Structure reduces lateral setdown motion, size, higher costs associated with competing designs
Nagan Srinivasan
Dreco
- Tension base TLP concept.
- TLP-based deepwater concepts.
- Comparing principles of different deepwater conceplts.
- Offshore installation sequence for TBTLP concept.
In the near future, major oil companies plan to continue investing heavily in deepwater exploration and development. Shell currently is the operator for 387 deepwater leases in the Gulf of Mexico alone. Of this number, 192 tracts are in water depths beyond 4000 ft. BP Exploration holds 115 deepwater leases with water depths exceeding 4000 ft. Texaco holds 42 deepwater leases in the Gulf of Mexico, with the water depths exceeding 4000 ft. The list of other top deepwater leaseholders includes Mobil, Exxon, Chevron, Amoco and Conoco.
This list is of US Gulf of Mexico offshore leaseholdings. If world lease or license holdings were considered, the figures would be multiplied manifold.
Field development system designs for water depths in excess of 4,000 ft are very limited. This paper introduces a new development concept known as a tension base tension leg platform (TBTLP) that is ideal for water depths over 4000 ft.
After installation of the Auger TLP in 2,860 ft deep water at the cost of $1.2 billion, many designers are more or less convinced that the conventional TLP concept is not very economic at water depth beyond 3,000 ft. The reliability of the conventional TLP system is also questioned in deep waters.
Engineers have found that after certain water depth (3,000 ft), the size of the TLP increases drastically with water depth. The magnitude of first and second order wave forces on the structure, consequently increase. Also, the effect of resonance due to second order forces is found to significantly affect the reliability of the conventional TLP at very deep water.
Deepwater problems
The TLP behaves like an inverted pendulum. The excess buoyancy of the hull maintains tension in the tether mooring system. The fundamental sway natural period of the deepwater TLP is in the order of 150-250 seconds.
With this large sway period, the TLP moves compliantly to waves. The TLP sway motion magnitude is in the order of 100-300 ft. This kind of large offset in the horizontal sway produces excess vertical setdown which causes a serious problem in a TLP design. The setdown reduces tension, resulting in slacking to the aft tethers. Slacking in any of the four leg tethers in the TLP significantly weakens the reliability of the TLP concept.
One solution by which the slacking can be avoided is to increase the TLP size, a cost increase. However, increasing TLP hull size also increases the wave force, sway amplitude, and thus again the setdown. Hence, it becomes clear that after a certain water depth, the TLP size increases exponentially with respect to water depth.
The second problem of water depths over 3,000 ft is that the vertical heave motion of the TLP becomes very severe. The vertical natural period can exceed 5 seconds, which is close to everyday wave periods. This causes a kind of vertical resonance called springing which is a potential disaster for TLP tethers.
New concepts
The principle of compliance to waves inherent in the conventional TLP concept seems attractive for deepwater. However, the excess compliance unavoidable in the deepwater TLP introduces other potential problems which are not good for very deep water applications.
Researchers have realized that the TLP size, wave force, sway period, and sway amplitude should be reduced significantly in order to make the TLP concept attractive and economic in deep waters. With this objective, recently, many research papers have proposed several new generation concepts.
The new generation concepts are based on the conventional TLP concept. They include the suspended tension leg platform (STLP), tension raft jacket (TRJ), hybrid compliant platform (HCP).
The tension based TLP is competitive with the conventional TLP in water depths greater than 3,000 ft. The TBTLP reduces the size of the water surface hull structure, the wave forces, sway period and amplitudes, compared with the conventional TLP in very deep water. Thus, the negative points of the TLP are eliminated in the TBTLP concept. Figure 1 shows the conceptual picture of TBTLP designed for a 4000 ft water depth.
The TBTLP consists of a reduced size TLP where tendons are moored on a deeply submerged buoyant base. The buoyant base is again vertically moored by another set of lower tendons to the seabed. Optional lateral resisting mooring lines are used for the submerged base to ease installation and to improve the in-place behavior. Hence, the TBTLP has two levels of tendons:
- Upper tendons used for the water surface hull
- Lower tendons for the submerged buoyant base.
