The Grouped SLOR: Design and Implementation

Abstract

Large deepwater developments 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 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 is avoided. The freestanding hybrid riser, in both the bundle and single line arrangements, is a configuration that has been widely used in West Africa. It has been selected due to its excellent fatigue performance, the decoupling of the vessel motion via jumpers and the ability to be pre-installed, thus taking this activity off the critical path. However, both the bundled hybrid riser and the Single Line Offset Riser (SLOR) have field layout problems: The bundled riser, whilst able to efficiently incorporate 10–12 lines in a single structure, poses practical problems at the bottom and top ends where connections need to be made to flowline and flexible jumpers respectively. Due to the large number of lines terminating in a small envelope the problem is how to acceptably route flowlines and their associated jumpers, whilst accommodating pipe expansion, movements and installation tolerances and is further compounded by thermal and other flow assurance issues. Similarly at the top end, the offtake of dynamic flexible jumpers to the vessel can be challenging to achieve an acceptable arrangement that both facilitates installation and prevents clashing during operation. The field layout challenge presented by the SLOR is primarily as a result of its large deflections as a result of current loading. This requires each SLOR to have a large spatial clearance with the adjacent SLOR, mooring line or umbilical. So whilst the SLOR facilitates easy access, at the top and bottom end, for connection of jumpers the maximum number of SLORs that can be accommodated within a given field layout is often limited and insufficient to meet initial and future project requirements. This paper provides an introduction to the Grouped SLOR, a riser solution that is developed specifically with a view to optimize the riser/vessel interface and seabed layout. It uses a buoyant frame to guide the freestanding risers which constrains all risers to move collectively and this effectively eliminates the risk of clashing. As the riser spacing can be greatly reduced compared to a conventional SLOR the arrangement allows seabed real estate to be optimized without losing the many benefits of the freestanding riser concept. The Grouped SLOR is discussed and compared with current riser solutions from component level, up to the technical and operational advantages of the system and on to its application in a typical West of Africa deepwater development.