Dry trees (DT) provide vertical well access from the water’s surface with well bore tools, and are especially useful for speedy interventions. Lower potential CAPEX helps, thus avoiding some of the more complex deepwater seafloor construction projects. However, in the ultra-deepwater, risers for DT become the demons of necessity. Deepwater pressures and high formation temperatures make for thick walled pipe. All of this requires large, extensive kit, and a challenge for existing DT designs such as a spar or a tension leg platform (TLP). Riser weight controls DT design choice. The common top tension machine is limited to about 35 feet of the up and down motion of the sea. When industry designed mobile offshore drilling units (MODU’s) larger with deeper draft to offset vertical riser motion, the structure itself became a cause of unwanted motion and vibration. After hull considerations, research focused on structural vortex induced vibration (VIV) and vortex induced motion (VIM), riser motion onboard, and lower weight risers. Deep draft designs studied by RPSEA and DeepStar? showed VIM and VIV reactions through simulated computer and tow tank testing. Such designs require mitigation strategies. Paired column semisubmersible floating vessels (PC-Semis) can be designed with some control over column size as part of the mitigation. Critical steps to complete are as follows: 1. Performance on the drill/production aspects, especially of the risers relative to fatigue, and safety in both normal weather and under extreme conditions. 2. Well bay designs to allow simultaneous ongoing drilling operations while producing [with risers], and keeping the area safe for workers. The results of nine years of study have shown that there were no major technical show stoppers. Since the original quest was to save money, the objective has become a more difficult moving target. Lessons learned suggest a possibility to use deep draft dry tree (DT) designs as less costly wet tree vessels.
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