Traditionally, shallow water wells have been drilled from fixed platforms, jack-ups or moored drilling rigs. Recently there has been increased interest in performing operations on these wells using new generation of Dynamically Positioned (DP) rigs, driven by available capacity of these rigs and environmental regulations that restrict laying anchors on the seabed. Shallow water offshore drilling operations present a set of unique challenges and these challenges are further amplified when operations are performed on older wells with legacy conductor hardware with newer DP vessels and larger BOPs.
The objective of the paper is to present challenges that occur during drilling in shallow water and discuss mitigation options to make these operations feasible through a series of case studies.
Key challenges to optimizing riser operability and rig uptime are discussed. Potential modifications to the upper riser stack-up and rig deck structure for maximizing operational uptime are discussed. Riser system weak point assessment is presented along with solutions for mitigating risks in case the wellhead or conductor structural pipe is identified as the weak link. Selection of the drilling rig can have significant impact on wellhead fatigue response. Some criteria for rig selection based on drilling riser and wellhead system performance is presented with the objective of optimizing the fatigue performance of the wellhead and conductor system. Wellhead fatigue monitoring solutions in combination with physical fatigue mitigation options are presented to enable operations for fatigue critical wells.
For ultra-deepwater subsea wells, a riser system is required to conduct completion, intervention/workover and end of life activities. For ultra-deepwater riser systems with high temperature and pressure requirements, the intervention riser system often requires vessel interface optimization to achieve acceptable design response. The upper riser can be configured in several different ways, each with its own benefit from a safety, risk and performance perspective. This paper compares the riser response for various vessel interfaces for ultra-deepwater applications.
As discussed above, intervention riser structural response is sensitive to the riser configuration at the vessel interface. For a typical intervention riser, due to ultra-deepwater and high tension requirements, the functional tension load may utilize up to 40% of yield strength thus decreasing the capacity available to accommodate bending and pressure loads. Vessel operators have options to modify the system configuration to improve the strength and fatigue response of the riser. The different vessel interface options include the tension lift frame (TLF) to vessel interface, the top tension application method and the use or otherwise of a surface tree dolly. Upper riser assembly (URA) loads may be optimized by use of rotary wear bushings, a cased wear joint assembly or flexjoints as a part of the stack-up.
The various riser-vessel interface options are evaluated and compared in this paper. This paper highlights the riser design challenges for ultra-deepwater applications.
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