Flexible Risers versus Steel Risers

Non-bonded flexible pipe, made to API 17 J specification, is a spooled product and typically competes with steel pipe in the offshore oil and gas sector for infield pipelines.  Flexible pipe is especially useful as risers to connect the flowlines on the seafloor up to floating production systems.  In this article, we will look at some of the differences between a flexible riser and a steel riser for the same specification requirements.  

Global Riser Analysis

The global, or dynamic, riser analysis for a flexible riser pipe is similar to that of a steel riser.  However, for the same conduit requirements of inner diameter (ID), pressure, water depth, temperature, insulation value, etc., there are some resulting design differences that will affect the global riser analysis.  Some of these differences include:  

Weight: Flexible pipe typically has as higher weight per foot in water than steel pipe.  

Minimum departure angle: The minimum departure angle (hanging from the hull) for flexible pipe is typically 4-8 degrees compared to 12-15 degrees for a steel riser; based on the expected motions and the minimum bend radius allowed at the touchdown point (TDP).  

Outer diameter: The outer diameter (OD) of the flexible riser is usually larger than the steel riser because of the multi-layer construction.  

The net effect of weight and departure angle is that the top hang-off loads of a flexible riser could be similar to that of a steel riser.  

A more significant difference can be in riser configuration where the TDP fatigue may be an issue for a steel riser making it necessary to change a steel catenary riser (SCR) configuration into a lazy wave SCR. Under the same conditions, the flexible riser can remain in a simple catenary configuration because the flexible pipe is less sensitive to fatigue than the SCR.  In this scenario, a lazy wave SCR would be more expensive in material and installation (pipe, buoyancy modules, analysis, and handling) and the lazy wave SCR would require a much larger foot print, or step out from the host platform, than the flexible pipe.  

Fatigue Capacity

A primary benefit of a flexible riser is its fatigue capacity.  The layer most subject to operational fatigue is the tensile armor layer.  The tensile armor layer consists of many wires within the layer, with space between the wires; typical fraction fill of the layer is 90-95%.  When the pipe is subject to cyclic bending/tension load, the wires slip within the cylinder formation relieving some of the bending stress, thus lowering the fatigue stress compared to a continuous cylinder pipe.  The carcass and pressure armor layers are less subject to fatigue because they are discontinuous layers, wrapped helically and nearly perpendicular to the pipe axis.  The overall pipe extension and contraction is limited by the tensile armor layer so that the carcass and pressure armor layers do not reach full extension, or “lock-up”, of the interlocking wraps.  The polymer layer design is based on strain and governs the pipe minimum bend radius.  

For reference, some major flexible pipe projects around the world include: Troll, Kristen in North Sea; Agbami in Nigeria; Bijupira Salema, Frade, Roncador in Brazil; Kikeh in Malaysia; Pyrenees in Australia; Who Dat in Gulf of Mexico and Terra Nova in East Canada.  

Ocean Flow International is an established subsea project management and engineering company founded in 2000, with clientele mostly being independent and multi-national oil and gas operators.  This includes field architecture planning, detail design and operations. OFI also provides overall fabrication, installation, and pre-commissioning oversight, as well as onshore and offshore inspection services.