WO1999005389A1

WO1999005389A1 - A floating system for a marine riser

Info

Publication number: WO1999005389A1
Application number: PCT/US1998/015200
Authority: WO
Inventors: Lou W. Watkins
Original assignee: Cuming Corp
Current assignee: Cuming Corp
Priority date: 1997-07-23
Filing date: 1998-07-23
Publication date: 1999-02-04
Anticipated expiration: 2000-01-23
Also published as: AU8580798A

Abstract

Buoyancy modules are integrally shaped externally to reduce VIV formation. The shaped modules are configured and dimensioned to pass intact through a rotary table, so that no time-consuming assembly or disassembly on board a drilling rig is required. Integral molding of the VIV interfering shapes does not add appreciable cost to the system. The integrally molded shaped externally modules of the present invention are inherently less volumetrically efficient, suffering a reduction of 20 % to 30 % less volume than conventional cylindrical modules of the same external diameter. This loss in volume is reflected in an equivalent reduction in buoyancy. The present invention obviates this problem by taking advantage of the fact that it is not necessary to cover the entire riser string with integrally molded shaped modules in order to effect the desired reduction in VIV. Ocean currents are most often restricted to relatively shallow depths, typically the uppermost 300 to 500 feet. In the case of the deep water wells, this amounts to only about 10 % of the total riser length being at risk of damage from currents. In many cases, the lower density of the uppermost modules can be used to advantage to offset some if not all of the buoyancy deficit resulting from their non-cylindrical contours. Thus, in even the most extreme cases, overall buoyancy loss with a system according to the present invention will be only 2.0 to 3.0 % less than that which might otherwise be achieved with conventional systems.

Description

A FLOATING SYSTEM FOR A MARINE RISER

BACKGROUND OF THE INVENTION 1. Field of the invention

This invention relates generally to marine risers connecting submerged well heads to floating drilling rigs, and is concerned in particular with a flotation system for such risers.

2. Description of the Prior Art In a typical deep sea drilling operation, syntactic foam buoyancy modules are employed to impart buoyant lift to the steel riser pipe, thereby eliminating most of the weight of the riser in sea water. The goal is to offset as much of the riser weight as possible without making the riser positively buoyant, which could lead to a dangerous condition. The target is to provide buoyancy equal to about 95% -98% of the riser weight in sea water. This enables safe operation of the drill rig in all kinds of sea states without undue wear and tear on its riser support mechanisms.

The major constraints on the amount of buoyancy that can be provided are (1) the density of the syntactic foam, which must necessarily increase as the water depth increases, and (2) the diameter of the opening in the rotary table on the drilling rig, through which the riser and its attached buoyancy modules must be lowered. Typically, syntactic foam density ranges from 26.0 lbs per cubic foot at a depth of 2,000 feet to 36.0 lbs per cubic foot at 10,000 feet. Rotary table diameter openings are standardized at 39.50", 49.50", and 60.00". In today's increasing water depths, it is often difficult to provide as much buoyancy as the drilling engineer requires, and the rig operator must therefore compromise between safety margins and operating efficiencies over a wide range of conditions.

A further complicating factor is presented by ocean currents which may arise in the drilling area. These currents, which typically range from 1.0 to 6.0 knots, cause two problems: (1) drag, which pulls the riser to one side and increases its apparent weight burden on the drilling rig, and (2) vortex-induced vibration ("VIV"), which can severely reduce the fatigue life of the riser and, in extreme cases, literally shake it to pieces. The threat of encountering ocean currents slows or retards oil exploration in many areas, and prevents it altogether in others. Drilling rigs which have been caught in unexpected currents have suffered severe and costly damage to the riser packages. The danger from unpredicted currents becomes increasingly severe as water depth increases.

