DNV: As dual gradient grows, system qualification will be needed to ensure safe operations

By Katherine Scott, associate editor

Qualification of DGD systems is dependent on the specific configuration of the DGD system, the drilling installation and the well conditions, DNV’s Francisco Chávez V. said at the IADC Dual Gradient Drilling Workshop on 9 May in Houston.

As industry gains experience in the deployment of dual-gradient drilling (DGD) systems on floating rigs, qualification of these systems will become necessary to provide documented evidence that the system will operate safely, Francisco Chávez V., principal project manager of the drilling & well section for Det Norske Veritas (DNV), said at the 2013 IADC DGD Workshop in Houston on 9 May. Such qualification would be dependent on three factors: the configuration of the DGD system, the drilling installation and well conditions, he said. “These three set the qualification boundaries, and changing any one of these will require a reassessment of qualification.”

Because new technologies begin with no defined standards that become developed as the technology is utilized, Mr Chávez said, a risk-based, systematic approach to system qualification would ensure that the technology functions reliably within specified limits. This approach should be applied to DGD technologies, he said, which remains new in terms of field deployment, particularly in deepwater environments. It’s also necessary to qualify DGD systems to extend the operational limits when existing DGD technology is used in more demanding operations, he said.

Mr Chávez cited the following as the current, relevant design standards for DGD on floating drilling rigs with DNV Class:

DNV OS E 101 “Drilling Plant”DNV OS A 101 “Safety Principles and Arrangements”DNV OS D 202 “Automation, Safety, and Telecommunication Systems”

Efforts are also ongoing to produce more specific guidelines for designing DGD systems:

The IADC Underbalanced Operations & Managed Pressure Drilling Committee has drafted an API Recommended Practice for managed pressure drilling, “Constant Bottom Hole Pressure using Applied Surface Back Pressure (Category 2 MPD) with Single Phase Fluid”Revised version of NORSOK D-010DNV revision of DNV-OS-E101

“The design of all equipment has to consider the regulations and the standards, which will eventually be used for qualifying such a system for the operation that is intended. Within that, it implies as well the need to assess the reliability of the different components within that part of the classification so the standards and the qualification methods contribute to building trust on the safe operations, the design and the performance, the reliability of a DGD system,” Mr Chávez said.

Paul Guirlet, Pacific Drilling, said at the workshop that the company had to make several modifications to integrate DGD on the Pacific Santa Ana but is incorporating lessons learned on the four rigs still under construction, which will be delivered “as DGD ready as we can.”

In another presentation at the workshop, Paul Guirlet, vice president of technical support for Pacific Drilling, highlighted the challenges and modifications associated with the integration of DGD on rigs, particuarly what the company has accomplished over the past three years working with Chevron on the Pacific Santa Ana.

He acknowledged that integrating DGD on that drillship has been a team-based effort among many companies, including Chevron, GE, AGR and National Oilwell Varco, adding that the biggest challenge from his perspective was finding space on the rig for the system. “Even if we have a very large vessel, the challenge is finding room.” Further, installation of DGD equipment required locations where it would be easy to access, easy to maintain and easy to use. “This is quite a big piece of equipment; consider it like a lower stack of a BOP that needs to be installed a part of the riser string… We’ve done a lot of work trying to optimize and make the best use of the space available on the vessel.” The DGD equipment includes a drilling riser cross-section, subsea rotating device, solids processing unit, MaxLift pump and drill string valve.

Other equipment on the rig, such as the piping system and the Christmas tree, also needed to be retrofitted due to the fluids used in DGD. “All of our vessel are coming with two storages for the BOP, but of course the MaxLift pump was not exactly similar to a BOP, so we needed to upgrade that. We needed to have a second guiding system to make sure the vessel motion would not create any problems.” Pacific Drilling also had to add two Mux reels, he said.

