Note: Descriptions are shown in the official language in which they were submitted.
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VIBRATION-ISOLATING ~PPARATUS
BACKGROUND 0~ THE INVENTION
1. Field of the Invention
The present invention relates to the protection of
sensitive equip~ent from damaging vibrations created by external
forces. More particularly, the present invention relates to a
vibration-isolating apparatus for transporting equipment during
subsea drilling operations.
2. Description of the Prior Art
The expanding search for oil and gas has extended
drilling operations from shallow coastal regions i~to deeper
water. As the depth of the water increases, the ability to
monitor or directly control submerged equipment becomes more
arduous. Divers are frequently used in shallow drilling opera-
tions, yet the problems of decompression and the deficiencies of
isobaric diving may limit efficient diving operations to depths
of 600 feet. While the range o~ conventional diving operations
is limited to shallow water, subsea drilling operations frequently
extend to depths greater tha~ 1000 feet.
At greater drilling depths, many drilling and production
operations are controlled remotely. Underwater television cameras
and other equipment are lowered from a drillship or platform to
perform a variety of services. Television cameras are routinely
used to observe the landing of subsea equipment, cementing, and
tool operations. ~ltrasonic equipment is utilized in nondestruc-
tive testing to identify corrosion and other deficiencies in
submerged equipment. The remote control of similar operations
has proven indispensable in directing efficient subsea drilling
operations.
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Various guidance systems have been developed to trans-
port eqùipment to the ocean floor. Guidelines attached between
the sea floor and a drillship or platform are commonly used to
guide equipment to a subsea wellhead. U.S. Patent No. 3,184,541
illustrates a television camera deployed along a system of four
guidelines anchored by a weighted guidebase to the ocean floor.
The use of guidelines is economical and reliable for
drilling depths approaching 1800 to 2000 feet. Yet guidelines
can prove impractical at greater drilling depths because of the
increased size of the guidelines and guideline handling equipment
required to manipulate a larger guideline system. ~arge guideline
systems are not only expensive and unwieldly, but at greater
depths the guidelines and drillpipe may become tangled due to
ocean currents and vessel drift.
Consequently, guidelineless drilling systems have been
developed for drilling in ocean depths greater than 2000 feet.
Guidelineless systems are particularly appropriate for a dynami-
cally positioned drilling vessel because of vessel movement,
within a few degrees of center, about the subsea well. In guide-
lineless drilling operations, equipment-carrying tools transport
equipment along the exterior of the drillpipe or casing. The
tools are commonly lowered along the drillpipe by a cable or line
controlled from the surface.
The deployment of equipment along the drillpipe in a
guidelineless system is satisfactory in a static subsea environ-
ment, but the presence of strong ocean currents may induce drill-
pipe vibrations which jeopardize vibration-sensitive equipment.
In a current, the flow of water around the drillpipe can create
opposing vortices which are alternately shed from each side of
the drillpipe. This vortex shedding creates pulsating forces on
the drillpipe transverse and parallel to the current. Under
certain circumstances, particularly where these forces correspond
with the natural frequencies of the drillpipe, the vortex-induced
forcing frequencies can cause the drillpipe to vibrate turbulently.
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This effect is more pronounced at greater drilling depths because
a long drillpipe is exposed to greater current forces and has
many natural frequencies.
The forcing vibrations and corresponding movement of
the drillpipe induced by current flow can destroy vibration-
sensitive e~uipment deployed near the drillpipe. A need, there-
fore, exists for an apparatus that will isolate equipment from
excessive vibrations produced in a dynamic environment. The
apparatus must attenuate a wide range of vibrational frequencies
impinging on the apparatus.
SUMMARY OF THE INVENTION
The present invention overcomes the above-described
disadvantages by providing an apparatus to transport equipment
while protecting the equipment from excessive vibrational forces.
The vibration-isolating apparatus includes a guide
member located about a directional guide which directs the deploy-
ment of the apparatus. A frame means is located about yet is
separated from the guide member. One or more shock-absorbing
members connect the guide member to the frame means.
