Production of oil and gas from offshore fields has created many unique engineering challenges. One of these challenges is dealing with effects of currents on cylindrical marine elements employed in a variety of applications, including, e.g., subsea pipelines; drilling, production, import and export risers, tendons for tension leg platforms, legs for traditional fixed and for compliant platforms, other mooring elements for deepwater platforms, and the hull structure of spar type structures. These currents cause vortexes to shed from the sides of the marine elements, inducing vibrations that can lead to the failure of the marine elements or their supports.

For short cylindrical elements that are adjacent convenient means for secure mounting, the marine elements and their supports can be made strong enough to resist significant movement by the forces created by vortex shedding. Alternatively, the marine element could be braced to change the frequency at which the element would be excited by vortex shedding.

However, strengthening or bracing becomes impractical when the application requires that the unsupported segments of marine element extend for long runs. Deepwater spar structures are typical of such applications.

Helical strakes and shrouds have been used or proposed for such applications to reduce vortex induced vibrations. Strakes and shrouds can be made to be effective regardless of the orientation of the current

to the marine element. But shrouds and strakes materially increase the drag on such large marine elements.

Thus, there is a clear need for a low drag, VIV reducing system for protecting the hull of a spar type offshore structure.

In accordance with the invention there is provided a spar structure for offshore hydrocarbon recovery operations having a vertically oriented elongated floating hull with a buoyant upper section and a ballasted lower section and an anchoring system connecting the hull to the ocean floor, the spar structure being provided with a vertically oriented fairing shaped profile section connected to the hull.

Prior efforts at suppressing VIV in spar hulls have centered on strakes and shrouds. However both of these efforts have tended to produce structures with having high drag coefficients, rendering the hull more susceptible to drift.

Fairings can provide low drag VIV suppression for cylindrical members. However, these have been best suited for relatively small diameter elements such as offshore risers. For a number of reasons, fairings have not been thought applicable to large marine elements. One reason is the correlation of the need for effective VIV suppression to Reynolds number. The Reynolds number for a stationary cylinder within a fluid moving perpendicular to the axis of the cylinder is approximated with the following expression: Re = VD/v where: Re is the Reynolds number; V is the current velocity; D is the outside diameter; and v is the kinematic viscosity

Thus, in a given medium, here seawater, the Reynolds number is proportional to the velocity time the diameter and the hull of a spar is several orders of magnitude greater in diameter than typical risers where fairings have been thought appropriate. Typical of prior applications are offshore production risers designed on the basis of Reynolds numbers on the order of 50,000 to 100,000 and drilling risers at one to two million. By contrast, spar structures would anticipate Reynolds numbers on the order of five to fifty million, and perhaps more, depending upon the size and con- figuration. Further, it has been common wisdom that the well correlated vortex shedding along a cylinder exhibited at high Reynolds numbers would require that effective VIV suppression also addresses reducing spanwise correlation. However, conventional fairings are not the choice in applications defined with in this manner when compared with helical strakes or shrouds which disturb such correlation spanwise as a natural side effect of breaking up the correlation of trans- versely passing seawater. In addition, it has been the conventional wisdom that changes in attack angle of environmental current to a fixed fairing would both limit the effectiveness in vortex shedding and subject the tail of the fairing to significant rotational ioads and increased drag. Thus, fairings in general and fixed fairings in particular have been thought inappli- cable to solve VIV problems for spar hulls.

The fairing shaped profile section can be fixedly connected to the hull, or be rotatable relative to the hull.

The invention will be described further in more detail and by way of example with reference to the accompanying drawings in which:

FIG. 1 is a side elevational view of a fairing shaped spar in accordance with one embodiment of the present invention; FIG. lA is a side elevational view of a fairing shaped spar in accordance with another embodiment of the present invention; FIG. 2 is a cross sectional view of the fairing shaped spar of FIG. 1, taken at line 2-2 of FIG. 1; FIG. 2A is a cross sectional view of the fairing shaped spar of FIG. 1A, taken at line 2A-2A of FIG. lA; FIG. 3 is a cross sectional schematic illustration of a fairing shaped spar under the influence of ocean current; FIG. 4 is a cross sectional schematic illustration of a fairing shaped spar under the influence of ocean current; FIG. 5 is a cross sectional schematic illustration of a fairing shaped spar under the influence of ocean current; FIG. 6 is a side elevational view of a multiple fairing staggered system deployed about a marine riser of a spar structure; FIG. 7 is a cross sectional view along line 2-2 of FIG. 6; FIG. 8 is a cross sectional view of the multiple fairing staggered system of FIG. 6, taken at line 3A-3A; FIG. 9 is a cross sectional view of the multiple fairing staggered system of FIG. 6, taken at line 3B-3B; FIG. 10 is a cross sectional view of a multiple fairing staggered system illustrating schematically an increased optimum angle of attack for effective VIV suppression;

