The dual-gradient SubSea MudLift Drilling (SMD) system will be used in 4,000-10,000-ft water depth, but the prototype system was installed on a second-generation semisubmersible contracted to drill a test well in 910-ft water depth. This test venue allowed the SMD teams to evaluate its subsea operation without incurring the high costs of drilling from a larger rig with greater water depth capacity. This test also confirmed that the SMD system could be retrofitted to a smaller rig, extending the rig's water depth capabilities, a potential benefit of dual gradient drilling.
The test rig Ocean New Era (Diamond Offshore) is a second generation 2449 ST VDL rig with 1,500-ft water depth capability. It was used as a training center for two years prior to contracting to drill the world's first SMD, dual-gradient well. The joint industry partnership (JIP) companies had a significant investment in the system. The partners contributed people into the project on a full-time basis. Creating an ideal mix of expertise covering the interest of designers and manufacturers, operators, contractors, and special interests (such as well control). With the rig selected, the process of integrating the SMD system to the rig began.
Meanwhile, SMD equipment designers, now apprised of the New Era's specifications and deck layouts, worked through the final configuration for the SMD mudlift pump (MLP), the subsea manifolding and the integration of the MLP with the lower marine-riser package (LMRP) from the New Era. The SMD operations team worked through surface manifold design and fabricated the MLP seawater power system, which included a filtration skid and pumps dedicated to providing seawater to the subsea pumping package. The weight of the SMD package had to remain within the New Era's handling capacity, and its height was a critical factor in lifting and installation.
Formal test of SMD equipment outside Hydril Technology Center
Before the SMD system could be transported to the rig docksite, it was subjected to a factory acceptance test (FAT) to prove it would function as designed and an endurance test to illustrate its robustness. Every valve was functioned, and all electronic and hydraulic controls were tested and fine-tuned. This was achieved by integrating the SMD in the shop with the seawater power system that would drive the MLPs after being installed on the rig.
SMD design for test rig
At the outset of the SMD JIP, Hydril's equipment design team evaluated several potential pumping methods for subsea application. These included gaslift systems, the use of glass beads to decrease the hydrostatic head in the return line, moyno (progressing cavity) pumps, centrifugal pumps, and positive displacement pumps.
Based on a comprehensive analysis of anticipated drilling needs and conditions in the deepwater environment, the team searched for a concept that could meet the criteria for hole cleaning, drilling hydraulics, and seafloor horsepower requirements. The gaslift and glass bead concepts were rejected. The moyno pumps pressure and capacity limitations made them unsuitable for the targeted water depths.
Centrifugal pumps had been used in similar applications, but their efficiencies are in the 60% range. A centrifugal pump of the size and specifications needed to meet the design basis for deepwater subsea pumping could require 6,000 hp at the seafloor, meaning 10,000 hp would have to be provided at the surface.
The team's research turned to mining technology, where large chamber positive displacement pumps are used to lift solids and slurries from the depths. The MLPs now being tested by the SMD JIP operate in a similar manner: hydraulic power fluid on one side of the diaphragm and drilling mud on the other. Because Hydril had accumulators, diaphragms, and chambers available, the development of a fit-for-purpose diaphragm pump proceeded immediately.
The first three months of the JIP were spent creating design criteria for the critical components. The MLP is the heart of the SMD system; other components were developed to cause the system's performance to appear as conventional as possible. The development of this pump and other specialized SMD components was pursued. All key components - the mud valve, the solids processing unit (SPU), the drillstring valve (DSV), and the subsea rotating diverter - were built and tested in the Hydril Technology Center prior to assembly of the field test unit. The MLP was tested for nearly two years in the Hydril Technology Center flow loop, built specifically for the JIP.
Hyperbaric chamber testing
In a formal test of SMD equipment outside Hydril Technology Center (HTC), the mud valves, pressure transducers, hydraulic valves, electric motor, hydraulic pump, and control electronics were tested to 4,000 psi in a flooded hyperbaric chamber at Southwest Research Institute, in the largest hyperbaric chamber available in the southwestern US. The components selected for the hyperbaric test were either housed in one-atmosphere chambers or compensated to ambient pressure conditions. The test concluded successfully with no accidents or lost-time accidents to personnel.
Solids processing unit offshore test
The SMD solids processing unit (SPU) was tested in the lab with a variety of materials, including rubber from cementing plugs, pieces of float equipment, and gumbo/clay balls, but the question of its ability to handle real gumbo attacks remained until the SMD test team organized an offshore test. Conoco permitted members of the test team, one of whom was the SPU designer, to install the unit in the 12-in. flowline ahead of the gumbo chain on Ensco jackup Rig 69, where anything coming down the flowline would pass through the unit.
The offshore test was completely successful. A subsea version of the SPU was built and installed on the SMD system for the field test well.
Drillstring valve flow loop test
The drillstring valve (DSV) was installed in the measurement-while-drilling (MWD) flow loop and subjected to the following tests:
- Lost circulation material tolerance: 60 ppb coarse grind walnut hulls
- Valve opening/closing repeatability: 10,000- ft flow loop to simulate 10,000-ft water depth
- Abrasion test: 3-4% sand content
- Ability to function without hindering MWD signals.
In the final phase of the flow-loop testing, a DSV with a metal seal and the same internal geometry used with the resilient seal was subjected to the tests once more. It was determined that making the flow nozzle and metal seal from tungsten carbide provided the best results. This is the DSV design that was prepared for the field test.
The SMD solids processing unit was tested in the lab with a variety of materials, including rubber from cementing plugs.