Mr Guirlet noted that Pacific Drilling is taking all the lessons learned from the Pacific Santa Ana, which was delivered in late 2011, to its new ultra-deepwater drillship, the Pacific Sharav. “We really believe in the DGD plan, so all of the four rigs that we’ve got currently under construction at Samsung will be as DGD ready as we can, because there is a lot of development still ongoing at this stage. And we will know more certainly after we drill the first well in not too long.” Samsung is expected to deliver the Pacific Sharav at the end of 2013.

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Different dual-gradient methods enable drilling in deepwater, depleted reservoirs

Increasing pore pressures and fracture gradients in target reservoirs in the Gulf of Mexico have motivated Chevron to use a seabed pumping dual-gradient drilling method, Ken Smith, Chevron, said at the 2013 IADC DGD Workshop on 9 May in Houston.

By Joanne Liou, associated editor

Dual-gradient technology continues to gain attention as an important solution to deepwater drilling and extraction of resources from depleted reservoirs. Chevron is months away from deploying its dual-gradient system in the deepwater Gulf of Mexico, where the environment is largely characterized by increasing pore pressures and increasing fracture gradients, Ken Smith, manager of the dual gradient drilling (DGD) project implementation at Chevron, explained. “We’re really driven by the environment we’re drilling, the rocks that we have to drill. We’re motivated to change the physics behind our drilling,” he said at the 2013 IADC DGD Workshop on 9 May in Houston.

Nonproductive time is a major challenge, averaging up to 30% in the deepwater GOM, Mr Smith noted, adding that one-third of Chevron’s well costs go toward fighting NPT. “It’s getting worse as we routinely drill 30,000-ft wells, and we have leases in up to 20,000 ft of water.” This type of drilling environment is changing the playing field, and DGD will help overcome the challenges, he said. From a well design standpoint, DGD takes water depth out of the equation.

Chevron’s DGD system uses seabed pumping with positive displacement to open up tight pressure margins. “It improves the detection and reaction of the downhole challenges,” Mr Smith explained. “It restores the riser margin and remains overbalanced at all times.” With a restored riser margin, fewer casing strings are needed to reach TD.

In DGD, the fluid in the riser is replaced with seawater-dense fluid, setting up a pressure profile that is aligned with nature’s pressures. “We’re not fighting (natural pressures) as much as we do in conventional drilling,” Mr Smith said. “We enhance operational performance with the MPD capabilities of our system being closed and pressurizeable, which leads to improved well integrity and ultimately well productivity.”

Dag Ove Molde, Statoil, discussed the different types of dual-gradient systems that have been classified under the categories of pre-BOP and post-BOP.

While Chevron’s DGD is an example of seabed pumping, other methods of DGD also were discussed at the workshop, including Dag Ove Molde, specialist drilling technology for Statoil. The IADC DGD Subcommittee recently classified dual gradient systems into two main categories, pre-BOP and post-BOP. Mud-line pumping is one method under pre-BOP, while seabed pumping, dilution and controlled mud level fall under post-BOP.

Mud-line pumping is a riserless concept that has been deployed in the Gulf of Mexico and in the Norwegian sector, Mr Molde said. The system may consist of an interface on the seafloor, a subsea pump, a control system and a return conduit. Subsea pumps return the drilling fluid to the rig through a small-bore riser, which allows the mud to be used in the top sections of the well.

When mud inside the riser is diluted, injecting a lower-density fluid into the drilling annulus reduces the hydrostatic head of the circulating fluid. The mixing process results in the required density to achieve a constant bottomhole pressure, Mr Molde explained. Dilution is applicable from intermediate to deepwater operations.

Controlled mud level systems also use two fluids to control the wellbore pressure gradient. “The main usage is to control equivalent circulation density limitations,” Mr Molde said. The system can be placed at different levels in the riser to achieve variable control over the wellbore pressure based on fluid density and placement. Controlled mud level systems are applicable to intermediate water depth.

Dag Ove Molde, Statoil, discussed the different types of dual-gradient systems that have been classified under the categories of pre-BOP and post-BOP.

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