Design Optimization of Top-Tensioned Risers for Deepwater HPHT Applications: Part II

Dry-tree solutions with top-tensioned risers (TTRs) have been successfully used with floating production systems (FPS), such as Spars and TLPs, in a wide range of deepwater applications. Both single-casing and dual-casing top-tensioned risers are field-proven in existing field developments. The top-tensioned risers can bring technical advantages and operational cost benefits. Moreover, recent oil and gas developments that have high pressure and high temperature (HPHT) in combination with severe environmental loads lead to more design challenges for steel risers in deepwater, pushing the design limits of conventional steel pipes in deepwater risers. High-strength steel pipes are therefore considered for both technical and economic reasons. The objective of the study that forms the basis for this paper is to provide top-tensioned riser system configurations that meet challenges of the extremely high operational pressure and environmental loads in deep and ultra-deep waters. Part I of the paper was published in OMAE 2010 [1], addressing strategies for top-tensioned riser sizing and weight management for HPHT applications in deep and ultra-deep waters, and also design considerations for TTR specialty joints. Part II here present spar top-tensioned risers and their support tensioning systems. The paper illustrates the HPHT riser global configuration on spar and the tensioning system performance optimization, as well as coupled motion compensation with the spar platform. The impact of riser loads on spar global performance is also discussed.