US20150016892A1 - TLP Pontoon - Google Patents

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.

Description

FIELD AND BACKGROUND OF INVENTION
• The invention is generally related to offshore floating structures and, more particularly, to a TLP (tension leg platform).
• 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.
• 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_eff, and an effective elastic vertical stiffness C_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.
• 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_n, can be calculated by equation 1 below.
• $\begin{array}{cc}{T}_N=2\pi ·\sqrt{\frac{M_{\mathrm{eff}}}{C_{\mathrm{eff}}}}& \mathrm{Equation}\left(1\right)\end{array}$
• 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.
• 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.
• 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.
• 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.
• The axial stiffness of a single tendon is determined by equation 2 below where C_Tendon is the axial stiffness of the tendon, E is the elastic modulus of the tendon material, A_eff is the effective cross sectional area of the tendon, and L is the length of the tendon.
• 
  C _Tendon =E·A _eff /L  Equation (2)
• It can be seen from equation 2, as the length L of a tendon increases, its axial stiffness decreases.
• 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
• 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.
• 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
• 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:
• FIG. 1 is a schematic illustration of a TLP.
• FIG. 2 is a Mass-Spring representation of a TLP and its tendons.
• FIG. 3 is a graph of a typical wave energy spectrum.
• FIG. 4 is an illustration of a TLP with four columns and four pontoons with extensions.
• FIG. 5 provides examples of pontoon cross sections.
• FIG. 6 depicts pontoon added mass vs. cross section height-to-width ratio.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
• 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.
• 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_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.
• 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.
• 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_a. An added mass coefficient of 0.8 indicates that the added mass of a hull is 80% of its displaced water mass.
• The present invention is directed to a particular shape of the TLP hull, more specifically the pontoons, to reduce the hydrodynamic added mass.
• 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.
• 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. 5 b). 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.
• 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.
• 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.

Claims (4)

What is claimed as invention is:

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.

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