so on moves of great distances. Heretofore, such semi-submersibles have not been self-propelled and have relied upon outside tugs to tow them. To make such moves safe and efficient, two or more tugs of large horsepower were usually required, and speed of advance has been on the order of 4 knots. On the other hand, a typical self-propelled drilling ship would advance at to 12 knots under its own power and not require outside tugs. The greater speed and saving of tug costs thus give the ships significant competitive advantage on long moves.

I This invention, while preserving the available operating characteristics of a semi-submersible, adds the capability of cruising under its own power for extended periods and will thus greatly reduce and even overcome the present advantages of mobility available to ship-type structures. In order, however, to assure maximum capability as a self-propelled unit, it is important in accordance with this invention, to incorporate certain features of design in the semi-submersible which together with the self-propulsion units result in a unit capable of optimum mobility and operating efficiency.

In accordance with this invention a semi-submersible unit is provided in which the principal supporting hull or base comprises a plurality of elongate, parallel, spaced-apart, tubular elements in the aft ends of at least some of which are mounted propulsion units driven by power sources aboard the vessel. The propulsion units are capable of propelling the rig at speeds comparable to those attained by ship-type units over long distances and add to the operating efficiency of the rig as a whole.

The hull elements have mounted thereon a superstructure comprising a plurality of large diameter vertical columns which are arranged in a generally circular configuration adapted to provide uniform stability in all directions and which are arranged to afford a maximum degree of nautical stability under all conditions. The superstructure includes a working deck, and the drilling location is disposed in the center of the structure, again affording maximum stability under all operating conditions.

The propulsion system comprises propulsion units mounted in at least two of the parallel hull members, preferably the inboard members. Each unit basically consists of a five bladed propeller enclosed within a Kort nozzle, a known type of accelerating nozzle, which is attached to the aft end of the hull member and made steerable for purposes to be described hereinafter. The propeller is driven by a straight shaft coupled directly to the DC. electric motor. The nozzle is pivoted in a support structure and turned by an electro-hydraulic steering system.

Electric power for the propulsion motors is provided by the main diesel-electric power plant required for the drilling operation. The propulsion system is economically adaptable to a mobile drilling unit since the main power plant is not needed during moves of the rig and thus becomes available for driving the propulsion motors. As the horsepower requirements are about the same for drilling and for propulsion, power supplied by a diesel-electric system can be readily transferred by a simple switching system from drilling to propulsion, thereby rendering the system relatively simple and economically attractive.

Other and more specific objects and advantages of the invention will become more readily apparent from the following description when read in conjunction with the accompanying drawing which illustrates a useful embodiment in accordance with this invention.

In the drawing:

FIG. 1 is a perspective view of the complete drilling rig;

FIG. 2 is a transverse sectional view taken generally along lines 2 2 ofFlG. 1;

FIG. 3 is a longitudinal sectional view taken generally in line 3 3 of FIG. 2 showing one of the propulsion units;

FIG. 4 is a detail of one of the steerable propeller units;

FIG. 5 is a side profile of the rig structure; and

FIG. 6 is an aft end elevational view taken on line 6 6 of FIG. 5.

Referring to the drawing, the drilling rig comprises the base or main hull member designated generally by the numeral 10, a superstructure, designated generally by the numeral 12, mounted on the base member and in turn supporting a working platform, designated generally by the numeral 14.

In the illustrative embodiment the base member comprises a plurality of parallel spaced-apart hull elements of hollow cylindrical form and includes a pair of inboard members 16 l6 and a pair of outboard members 18 18 of lesser length than the inboard members and centrally disposed longitudinally with respect to the inboard members.

These hull members are of quite large diameter, for example 25 feet 6 inches, and together provide substantially all the required buoyancy for floating the structure. Although not illustrated, it will be understood that these hull members will be divided by means of suitable bulkheads into a plurality of compartments which will be connected to a suitable system for ballasting and de-ballasting the compartments, all in accordance with known practices, to effect submergence and raising of the rig as required in connection with drilling and towing operations to be conducted therewith. Some of the compartments in the hull members may be employed for storage of fresh water, fuel oil and the like.