Equipment can be fastened to the frame means for trans-
port along the directional guide. The equipment will be isolated
from resonant vibrations in the guide member by the shock-
absorbing members. The vibration-isolating apparatus attenuates
a wide range of vibrational amplitudes and frequencies which may
be deleterious to equipment. The vibration-isolating apparatus
is particularly suited to varying directional guides and to con-
ditions where the vibrational forces vary along the directional
guide.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the features of this invention may be
better understood, a detailed description of the invention as
illustrated in the attached drawings follows:
FIG. l depicts an isometric view of the vibration-
isolating tool disposed about a directional guide;
FIG. 2 is a partial sectional view taken along line 2-2
of FIG. l, and illustrating a detailed guide member;
FIG. 3 is a plan view illustrating the relationship
between the guide member, frame means, shock-absorbing member,
and hinged provisions for installing the apparatus aboùt a direc-
tional guide; and
FIGS. 4 ~A & B) illustrate line graphs with a horizontal
scale depicting shock-absorbing member stiffness. The graphs
correlate the relationship between shock-absorbing member stiff-
ness and regions of vibrational resonance and contact probability
for two specific directional guides.
DESCRIPTION OF THE PREFERRED EMBODI~IENT
With reference to FIGS. 1-3, and with particular refer-
ence to FIG. l, a vibration-isolating apparatus is illustrated.
A guide member l0 is mounted in operational relationship with a
directional guide ll having a longitudinal axis 13. The direc-
tional guide ll is used to position the guide member l0 as the
guide member 10 is deployed. Located about, yet spatially
separated from the guide member l0 is a frame means 14. The
guide member lO is connected to the frame means 14 hy a plurality
of shock-absorbing members 16 which allow relative movement
between the guide member l0 and frame means 14 while mitigating
contact therebetween. The shock-absorbing members 16 are attached
at each end by suitable shackle means 18.
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The present invention is especially suitable for the
transport of equipment 19 along a directional guide 11 in a
subsea environment. The equipment 19 is attached to the frame
means 14 by any suitable fastening means 20. The invention
protects the equipment 19 by isolating it from excessive vibra-
tions or shock caused by violent motion of the directional guide 11.
The equipment 19 deployed can include television cameras, lights,
ultrasonic or data gathering instrumentation, or any other equip-
ment. An umbilical cable 21 can be operated from a console on a
floating vessel (not shown) to supply power to the equipment 19.
Referring to FIG. 2, the guide member 10 is preferably
an elongated cylinder, yet its shape need not be limited to a
cylinder or an elongated member. The guide member 10 can be
deployed along the directional guide 11 to transport the equipment 19.
The ends of guide member 10 may preferably be flanged to traverse
pipeline joints along the directional guide 11 that are encountered
by the guide member 10.
The guide member 10 is depicted in FIGS. 1-3 as being
in an operational relationship with the directional guide 11. To
achieve this operational relationship, the guide member 10 is
preferably constructed of two or more detachable segments to
permit its ready installation about the directional guide 11.
~or example, the guide member 10 may be parted lengthwise into
sections as illustrated in FIGS. 2 and 3 with an opening sufficiently
large to permit insertion of the directional guide 11 therethrough.
Following insertion of the directional guide 1] into the sectioned
guide member 10, the directional guide 11 can be fastened in a
fixed operational relationship with the guide member 10 by binding
the sections of the guide member 10 into a unitive member. Any
suitable fastening means such as bands 22 can be used to bind the
guide member 10 sections. The bands 22 can preferably be manu-
factured from galvanized steel, stainless steel, or Kevlar (a
Dupont Trademark fiber with a strength to weight ratio five times
that of steel).
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The exterior surface of the guide member 10 can be
faced with a resilient material 24 such as rubber or an elastomeric
material. The resilient material 24 is secured to the guide
member 10 by means of bands 22 as described above. The resilient
material 24 performs two functions. First, it dampens internal
resonant vibrations within the guide member 10 created by impact
between the ~uide member 10 and a vibrating directional guide 11.
Second, extraordinary displacement of the frame means 14 in
relation to the guide member 10 could create an impact between
these elements. In such an event, the resilient material 24
would cushion the impact between the guide member 10 and the
frame means 14.
