Novel TLP Concept for Ultra-Deepwater Field Development

The tension leg platform (TLP) is an ideal choice for supporting dry trees in deepwater, mainly due to its favorable motion characteristics and limited stroke requirement for riser tensioners. However, as the developments move into the ultra-deep waters (beyond 2000m), conventional TLP’s begin to show signs of technical and economical disadvantages compared to other floating systems. This is due to the fact that the natural period of the TLP system begins to grow beyond the practical limit (∼4–4.5sec) required to mitigate fatigue problems. SBM Atlantia has developed a new TLP concept, called the Pontoon-less TLP (or PLT), that resolves the above issue in a simple and effective way and enables ultra-deepwater development with TLP technology. The PLT concept eliminates the pontoons from the hull and replaces them with a rigid top-of-the-column frame. This change in the geometry significantly reduces the added mass of, and forcing on, the TLP hull. The resulting improvements are two fold: (1) The natural period of the PLT is reduced into the comfort zone (4–4.5sec), therefore the need for additional tendon steel to adjust the stiffness of the system is minimized (in most cases eliminated); and (2) The dynamic tendon tension is significantly reduced, hence the tendon system is optimized. This paper presents the development of the PLT system with emphasis on the hydrodynamic aspects that are unique to the concept. A case study is presented, where the performance of a dry-tree PLT system designed for ultra-deepwater production is compared to that of a conventional four column TLP system.