FIG. 11 is a cross sectional view of an alternate embodiment of a staggered fairing system; and FIG. 12 is a graph plotting RMS transverse acceleration against Reynolds number for tests of a cylinder without VIV suppression and the same cylinder with a staggered fairing system subjected to currents at various angles of attack.

FIG. 1 illustrates the environment of the present invention, with a spar 10 having a deck 12 above ocean surface 14. Spars are a broad class of floating, moored offshore structures which are resistant to heave motions and present an elongated, vertically oriented hull 22 which is buoyant at the top 21 and is ballasted at its base 23. Such spars may be deployed in a variety of sizes and configuration suited to their intended purpose ranging from drilling alone, drilling and production, or production alone.

A plurality of risers 16 extend from the deck to the ocean floor 18 at wells 20 to conduct well fluids.

Deck 12 is supported at the top of spar hull 22. The hull is elongated and vertically oriented with a buoyant top section and a ballasted lower section. A plurality of mooring lines 24 are connected to a spread of anchors (not shown) set in the ocean floor to hold spar 10 in place over wells 20. In other embodiments, the risers may act alone as tethers to form the anchoring system securing hull 22 in place while providing conduits for conducting produced oil and gas.

The upper end of risers 16 are connected to production facilities supported by deck 12 and, after initial treatment, the hydrocarbons are directed through an export riser to a subsea pipeline, not shown.

In this embodiment, risers 16 are arranged within a moonpool 26 along the interior periphery of hull 22.

See also FIG. 2. Further, a slot 28 in hull 22

provides an opportunity to pass risers 16 from an auxiliary drill and completion vessel (not shown) to the moonpool within the structure.

FIGS. 1 and 2 illustrate an embodiment of a production spar, but appropriately adapted spar configurations are suitable for drilling operations or for combined drilling and production operations as well in the development of offshore hydrocarbon reserves.

The elongated, usually cylindrical hull or caisson 22 is susceptible to vortex induced vibration ("VIV") in the presence of a passing current. These currents cause vortexes to shed from the sides of the hull 22, inducing vibrations that can hinder normal drilling and/or production operations.

In order to reduce such vibrations a vertically oriented fairing shaped profile section 30 is fixed on the hull. In the embodiment of FIGS. 1 and 2, this is provided by the shape of the outer wall of hull 22 itself. The fairing shaped profile section need not necessarily extend all the way the surface, nor necessarily to the bottom of the hull. Further the fairing shaped profile may have multiple orientations.

Returning to the embodiment of FIGS. 1 and 2, fairing shaped profile section 30 first projects from hull 22 some distance below ocean surface 14. This provides a horizontal surface at the top of the projection that will serve to entrap mass in the form of the overlaying seawater when the hull is driven to rise with a passing wave. The inertia of this mass furthers the heave resistance of the overall spar structure 10.

Here also, the fixed fairing shaped profile 30 is provided with a vertical slit or slot 28 which runs substantially the length of the hull and allows passage of production risers 16 from an auxiliary drilling

vessel to well slots inside moonpool 26. In this embodiment, slit 28 is adjacent the tip of the tail in a manner shielded from the current attack angle (see FIG. 3) with minimal impact on this flow about hull 22.

Other slot configurations may be selected which coordinate in contributing to the VIV suppression.

Further, slot 28 may be temporary with structural spanning members deployed except when risers are being passed or may be paired with exterior and interior structural spanning members to be sequentially removed and replaced in passing risers.

FIG. 2 illustrates the important chord "c" and thickness "t" dimensions of the fairing shaped profile, which ratio is preferably between about 1.5 and 1.20, or more preferably between about 1.20 and 1.10. In this embodiment, fairing shaped profile 30 formed by hull 22 is provided with a tail section 38 which is essentially a plate extending the trailing edge of the fairing shaped profile section beyond the terminus 40 of the angled converging sides 42. Adding tail 38 extends the cord length c with a minimum of materials.

