The biggest advantage in using the rig for Field development is to eliminate the need for an additional construction vessel such as a derrick barge or pipelay barge. The capabilities of the Discoverer Enterprise as a dedicated deepwater pipelay vessel actually exceeds that of many specialized lay barges. By using the rig for this function, more schedule flexibility is realized. It is not necessary to coordinate the arrival of a separate construction vessel with the other field activities such as drilling or host installation. No additional mobilization of offshore vessels is required. The rig can even be used to drill anchor piles and install the mooring lines. The rig can lay pipelines in a piecemeal manner when needed rather than in one operation. The rig can be used at anytime within the development schedule for lay purposes or other tasks. This permits a more rational infrastructure development for the field rather than that found when the lines have to be laid all at once in a single operation.
Finally, there is the capability to increase the overall efficiency of the operation by changing the basic culture on the rig floor. Because of the second drilling center more focus and attention will be given to planning the subsequent phases of the operation. One complete rig exists to support the other, and just keeping that operation running will improve organization and help eliminate the unidentified unproductive time that is found in analysis of the well construction process. This potential savings due to better planning is real and can be seen in investigation of the improvements in efficiency in batch-drill operations. A significant amount of the batch-drill improvements cannot be tied to the operations eliminated by this process alone. Much of these improvements are from abolishing the need to plan between stages of the operation. The second rig causes better planning and coordination between stages because its primary function is to carry out the work for the next phase.
The Discoverer Enterprise
The Discoverer Enterprise is an extremely large drillship incorporating a multi-activity derrick and drillfloor. The hull is modeled on a standard 100,000 ton North Sea dynamically positioned (DP) shuttle tanker outfitted with six 7,000 hp azimuthing thrusters. Total stationkeeping thrust totals 42,000 hp, and the variable deck load is 20,000 tons.
The large stable platform provided by this hull allows use of a dual capacity derrick with an 80 ft x 80 ft drill floor and a 220 ft height. The derrick is fully outfitted with two drilling packages including topdrives and motion compensators. Figure I shows the basic floor arrangement. The allowable simultaneous hook loads are 2,000 kips each and the two rigs are outfitted to rack casing in triples and drillpipe in quadruples (125 ft stands). The dual derrick permits parallel and shared operations and places many rotary table activities out of the critical path. The continuous setback can feed either rotary table and allows casing, bottom hole assemblies, and running tools to be made up and stood back with one rig while drilling hole with the other. The dual activity also permits the auxiliary rig to drill and case the top hole while running the BOP/Riser with the main rig. The derrick is designed to transit with two full drill pipe setbacks.
The drillship is outfitted with two mud systems, quarters for up to 200 men, and storage for 20,000 ft of riser. There is space for staging and running subsea trees, and for the normal shorebase storage and office facilities. The vessel's sleek lines and high power enable economical moves between operating areas, and the ship can carry variables for up to five wells. There is 100,000 bbl crude storage potential for well testing or completion.
The motion response characteristics of the drillship are extremely good and are in fact better than those for a semi- submersible for waves in the 9-14 second head seas.
The design is especially suitable for field development work. The stationkeeping particulars, the high hoisting capacity and the ability to work subsea in parallel with the drilling operation lends the opportunity to economically carry out work that would normally be done with a dedicated construction vessel.
Dual-Activity Drilling Operations
For single exploratory wells there are three main areas where dual activity can be carried out: running the BOP stack and top-hole operations; make up and testing of bottom hole assemblies and downhole tools; and make up and running of casing. Up to the time the BOP stack is landed both rigs are 100% engaged, thus halving the time required to drill that section. One rig runs the BOP and riser while the other drills the top hole and runs casing down to 20 in. One rig does all tophole operations, pilot hole, drilling, tripping, running pipe etc. while the other is running the BOP and riser. Once the wellhead running tool is released the rig moves 20 ft and the stack is landed.
At the end of the well the auxiliary rig can be used to remove the wellhead or to run abandonment gear while simultaneously pulling the BOP stack.
After the stack is landed the auxiliary rig is devoted to preparing bottom hole assemblies and down hole tools for the main rig. In addition to the time saved simply from moving these activities out of the critical path there is the benefit of reduction of downtime from testing faulty tools. This can be done with the auxiliary rig and allows time to troubleshoot problems that would have normally been charged against the spread costs. This is especially important in deepwater or in applications requiring special tools. It is possible to build stands outside the main rotary table with a special mouse hole but it is difficult to test running tools in this area while the drill string is rotating or while tripping pipe.
In addition to servicing downhole tools the auxiliary rig makes up the casing string in 125 ft stands and racks back for running. Time is saved in making up the shoe joints and float collars, and half of the casing joints are made-up out of the critical path. Tubing strings, gravel pack and other special strings are made up by the auxiliary rig as well as drill pipe for the main rig or for inspection.