Solutions to the ocean current problem have heretofore been of two types: (1) streamlined shrouds commonly referred to as "fairings", which pivot or "weathervane" into the current to reduce both drag and VIV, and (2) helically wound bands called "strakes", which do nothing to reduce drag but do reduce VIV by interfering with the regular formation of vortices in the von Karman vortex street which trails the riser. Both types of devices are attached to the buoyancy modules in the "moonpool" under the drill deck, since they are too large to pass through the rotary table. This assembly operation, which must later be reversed when the riser is retrieved, slows drilling operations and therefore is very costly. The costs of the extra fairing or strake parts adds still further to drilling expenses.

SUMMARY OF THE INVENTION

The present invention is an improvement whereby the buoyancy modules are integrally shaped externally to reduce VIV formation in a way analogous to the strakes referred to above. The shaped modules are configured and dimensioned to pass intact through the rotary table, so that no time-consuming assembly or disassembly on board the drilling rig is required. Integral molding of the VIV interfering shapes does not add appreciable cost to the system.

The integrally molded shaped externally modules of the present invention are inherently less volumetrically efficient, suffering a reduction of 20% to 30% less volume than conventional cylindrical modules of the same external diameter. This loss in volume is reflected in an equivalent reduction in buoyancy.

The present invention obviates this problem by taking advantage of the fact that it is not necessary to cover the entire riser string with integrally molded shaped modules in order to effect the desired reduction in VIV. Ocean currents are most often restricted to relatively shallow depths, typically the uppermost 300 to 500 feet. In the case of the deep water wells, this amounts to only about 10% of the total riser length being at risk of damage from currents. In many cases, the lower density of the uppermost modules can be used to advantage to offset some if not all of the buoyancy deficit resulting from their non-cylindrical contours. Thus, in even the most extreme cases, overall buoyancy loss with a system according to the present invention will be only 2.0 to 3.0% less than that which might otherwise be achieved with conventional systems.

These and other features and advantages of the present invention will now be described in greater detail with reference to the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a somewhat diagrammatic illustration, not to scale, of a drilling installation with a marine riser flotation system in accordance with the present invention; Figure 2 is a three dimensional view of conventional cylindrical buoyancy modules;

Figure 3 is a horizontal sectional view through the buoyancy modules shown in Figure 2;

Figure 4 is a three dimensional view of a buoyancy module in accordance with the present invention;

Figure 5 is a horizontal sectional view through the buoyancy module of Figure 4;

Figure 6 is a side view looking upwardly with respect to Figure 5; and

Figure 7 is a side view at 90° with respect to Figure 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference initially to Figure 1 , a marine riser flotation system is generally depicted at 10. The flotation system is used to encase and impart buoyant lift to a riser string extending downwardly from a drilling rig 12 floating on the ocean surface to a submerged well head 14 at the ocean floor.

In a conventional buoyancy system, as depicted in greater detail in Figures 2 and

3, generally C-shaped syntactic foam module halves 16a, 16b encase the drill pipe 18 and its associated service conduits 20, 22. The module halves are secured together by external straps 24, and are externally configured to provide a cylindrical exterior surface exposed to the surrounding water.

In a typical drilling installation, the riser string is exposed to ocean currents which are most often restricted to relatively shallow depths, typically the uppermost 300- 500 ft as indicated at 26 in Figure 1. As the current flows past the riser string, vortices 28 (see Figure 3) are shed by cylindrical exterior surfaces of the conventional buoyancy modules. If the vortices are allowed to form in regular and repetitive "streets", as shown in Figure 3, VIV formation can result. In accordance with the present invention, as shown in Figures 4-7, the external shape of the module halves 30a, 30b are changed from simple cylinders to a new geometry which prevents the formation of regular and repetitive streets of vortices. Preferably, the new shape is kept as close as possible to the prior art cylindrical configuration so as to conserve volumetric displacement. In the preferred embodiment herein illustrated, the new external shape defines a twisted hexagon having six flat sides , each of which twists to thereby define a helical facet 32.

As can best be seen in Figure 5, the six helical facets are tangent to an inner reference circle 34 and comprise chords of an outer reference circle 36, with the inner and outer reference circles being concentric with respect to a common central axis "A".

