CA1052193A - Riser spacers for vertically moored platforms - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This invention relates to a structure floating on a body of water, and particularly a structure for drilling or producing wells from below the water. Boyant members support at least a part of the struc-ture above the surface of the water. The structure is connected to anchors in the floor of the body of water by a series of parallel leg members. Each leg member is composed of a plurality of elongated mem-bers, such as large diameter pipe usually called risers. These risers are parallel. Vertically spaced spacers are provided along the risers of each leg to (1) maintain the risers a fixed distance apart and (2) change the natural or resonant frequency of the individual riser pipes to be greater than the flutter frequency caused by the motion of the water past the risers.

Description

~s?~ 3 BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to a structure floating on a body of water. More particularly, the invention relates to a floating structure from which drilling or production operations are carried out. In its .
more specific aspects, the invention concerns a floating structure -having buoyancy means for supporting the structure and anchored to the ocean floor by parallel elongated members.

2. Setting of the Invention : ,: , In recent years there has been considerable attention attracted to the drilling and production of wells located in water. Wells may be -~
drilled in the ocean floor from either fixed platforms in re~atively . ~ .
shallow water or from floating structures or vessels in deeper water.
The most common means of anchoring fixed platforms includes the driving or otherwise anchoring of long piles in the ocean floor. Such piles extend above the surface of the water and support a platform attached to the top of the piles. This works fairly well in shallow water, but as the water gets deeper, the problem of design and accompanying cost becomes prohibitive. In deeper water it is common practice to drill ; ~-~
from a floating structure.
In recent years there has been attention directed toward the ;~
many different kinds of floating structures from which underwater wells ;can be drilled. One such drilling structure is referred to as the "ver-tically moored platform" and is described in U.S. Patent 3,648,638, ^-~ ~
issued March 14, 1972, Kenneth A. Blenkarn, inventor. In the vertically ~;
" moored platform a structure is supported above the surface of the water -~
by buoyant members. The buoyant members are connected to anchors in the floor of the body of water by long, elongated leg members which ar parallel. There are no other anchoring means for the vertically moored platform. ;~

1-- ~

` ~

BRIEF DESCRIPTION OF THE INVE~TION
This invention concerns a method and apparatus of anchoring a floating offshore structure at a selected location in a body of water having a maximum design wave and current at such location. An anchor means is provided at the bottom of the body of water at the ~elected location. Then a plurality of parallel-spaced legs connect the offshore floating structure with the anchor base means. Each of the legs include~
a plurality of parallel elongated members, commonly called risers, under tension. A plurality of spacing or centralizing means at vertically 6paced intervals is provided for several purposes, including: (1) to hold the elongated members in a fixed position with respect to each other at the level at which the spacing means is provided, and (2) to ;~
cause the risers to have a natural frequency different from the flutter frequency. The intervals between the spacing means are set so tha~ the natural or resonant frequency of each individual span of each riser ,. ^.
between the spacing means is greater than the maximum anticipated flutter frequency of the corresponding individual span. By the use of this invention the risers are prevented from damaging one another and the potential of fatigue dam~ge in the risers as a result of resonant motion :, - . ,.. -, ln the risers due to flutter ~s reduced.
In one aspect of the present invention there is provided a method of anchoring a floating offshore structure -~
at a selected location in a body of water having a maximum design wave and current. According to the method an anchor means is provided at the bottom of the body of water; a plurality of parallel s~aced legs are provided to connect and anchor the offshore structure with the anchor means,each leg . comprising a plurality of parallel elongated anchoring members -~ :
under tension. At least three spacing means spaced vertically ;
along the elongated members are provided for holding the :,, ~ 2 -1 ~ ~
~.
- `. i '- `

