New Tendon and Riser Technologies Improve TLP Competitiveness in Ultra- Deepwater
Authors: Hanna Shaddy Y. , Salama Mamdouh M. , Hannus Henrik 
Published: October 2008

The extensive industry experience with design, fabrication, installation, and operation of TLPs in water depths up to 4,000 ft has demonstrated the flexibility, reliability, and robustness of the TLP concept. However, the TLP concept is falling out of favor for ultra-deepwater development because of the challenges of its tendon system. The hydrostatic pressure imposed on ultra-deepwater tendons makes conventional steels tendons very heavy. In this paper a lightweight composite tendon system is described. The benefits of the composite tendon are illustrated by a case study for developments in both the Gulf of Mexico and West of Africa. In addition to the composite tendons, the paper presents the economic benefits of also using composite production risers instead of the steel risers. A review of the current industry effort to qualify both composite tendons and risers is also presented. Results show that by applying this new tendon and risers technologies, the competitiveness of the TLP concept is extended to ultra-deepwater developments.

Introduction

The Minerals Management Service (MMS) classifies water depths greater than 1,300 feet as deepwater and water depths greater than 5,000 feet as ultra-deepwater. Many experts believe that deepwater reservoirs in the Gulf of Mexico has the potential to provide as much oil and natural gas as the North Slope of Alaska. It is also believed that ultra-deepwater Gulf of Mexico holds enormous potential for oil and gas development [1]. Deepwater developments are, however, unforgiving, physically, technically and economically. There is no room for failure in any area: reservoir understanding, technical judgement, operating performance or economic discipline. A key imperative to deepwater development is continued technological advances to reduce the development cost while improving reliability. Since the early deepwater development in 1993-94 (Conoco's Jolliet and Shell's Auger), a cost per barrel has been reduced by nearly two-thirds. A key element in this reduction is focusing on life cycle cost. This is particularly true in deepwater where the costs of maintenance and well intervention are high and the penalties are severe.

The main concepts that are being considered for deepwater developments are tension leg platform (TLP), Spar, and floating production units with and without Storage (FPS and FPSO). The TLP and Spar concepts that are considered the most attractive for deepwater development because they allow for easy access to the wells. Cost effective development of deepwater fields requires flexibility in concept selections because it is very unlikely that a single concept will prevail as the optimum solution. A combination of concepts may offer the optimum cost-effective solution. As an example, a combination of a SPAR or an FPSO with satellite TLPs may represent a viable option. While the reliability and ease of operability of the TLP concept has been demonstrated by the many TLPs currently in operations, the application of TLPs for ultra-deepwater becomes inhibited by the heavy weight of the conventional steel mooring system.