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

By Katherine Scott, associate editor

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-010
• DNV 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.

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.