Composite Tethers and Risers in Deepwater Field Development: Step Change Technology
Authors: Bøtker Stig , Storhaug Turid , Salama Mamdouh M. 
Published: October 2008

A key imperative to deepwater development is continued technological advances to reduce the development cost while improving reliability. Therefore, composite materials are being considered for several water depth sensitive systems such as risers and tethers. This paper focuses on the application of the composites technology that is being developed through the Conoco/Kværner Composite Alliance as part of the effort to promote the commercialisation of composite risers and tethers. The paper demonstrates the economic impact and the enabling capability of composite tethers and risers for ultra deepwater TLP's. Benchmark cost of conventional TLP solutions with TLP solutions based on composite tethers and risers is being presented.

Subjects to be covered in details are:Feasibility of composite tethers and risers for deep and ultra deep water TLP applications,Composite risers and tethers impact on overall TLP design,Design and fabrication of composite tethers. Also presented, are the results of a composite tether prototype development that is being performed as part of a DEMO 2000 project.

Introduction

For deepwater developments with floating structures, the use of lightweight and high-strength materials has become increasingly important. Increased water depth generally means increased weight and tension from risers and station keeping systems. These effects have to be compensated for with additional buoyancy; which results in increased displacement and dynamic mass of the floating structure. This again requires larger station keeping systems, and the design spiral continues until convergence is achieved.

Generally, the use of composites and advanced materials reduces the requirement for increasing the displacement. This effect becomes more important as the water depth increases. The cost benefits gained by introducing composites are dependent on the type of concept and where the substitution takes place. The more weight-sensitive the concept is, the larger the gain will be. The most sensitive floating concepts are the TLP followed by the Semi, and the least sensitive is the FPSO.

Through the Conoco/Kværner Composite Alliance, both Kvaerner and Conoco have performed significant work to assess the global impact of the use of composites on overall platform configuration. The costs associated with composite applications, and the cost reduction for the entire system when substituting composites for steel, has also been extensively investigated. The reduction of the system cost is important since the procurement costs for composite components are generally higher than equivalent steel components by a factor of 1.5 to 2.0.

Although several studies have shown that the use of composite materials in risers and tethers results in many benefits and that the market potential is large, several hurdles remain before the technology becomes commercially available. One of the main objectives of the Kværner/Conoco Composite Alliance is to overcome these hurdles. The current plan is to complete by the end of 2001 programmes for the demonstration of full scale fabrication and installation of both composite risers (CompRiser?) and composite tether (CompTether?). In addition, systems for certification and quality control will be established according to DNV design guidelines for composites offshore.