The superstructure comprises a plurality of large diameter hollow cylindrical columns 19 totaling eight in number, two of which are mounted on each of the hull members spaced near the opposite ends thereof. These columns 19 which provide nautical stability, particularly in raising and submerging of the rig, also provide support for the working platform and its loads, and are arranged in an annular configuration defined by a circle C (shown in broken lines in FIG. 2) which passes through the centers of all of the columns 19 and has its own center coincident with the center of the drilling location indicated at 20 in FIG. 2, which is located on the intersection of the longitudinal and transverse center lines of the rig. The angular spacing of columns 19 on the circle, while not necessarily equal in all cases, generally provides a symmetrical arrangement about center 20. Opposed columns 19 mounted on outboard members 18 are connected by hollow cylindrical cross members 21 21 disposed at a substantial elevation above the several hull members and are secured inboard members 16 by risers 22. The columns 19 mounted on inboard hull members 16 are likewise interconnected by hollow cylindrical cross members 25, likewise located at a substantial height above the hull elements. The height of the several cross members above the hull members will preferably be selected such that they will be well above the water surface when the hull members are submerged to normal towing draft to reduce the drag during towing of the rig. A number of additional columns 24 substantially smaller in diameter than stabilizing columns 19 are mounted to extend between the hull members and the platform to provide additional support therefor and to generally strengthen the structure. A few additional horizontally disposed cylindrical members 25 are employed to connect the stabilizing columns as bracing members; however, it will be evident that with the geometric configuration illustrated, the superstructure involves the use of a minimum number of structural elements thereby providing maximum wave transparency for the structure while assuring adequate strength under all conditions.

By providing the generally circular configuration of the main stabilizing column a number of advantages result. This configuration eliminates any directional weakness when considering the energy required to overcome wind heeling force for stability considerations. Also, the amount of topside weight allowables are increased by this configuration.

By locating the rotary table of the drilling rig in the center of the vessel, both longitudinally and transversely, lesser pitch and roll motions at the rotary table during drilling operations will result.

By making the various cross members circular in shape, drag effects due to current are minimized as well as inertia and drag forces due to waves while at drilling draft.

Mounted to the aft end of the inboard hull members are rearwardly tapering conical extensions 26 .26 which house and support the propulsion units. The latter, as best seen in FIGS. 3 and 4, comprise five bladed propellers 30 mounted for rotation on straight shafts 31 extending through the centers of extensions 26 for connection to power units 32 which are preferably D.C. electric motors. The current will be supplied to the motors from conventional diesel-electric generating units (not shown) of generally conventional form, mounted on the working platform. Each propeller is enclosed within an accelerating type flow nozzle 33 which may be of the well known Kort form. The nozzle is mounted on upper and

lower shafts

34 and 35 respectively journaled in a bracket 36 mounted on the aft end of extension 26 to render the nozzle steerable. Upper shaft 34 extends into a housing 37 and is secured therein to the center of a tiller arm 38, the opposite ends of which are connected to fluid pressure-operated steering cylinders 39 39 which are powered by a conventional steering motor and pump 40 mounted in extension 26 (FIG. 3.).

By making the Kort nozzle 33 steerable it becomes an effective steering device, being referred to as a Kort rudder, which directs the nozzle thrust as required to create a turning moment in the vessel. This directed thrust is not dependent on the vessel speed as with a conventional rudder. By the use of these rudders the vessel may be steered by varying and reversing the speeds of the propellers in the opposite hulls. Thus, these steerable nozzles provide a safety measure in case one propeller is damaged in which case one of the rudders could be used to hold the vessel on a straight course. Moreover, these rudders are also highly useful in maneuvering on location. For example, the propulsion motors will be used while the vessel is moored on drilling location to reduce the loads on the moorings during extreme weather thus reducing the likelihood of anchor or chain failure. Another advantage of these propulsion devices is that the installation of the anchor system at the drilling site is facilitated by the unit's capability of moving itself about or maintaining its position without assistance from tugs while auxiliary vessels stream the anchors. Moreover, the propulsion devices will also prove useful for assisting with dynamically positioning the unit when drilling in deep water where conventional moorings are not feasable.

Working platform 14 will be provided with the various housings for men and equipment as shown in FIG. 1. Drilling derrick D is shown mounted over the center of the vessel through which drilling will be conducted as required.

in operation, the rig has variable draft capability. Vessel motions (depending on the height and period of the seas) in any particular location will dictate what operating draft results in the least vessel motion. The propulsion units are sufficiently powered to self-propel the rig for any required distance at a satisfactory rate of speed, say 8 knots per hour. The system described is usable for propulsion at drafts varying from that at which the vessel is normally towed (about 19 feet) down to and including the normal drilling draft (about 70 feet). While in transit the vessel can submerge to drafts deeper than the normal towing draft to minimize adverse motions resulting from heavy seas, and still maintain headway. Such drafts could be expected to vary from about 45 to about 60 feet depending on sea conditions.

By adding additional propellers or thrusters oriented transverse to the fore and aft axis of the vessel, additional chain tension relief in that direction can be gained. Further, addition of a central control system would then result in a self-positioning vessel, capable of operating in deep water without mooring chains.