In FIGS. 1A and 2A, fairing shaped profile section 30 extends the vertical length of hull 22 of spar 10 and the trailing edge of the fairing shaped profile section ends at the terminus 40 of converging sides 42. Further, mooring lines 24 radiate sym- metrically about the central axis of hull 22 as opposed to the asymmetrical mooring shown in dotted outline at 24'. No slot is provided in this embodiment for laterally passing risers from outside the spar to moonpool 26 through the vertically floating hull. This suggests the wells be drilled from within the moonpool, that wells 20 be pre-drilled and risers 16 installed from within the moonpool, or that more complicated riser handling techniques be employed to bring the

risers inside. In the illustrated embodiment, a drilling facility 44 has been deployed an a rather large spar structure. Note too that the drilling rig may be off center, taking advantage of additional buoyancy in the fairing shaped profile section. Alter- natively, fairing shaped profile section 30 may be ballasted to neutral buoyancy with, e.g., equipment, seawater or oil storage. Exterior import and/or export risers may be conventionally employed passing down the sides of the hull without significantly affecting VIV suppression or drag reduction effects on the spar hull from the fairing profile.

Further, any detriments in an asymmetrical arranged mass, effective mass, or even buoyancy is minimized with a either a short fairing or an ultra- short fairing in combination with base ballast. "Short fairings" are defined as having a chord to thickness ratio between about 1.50 and 1.20 and "ultra-short fairings" are those between about 1.20 and 1.10.

US patent No. 5,410,979 discloses fairings having a chord to thickness ratio of between about 1.50 and 1.25.

FIGS. 3-6 illustrate the effects of ocean current on the fairing shaped profile section 30 of spar hull 22 and methods to utilize and/or respond to these effects. FIG. 3 illustrates genericly a probability gradient of historical current data shown with azimuth and magnitude with arrows 50. It is common for patterns of prevailing currents for given locations to have significant year round correlation.

The fixed fairing shaped profile is oriented on deployment to align with the prevailing current gradient alpha and can be effective for short fairings at angles of attack up. to about 52.5 degrees on either side of the nominal current orientation. Further,

ultrashort fairings can expand this range significantly without losing effective VIV suppress ion and while retaining net drag reduction at high Reynolds numbers.

Brief periods somewhat outside of these ranges may be tolerable if VIV problems response to the current is primarily an issue of fatigue failure which itself is a function of time, the majority of which will find spar 10 in an effective orientation. Alternatively, the orientation can be altered to rotate spar 10 to a new orientation by using playing out and taking in asymmetrical mooring lines 24.

FIG. 5 illustrates another possibility, parti- cularly with somewhat longer fairings, in which the spar is deliberately rotated out of alignment with the current to "fly" the spar with the fairing shaped profile section acting as a wing, aiding to bias (see arrows 51) the spar to an offset position where it is retained by current and mooring 24. This may be useful to provide vertical access over additional wells for an auxiliary drilling vessel.

FIG. 6 is illustrates several fairing arrays 110 grouped as staggered fairing systems secured to a substantially cylindrical marine element 112, here riser 112A, of a schematically illustrated production mini-spar 114. Three fairing arrays are shown, denoted as staggered fairing systems 110A, 1105 and llOC, to illustrate a range of possible embodiments. The middle array is formed from two fairings 108 arranged in different azimuthal orientations. Here fairings 108 of staggered fairing system 110B are mounted adjacent or even as a single unit about riser 112A. Gaps may be left along the marine element, both between the arrays of staggered faring systems and between the individual fairings within an array.

FIG. 7 is a top view of a single fairing 108 secured about the marine element in a substantially fixed, non-rotative manner. It can be connected directly to the riser, e.g. in a tight circumferential friction engagement, or indirectly e.g. connected to buoyancy modules which are themselves connected to the riser. Some rotational slippage may be allowed in some embodiments provided not all individual fairings are free to rotate to effectively weather-vane about marine element 112, or fairings are secured to one another to maintain relative alignment even if the array rotates.

Relative engagement of adjacent fairings 108, such as in staggered fairing system 110A, may provide direct interconnection of fairings endwise to ensure an appropriate spread of orientations. The fairing has a leading edge 116 generally directed toward a possible current direction. The leading edge of fairing 108 follows the circular profile of marine element 112A, departing therefrom with two shaped sides 118 con- verging at trailing edge 120. The trailing edge may or may not include a tail 122.

