RISER SPACERS FOR VERTICALLY MOORED PLATFORMS

ESPACEURS DE COLONNES MONTANTES INSTALLEES SUR DES PLATES-FORMES AMARREES VERTICALEMENT

English 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.

Claims

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:
<IMG> ,

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

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
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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~ -
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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
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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.
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?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.
-5~
' ~ :

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.
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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. -
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