Qualification of the Grouped SLOR Riser System
Authors: Karunakaran Daniel N. , Lee Dan , Mair John 
Published: July 2009

The growing trend of deep and ultra deepwater developments necessitates the use of risers that will give good stress response and fatigue performance, and be able to optimise field architecture to accommodate complex and congested seabed layouts. In order to achieve this, Subsea 7 and 2H Offshore have developed the Grouped SLOR, a hybrid riser solution which captures the above stringent riser requirements and maintains maximum operability in deepwater developments at water depths greater than 700m.

The Grouped SLOR consists of individual free standing risers, SLORTM and/or CORTM grouped together by a buoyant guide frame tethered down at either ends to suction piles. Connection between the host vessel and the SLORTM or CORTM is provided by a flexible jumper from a gooseneck located at the top of the riser assembly.

The paper describes the technical developments, key features of the riser system, and the qualification programmes that have been performed to validate the robustness of this concept and design, namely:Finite element analysis assuming constant hydrodynamic properties of the air cans;Computational Fluid Dynamics analysis to validate the assumed hydrodynamic coefficients of the air can arrays;Model test in tow tank for air can arrangements with varying currents and varying current incidence directions to validate the hydrodynamic coefficients of the air can arrayModel test on the complete Grouped SLOR assembly, including air cans, guide frame, risers and tethers to investigate the flow phenomena around the air can assembly;Full installation assessment.

Introduction

Grouped SLOR is an " open bundle?? riser solution developed specifically to optimise the riser/vessel interface, production vessel approaches and seabed layout. It uses a buoyant truss frame to guide the freestanding risers, constraining all risers to move collectively, and thus eliminating the risk of clashing.

The Grouped SLOR has great potential for large deepwater developments, which typically have a complex and congested seabed layout immediately adjacent to the production vessel. This is due to the large number of risers and umbilicals often required to meet production, injection and export requirements, and the spatial constraints imposed by mooring lines and vessel offsets. This poses significant constraints on the riser design to achieve an acceptable riser arrangement whilst ensuring that clashing and interference are avoided. In addition, the fatigue requirements, stringent insulation and gas lift requirements (met by the use of a concentric riser system) greatly favour the use of Grouped SLOR.

In order to qualify the application of Grouped SLOR for deepwater environments, a series of qualification projects had been performed to validate the robustness of the concept and design, and they are listed as follows:Finite element analysis based on assumed constant hydrodynamic properties of the air cans;Computational Fluid Dynamics analysis to validate the assumed hydrodynamic coefficients of the air can arrays;Model test in tow tank for air can arrangements with varying currents and incidence directions to validate the hydrodynamic coefficients of the air can arrays;Model test on the complete Grouped SLOR assembly, including air cans, guide frame, risers and tethers to investigate the flow phenomena around the air can assembly;Full installation assessment.