Intervention Riser Analysis

Intervention Riser Analysis

Intervention Riser Analysis helps optimise limits and fatigue life of exisitng systems and assists in the design of new intervention riser systems.

Intervention risers allow a range of downhole operations to be carried out on subsea wells using both wireline and coiled tubing equipment deployed from monohull vessels or semi-submersible rigs.

Typical down-hole workover operations performed through intervention risers include:

            • Logging
            • Setting and removing plugs
            • Circulation
            • Perforations
            • Patching
            • Fishing
            • Milling

Conventional workover risers were made of casing welded to proprietary forged connections.  This was limiting in relation to the material grade suitable for welding (i.e. 75 – 80 ksi) and in deep water applications high wall thicknesses were required for riser pipe.  Furthermore, the presence of welds to connect the riser pipe to the forged connectors attracted high fatigue damage at surface close to the wave and vessel motions.

Modern mono-bore intervention risers utilise high strength threaded and coupled casing connections where the absence of welds means that higher yield materials can be tolerated. Although, there is a limit to the material yield particularly in sour-service applications.  Furthermore, recent developments in casing coupling design for open water applications have significantly improved the fatigue performance of the connections.  Most systems incorporate several special joints which are normally forged to eliminate the need for welds.

Typical special joints required for intervention risers are:

            • Surface Stress Joint
            • Moonpool Joint
            • Subsea Stress Joint

In some cases, it may be possible to incorporate a high pressure flex joint in place of the surface stress joint. The requirement for a surface flex joint depends on the type of Coiled Tubing Lift Frame and its constraint to the vessel.

Global riser analysis is performed to determine the loading on riser systems and subsea equipment.  The loading of the riser can be external from the vessel and the environment or internal from pressure. Analysis is carried out to verify the system for the following design conditions:

            • Normal Connected Operations
            • Extreme Environmental Loading
            • Accidental Vessel Excursions during Drift Off
            • Stroke out of the Compensator
            • Excessive Tension Overload during Compensator Lock up
            • Weak Point Assessment
            • Storm Hangoff
            • Fatigue Assessment (Wave and VIV damage)

AS Mosley performs global riser assessment and detailed finite element analysis to design new intervention riser systems or optimise limits and fatigue life to existing systems.  AS Mosley performed global riser analysis and detailed FEA to develop the first CT intervention riser system deployed from a small monohull vessel.

For more information contact AS Mosley directly.