3 : .
elongated members in a spaced apart position at the level at which the spacing means are provided. The spacing means are free of structural connection to the anchor means or the off-shore structure except for the spaced legs.
In another aspect of this invention there is also provided an offshore structure for use in a body of water. The structure comprises a platform means buoyancy means for supporting the platform means; and an anchor means. The structure is also provided with a plurality of parallel legs connecting and anchoring the buoyancy means and the anchor means.
Each leg comprises a plurality of parallel elongated anchoring members all under tension. The structure is further provided `;
with at least three spacing means spaced vertically along each -~ `
leg. Each spacing means holds the elongated anchoring members of the leg in a spaced relationship laterally with each other at the level of such spacing means. The spacing means are free of direct structural connection to the anchor means or the buoyancy means.
In a preferred embodiment, the vertical spacing between two adjacent spacing means are such that:

< ~d ~1 ~ .5ax , where D = diameter of the elongated anchoring member, c = the string wave velocity x = distance from the bottom of the body of water, and ~ .. ,,.
a = 2LP

where - 2(a~ -~J

P = wave period, H = wave height, L = length of elongated anchoring member.

Various objects and a better understanding of the invention can be had from the following description taken in conjunction with the drawings. ;~
DESCRIPTION OF THE DRAWINGS
FIGURE l is a view of a floating structure equipped `
with this invention; `:
FIGURE 2 is an enlarged perspective view of spacer ~`
means useful in the device of FIGURE l.
DETAILED DESCRIPTION OF THE DRAWINGS ~ ;
Referring to the drawings as shown, vessel 10 is ~: ~
supported on a body of water 18 r having a bottom 16. The `~ `
structure 10 generally `: J :

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;~.' -" , ,.": . '-'. ':

. ~ , , .

' :.-:; ::~,:''.,.' ,' , ,: :....

; ' : ~:
, . . . ~ . .
:.. . ~ ~ . . ...
: ~ ~;: :
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includes a float means 14 which supports a working deck 12 above the surface 26 of the body of water. An anchor means 28 ls affixed to the water bottom 16 in any desired manner. The only anchoring means is the legs 20 which connect the floating me~bers 14 with the anchor 28. This is what is commonly called a vertically moored platform and is described in detail in U.S. Patent 3,648,638, supra. Each leg member 20 is com-posed of a plurality of elongated members or riser pipes 22. Riser `
pipes are normally made of high quality steel and typically are 20 inches in diameter. Each leg 20 is a group or bundle of several riser pipes 22 which typically vary in number from 4 to 8l depending on design `-conditions, and extends from its respective float member 14 to the anchor 28. These pipe risers 22 are parallel and are in tension.
Typical lengths for these riser pipes 22 may be from 500 feet upward to several thousand feet from the base of the float member 14 of the ver~
tically moored platform to the sea floor 16.
The vertically moored platform is sub~ected to horizontal `
motion under the influence of waves, wind and current. Large relative ` `
motion of the risers with respect to the water, upward to 50 to 100 feet, may be expected. Centralizers or spacing means 24 are provided ~1 along the length of each leg 20. These spacing devices are for two purposes. The first purpose is to maintain the individual tensioned riser pipes parallel and separated at all times so as to prevent one riser from damaging another. As will be explained in greater detail, these centralizing devices 24 are spaced at predetermined vertical ~`
intervals such that under the influence of waves and currents, the relative displacement of individual risers 22 within an individual leg -20 will not be sufficient to cause one riser to damage another. The second purpose of the spacers 24 is to force the individual tensioned " ~ ~
risers 22 to have a fundamental natural frequency of lateral v-lbration ~ ` `
which is higher than the highest anticipated frequency due to vortex shedding.
-3- ` ;