Note that both the upper and the lower tendons are retained always in tension due to excess buoyancy of the hull as well as the submerged base. The submerged buoyant base is located at 1,000 ft below the water surface and thus the wave forces on the base are negligible.
TBTLP principles
The TBTLP concept is mainly aimed at avoiding tether slacking in very deep water without increasing the size of the hull near the water surface. In fact, TBTLP is designed to use a much smaller size TLP, similar to a TLP designed for a shallow water. The TBTLP design provides large pretension for the bottom tethers, which support the submerged buoyant base to the seabed.
Because of the reduced size of the water surface hull, the wave forces on the TBTLP are much smaller than in the case of a conventional deepwater TLP. The TBTLP wave offset, setdown, and consequently the tendon slacking effect are reduced to a much greater extent. Thus, the reliability of the TBTLP is enhanced in deep waters. The tendons supporting the submerged base are subjected to two tensile loads: one due to the excess buoyancy of the base and the second due to the excess buoyancy of the surface hull.
Thus, the lower set of tendons is subjected to a larger tension than that of the upper tendons. This action increases the global stability of the TBTLP and reduces the lateral motion of the submerged base. Hence, the slacking effect of the lower tendons is eliminated.
The TBTLP concept uses a lateral resisting mooring system to the submerged base to reduce offsets due to underwater currents. The lateral resisting moorings are more effective for the underwater current loads than compared to the surface wave loads. Thus, the submerged base motions are well controlled in the TBTLP concept.
The submerged base of the TBTLP almost behaves like a floating foundation to the surface floating TLP. Thus, the effective water depth for the surface TLP has reduced from 4,000 ft to 1,000 ft. Consequently, the sway period and amplitude of the surface TLP are much less compared to the conventional TLP.
Comparing concepts
Figure 2 shows the three-dimensional view of the conventional TLP, the suspended TLP, the tension raft jacket, and the hybrid compliant platform concepts. The merits and demerits of TLP are widely known. The merits and demerits of the other three concepts can now be examined in conjunction with the TBTLP concept.
- Suspended TLP: The suspended tension leg platform, proposed by Jagannathan, (1992), consists of a regular TLP with suspended foundation. The suspended foundation is called the lower platform. It is a submerged heavy weight balancing the excess buoyancy of the water surface TLP hull.
Because of its floating characteristics, the STLP is conceptually applicable for any water depth. However, it is difficult to balance the weight of the lower platform to the required tether tensions. Hence, a STLP demands a dynamic ballasting system which increases the cost and the complexity of the concept.
The draft of the surface TLP is sensitive to the variable weight of the deck during operation. The STLP has poor stationkeeping system to wave drift and current forces. The platform tend to drift. The upper and lower platform masses can move towards each other, due to inertia during heave oscillation, and that can slack the tendons. This causes catastrophic failure to the tendons. Thus, even though the STLP is applicable for any water depth, the reliability and the functionality are not well suited for deepwater operations.
- Tension Raft Jacket: The tension raft jacket was introduced by Abbott et al. (1994). The platform concept uses a submerged pontoon with vertical tension moorings. A conventional jacket type platform is placed over the buoyant pontoon. The concept basically has replaced the columns of the conventional TLP with 430 ft tall slender jacket legs.
The main advantage of TRJ is that the wave force on the jacket near the water surface is much reduced than that of a TLP designed at this water depth. However, the moment lever arm for the TRJ has significantly been increased. Thus, the TRJ will be subjected to large and possibly uncomfortable pitch and roll motions. This affects the tendons significantly.
In order to have a reliable TRJ, the submerged pontoons should be oversized and the tendons should be kept at very high tensions. Consequently, this can lead to an uneconomical structure.
- Hybrid compliant platform: The hybrid compliant platform (HCP) concept, developed by Srinivasan, (1994), is aimed at reducing the size of the structure for large deckload capacity for 3,000 ft deep waters. The structural sway motion amplitude is controlled in the HCP concept.
