Workover Riser Analysis
Workover operations are performed to re-enter the well through the subsea tree, often using coiled tubing (CT) or wireline (WL) equipment. These operations are performed from semi-submersible rigs with CT or WL run through the workover riser. Typically, a lower riser package, emergency disconnect package and subsea stress joint make up the lower stack and land on the subsea tree.
The workover riser extends from the subsea tree up to the vessel with surface equipment above the drill floor with the following main components:
- Coil Tubing lift frame (CTLF) or bails in the derrick containing:
> Injector head, stripper and surface BOP for CT operations
> Lubricator and wireline BOP for WL operations
- Surface Test Tree below lift frame
- Cased Wear Joint (CWJ) with protective casing spanning the drill floor
- Tension Joint
- Workover riser joints including umbilical
- Subsea Stress Joint
- Emergency Disconnect Package (EDP)
- Lower Riser Package (LRP) connected to subsea tree providing well barriers
AS Mosley performs global riser analysis of the workover riser string to determine the operating limits and fatigue life of the system. The subsea stack (LRP and EDP) can be around 50 Te which does not typically lead to the high loading on the wellhead and conductor that can be seen on marine riser systems. Some of the main challenges for workover risers are:
- Surface equipment dynamics
- Fatigue damage due to wave action and vortex induced vibration (VIV)
- Stroke-out or lock-up of the motion compensator
- Storm Hang-off
High fatigue damage can be induced in the riser at surface above or below the CWJ. The CWJ protects the riser and umbilical as it passes through the rotary table. The cased section of the CWJ is relatively stiff and causes higher bending in the riser pipe outside the protected region. AS Mosley has experience in designing surface stress joints to increase fatigue life, as well as using the latest modelling techniques to ensure realistic loading is predicted.
Vortex Induced Vibration (VIV) can also contribute to fatigue damage, especially in high current regions such as West of Shetland. The addition of helical strakes to the workover riser can significantly reduce VIV fatigue damage and is something we specialise in modelling.
A weak point assessment is performed to determine the most likely point of failure in the system in the event of motion compensator lock-up resulting in excessive tension through the riser. It is important that the weak point is above the critical barriers of the LRP, therefore this assessment is performed with the riser vented and pressure shut-in below the LRP. This ensures the equipment to be protected is in its most highly loaded state, while the riser has its maximum capacity. The ideal weak point is just above the subsea stress joint so that in the event of a failure, the whole riser is not lost.
AS Mosley's workover riser analysis delivers optimised operating windows and fatigue lives, extending operations and reducing time spent waiting on weather.