Marine Drilling Riser Analysis
Marine riser systems provide the pressure barriers to enable the drilling of subsea wells for the oil and gas industry.
The marine riser extends from the subsea BOP up to the vessel with the following main components:
- Upper Flex Joint – connected to Diverter Housing
- Telescopic Joint
- Tension Ring
- Riser Joints – including choke, kill and booster lines and any buoyancy
- Lower Flex Joint
- LMRP (Lower Marine Riser Package)
- BOP (Blow Out Preventer) – connected to wellhead housing or Horizontal Tree
The riser joints include sections of pipe with surrounding additional lines (choke, kill and booster). The upper flex joint and the telescopic joint reduce the effects of pitch, roll and heave vessel motions from the riser and the lower flex joint provides rotational freedom just above the BOP which reduces the bending moment on the wellhead for high vessel offset. Each of these marine riser components greatly reduce the loading imposed on the system. Nevertheless, during accidental vessel excursions the loading on the wellhead caused by mooring line failure or DP drift-off can be substantial. Moreover, with a weak soil foundation the higher bending loads are pushed deeper into the conductor significantly reducing operability. Dynamics caused by heavy modern BOPs from 5th and 6th generation rigs can exacerbate the problem and cause very high fatigue damage, therefore, the system as a whole needs to be thoroughly investigated.
AS Mosley performs global riser analysis to completely understand the complex loading in the riser, wellhead and conductor for all design conditions. The analysis will establish optimum tension and buoyancy configurations to reduce loading and meet code compliant flex joint angle limits. The model extends down to the bottom of the conductor and incorporates an accurate representation of the soil. Furthermore, a high level of detail is included in the region of the wellhead housings and extensions. The detail in the wellhead system and soil ensures the determination of accurate loading on the wellhead and conductor. The global analysis considers many of the challenges that a Marine Drilling Riser system will face during all design conditions.
A summary of some of these challenges has been presented below:
- Over load of the system components and wellhead and conductor
- Fatigue damage due to wave loading and loading from vortex induced vibration (VIV)
- Stroke out of the telescopic joint/tensioning system
- Dynamic motions of the BOP stack
- Weak soil conditions causing high loads in the conductor
- Storm Hangoff
- Accidental loss of marine riser tension
- Accidental loss of vessel position from single mooring line failure or Drift off (DP failure)
- Weak point assessment during accidental offsets incorporating Flex Joint lock up
AS Mosley provides valuable experience in maximising operating limits and reducing fatigue damage for the field specific challenges of each well. We optimise each system accordingly and provide rationalised drilling and non-drilling envelopes to ensure that the system is never overloaded even during accidental vessel excursions or loss of tension. The recommendations that we provide will extend the range of operating limits and thus reduce costs.
For more information on AS Mosley’s Marine Drilling Riser Analysis contact contact us directly.