FIG. 3 illustrates a plan view of the preferred embodi-
ment. As shown, the frame means 14 can be constructed in a cage-
like design encircling the guide member 10, yet it need not belimited to this configuration~ The frame means 14 is preferably
extended in its construction to enclose the equipment 19. This
extension 25 of the frame means 14 would protect the equipment 19
from damaging contact with structures or other objects present in
the subsea environment.
The vibration-isolating apparatus is particularly
suited for use in a dynamic aqueous environment such as an ocean
current. As previously noted, an object placed in a current will
be subjected to drag forces induced by the current. To reduce
the drag forces exerted on the vibration-isolating apparatus, the
frame means 14 may preferably be shaped as a fairing. As illus-
trated in FIG. 3, fairings are bodies generally tapered in shape
from a blunt end to a narrow end and are well-known in the art as
a means of reducing the drag forces exerted on an obiect exposed
to a current. A frame means 14 shaped as a fairing would there-
fore reduce the drag exerted on the frame means 14. As the
directional guide 11 vibrates, the frame means 14 would tend to
rotate about the directional guide 11. A frame means 14 shaped
as a fairing would prevent this rotation. Fixed or detachable
fins 26 are preferably attached to the frame means 14 to stablize
the rotation of the frame means 14 about the directional guide.
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The shock-absorbing members 16 may be fashioned from
any flexible medium not limited to springs or other elastic
materials. A singular shock-absorbing member 16 composed of an
elastic medium such as a foam may be disposed between the rame
means 14 and guide member 10. A plurality of another type of
shock-absorbing members 16 can be utilized as illustrated in
FIGS. 1-3. The shock-absorbing members 16 selected for the
preferred embodiment are commonly known as "bungee cords" and may
be purchased from Thomas Taylor & Sons of Hudson, Massachusetts.
Each bungee cord is composed of an inner core of pliable rubber
surrounded by a protective covering of woven fabric.
The shock-absorbing members 16 can be connected in
various orientations between the guide member 10 and frame means 14.
For example, the shock-absorbing members 16 can be oriented
diametric about the guide member 10. As illustrated in FIGS. 1
and 2, the shock-absorbing members 16 can be fastened in pairs in
a decussate fashion between the guide member 10 and frame means 14.
This orientation would prevent rotation of the frame means 14
about a plane perpendicular to the longitudinal axis 13 of the
directional guide 11. Furthermore, this decussate configuration
allows the advantageous use of longer shock-absorbing members 16
in the vibration-isolation apparatus. Longer shock-absorbing
members 16 increase the damping capability of the apparatus
because a longer shock-absorping member can be maintained in
tens~ion throughout a wide range of displacements of the frame
- means 14 relative to the guide member 10.
The shock-absorbing members 16 are preferably pre-
tensioned as they are installed. This pre-tensioning ~in;m;7~es
the possibility of slackness in the shock-absorbing members 16
and corresponding damage to the apparatus upon the extraordinary
displacement of the frame means 14 in relation to the guide
member 10.
The vibration-isolating apparatus protects e~uipment by
attenuating vibrations induced by the directional guide 11. In
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a current, vortices alternately shed from both sides of a direc-
tional guide 11 will vibra~e the directional guide 11. As the
directional guide 11 vibrates it will induce a range of vibrational
~requencies within the apparatus as more fully described below.
Because this range of vibrations will vary according to the
dimensions and composition of each directional guide 11 as well
as the velocity of the current, the shock-absorbing members 16
must be tailored to each application of the vibration-isolating
apparatus.
The appropriate shock-absorbing members 16 used in the
preferred embodiment are selected according to their stiffness.
Stiffness is measured by determin;ng the distance each shock-
absorbing member 16 elongates when exposed to a given tensile
force. Units of stiffness may therefore be expressed in units of
pounds per inch (lb/in). If the elasticity of the rubber core
and the weave for the woven covering of each shock-absorbing
member 16 is constant, the stiffness for each shock-absorbing
member 16 in the preferred embodiment will vary according to its
outside diameter. In general, the stiffness of a shock-absorbing
members 16 will increase as its diameter increases.