What I claim and desire to secure by Letters Patent is:

l. A self-propelled semi-submersible drilling rig, comprising:

a. a submersible base structure comprising a plurality of hollow, cylindrical, elongate, spaced-apart parallel hull members; said base structure comprising:

i. a pair of inboard members; and

ii. a pair of outboard members shorter in length than the inboard members and substantially centered longitudinally with respect to the latter;

b. a wave-transparent stabilizing superstructure mounted on the base structure; said superstructure comprising;

i. a plurality of vertical hollow cylindrical stabilizing columns disposed in angularly spaced-apart relation in a configuration defined by a circle passing through the centers of said columns and whose center coincides with the intersection of the longitudinal and transverse center lines of the rig;

ii. hollow cylindrical cross members extending transversely of said rig and connecting opposed ones of said columns at elevations above said hull members; and

iii. cylindrical risers connecting said inboard members to at least some of said cross members; i c. a working platform supported on said superstructure; and

d. steerable propulsion means mounted on at least two of said hull members for propelling said rig through a water body.

2. A drilling rig according to claim 1 wherein said superstructure comprises:

a. two of said stabilizing columns mounted on each of said hull members disposed adjacent the opposite ends of said hull members; and

b. additional vertical columns smaller in diameter than said stabilizing columns supporting said platform from said hull members at points intermediate said stabilizing columns.

3. A drilling rig according to claim 1 wherein each of said propulsion means comprises:

a. a propeller rotatably mounted on the aft end of the hull member;

b. a power unit enclosed within said aft end of the hull member drivingly connected to said propeller;

c. a steerable accelerating nozzle enclosing said propeller and mounted to the hull member for pivotal movement about said propeller to steer said rig; and

d. means on the hull member for steering said nozzle.

4. A drilling rig according to claim 3 wherein said means for steering said nozzle comprises:

a. a rudder post connected to said nozzle;

b. a tiller arm secured to said post; and

c. fluid pressure-operated means operably connected to said tiller arm for turning the same.

5. A drilling rig according to claim 1 having a well drilling unit centered on the intersection of the longitudinal and transverse center lines of the rig.

6. In a drilling rig according to claim 1:

a. rearwardly tapering conical extensions on the aft ends of said inboard members for housing and supporting said propulsion means.

7. A drilling rig according to claim 3 wherein said nonle is a Kort nozzle.

Claims

1. A self-propelled semi-submersible drilling rig, comprising: a. a submersible base structure comprising a plurality of hollow, cylindrical, elongate, spaced-apart parallel hull members; said base structure comprising: i. a pair of inboard members; and ii. a pair of outboard members shorter in length than the inboard members and substantially centered longitudinally with respect to the latter; b. a wave-transparent stabilizing superstructure mounted on the base structure; said superstructure comprising; i. a plurality of vertical hollow cylindrical stabilizing columns disposed in angularly spaced-apart relation in a configuration defined by a circle passing through the centers of said columns and whose center coincides with the intersection of the longitudinal and transverse center lines of the rig; ii. hollow cylindrical cross members extending transversely of said rig and connecting opposed ones of said columns at elevations above said hull members; and iii. cylindrical risers connecting said inboard members to at least some of said cross members; c. a working platform supported on said superstructure; and d. steerable propulsion means mounted on at least two of said hull members for propelling said rig through a water body.

2. A drilling rig according to claim 1 wherein said superstructure comprises: a. two of said stabilizing columns mounted on each of said hull members disposed adjacent the opposite ends of said hull members; and b. additional vertical columns smaller in diameter than said stabilizing columns supporting said platform from said hull members at points intermediate said stabilizing columns.

3. A drilling rig according to claim 1 wherein each of said propulsion means comprises: a. a propeller rotatably mounted on the aft end of the hull member; b. a power unit enclosed within said aft end of the hull member drivingly connected to said propeller; c. a steerable accelerating nozzle enclosing said propeller and mounted to the hull member for pivotal movement about said propeller to steer said rig; and d. means on the hull member for steering said nozzle.

4. A drilling rig according to claim 3 wherein said means for steering said nozzle comprises: a. a rudder post connected to said nozzle; b. a tiller arm secured to said post; and c. fluid pressure-operated means operably connected to said tiller arm for turning the same.

6. In a drilling rig according to claim 1: a. rearwardly tapering conical extensions on the aft ends of said inboard members for housing and supporting said propulsion means.

7. A drilling rig according to claim 3 wherein said nozzle is a Kort nozzle.