COMPLIANT STEM BUOYANCY CAN AND GUIDE

BOUEE PLATE ET GUIDE DE FLOTTABILITE MASSIFS SOUPLES

English Abstract

A compliant stem buoyancy can and guide for a floating
offshore structure or vessel. The buoyancy can is supported at
the upper stop and the various heave plates by a slip ring that
allows axial displacement but constrains any radial motion.
The buoyancy can behaves as a complaint beam with a dynamic
response like a spring/mass/damper. The stiffness and mass
acts to govern the amplitude and frequency of the motion. The
water in the well bay reacts to the relative speed of the
vessel motion and the buoyancy can. The water thus acts as a
damper. The vessel, stem, and buoyancy can are designed so the
clearance between the buoyancy can and hull is larger than the
maximum amplitude of the oscillation. Therefore, the can will
not impact the hull. The kinetic energy is dissipated by the
compliant response of the buoyancy can and stem structure.
Because the buoyancy can never hits the hull there are no
impact loads or fatigue associated with impacts.

Claims

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What is claimed as invention is:
1. In a floating offshore structure having a center well and a
riser received in the center cell, a compliant stem buoyancy
can and guide, comprising:
a. a buoyancy can attached to the upper portion of the
riser, said buoyancy can having a predetermined compliant
dynamic radial response to environmental forces;
b. a stem attached to said buoyancy can and extending
through the offshore structure, said stem receiving the riser
inside the stem and having a predetermined compliant dynamic
radial response to environmental forces; and
c. a plurality of slip rings attached along the length of
said stem to the offshore structure, said slip rings closely
receiving said stem and allowing vertical movement of said stem
and the riser.

Description

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a
COMPLIANT STEM BUOY~~N'CY GAIT AND GUIDE
BACKGROUND OF THE INVENTTON
1. Field of the Invention
The invention is generally related to risers in offshore
floating structures and more particularly to buoyancy can
guides for risers.
2. General Background
In offshore floating structures or vessels used to drill
for and produce oil and gas, such as spar type structures and
TLP's, buoyancy cans are used to support the weight of drilling
and production risers. Buoyancy can guides are placed in the
body of the vessel. Environmental forces such as wind, waves,
and currents cause the buoyancy cans to move and impact wear
plates against the guides ir~ the hull.. Two design. approaches
have typically been followed to address the wear associated
with the impacts. One approach is compliant guides. In this
approach an elastic material such as rubber that deforms under
load is used to absorb the impact energy. This approach does
not present a long-term solution because the rubber guides are
unable to withstand the loads over a long period of repeated
impacts before they fall apart. A second approach is to
provide a near zero gap between the buoyancy guides and the
vessel. In this approach the tolerances are kept to a minimum
to impede the acceleration of the buoyancy can, therefore
reducing the impact load. The hull and buoyancy cans are
designed to withstand the impact loads. 'The problem with this
approach is that construction tolerances make it very difficult

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to achieve the small gaps that are required to impede the
acceleration of the buoyancy can to an acceptable level.
SUMMARY OF THE TNVENTION
The invention addresses the above needs. What is provided
is a compliant stem buoyancy can and guide . The buoyancy can
is supported at the upper stop and the various heave plates by
a slip ring that allows axial displacement but constrains any
radial motion. The buoyancy can behaves as a complaint beam
with a dynamic response like a spring/mass/damper. The
stiffness and mass acts to govern the amplitude and frequency
of the motion. The water in the well bay reacts to the
relative speed of the vessel motion anc-.~ the buoyancy can. The
water thus acts as a damper. The apparatus is designed so the
clearance between the buoyancy can and hull is larger than the
maximum amplitude of the oscillation. Therefore, the can will
not impact the hull. T'ne kinetic energy is dissipated by the
compliant response of the buoyancy can and stem structure.
Because the buoyancy can never hits the hull there are no
impact loads and no fatigue associated with impacts.
BRIEF DESCRIPTION OF THE DRAWINGS
For a further understanding of the nature and objects of
the present invention reference should be made to the following
description, taken in conjunction with the accompanying drawing
in which like parts are given like reference numerals, and
wherein:
The sole drawing is a s;de section view of the invention
in a spar type structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