The drilling and production risers are supported throughout the depth of the water and conventional well drilling system is used. The reliability of the concept at deepwater seems improved. HCP uses a relatively smaller size TLP and manageable size sections of a smaller size compliant tower. The tower is installed in the middle opening of the four leg TLP. The TLP moves independent of the tower in the vertical mode and couples with the tower in the horizontal sway mode.
The drilling modules are separated from the production and facility modules at two different levels. The design uses a special installation procedure which is cost effective compared to the installation of a 3,000 ft long single piece compliant tower. The fabrication, installation, drilling and production costs are reduced in the HCP concept. HCP is attractive for large deck payloads. For smaller deck pay loads and for water depth over 4,000 ft, the HCP will not be as economic as the TBTLP.
Figure 3 schematically illustrates the principles of the discussed deepwater concepts. The TLP with single tension tether at the top loses its stiffness as water depth increases, unless the hull size is increased drastically.
The STLP, first of all, has poor stationkeeping. Secondly, it collapses when the two masses happen to move towards each other in its dynamic vertical motion. The rigid connection between the top and the bottom masses inherent in the TRJ concept causes unwanted pitch and roll moment and weakens the life of the tension members. The HCP seems to be uneconomical for smaller deck load and deeper water.
Among the above discussed deepwater concepts, the TBTLP concept appears to be attractive and cost effective in very deep depths. This is because of its simplicity, reduced size and material weight, proven installation technique, and reduced sway motion behavior.
The TBTLP uses conventional and well proven installation techniques. Figure 4 shows the sequence of installation for the TBTLP concept. First, the foundation and well templates are pre-installed. Then the buoyant base is towed to the location. By partially floating, the base is lowered to the desired depth. Optional buoys, not shown in the figure, can be used to hang the base underwater.
Laterally resisting mooring systems used here reduce the motion of the base and helps by installing the lower tendons smoothly. Because the moorings are under water below 1,000 ft and not normally subject to waves, they are also effective in controlling the in-place motions during production. Because of deep water, the base tendons are assembled and installed vertically.
After the lower tendons are fully fixed, the base is ballast controlled for the designed excess buoyancy. Next, the TLP can be towed to the location. Since the distance between the base and the TLP is only 1,000 ft, the upper tendons can swing down and connect the TLP to the submerged buoyant base. The TLP is now ballast controlled to the designed excess buoyancy. Thus the TBTLP is fully installed.
Conclusions
In a deepwater production project, structural costs generally become the major contributors to total field-development cost. The selection of an appropriate concept for a deepwater field development, with limited choice, is crucial to controlling the cost of the project. The reliability of the concept is another important factor to be considered in the selection.
Considering this, the TBTLP appears to be the best suited concept for very deepwater. The TBTLP technical features such as size, payload capacity, and motion behavior to waves and winds are beyond the scope of this paper. One of the important advantages of the TBTLP concept is that it can be extended to water depths over 5,000 ft or more by providing more than one intermediate submerged buoyant base. Also, the surface TLP can easily be unlinked from the base then reused in another location.
Further development and research on the TBTLP concept are in progress, and detailed mathematical and physical model studies can be used in joint industry projects.
References
Jagannathan, S. K., (1992), "The Suspended Tension Leg Platform (STLP): A New Platform Concept for Deepwater Exploration and Production", 11th International Conference on Offshore Mechanics and Arctic Engineering, Calgary, Canada, 1992, Volume I-B, pp 517-525.
Srinivasan, N., (1994), "Hybrid Compliant Platform: An innovative Concept for Deepwater Production", Behavior of Offshore Structures (BOSS94), MIT, Cambridge, July 1994.
Abbott, P., Nygaard, C., Huang, E., Johnson, B., DSouza, R., Dove, P., Datta, B., (1994), "Tension Raft/Frame Reduces TLP Motions, Costs" Offshore, PennWell Publication, August 1994, pp. 40-62.
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