FAIRING FOR MARINE DRILLING RISERS

Abstract

A fairing includes two half-shells of fiber reinforced plastic or fiberglass. The shell halves are shaped to close around the buoyancy modules (e.g., cylindrical syntactic foam buoyancy modules) commonly used on deep-water risers, but with an extended trailing edge. The fairing includes a tail that streamlines the riser and reduces/prevents formation of the vortex, which causes drag and vibration. Hinges at the tail join the halves and enable them to be opened for assembly around the riser pipe as it is being run into the water. The fairing may be fastened together by bolted stainless steel tensioners that tightly draw together straps of high strength aramid fiber, such as KEVLAR®. The fairing is positioned on the riser such that the fairing is free to swivel and rotate into the current, while integral buoyancy blocks support the tail. To prevent movement up or down the riser string, the fairing is held in place by a channel clamp gripping the outside diameter of the syntactic foam buoyancy module.

Description

PRIORITY INFORMATION

This application claims priority under 35 U.S.C. §119(e) from U.S. provisional application Ser. No. 60/834,219 filed Jul. 28, 2006, which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

As offshore oil and gas exploration moves into deeper and more inhospitable waters, the danger of equipment failure increases. One of the hazards now being encountered is that of high-speed (e.g., 3 knots and greater) currents damaging the riser pipe that connects a floating drill rig to its wellhead on the ocean floor. Such currents exert a massive drag force on the pipe while causing it to shake violently in a phenomenon called vortex-induced vibration (VIV).

Fairings, strakes, shrouds, and similar devices have been used for many years in an attempt to prevent riser damage by currents. These devices have been only partly successful because they were (1) overly complex and expensive; (2) difficult and time-consuming to assemble on board the rig; and (3) lack the necessary robustness to withstand the rigors of shipboard use.

There is a need for a fairing that decreases drag and vortex induced vibration to marine risers.

SUMMARY OF THE INVENTION

A fairing includes two half-shells of fiber reinforced plastic or fiberglass. The shell halves are shaped to close around the buoyancy modules (e.g., cylindrical syntactic foam buoyancy modules) commonly used on deep-water risers, but with an extended trailing edge. The fairing includes a tail that streamlines the riser and reduces/prevents formation of the vortex, which causes drag and vibration. Hinges at the tail join the halves and enable them to be opened for assembly around the riser pipe as it is being run into the water. The fairing may be fastened together by bolted stainless steel tensioners that tightly draw together straps of high strength aramid fiber, such as KEVLAR®. The fairing is positioned on the riser such that the fairing is free to swivel and rotate into the current, while integral buoyancy blocks support the tail. To prevent movement up or down the riser string, the fairing is held in place by a channel clamp gripping the outside diameter of the syntactic foam buoyancy module.

A number of different clamp and fairing arrangements are of course possible. The half-shells may be molded. The integral buoyancy module may include syntactic foam.

Significantly, the channel clamp is secured in place around a buoyancy module by a strap, such a KELVAR synthetic fiber strap. These straps may be tightened securely in place by a tension rod. The tension rod may include a thread bar and nut such that as the nut is run up the strap is tightened to secure the clamp to the buoyancy module.

In one embodiment, the adjustable clamp may be mounted such that a fairing is mounted around the clamp so the fairing weathervanes with the current. The adjustable clamp may also be mounted externally to the fairing(s), for example to hold one or more fairing in place along the buoyancy modules. For example, a plurality of fairings may be securely positioned between first and second adjustable clamps.

These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a top view and several side views of the fairing;

FIG. 2 illustrates a perspective view of the fairing in the open position; and

FIGS. 3A-3C illustrate several different fairing and clamp arrangements on a riser.

DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a top view and several side views of a fairing 10. The fairing 10 includes first and second fairing halves 12, 14 that are hinged and operatively secured together. The first and second fairing halves 12, 14 are preferably constructed of fiberglass. The first fairing half 12 includes a first sidewall 16 (e.g., semi-circular), while the second fairing half 14 includes a second sidewall 18 (e.g., semi-circular). The first and second fairing halves 12, 14 include first and second tails 20, 22 respectively, which extend from their associated fairing half and are joined together at a hinge 24.

FIG. 2 illustrates a perspective view of the fairing 10 in the open position. The fairing also includes first and second synthetic straps 26, 28, such as for example KEVLAR straps that are used with associated stainless steel tensioners and suitcase latches to securely close the fairing in an operable position around the buoyancy module. Strong and flexible straps of aramid fiber attach the fairings tightly, yet have enough elasticity to accommodate riser motion under the most difficult conditions of bad weather and high current.

Buoyancy material 30, 32 such as syntactic foam may be placed along interior sidewalls of the first and second tails 20, 22 to increase the buoyancy of the fairing.

To secure the fairing 10 to the riser buoyancy module an adjustable channel clamp 40 (e.g., metallic such as aluminum) that includes first and second halves 42, 44 is placed around the riser buoyancy material and closed. The clamp is secured in the closed position with a synthetic fiber strap 46 (e.g., a KEVLAR strap). The strap 46 may be tightened by a tension rod to secure the channel clamp in place on the riser buoyancy module. The tension rod may include a thread bar and nut, such that as the nut is run up the strap 46 is tightened securing the clamp 40 to the riser buoyancy module. Advantageously, the strong and flexible straps of aramid fiber attach the fairings tightly, yet have enough elasticity to accommodate riser motion under difficult conditions of bad weather and high current.

The adjustable channel clamp 40 may be mounted such that the fairing 10 rotates around to weathervane with the current, thus reducing vortex induced vibration. As shown in FIGS. 1 and 2, exterior surfaces 50 of the clamp 40 rest within a channel 52 on the fairing 10 to allow the fairing to rotate with the ocean current.

FIGS. 3A-3C illustrate several different fairings and clamp arrangements on a riser. FIG. 3A illustrates a single clamp that may be used per fairing, with the clamp 40 being positioned within the channel of the fairing. FIG. 3A illustrates an embodiment comprising six fairings and four clamps, where two fairings comprising clamps sandwich a fairing that does not include a clamp. FIGS. 3A-3C illustrate an embodiment comprising eight fairings and five clamps, where two fairing are sandwiched between a fairing mounted externally to the fairings. Advantageously, the adjustable channel clamp may be located within or outside the fairing depending upon the fairing configuration being employed. In each embodiment the fairings are free to rotate with the current.

Hydrodynamic testing indicate that fairings according to the present invention reduce drag on a marine riser by as much as 75% and significantly reduce/virtually eliminate the harmful effects of vortex induced vibration.

Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.

Claims

1. A fairing for use on with a riser, comprising:

an adjustable channel clamp that includes first and second clamp halves that are longitudinally and rotatably secured together around the riser with a first synthetic fiber strap;

a fairing that includes first and second fairing halves secured around the riser, where the first and second halves form a channel around an interior periphery of the first and second halves such that the adjustable channel clamps rides within the channel; and

a second synthetic fiber strap that co-axially encircles the fairing to hold the first and second fairing halves in a closed position.

2. The fairing of claim 1, where the each of the first and second fairing halves includes a semi-cylindrical component that partially surrounds the riser and a tail that tapers downward to a point at a tail end of the fairing.