In selecting the appropriate stiffness of shock-
absorbing members 16, two primary factors jeopardize the survival
of equipment 19 transported by the present invention. First, the
shock-absorbing members must be sufficiently stiff to mitigate
contact between the frame means 14 and guide member 10. Second,
the shock-absorbing members 16 must be sufficiently flexible to
dampen excessive vibrations induced by the directional guide 11.
As the stiffness of the shock-absorbing members 16 is reduced,
the probability of contact between the frame means 14 and guide
member 10 will increase. Such contact could damage the vibration-
isolating apparatus and the equipment 19 transported by the
apparatus. The range of shock-absorbing members 16 insufficiently
stiff to nimi7e contact between the frame means 14 and guide
member 10 is labeled herein as a range of "Contact Probability"
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g
and is illustrated in FIGS. 4A and 4B. ~hile contact can occur
at nearly all stiffnesses of shock-absorbing members, this Contact
Probability range includes shock-absorbing member 16 stifnesses
sufficiently flexible that the frame means 14 is excessively
offset from its equilibrium position relative to the guide member 10
when exposed to a current. Because this offset would reduce the
distance between the frame means 14 and guide member 10, an
offset would increase the possibility of contact between the
frame means 14 and guide member 10. As the stiffness of the
shock-absorbing members 16 is increased, the probability of
contact between the frame means 14 and guide member 10 will
increase, but the shock-absorbing members 16 will attenuate fewer
vibrational frequencies than will more flexible shock-absorbing
members 16.
With regard to this second factor of vibrational damping,
two sources of vibrations should be analyzed. The first source
of vibrations occurs as the vibration-isolating apparatus is in
contact with and vibrates at the frequency of the directional
guide 11~ This resonance imparted to the vibration-isolating
apparatus correlates with the base vibrational modes of the
vacillating directional guide`ll and is termed herein as the
"Base Excitation On Mode Resonance." A secondary source of
vibrations occurs as the directional guide 11 impacts the inner
surface of the guide member 10. This impact, or impulse, creates
internal resonant vibrations within the guide member 10 which are
then transferred to the connected shock-absorbing members 16.
This source of vibrations is termed herein as the "Impulse Reso-
nance" of the vibration-isolating apparatus.
The Contact Probability factor and vibrational sources
of Base Excitation On Mode Resonance and Impulse Resonance (as
previously defined) will vary in magnitude and effect according
to the particular stiffness of each shock-absorbing member 16.
As the stiffness of the shock-absorbing members 16 is increased,
the probability of contact between the frame means 14 and guide
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member 10 will be reduced, yet the shock-absorbing members 16
will attenuate a narrower range of vibrations. As the stiffness
of the shock-absorbing members 16 is reduced, a wider range of
vibrations wi~1 be attentuated, yet the probability of contact
between the frame means 14 and guide members 10 will be increased.
To determine the optimal stiffness obtained from balanc-
ing these competing factors of Contact Probability and vibrational
damping, each directional guide 11 should be tested to determine
the magnitude of the factors at differing shock-absorbing member 16
stiffnesses. The results of each test can then be plotted -to
graphically illustrate the relation of each competing factor to
shock-absorbing member 16 stiffness.
As illustrated in FIGS. 4A and 4B, the performance of
the preferred embodiment has been analyzed. Tests of a prototype
vibration-isolating apparatus have verified this analysis. In
the tests, a five inch tinside diamter) drillpipe was vibrated at
3-4 Hz and the results were plotted on a bar graph in FIG. 4A.
As illustrated, the range of Base Excitation On Mode Resonance
vibrations extends over a greater range than the more narrow
range of Impulse Resonance vibrations. The range of Impulse
Resonance vibrations will therefore be relatively insignificant
when compared to the more dominant range of Base Excitation On
Mode Resonance vibrations. As illustrated in FIG. 4A, the Base
Excitation On Mode Resonance vibrations range from a stiffness
near no support to a shock-absorbing member 16 stiffness of 4800
lb/in. The range of the competing factor of Contact Probability
extends from no support to a shock-absorbing member 16 stiffness
of 400 lb/in.
~sing the graph in FIG. 4A, the stiffness of the shock-
absorbing members 16 can be selected to achieve varying balances
between the factors of Base Excitation On Mode Resonance, Impulse
Resonance, and Contact Probability. To determine the optimal
balance between these competing factors, a stiffness should be
selected in the region where the ranges of these factors intersect.
