Lift and Pull. Utilizing both drawworks allows one to lift a pipeline and the other to pull in the pipeline. This helps in executing pipeline deflection and connection operations.
Torque. The rig can apply and hold torque to the pipeline when landing the second end. This enables the rig to steer the pipeline to a desired lay direction or to land the pipeline base in the correct vertical hub orientation.
Ability to Drill or Pile Pipeline Bases. The drillship is well suited to drill in piles for supporting pipeline bases or manifolds. By offsetting the rig and allowing the riser to take a small angle the second rig can drill piles while the primary rig is drilling through the BOP. In 7,000 ft of water these piles can be as far away as 250 ft from the BOP stack. By drilling off-line, only the material cost of the pile adds cost to the project.
ROV and Survey. The drillship is equipped with two full ROV spreads including crews. Positioning reference systems include twin DGPS, short baseline, long baseline, and combined ultra-short/long baseline acoustic systems.
Timing with Drilling Operations
The Discoverer Enterprise is actually a triple-activity rig for floor operations. There are two fully functional rotary tables and drawworks rigged up in the dual derrick, plus there is a mechanized mousehole (smart mousehole) that, while using the aft pipe handler, can make up and break down bottom hole assemblies and make up and rack casing and completion strings. A more accurate description of the drillship is a dual activity unit for below water operations and triple-activity unit for drillfloor operations. As such the second rotary is available whenever there is no need for dual in-water operations. This is a substantial amount of time and includes all time after landing the BOP stack up until the tree is moved to the center of the auxiliary rotary table for rigging up. And includes the time from when the tree running string is retrieved up until abandonment. This is roughly 80% of the entire time on location. The mousehole operation does however, require use of one of the pipehandling machines and will interfere with the auxiliary rig operations when the second pipehandler is needed. To be conservative it is assumed that the mousehole time interferes 100% with non-drilling construction activities. This brings the available time for these operations to 65% of the location time.
Figure 4 depicts a detailed schedule for a theoretical deepwater well with no trouble time. a perfect well. The drilling and completion time is 37 days. This is a challenging schedule for planning concurrent field construction activities. In reality the perfect well gives the most pessimistic template for structuring the simultaneous operations. The auxiliary rotary table operations are a fixed set of tasks and are not subject to changes owing to drilling problems. Thus, if the program is delayed, then the extra drilling time can be considered as more time available to the auxiliary rotary table for construction activities. Figure 5 is the perfect well schedule filtered for all tasks involving the auxiliary rotary table and the mouse hole. The schedule has been further enhanced by making two tasks to start as late as possible. This enables the available auxiliary rotary table time to be arranged in two continuous blocks and yet not affect the overall drilling and completion time. The schedule also shows the slack time where neither the auxiliary rotary table nor the mouse hole is being used. The first large block occurs between the 17-1/2 in. section and running 9-5/8 in. pipe. There are 7.15 days shown where no auxiliary rotary table or mouse hole activity is scheduled. The second period is after landing the subsea tree and comprises almost 14 days. Thus, without reference to the well operation, there are 21 days available out of the 37 in two large blocks. There are smaller segments of available auxiliary rotary table time earlier on which make up the two days needed for the 65% claim.
There are two types of field development work identified as candidates for installation with the drillship while drilling. Installing seabed equipment such as trees, manifold bases, jumpers, and support piles comprise one group. These tasks are normal drill rig activities and are done at the auxiliary rotary table without interference to the normal drilling operations at the main rotary table. The second group includes pipeline construction and involves welding on the rig floor. This type of work should be undertaken during the first of the time blocks, while drilling the well, rather than during the well completion phase when the well is live.
Drillfloor Welding. The drillfloor is classified as a Zone II hazardous area. As such welding is allowed if contained in an area positively ventilated with air from outside the hazardous zone. This is the setup which allows use of direct current (DC) drilling motors and the other rig floor electric equipment on the drillfloor. Additionally it is desired to keep hot welding material from falling into the moonpool and to isolate the welding flash from the adjacent drilling crew. The rotary tables are 40 ft apart. Had this separation distance been 50ft then welding could be undertaken without the need for the positive ventilation. To positively ventilate and to keep the flash off the drill crew a small house will enclose the pipe while welding. This house is equipped with supply fans that are ducted to take air from a safe area. The roof and fore and aft walls of the house open up to allow the pipe to be picked up and set in position as seen in Figure 6. While aligning the pipe the house is closed and the ventilation is started. The house can be as large as 10 ft x 15 ft, and will have a floor to keep sparks and molten metal from falling into the moonpool.
Subsea Equipment Installation with Dual-Activity Drillship
The subsea installation most evident for the offline rotary table is installation of the subsea tree. While the BOP is down and drilling operations are being conducted with the main rig. the tree is brought to the secondary rotary table and rigged up for running.