' ~ ' ?S~1~3 We will next describe the spacer shown in detail in FIGVRE 2 and then explain how the correct spacing is obtained to prevent damage when the risers bump into each other and also the potential damage due to fatigue if the resonant frequency of the individual risers is less ~ -than the frequency of vortex shedding. Attention is now directed to FIGURE 2 which shows in detail the spacer 24 of FIGURE 1. Shown thereon are a plurality of sleeve segments 30. These segments 30 fit about the individual riser pipes 22, as indicated by dotted lines 22A in one of the sleeve segments 30. Sleeve segment 30 is fixed to its respective riser 22 by some mechanical means. The individual sleeve segments 30 are braced from each other by braces 34 and by cross radial braces 36.
Typically, these sleeve segments 30 are approximately the same size in internal diameter as the external diameter of the riser pipe, which is normally about 20 inches, and the longitudinal dimension would typically be about 3 to 4 feet. Bracings 34 and 36 are typically I-beams or T-beams and are so designed as to add or increase the drag effect to the overall leg system. The centralized risers can flutter as a group. To dissipate this flutter energy a high drag is designed in the spacers. -~
This is done by using small angular members as shown in FIGURE 2 andjor ;
by mounting perforated plates or screens on the centralizer. A benefi- -ciary secondary effect of this added drag is to increase damping of platform motions.
5pacing of Centralizin~ Devices -:.. ' , .
We shall now give a detailed consideration as to the spacing for the vertical intervals between the centralizing or spacing devices. ~
As mentioned above, one function of the centralizing devices is to keep -the riser pipes of each leg apart, that is, when the wave motion causes the riser pipes to want to deflect, the spacing of the centralizer `~
should be close enough so that the riser pipes will not bang into e~ch other, thus causing damage. This can be accomplished by the use of ;

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~S'~ 3 simple beam theory. For example, a structural engineer can use beam theory to determine the deflection of a riser between two supported points, in this case, the spacer element, which is shown in FIGURE 2.
The spacing devices 24 are apced sufficiently close vertically so that the lateral spacing of the risers is greater than the calculated rela-tive deflection of the risers. ~nother criteria is that the maximum deflection of two risers without yield is such that they cannot ~ake contact.
We will next consider the selection of the spacing of the cen-tralizing devices to control the natural frequency of the riser to be higher than the maximum flutter frequency which would develop due to the ~-shedding of von Karman vortices. In many instances, due to the relative motion between the tensioned riser pipes and the surrounding water, lateral flutter of the pipe risers will develop due to the shedding of the von Karman vortices. The frequencies of vortex shedding will norm- ` ;
ally lock onto the nearest natural frequency of lateral vibration of the tensioned riser. In such an event, flutter will induce resonant lateral ;
motion of the tensioned riser, which, in turn, could result in a large ~`
cyclic stressing of the riser and subsequent failure due to fatigue.
This will be avoided by our invention. It is shown by classical flutter theory that the flutter frequency is proportional to the relative water particle velocity. Therefore, higher relative velocities are required `~
to develop flutter at higher frequencies. We therefore desire to design a tensioned riser pipe in such a manner that the relative velocity re~uired to set up vortex shedding at the lowest natural frequency of the riser is sufficiently high that 1t cannot be encountered under `~
normally anticipated conditions. Stated differently, ~e design the ~ ~
riser pipe for a selected location such that the natural resonant fre- ~ `
quency of the riser pipe FN is greater than the flutter frequency FF.
We shall now give ~ consideration for determing FF and FN. Equation 1) is generally accepted for determining FF.

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1) FF = .22 V/D, where ~ ?
V = relative velocity of the riser and water D = diameter of the riser.
We then perform the following steps.
a) Calculate the maximum relative velocity at each point along the riser span. ~
b~ Calculate the maximum flutter frequency at each point ~ ;
along the span.

c) Choose the spacer position such that the natural fre~
quency of the individual riser span is larger than the maximum flutter frequency at this point.
In regard to step a) for exa~ple, if the maximum relative `
velocity of the water with respect to the riser is given approximately by equation 2).
2) Vmax(x) = ax, ; ~
where ~`` `
x = distance of a particular point from the bottom of the `;
riser pipe and ~ ~

a = constant. ; ~;
Then, the maximum FF is given by equation 3). ~`
3) FF(x) = .22 a x/D.
Let us assume tha~ the frequency FN of the individual span of the riser pipe between spacers or centralizers is given by equation 4).
4) FN = c/2Q, ~`
where -~
Q = length of the span or vertical distance between spacers, c = a constant which is the string wave veloclty.
We can develop this equation beginning from a basic equation 5) appearing on page 458 of a book entitled, "Engineering Vibrations", by Jacobsen and Ayre, published by McGraw Hill Book Co~pany, New York, ~ -1~58.

