U.S. patent application number 13/939889 was filed with the patent office on 2015-01-15 for tlp pontoon.
The applicant listed for this patent is Floatec, LLC. Invention is credited to Surya Prakash Banumurthy, Guibog Choi, Edmund Otto Muehlner.
Application Number | 20150016892 13/939889 |
Document ID | / |
Family ID | 52277216 |
Filed Date | 2015-01-15 |
United States Patent
Application |
20150016892 |
Kind Code |
A1 |
Muehlner; Edmund Otto ; et
al. |
January 15, 2015 |
TLP Pontoon
Abstract
A TLP design with improved motion characteristics and that is
drawn to a means of reducing the required tendon stiffness and
thereby reducing the overall cost of deepwater TLPs. The invention
reduces the hydrodynamic added mass of the TLP hull. The horizontal
pontoons that connect the vertical columns of the TLP are shaped to
reduce the hydrodynamic added mass of the structure in the vertical
direction.
Inventors: |
Muehlner; Edmund Otto;
(Houston, TX) ; Choi; Guibog; (Houston, TX)
; Banumurthy; Surya Prakash; (Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Floatec, LLC |
Houston |
TX |
US |
|
|
Family ID: |
52277216 |
Appl. No.: |
13/939889 |
Filed: |
July 11, 2013 |
Current U.S.
Class: |
405/223.1 |
Current CPC
Class: |
B63B 21/502
20130101 |
Class at
Publication: |
405/223.1 |
International
Class: |
B63B 21/50 20060101
B63B021/50 |
Claims
1. A floating tension leg platform for offshore production and
drilling, comprising: a. a deck; b. a plurality of columns attached
to and extending downwardly from the deck; and c. pontoons spanning
the lower ends of the columns and rigidly attached to the columns,
with the pontoons having a height-to-width ratio of at least
1.2.
2. The TLP of claim 1, wherein each pontoon has a semi-circular
rounded top.
3. The TLP of claim 1, wherein each pontoon has a semi-circular
rounded bottom.
4. A floating tension leg platform for offshore production and
drilling, comprising: a. a deck; b. a plurality of columns attached
to and extending downwardly from the deck; and c. pontoons spanning
the lower ends of the columns and rigidly attached to the columns,
with each pontoon having a height-to-width ratio of at least 1.2
and having a semi-circular rounded top and bottom.
Description
FIELD AND BACKGROUND OF INVENTION
[0001] The invention is generally related to offshore floating
structures and, more particularly, to a TLP (tension leg
platform).
[0002] TLPs are floating structures permanently moored to the
seafloor by vertical mooring members, called tendons (FIG. 1).
Tendons restrain the platform in such a way that heave, pitch, and
roll motions are small. Small vertical motions allow the platform
to support vertically arranged top-tension risers (TTRs). For
application in deepwater the length of the tendons have to
increase, which adversely affects the dynamic behavior of a TLP and
also increases the costs. For these reasons, TLPs become less
attractive with increasing water depth.
[0003] A TLP moored by its vertical tendons represents the dynamic
system depicted in FIG. 2, which is a mass-spring representation of
a TLP and its tendons. It has an effective mass M.sub.eff, and an
effective elastic vertical stiffness C.sub.eff. The majority of the
vertical stiffness is provided by its tendons. Only a small
stiffness contribution comes from the hydrostatic stiffness due to
the hull's water plane area. The effective mass of a TLP is
composed of the total body mass of hull and topside, the
hydrodynamic added mass of the surrounding water, and a portion of
the tendon mass.
[0004] The system in FIG. 2 will oscillate following an excitation
by an impulsive load. The cycle period of the ensuing oscillation
is called the natural period. The natural period of the system in
FIG. 2, T.sub.n, can be calculated by equation 1 below.
T N = 2 .pi. M eff C eff Equation ( 1 ) ##EQU00001##
[0005] The natural period of a TLP is an important property since
it influences the TLP's dynamic response to ocean waves. In the
TLP's nominal position the buoyancy of the hull keeps the tendon
under constant tension. When exposed to ocean waves, a TLP
undergoes dynamic motion response which gives rise to fluctuating
tendon tensions. If the tendon tension fluctuations become too
large, the tendons may fail. A primary objective in TLP design is
therefore to keep the dynamic tendon loads within acceptable
limits.
[0006] The magnitude of a TLP's dynamic response to waves is
determined by the magnitude of the exciting load and by the ratio
between the excitation period to the natural period of the TLP. The
response is largest when the period of the wave excitation is equal
to the natural period of the TLP. The dynamic response becomes
smaller when the natural period is well separated from the period
of excitation. A fundamental design principle for TLP design is
therefore to keep the vessel's natural periods well outside from
the wave energy range.
[0007] Ocean waves are typically composed of a series of waves
whereby significant energy is contained in waves with periods
between about 5 and 25 seconds. TLPs are therefore designed to have
their natural periods outside the wave energy range, i.e. below
about 5 seconds and above 25 seconds, as indicated in FIG. 3.
[0008] Keeping a TLP's natural periods for heave, pitch, and roll
below the wave energy range becomes increasingly difficult when the
water depth increases. The challenge stems from the fact that a
tendon's axial stiffness decreases when it gets longer. As seen
from equation 1 above, decreasing tendon stiffness causes the
natural periods of the TLP to increase and thereby to encroach on
the wave energy range.
[0009] The axial stiffness of a single tendon is determined by
equation 2 below where C.sub.Tendon is the axial stiffness of the
tendon, E is the elastic modulus of the tendon material, A.sub.eff
is the effective cross sectional area of the tendon, and L is the
length of the tendon.
