L-riser: Steel Dynamic Riser for Severe Vessel Motions
Authors: Borgen Jon Erik , Fosterud Einar Kjelland , Larsen Truls Jarand 
Published: October 2008

The "L-riser" is a new steel production and export riser concept for floating production vessels that avoids the high cost, integrity problems and size limits of flexibles.

It consists of two initially straight, constant wall thickness, steel pipe sections joined to form an "L"; the steeply inclined upper section is terminated at the vessel, the lower section is tangent to the seabed at a point below the upper termination. An elastic anchor line is attached at the 90 degree corner and to an anchor on the seabed. The corner is a double planar stress joint of novel design incorporating a long radius bend (see Figure 1 and 2).

The "L-riser" configuration allows for large vessel motions in typical harsh environments. Motion components parallel with the upper riser section will be transferred almost undamped to the corner, and set up transverse oscillations that are damped out in the lower section. The 90 degree corner gives the upper steel pipe section dynamic flexibility. The anchor line maintains near constant tension in the lower section, minimising fatigue at the touchdown point. In addition, the flexibility of the lower pipe section allows large static platform offsets.

This paper present the feasibility study of the "L-riser" designed as a 28- ID export riser (see table 2 for pipe properties) in 850m and 1750m water depth, for semi submersible production unit in the Norwegian Continental Shelf (NCS) area. Installation, component design, static and dynamic analysis and fatigue life estimates are covered.

Estimated fatigue life of all parts of the system exceeds 250 years. Material was X65, weld class D. Installation may require only one vessel. The main analysis tool is Visual Orcaflex, by Orcina Limited, UK, Ref.[4]. The analysis results indicate that the design is not close to any limits, and that the "L-riser" is a feasible, robust and highly competitive riser system over a wide range of applications.

Estimated cost for one installed 28-ID riser at 850m water depth is 300MNOK. Cost for 1750m water depth is 450MNOK.

Major deepwater offshore development areas like NCS, as West Shetland, Canada, GOM and Brazil experience challenging environmental conditions as discussed in this paper. Other areas like West Africa have other riser challenges like temperature and insulation.

Introduction

This paper includes the results of the feasibility study of a steel riser system for export risers for floating production semi submersible. The riser system called "L-riser" has been developed by Inocean in Oslo, Norway. It avoids the limiting size problems in combination with choice of floater type, of other riser systems. The "L-riser" allows for large vessel motions in typical harsh environments. In addition, the elasticity of the riser configuration allows for relative large static platform offsets.

Ormen Lange and Kristin field design data are the design basis for this study. The work was initiated in June, 2002, for Norsk Hydro and Statoil towards the deep offshore field developments in NCS, Ref.[1].