In all tests, it was determined that the MWD signal could be successfully pulsed through the DSV.
Factory acceptance test
The objective of the FAT was to qualify the SMD equipment and its integration for use in drilling the test well. The process included testing all sensors, cables, and the pumping cycle operation at all parameters expected to be encountered during the drilling process.
The FAT targeted the following areas of operation:
- Initial pump startup
- Pump in constant flow rate mode
- Pump in constant inlet pressure mode
- Setpoint step changes
- Simulate making a connection
- Apply and remove annular friction pressure
- Simulate pumping out a kick
- Verify operational features of the SMD system.
The FAT proved that the operational requirements of the SMD system were met.
Beyond the factory acceptance test, the SMD JIP project advisory group requested an endurance test of 30,000 cycles over an eight-day period. This test allowed further simulation of rig activity and provided an opportunity for MLP operators and the test-well team to further familiarize themselves with the SMD system on a 24-hr/day-rotation schedule.
The test ran from May 23 to June 5, 2001. Several field-test objectives were covered by the test, including:
- Proper operation of the MLP with feed from a positive displacement pump,
- Ability of the MLPs to pump gas (nitrogen),
- Ability of the MLPs to run in triplex and duplex modes,
- Proper method for setting and releasing the SRD,
- Ability to increase or decrease MLP inlet pressure proportional to pump rate,
- Execute critical sections of the field test well drilling program.
The endurance test provided an opportunity to practice the operation of the system and build confidence that the system would be mechanically robust for the duration of a drilling operation.
All the testing within the available duration that could be done with the SMD system onshore was accomplished. The results were more than encouraging; they satisfied the operations, well control, and equipment design teams to the extent possible without placing the unit on the seafloor. The SMD system was disassembled in preparation for its trip to the rig and its debut on the seafloor.
The initial concerns about integrating the SMD system onto the New Era revolved around the weight and dimensions of the MLP package. The weight of the MLP package alone is 185,000 lbs. When installed on the LMRP, the total weight of these two components is 272,000 lbs. Add to this the tight clearances through the New Era's moonpool, and it is easy to see there was a difficult test lying ahead for the SMD test team.
The workscope for the Diamond, 2H, and other SMD personnel charged with preparing the rig to accept the system included the following modifications:
- Surface piping
- Mud return line
- Seawater supply line to seawater filtration skid
- Seawater power line
- Valve control panel
- DSV monitoring line (vacuum breaker),
- Dual trip tanks
- Riser fill-up (seawater side)
- Rig trip tank system (drilling fluids side)
- Cellar deck modifications/strengthening
- BOP bridge crane modifications
- Additional cabins for SMD team and obser-vers
- Seawater filtration skid
- Riser modifications
- PZ-7 seawater power fluid pumps
- Generator for clean power to drive MLP elec- tronics.
The overriding conclusion reached by all those involved with the testing was clear: extensive testing enabled the success of the first dual-gradient well.
The time spent at HTC working through the FAT and the endurance test was valuable training, as intended. Further, the factory acceptance test is an essential step toward producing a commercial SMD system.
Though nothing was discovered that hindered the plan to put the SMD in the water, the test team discovered a great deal about the nuances of the system's operation and the design team were afforded the time to refine its performance. The testing was a success in terms of "qualifying the SMD equipment for use in drilling the test well," and it also increased the confidence of those who were taking it offshore and the participating companies.
The SMD JIP would like to acknowledge the contributions of many people, without whom the SMD system would not have reached its current stage of development. Their untiring efforts and willingness to collaborate effectively with all the JIP personnel should not go unrecognized:
Hydril Equipment Design Team
Greg Bailey, Ron Brooks, Ralph Copenhaven, Joe Frederick, Hari Hariharan, David Manrique, Dat Nguyen, Al Olvera, Ken Pelata, Charlie Peterman, Mario Rodriguez, and John Scoggins.
SMD Implementation Team and contributors to the rig integration effort
Bob Blank, Bill Brossman, David Bruce, David Dowell, Gary Owen, David Patrick, Ken Smith, Ricky Theti, Curtis Weddle, and Dana Witt.
Smith, K., Gault, A., Witt, D., and Weddle, C.: "SubSea MudLift Drilling Joint Industry Project: Delivering Dual Gradient Drilling Technology to Industry," paper SPE 71357 presented at the 2001 SPE Annual Technical Conference and Exhibition, New Orleans LA, Sept. 30-Oct. 3.
Schumacher, J., Dowell, D., Ribbeck, L., and Eggemeyer, J.: "SubSea MudLift Drilling: Planning and Preparation for the First Subsea Field Test of a Full-Scale Dual Gradient Drilling System at Green Canyon 136, Gulf of Mexico," paper SPE 71358 presented at the 2001 SPE Annual Technical Conference and Exhibition, New Orleans LA, Sept. 30-Oct. 3.
Editor's Note: This is a summary of the SPE 71359 paper that was prepared for presentation at the 2001 SPE Annual Technical Conference and Exhibition held in New Orleans, Louisiana, Sept. 30-Oct. 3, 2001. Also, this paper is the last of a series of three discussing the SubSea MudLift Drilling Joint Industry Project (SMD JIP) evolution of dual gradient drilling (see SPE 71357, "SubSea MudLift Drilling Joint Industry Project: Delivering Dual Gradient Drilling Technology to Industry"; and SPE 71358, "SubSea MudLift Drilling: Planning and Preparation for the First Subsea Field Test of a Full-Scale Dual Gradient Drilling System at Green Canyon 136, Gulf of Mexico").