Short fairing elements with a short chord to thickness ratio of about 1.5 to about 1.2 and ultra- short fairings with a chord to thickness ratio of about 1.20 to 1.10 are particularly useful for combination into arrays of staggered fairing systems. A third parameter illustrated in FIG. 7 is the orientation of the fairing. Currents and relative position between fairings are defined in angular relationship (a) from a line taken from the longitudinal axis of the cylindri- cal marine element to trailing edge 120 of the fairing.

FIG.8 is a cross section of marine element 112 immediately above staggered fairing system 1105 which employs two fairings,- an upper fairing 108A and a lower fairing 108B, connected about riser 112A. In this

illustration, the fairings are arranged 30 degrees on either side of the nominal design current orientation, see current vector V. Thus -there are 60 degrees between orientations of the respective upper and lower fairings. This is consistent with a preferred spread between adjacent fairing elements of between 20 and 60 degrees.

However, the current is not always aligned with the nominal design orientation. FIG. 9 is a similar cross section, here taken through staggered faring system llOC illustrated in FIG. 1. Here current vector V deviates substantially from the nominal orientation.

Fairing 108B is itself 75 degrees out of alignment with the current. Acting alone, this would be out of the range of effective VIV suppression. However, fairing 108B is but one component of the system and fairing 108A is well within the range for angle of attack for which effective VIV suppression will be provided the cylindrical marine element 112. Within a range, this is a trade-off of some increases in drag from non-aligned fairings as other fairings in the array remain or enter into more effective VIV suppression alignment.

As a system, it appears that very effective VIV suppression is possible across at least 90 degrees of possible current variance with drag increases which remain acceptable for many of the offshore applications where VIV suppression is important. See FIG. 10 in which increased variance is denoted by areas 130 and 132 over the nominal optimal variance 134 schematically illustrated for a single fairing.

Where drag is less critical, the system can be pushed to the effective limits of individual fairings within the array, with orientations that are repeated systematically down the riser. See FIG. 11, where the array presents orientations across about a 120 degree

range between fairings within the array. Here the nominal orientation is met with the whole array within an effective orientation, i.e., within 60 degrees for ultrashort fairings 108A that are most mis-aligned.

However, in this embodiment, the chord to thickness ratio increases as individual fairing elements are less eccentric to the nominal current orientation. Thus, fairing element 108C which is aligned with the nominal current orientation is outside the limited "ultrashort" range. This places those fairings which are least susceptible to net drag increases and most forgiving to angle of attack on the periphery, while those that can best provide a drag reduction but with limited angular response are more nearly aligned with the nominal design current. Dotted line 136 illustrates this aspect of this embodiment. Further, some locations may have secondary as well as primary design nominal current orientations, e.g., prevalent seasonal shifts. Again, the array may be constructed to optimally address these prevalent currents as well as a range of deviant current orientations. It should also be noted that more than one staggered fairing system may be deployed on a single marine element and that it may be useful to have these disposed to different orientations. For example, a given location may be routinely subject to different currents as a function of depth in the water column. In this circumstance, different prevailing currents could be optimally addressed with staggered fairing systems deployed at various levels which are designed for the orientation, magnitude, and projected variance expected along the marine element.

FIG. 12 is a graph plotting RMS transverse acceleration A(m/s2) against Reynolds number Re for tests on a staggered fairing system configured like those illustrated in FIGS. 8, 9 and 10. This is a

system of two fairings about a cylindrical marine element oriented to plus or minus 30 degrees from the design nominal current orientation which is designated as 0 degrees for FIG. 12. VIV excitement was measured for the staggered fairing system at five different angles (a) of attack, from 0 to 90 degrees. The base line (B) for a bare pipe test is also illustrated on the graph. Significant VIV suppression is still observed for this staggered fairing system even at an angle of attack of a = 90 degrees.

Although the staggered fairing system has been described for application on a marine riser, it can be applied to a full range of other cylindrical marine elements, including, but not limited to subsea pipelines; drilling, import and export risers; tendons for tension leg platforms; legs for traditional fixed and for compliant platforms; cables and other mooring elements for deepwater platforms; and notably to the hull structure of spar type structures.

Other modifications, changes and substitutions are intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features.