The facets 32 extend helically with respect to the axis A. The diameter of the inner reference circle 34 is preferably about 80-90% of the diameter of the outer reference circle 36. The twisted configuration of the facets 32 staggers the formation of vortices

28, thereby avoiding VIV and its damaging consequences. Although six facets have been illustrated in the drawings, any number between about 5 to 8 should operate satisfactorily. The helical angle of twist of the facets should preferably be about 3-6°, with the helically extending facets of successive module assemblies defining continuous helixes extending along the length of the riser encased by the shaped modules. Again with reference to Figure 1 , it will be understood that because only the upper section of the riser string in the region 26 is exposed to potentially dangerous currents, the use of externally contoured module halves 30a, 30b can be limited to that region, with conventional cylindrical module halves 16a, 16b being employed at greater depths, where currents are not a problem. Thus, the higher syntactic foam densities of the module 16a, 16b is offset by the greater volume afforded by their cylindrical configurations. By the same token, the decreased volumes of the uppermost modules 30a, 30b is also offset by their relatively lower syntactic foam densities. Among the advantages of the present invention are its inherent simplicity, ruggedness and economy. Integrally molding vortex-shedding capability into the external surfaces of the module halves adds a margin of safety and operating latitude to the riser with no extra parts and at little if any extra cost. The new shapes are as rugged as conventional cylindrical shapes, and far more durable than separately applied fragile fairings or strakes. Since no shipboard installation and removal of separate components is involved, valuable rig time is more efficiently utilized.

In light of the foregoing, it will now be appreciated by those skilled in the art that other integrally molded vortex-shedding geometries are possible, including pentagons, octagons, ribs, bumps, grooves and dimple arrays similar to those found on the surfaces of golf balls. It is my intention to cover these and any other changed or modifications to the embodiments disclosed herein which do not depart from the spirit and scope of the invention as defined by the appended claims.

I claim:

Claims

CLAIMS 1. A flotation system for a marine riser extending upwardly through a body of water from a submerged well head to a floating drilling rig, said system comprising: a plurality of syntactic foam buoyancy modules; and, means for interconnecting mating pairs of said modules to form assemblies encasing progressively deeper unit lengths of said riser, the modules of at least some of the uppermost of said assemblies having contoured integrally molded exteriors exposed to the surrounding water and defining non-cylindrical shapes configured to suppress vortex-induced vibration of said riser.

2. The flotation system of claim 1 wherein said mating pairs of modules have confronting interior surfaces defining cylindrical axially extending bores sized to accommodate said riser.

3. The flotation system of claim 1 wherein said non-cylindrical exterior surfaces define multiple facets, said facets being tangent to an inner reference circle and comprising chords of a outer reference circle, said inner and outer reference circles being concentric to a common central axis.

4. The flotation system of claim 3 wherein said facets extend helically with respect to said axis.

5. The flotation system of claim 3 wherein the diameter of said inner reference circle is about 80 - 90% of the diameter of said outer reference circle.

6. The flotation system of claim 3 wherein said facets range in number from about 5 to 8.

7. The flotation system of claim 4 wherein said facets extend helically at an angle of twist of about 3 - 6° .

8. The flotation system of claim 1 wherein the modules of at least some of the lowermost of said assemblies have exteriors defining cylindrical shapes.

9. The flotation system of claim 8 wherein the buoyancy provided by the modules of said uppermost assemblies is not less than about 80% of that provided by the buoyancy modules of said lowermost assemblies.

10. The flotation system of claim 4 wherein the helically extending facets of successive assemblies defme continuous helixes extending along the length of said riser.

11. A flotation system for a marine riser submerged in a fluid medium, said system comprising: a plurality of buoyancy modules integrally molded from a syntactic foam material, and means for interconnecting mating pairs of said modules to form assemblies encasing unit lengths of said riser, the modules of each of said assemblies having confronting interior surfaces and non-cylindrical contoured exterior surfaces exposed to said fluid medium.