-6- ' `~

~3~ ;3 5) p = n~ ~T~
where Pn = natural frequency in radians ~~2~FN~
T = tension on the riser, g = acceleration oE gravity, W = weight per unit length of the riser.
6) Let c = ~ Tg/W, which is the string wave velocity.
By substitution and elimination of Pn, we get equation 7).

7) N 2L ~
and if we let the length of the span Q = Ltn, where L is the total length ~`;`
of the riser and n-l is the number of spacers or centralizers, then we have

8~ N 2Q
We will llOW discuss the development of an equation for the determination of Q, the distance between spacers, so that ) N F
By substitution we get 10) c/2Q > .22 a D 1 11) Q < cD

We would then choose Q as follows~
12) Q < .5 ax where 13) a ~ L P ~ 2 where P = the maximum expected wave period, H = the maximum expected wave height. `~
Our main consideration in selecting the spacing of the vertical interval between the centralizing devices is the determination of a spacing ~ ~ ;
which will eliminate fl~tter of cause FN to be greater than FF. ~ -While the preferred embodiment has been described in a great deal of detail, it is possible for various variations therein without departing from the spiri~ or scope of the invention. -

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of anchoring a floating offshore structure at a selected location in a body of water having a maximum design wave and current which comprises:
providing an anchor means at the bottom of the body of water;
providing a plurality of parallel spaced legs connecting and anchoring the offshore structure with said anchor means, each said leg comprising a plurality of parallel elongated anchoring members under tension;
providing at least three spacing means spaced vertically along said elongated members for holding the elongated members in a spaced apart position at the level at which the spacing means are provided, said spacing means being free of structural connection to said anchor means or said offshore structure except for said spaced legs.

2. A method as defined in Claim 1 including the steps of:
determining the flutter frequency of the individual elongated members;
determining the vertical spacing of said spacing means such that the frequency FN of each individual vertical span between said spacing means is greater than the flutter frequency at the corresponding individual span; and placing the spacing means in accordance with such deter-mination.

3. A method as defined in Claim 1 including the step of selecting the vertical spacing ? between two adjacent spacing means such that:

, where D = diameter of the elongated member c = the string wave velocity, x = distance from the bottom of the body of water and , where P = wave period, H = wave height, L = length of riser.

4. A method as defined in Claim 1 including providing drag increasing means on said spacing means to increase drag through said water.

5. A method as defined in Claim 2 including providing drag increasing means on said spacing means to increase lateral drag through said water.

6. A method as defined in Claim 3 including providing drag increasing means to increase lateral drag through said water.

7. A method as defined in Claim 1 including the steps of:
determining the vertical spacing of said spacing means such that the maximum deflection of a riser between two spacing means is less than the lateral spacing of the riser; and placing the spacing means in accordance with such deter-mination.

8. An offshore structure for use in a body of water which comprises:
a platform means;
buoyancy means for supporting said platform means;
an anchor means;

a plurality of parallel legs connecting and anchoring said buoyancy means and said anchor means, each said leg comprising a plurality of parallel elongated anchoring members all under ten-sion; and at least three spacing means spaced vertically along each said leg, each spacing means holding the said elongated anchoring members of said leg in a spaced relationship laterally with each other at the level of such spacing means, said spacing means being free of direct structural connection to said anchor means or said buoyancy means.

9. An apparatus as defined in Claim 8 in which the vertical spacing ? between two adjacent spacing means are such that:

, where D = diameter of the elongated anchoring member, c = the string wave velocity x = distance from the bottom of the body of water, and , where P = wave period, H = wave height, L = length of elongated anchoring member.