C.sub.Tendon=EA.sub.eff/L Equation (2)
[0010] It can be seen from equation 2, as the length L of a tendon
increases, its axial stiffness decreases.
[0011] In order to counter the effect of reduced tendon stiffness
in deeper water, either the size or the number of the tendons has
to be increased. The additional tendon weight then also requires a
larger hull. As a result, the overall cost of TLPs increases
significantly with water depth.
SUMMARY OF INVENTION
[0012] The present invention mitigates the adverse effects
referenced above and is drawn to a means of reducing the required
tendon stiffness and thereby reducing the overall cost of deepwater
TLPs. The invention reduces the hydrodynamic added mass of the TLP
hull. The horizontal pontoons that connect the vertical columns of
the TLP are shaped to reduce the hydrodynamic added mass of the
structure in the vertical direction.
[0013] The various features of novelty which characterize the
invention are pointed out with particularity in the claims annexed
to and forming part of this disclosure. For a better understanding
of the present invention, and the operating advantages attained by
its use, reference is made to the accompanying drawings and
descriptive matter, forming a part of this disclosure, in which a
preferred embodiment of the invention is illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] In the accompanying drawings, forming a part of this
specification, and in which reference numerals shown in the
drawings designate like or corresponding parts throughout the
same:
[0015] FIG. 1 is a schematic illustration of a TLP.
[0016] FIG. 2 is a Mass-Spring representation of a TLP and its
tendons.
[0017] FIG. 3 is a graph of a typical wave energy spectrum.
[0018] FIG. 4 is an illustration of a TLP with four columns and
four pontoons with extensions.
[0019] FIG. 5 provides examples of pontoon cross sections.
[0020] FIG. 6 depicts pontoon added mass vs. cross section
height-to-width ratio.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] FIG. 1 illustrates a typical TLP 10 which includes columns
12, pontoons 14, deck 16, and tendons 22. The TLP hull is the
combination of the columns and pontoons and, if present, pontoon
extensions. As seen, the columns 12 support the deck 16 above the
water and the pontoons 14 are rigidly attached to the columns 12 to
hold them in their spaced apart relationship and may provide
buoyancy to the columns 12 and deck 16. The upper ends of the
tendons 22 are attached to the pontoons 14 or columns 12 and the
lower ends of the tendons 22 are anchored to the sea floor to hold
the TLP in the desired position for drilling and/or production
operations.
[0022] From equation 1 above it can be seen that the natural period
is not only determined by the effective stiffness but also by the
effective mass, M.sub.eff. If the mass is reduced by the same rate
as the stiffness is reduced, the natural period remains unchanged.
Light weight design is therefore of increasing importance for
deepwater TLPs.
[0023] Another way to reduce the effective mass in equation 1 is to
reduce the hydrodynamic added mass of the hull. As stated above, a
portion of the total effective mass is contributed by the
hydrodynamic added mass due to the water surrounding the hull.
[0024] The hydrodynamic added mass of a TLP is typically in the
same order of magnitude as the vessel's displacement. It varies for
different hull shapes and is expressed by an added mass coefficient
C.sub.a. An added mass coefficient of 0.8 indicates that the added
mass of a hull is 80% of its displaced water mass.
[0025] The present invention is directed to a particular shape of
the TLP hull, more specifically the pontoons, to reduce the
hydrodynamic added mass.
[0026] FIG. 4 illustrates a TLP 10 with four columns 12 and
pontoons 14 connecting the columns 12 together. The pontoons 14
span the lower end of the columns 12 and are rigidly attached to
the columns 12. A deck 16 is attached at the upper end of the
columns 12 and is above the water line during normal operations
offshore. The deck 16 normally includes living quarters as well as
production and/or drilling equipment not shown. The TLP hull may
also have pontoon extensions 20 extending outwardly from the
columns 12, providing additional buoyancy and stability. It should
be understood that the TLP drawing is only one example of a TLP
configuration and that more or fewer columns may be used.
[0027] FIGS. 5 A-D illustrate examples of different cross sections
of pontoons 14. The hydrodynamic added mass coefficient of a
pontoon is dependent on the shape of the pontoon cross section.
FIG. 6 depicts the hydrodynamic added mass coefficient in the
vertical direction for a rectangular pontoon cross section (i.e.,
FIG. 5b). As seen in FIG. 6, a larger height-to-width ratio of the
pontoon cross section leads to a reduction of the hydrodynamic
added coefficient. Selecting pontoons with large height-to-width
ratios are therefore beneficial to keep the heave, pitch, and roll
natural periods of a TLP separated from the wave energy range.
[0028] Thus, FIG. 5 D illustrates the generally preferred type of
pontoon cross section for the use of TLPs in deeper water, as
opposed to FIGS. 5 A and C where the height-to-width ratio is
essentially one or FIG. 5 B where the height-to-width ratio is less
than one. It may also be preferable that the pontoon cross section
have a semi-circular rounded top and bottom as seen in FIG. 5 D
with a height-to-width ratio of at least 1.2. The semi-circular
rounded top and bottom contribute to a reduction of the vertical
hydrodynamic added mass.
[0029] While specific embodiments and/or details of the invention
have been shown and described above to illustrate the application
of the principles of the invention, it is understood that this
invention may be embodied as more fully described in the claims, or
as otherwise known by those skilled in the art (including any and
all equivalents), without departing from such principles.
* * * * *