Marine Drilling Riser Analysis

Marine Drilling Riser Analysis

Marine drilling riser systems provide pressure barriers, enabling the drilling of subsea wells.

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

 

Riser System Loading

Riser joints include sections of pipe with surrounding lines (choke, kill and booster). The upper flex joint and telescopic joint reduce the effects of pitch, roll and heave vessel motions on the riser. The lower flex joint provides rotational freedom above the BOP.

All of this reduces the bending moment on the wellhead during extreme conditions. Nevertheless, during accidental vessel excursions, the loading on the wellhead, caused by an accidental mooring line failure or a DP drift-off event can be substantial. With a weak soil foundation, the higher bending loads push deeper into the conductor. This reduces operability.

Dynamics caused by heavy modern BOPs from 5th and 6th generation rigs can exacerbate the problem. This can cause very high fatigue damage. The whole system, therefore, needs to be thoroughly investigated.

To completely understand complex loading in the riser, wellhead and conductor for all design conditions, AS Mosley performs Global Riser Analysis. This analysis establishes optimum tension and buoyancy configurations to reduce loading and meet code-compliant flex joint angle limits, as per API RP16Q or ISO13624-1. The model extends below mudline 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.

Global analysis considers many of the challenges that a Marine Drilling Riser system will face during all design conditions.

 

Design Challenges

A summary of some of these challenges are as follows:

• • • 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 has valuable experience in maximising operating limits and reducing fatigue damage for field-specific challenges of each well. We optimise each system and provide rationalised drilling and non-drilling envelopes. This process ensures the system is never overloaded. This includes during accidental vessel excursions or loss of tension. Our recommendations will extend the range of operating limits and thus reduce costs.

For more information on AS Mosley’s Marine Drilling Riser Analysis please contact us.