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Referring to the drawing, it is seen that the invention is
generally indicated by the numeral 10. Oompliant stem buoyancy
can and guide 10 are comprised of a buoyancy can 12, a stem 14,
and a plurality of slip rings 16.
The drawing generally illustrates a floating offshore
structure 1$, such as a spar type structure, that is provided
with a center well 20 sized to receive drilling and/or
production risers G2.
The buoyancy can Z2 is attached to the upper portion of
the stem 14 and provides buoyant support to the stem 14 and
risers 22. The stem 14 extends upward from the buoyancy can I2
to controls 24 on t'ne top of the offshore structure ~.8.
A plurality of slip rings 16 are sized to closely receive
the stem 14 and are spaced along the length of the stem. One
slip ring 16A is preferably positioned around each stem 14 at
the upper end of the offshore structure 18. The remaining slip
rings 16 are positioned below the buoyancy can 12 and are
attached at the lower end of the center well 20 and to heave
plates 26 that are attached to the offshore structure 18.
The stiffness of the buoyancy can 12, stem 14, and riser
22 axe designed to work in conjunction with the slip rings 16
to prevent the buoyancy can 12 from contacting the offshore
str~~:.cture 18 during normal movement in response to
environmental forces.
The stiffness of the buoyancy can 12, stem, 14, and riser
22 is selected to control the compliant dynamic response of
these structures. The slip rings 16 closely receive the stem
14 and riser 22 to allow vertical movement as indicated by

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arrows 28 but limit radial movement.
The combination of the slip rings 1~ and the predetermined
compliant dynamic response limit the radial movement of the
buoyancy can 12 to a range that is less than the inner diameter
of the center well 20, as indicated by arrows 30. Thus, the
buoyancy can 12 behaves as a compliant beam with a dynamic
response like a springfmass/damper system. The stiffness and
mass of the configuration governs the amplitude and frequency
of the motion. The water in the center well reacts to the
relative speed of the offshore structure and the can, thus
acting as a damper. The drag of the buoyancy can 12 is a
function of VZ (velocity squared). Also, the acceleration of
the water in the center well induces buoyancy that damps the
motion. The configuration is designed so that the clearance
between the buoyancy can and inner wall of the center well is
larger than the maximum amplitude of the oscillation of the
buoyancy can. Therefore, the buoyancy can will not impact the
hull of the offshore structure. The kinetic energy is
dissipated by the compliant response of the buoyancy can and
stern structure.
The configuration will depend upon a variety of factors.
These factors include the depth of the structure, the minimum
natural period of the structure and the buoyancy can, the
buoyancy required, the diameter of the cen'~er well, the
diameter of the buoyancy can, the diameter of the stem, and the
diameter of the riser. fin example of one possible
configuration follows. The controls, buoyancy can, and stem
that run through a spar structure such as that described in

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.. S -
U.S. Patent No. 5,558,467 may have a total length of
approximately seven hundred fifty-four feet. The controls and
stem would extend approximately one hundred sixty-eight feet
above the normal water line. The buoyancy can would begin at
approximately five feet below the normal water line and extend
downward to approximately one hundred sixty-three feet below
the normal water line. The stem extends the remainder of the
distance through the structure and the riser extends beyond the
keel of the structure to the sea floor. For such a structure
having a center well with a diameter of thirteen feet, a
required tension (buoyancy) of one thousand kips is preferred
for a water depth of approximately five thousand six hundred
ten feet (one thousand seven hundred ten meters) . Seven slip
rings, as indicated, would be spaced along the length of the
center well. An example of one spacing arrangement is as
follows. The slip rings may be placed at intervals of fifty
and twenty-one feet above the mean water level and at five, two
hundred five, two hundred sixty-two, three hundred forty, four
hundred eighteen, and five hundred thirty-six feet below the
mean water level. It is preferable that a slip ring be
positioned at or near the keel joint of the offshore structure.
The invention provides several advantages. There are no
impact loads to the walls of the buoyancy can. Therefore; the
wall thickness and other structural steel can be reduced. This
provides a positive cost impact and increases the net buoyancy.
The slip rings are placed around the stem and the riser instead
of the buoyancy can. Because the diameter of the slip ring is
substantially smaller than if it were placed around the

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buoyancy can, this allows the use of more sophisticated devices
and materials to control the gap and the wear.
Because many varying and differing embodiments may be made
within the scope of the inventive concept herein taught and
because many modifications may be made in the embodiment herein
detailed in accordance with the descriptive requirement of the
law, it is to be understood that the details herein are to be
interpreted as illustrative and not in a limiting sense.

Representative Drawing