This paper highlights the challenges to drilling risers and running equipment for offshore drilling operations from dynamically positioned mobile offshore drilling units (MODUs) in extreme water depths. Whereas nearly all exploratory drilling has occurred in water depths shallower than 3,000-m, Total recently set the water depth record with its Raya-1 well (3,400-m water depth, offshore Uruguay) using the Maersk Valiant drillship. Petrobras has also set a new Brazilian record for exploratory drilling by reaching a water depth of nearly 3,000-m. The 3-BRSA-1296-SES well was drilled to a water depth of 2,988-m the Moita Bonita area, located in the Sergipe Alagoas basin off northeastern Brazil. Several operators have leases that extend into significantly deeper water depths. This paper discusses approaches and criteria to evaluate the suitability these systems for operations in such depths.
As drilling water depths increase, candidate MODUs may require additional and/or upgraded subsea and running equipment to perform drilling operations. Some of the main concerns faced at these sites by the existing riser systems include but are not limited to:
riser deployment and retrieval
challenges to handling and lift capacity caused by running weight and axial dynamic response,
diverter housing contact
rerating / requalification of subsea BOP equipment (particularly control components);
utilization of the riser system's rated capacity,
riser recoil following planned or emergency riser disconnect,
collapse pressure rating,
storm hangoff of the riser and LMRP,
fatigue due to vortex-induced-vibration or wave loads.
In such depths, longer and heavier riser equipment, rig motions, submerged weight to mass ratio, hydrostatic pressure, mud flow, stiffness, drag, waves, and current all play important roles on riser system integrity, from the wellhead to rig equipment. In addition, many of these wells are located in remote frontiers and are typically characterized by difficult logistics and harsh environment. These factors exacerbate the complexity and potential risk of already challenging operations.
This paper provides general guidelines to evaluate and understand the performance and limitations of existing drilling riser systems on MODUs in moderate and harsh conditions in extreme water depths. The paper will discuss evaluation methods, typical findings and potential mitigations for these issues.