TLP Pontoon
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.
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 (
A TLP moored by its vertical tendons represents the dynamic system depicted in
The system in
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
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 CTendon is the axial stiffness of the tendon, E is the elastic modulus of the tendon material, Aeff is the effective cross sectional area of the tendon, and L is the length of the tendon.
CTendon=E·Aeff/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 INVENTIONThe 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.
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:
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, Meff. 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 Ca. 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.
Thus,
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
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.
Type: Application
Filed: Jul 11, 2013
Publication Date: Jan 15, 2015
Inventors: Edmund Otto Muehlner (Houston, TX), Guibog Choi (Houston, TX), Surya Prakash Banumurthy (Houston, TX)
Application Number: 13/939,889
International Classification: B63B 21/50 (20060101);