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As previously noted, the range of Impulse Resonance is insignifi-
cant for the five inch drillpipe when compared to the range of
Base Excitation On Mode Resonance. A shock-absorbing member 16
stiffness of 200 lb/in would attenuate a wide range of vibrations
but is in the Contact Probability range and would risk some
contact between the frame means 14 and guide member 10. A stiff-
ness of 400 lb/in would prevent contact between the frame means
14 and guide member 10 while attenuating a somewhat narrower
range of vibrations.
The stiffness of the shock-absorbing members 16 should
be selected according to the design criteria considered most
important. If the prevention of contact is more important to the
survival of the vibration-isolating tool than is the attenuation
of a wider range of vibrations, this stiffness of 400 lb/in will
preferably be the critical damping stiffness of the apparatus
when adapted to a five inch drillpipe. As illustrated in FIG.
4B, an analysis of a twenty inch (outside diameter) casing pipe
establish a preferred stiffness of 200 lb/in. for critical damping.
Tests of bungee cords demonstrated that cords 3/4 inch in diameter
yield a stiffness of approximately 500 lb/in., and this diameter
was selected for the prototypè apparatus.
As a final step in the ~esting process, the vibration-
isolating apparatus should be tested to determine whether the
natural frequencies of the assembled apparatus are equivalent to
any dominant forcing frequencies of the drillpipe. If such
natural frequencies and forcing frequencies are coincident, the
forcing frequencies vibrating the equipment 19 will be amplified
.nstead of reduced by the vibration-isolating apparatus.
To deploy the vibration-isolating apparatus in the
preferred embodiment, the guide member 10 is attached in operational
relationship with the directional guide 11 as previously described.
The frame means 14 is positioned about the guide member 10 and is
connected to the guide member 10 by a plurality of shock-
absorbing members 16 attached in tension. Equipment 19 is attached
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~2~
to the frame means 14. A control means 28 or any suitable means
of locomotion is attached to the vibration-isolating apparatus to
transport the apparatus along the directional guide 11.
In an alternative embodiment, the vibration-isolating appara-
tus need not be dismantled for installation about the directional
guide 11. As illustrated in FIG. 3, the frame means 14 and guide
member 10 can be sectioned at two corresponding planes parallel
to and extending radially outward from the longitudinal axis 13
to form a section assembly 34. The section assembly 34 as illus-
trated forms a truncated pie shape comprised of partial sectionsof the frame means 14 and guide member 10 attached by shock-
absorbing members 16. A hinge 30 or similar means preferably
located on the frame means 14 allows the section assembly 34 to
be opened. This permits installation of the apparatus about the
directional guide 11 without dismantling the apparatus. Eollowing
installation, the section assembly 34 can be fastened by means of
a latch 35 or other suitable means. Temporary spacing bars 36
located between the frame means and guide member sections prevent
collapse of the section assembly 34 caused by the tensioned
shock-absorbing members 16 during installation of the vibration-
isolating apparatus. The spacing bars 36 are removed following
installation of the apparatus about the directional guide 11.
In an additional embodiment, the frame means 14 and
guide member 10 can be sectioned between shock-absorbing members 16
to leave an opening sufficiently large to permit the insertion of
the directional guide 11 therethrough. The guide member 10 is
then bound about the directional guide 11 by bands 22 or other
fastening means. In this embodiment, the apparatus could be
installed without removing the shock-absorbing members 16.
The vibration-isolating apparatus protects equipment
from damaging vibrational forces as the apparatus is deployed
along the directional guide. The vibration-isolating apparatus
is designed to permit both longitudinal and rotational movement
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about the directional guide and is adaptable to directional
guides of varying sizes. In addition to providing ease of instal-
lation about the directional guide, the design of the vibration-
isolating appàratus is particularly suited to conditions where
the directional guide forcing frequencies are erratic as well as
variable along the length of the directional guide. Because the
present invention is adaptable to a variety of applications and
differing constructions, it is intended that all subject matter
discussed above or shown in the accompanying drawings be interpreted
1~ as illustrative and not in a limiting sense.
