Achieving a Step Change in Ultra-Deepwater TLP Capital Costs with the Innovative Step-Diameter Tendon System
Authors: Hannus Henrik , Engebretsen Knut , Borresen Ragnvald , Schamaun Paul 
Published: October 2008

This paper demonstrates how significant reductions in TLP Capital Costs are achievable in water depths from 4,000 to 10,000 feet. The key is utilizing the innovative step-diameter tendon that is nearly neutrally buoyant in any water depth versus the conventional constant diameter and wall thickness tendon system currently employed.

The step-diameter tendon system has the potential to competitively extend the application of TLPs well beyond what industry currently believes to be the commercial water depth limit of about 4,000 feet in the Gulf of Mexico.

In this paper the cost benefits of the stepped diameter tendons are demonstrated by a case study of full size TLP in 4,000, 6,500 and 9,000 feet. Overall system costs are benchmarked against a TLP with conventional tendon system. A step change in TLP size is achieved by innovative use of proven components that result in a neutrally buoyant tendon.

Introduction

The TLP platform started as a "high tech" concept, but has now by a long range of installations become a workhorse for the deepwater. The concept offers limited movement and therefore lends itself as a dry completion unit. One consequence of the TLP maturity has been that the concept has been perceived as having certain application limits. This relates in particular to the water depth, for which claims have been made for a technical limit at around 6,000 feet and a commercial limit at around 4,500 feet. This paper will show that these limitations are a consequence of stretching the conventional design beyond its application and that the limitations can be offset by re-newing the tendon system.

The API recommended practice for TLP design [1] illustrate the interdependence between the various elements of the platform as the "design spiral". As the riser tension increases, this requires more deck steel, and the weight requires larger displacement that again requires increased hull weight and increased tendon tension.

The tendon string consists of steel pipe. The pipe diameter to wall thickness ratio is generally chosen so that the pipe has near to neutral weight in water. But in deep water, the hydrostatic head produces large compressive loads on the pipe. By increasing the wall thickness the pipe capacity can be increased, but this results in a tendon that is heavy in water.

The weight of the tendon will have to be carried by the platform, resulting in increased displacement and therefore increased hull weight and tendon loads. The knock-on effects result in a steep cost gradient beyond 4,000 feet, which challenges the commercial viability of the TLP in ultra-deepwater.

A number of ideas to overcome the tendon weight penalty in ultra-deepwater have been presented over the years. These include filling the tendon with lightweight foam, pressurizing the inside with air and replacing the steel pipe with high modulus composite fibers. The foam filled pipe and composite tendons still await a commercial breakthrough, while the pressurized tendon is held back by "hard to define" safety concerns.

This paper presents a step-diameter tendon [2] application.