The Single Leg Tension-Leg Platform (STLP) is a new concept for a compliant, deep water platform. This evolution of the TLP concept utilizes a hybrid mooring system with a central tension-leg to restrain heave motions while pitch and roll are limited, by proper hydrodynamic design of the hull. Like a conventional TLP, the STLP allows direct surface access to wells. The new concept limits the cost of the vertical tension-leg mooring system by reducing the number of tendons to a minimum. It also greatly reduces foundation costs because of the central tendon location and reduced tendon loads. The hybrid mooring, which uses a permanent spread mooring to supplement the vertical tension-leg mooring, provides station-keeping benefits for operational and extreme conditions. Analytical studies and model tests have demonstrated favorable performance characteristics. The concept is applicable for any water depth and field size that would be appropriate for conventional floaters or TLP's.
The STLP is a new evolution of the TLP concept. It features a hybrid mooring with one tension-leg attached to a column at the center of the platform and a spread mooring from the corner columns (Figure 1). Whereas the conventional TLP mooring stiffly restrains all vertical motions, the central tension-leg of the STLP suppresses only heave motions. The pitch and roll motions are controlled by proper hydrodynamic design of the hull these features significantly reduce tension loads in the vertical mooring. Therefore, the STLP does not need as many tendons or as much tendon strength (cross-sectional area) as a conventional TLP.
Initial feasibility studies for the STLP focused on a design for 4000 ft. (ca. 120Om) water depth in the Gulf of Mexico. The results of these studies indicated that a STLP is a technically viable alternative to a Tension Leg Wellhead Platform (TLWP) for deep water developments. For the great water depth targeted, fewer tendons and reduced costs for the foundation template, gave the STLP a cost advantage over the TLWP. Upon achieving this result, it was decided to verify the performance of the STLP and the accuracy of the analytical/design methods through model testing.
This paper presents the principal features of the STLP concept, reviews the results of initial engineering and model testing, and discusses the economic potential of the new platform for deep water development.
The main features and benefits of the STLP concept are listed in Table I and discussed below.
The primary feature of a STLP is its single, central tension-leg which acts to restrain the platform against heave motion. Since the tendons do not restrain pitch or roll, total dynamic tension variation is reduced, as can be seen in Figure 2. In this figure, the amount of tension variation due to pitch and roll restraint is compared to fluctuations due to heave alone for a centrally located tendon. Decoupling angular motions from tendon loads permits longer heave motion periods, further reducing tendon material requirements.
