Dynamic Performance Comparison of Deepwater Riser Systems for A Turret-Moored FPS
Authors: Alexander Walter L. , Wu Mason , Chang S.-H. Mark 
Published: October 2008

Abstract
As floating production systems move into deeper water applications, flexible risers are reaching hydrostatic collapse and axial tension design limits. It may be expensive to extend the shallow water flexible riser designs to deepwater applications due to its limited capacity in the deepwater applications. Therefore, a catenary or wave shaped steel risers offer an innovative alternative to flexible risers for these cases [1-2].
This paper presents the dynamic behavior comparison of two deepwater riser systems for a floating production system (FPS) with a Turret Mooring in the Gulf of Mexico (GOM) up to 4000 ft water depth. The deepwater riser configurations include a Lazy-Wave flexible riser and a Tension Leg Risers (TLR) with the combinations of Steel Catenary Riser (SCR) and a flexible riser. The work includes 6-inch and 8-inch risers for flowlines and water injection lines and 8-inch and 10-inch for gas export risers.
The design methodology and design criteria for these riser configurations are presented. Other than the flexible riser design criteria, the sizing and wall thickness of SCRs are based upon the design pressure, collapse resistance, hydro-test and installation requirement.
The configurations developed in these cases are very promising, with well acceptable stress levels. The estimated cost and proposed installation methods are also presented and compared. Furthermore, the concepts that minimize possible drawbacks related to high top hang-off angle, riser compression and soil-riser interaction are given.

Introduction
The objective of this study is to investigate the dynamic behavior comparison of two riser systems in a FPS with a Turret Mooring in the GOM up to 4000 ft water depth. The deepwater riser configurations include a Lazy-Wave flexible riser and a Tension Leg Risers (TLR) with combinations of Steel Catenary Riser (SCR) and a flexible riser. The work includes 6-inch and 8-inch risers for flowlines and water injection lines and 8-inch and 10-inch for gas export risers.
The FPS selected is a typical mono-hull vessel employing Single Point Mooring (SPM) turret production buoy. The FPS can support oil and gas processing facilities and export the separated and treated oil and gas through pipelines to shore.
This paper describes the riser system configurations, static and dynamic riser analysis results, installation procedures, schedules, and costs.

Design Basis

Environmental Conditions and Load Parameters
The riser system is designed to operate in the Gulf of Mexico up to 4000 ft water depth. Four different environmental conditions as shown in Table 1 are considered in the riser analysis. Figure 1 shows how the environmental conditions are applied to the FPS. The current profiles assume piecewise linearity and varying with water depth, while the direction is constant with water depth. Table 2 lists drag and added mass parameters applied in the riser analysis.

Vessel Data
The riser configurations are studied for maximum vessel offset of 12% water depth associated with 100-year hurricane condition and 11.5% water depth associated with maximum loop current. The offsets apply to both near and far positions of the FPS relative to the riser touch down point. Wave-induced vessel motion are prescribed by a set of Response Amplitude Operators (RAO) at the center of the FPS.

Riser System Configurations
Riser systems evaluated are Lazy Wave Riser (Option 1) and Tension Leg Riser (Option 2). The risers are connected to the FPS, which is moored in 3,700 ft water depth. The overall ship length is 781 ft with 138 ft beam and 67 ft draft.