Feature article – Industry leading drill riser buoyancy design advancements deliver in high current operations

Meeting global demand for oil is achieved through the expansion of existing reservoirs both onshore and offshore, enhancements in technology, and a persevering push to drill and produce in progressively challenging environments. Increasingly, offshore exploration and production of new oil and gas reserves is being achieved in locations and conditions the industry defines as harsh weather. These regions are characterised by onerous wind and waves, along with high currents, typically in remote areas of the world, and frequently in deepwater.  

Expanding operations in these high current locations continuously pushes the capabilities of drilling rigs and their equipment to the outer limit of operating envelopes. Success in this arena depends on the advancement of new equipment, technology and designs specifically targeted for deployment and operation in severe conditions. Receiving industry-wide attention is one particular area of research and development aimed at expanding rig operating envelopes in high currents. This is the suppression of current induced hydrodynamic effects on the drilling riser.  

Marine drilling risers characteristically have large outer diameters, an attribute exacerbated by the required use of buoyancy modules in deepwater operations. Hydrodynamic diameters regularly reach sizes of over 50 inches, causing high drag forces, and in instances where current velocities reach certain magnitudes, vortex induced vibration (VIV) of the riser. Both of these result in limits on the conditions in which the riser can be safely deployed, operated and retrieved, as these reactions drive the controlling riser system parameters of flex joint angles, drill string rotation, and fatigue damage of the riser and subsea stack, specifically the wellhead and casing program.

Spearheading this industry challenge is a new buoyancy module technology; CRP Subsea’s (formerly Trelleborg Offshore) Drill Riser Buoyancy Module Helically Grooved (DRBM HG). Designed with an integrated tri-start helical groove the buoyancy design decreases the influence of in-line drag and crossflow lift forces through manipulation of the fluid-structure interaction. Passive variation in the point of fluid separation along the profile of the riser resulting from inconsistent sequential cross-sectional geometries and a change to fluid momentum traveling through the helical groove geometry, works to suppress vortex formation in a similar way to the mechanism employed in helical fin strakes. This process results in suppression of riser cross flow oscillations by up to 95% in the range of flow speeds which are responsible for the most severe riser vibrations for traditionally shaped buoyancy.

Importantly, these VIV suppression capabilities are achieved without increasing the effective diameter of the riser, as the helix is formed through a groove rather than external fins. This fact, coupled with the altered streamline trajectories and delayed fluid separation along the grooves, produces a strong independence of the drag coefficient with respect to Reynolds number (i.e. current flow speeds). DRBM HG has an industry-leading average static drag coefficient value of 0.63 across the range of relevant Reynolds numbers for offshore operations.

A number of peer-reviewed industry publications were issued to document the physics behind the hydrodynamic performance discussed above+. These findings were produced with industry support, and later accepted by a number of key operators who have subsequently begun including requirements for ‘built-in’ VIV suppression in their tenders to drilling contractors.

The encompassing benefits of the DRBM HG design are targeted at drilling operations in high current conditions. Inclusion of the suppression mechanism into the body of the buoyancy modules eliminates the requirement for ancillary suppression equipment, such as fairings or riser fins. This results in an increase in free deck space and reduces the number of required support vessel supply runs, a significant time and cost saving measure for campaigns in remote areas and challenging sea conditions.

The elimination of ancillary suppression equipment additionally speeds up running and retrieval times for a riser to an equivalency of a standard riser string with traditional buoyancy. It also removes the possibility of dropped objects in the moonpool and lost equipment during operation, as well as maximising the available time for a drilling rig to react to a weather event.

Commercialisation of this innovative technology is the product of three years of joint development between Diamond Offshore Drilling and CRP Subsea (formerly Trelleborg Offshore). The equipment design and qualification project included years of computational fluid dynamic studies and optimising of groove geometry. It culminated in a comprehensive large-scale test program and a monitored full-scale field deployment.

Harris Reynolds, Director of R&D for Diamond Offshore, states: “We are committed to delivering fresh perspectives and solving complex deepwater challenges. For efficient operations in all conditions, especially high-current environments around the globe, this means equipping our vessels with the best technology. Our development of helically grooved buoyancy with CRP Subsea (formerly Trelleborg Offshore) is an extension of our commitment to advance the capabilities of the offshore drilling industry through innovation.”

DRBM HG has to date been manufactured for and installed on six drilling rigs, both drill ships and semi-submersibles, representing over 15,000 feet of upgraded buoyancy, with additional depth breaks scheduled for manufacture in 2020. A track record has emerged that demonstrates CRP Subsea’s (formerly Trelleborg Offshore) ability to produce helically grooved buoyancy modules, which are equivalent in stacking strength, incorporate identical important safety features (such as the module integrity system, MIS), and do not sacrifice on uplift compared to traditional buoyancy designs.

Numerous successful deployments have occurred in multiple deepwater regions around the globe, including the Gulf of Mexico, the North Sea, and South Africa. Simplification of drilling riser operations in these regions stemming from the use of DRBM HG has shown significant benefits with unanimously positive feedback.

Harris Reynolds continues: “Since 2016 we have engaged in a multi-million dollar program to upgrade the buoyancy on each of our four ‘Blackship’ vessels and our most advanced sixth generation semi-submersible, the Ocean GreatWhite, with helically grooved buoyancy. These improved buoyancy depth breaks help to minimise the adverse effects of operations in high current regions, expanding on our leading offshore drilling capabilities.”

Marine drilling risers deployed with this unique technology expand a vessel’s drilling operating envelope. This results from the dual effects of suppressed riser oscillations and decreased drag forces, reducing fatigue damage of the riser and subsea stack, minimising flex joint angles and improving riser deployment and retrieval in high current regions. All these factors culminate in an improved ability for drilling rigs to operate and deliver in demanding high current environments.

Odfjell drilling selected DRBM HG as part of their well-publicised success at the Brulpadda prospect offshore South Africa, a location that is widely regarded as one of the most challenging in the world.

“Successful drilling operations in the harsh conditions of the Brulpadda prospect with the Deepsea Stavanger semi-submersible, resulting in significant oil and gas discoveries and a subsequent multi-well drilling contract, substantiate Odfjell’s segment-leading expertise in the field of ultra-deepwater and harsh weather environments.”

“Drilling operations in such harsh conditions as experienced on the Brulpadda field requires engineering solutions with focus on providing redundancy for all critical functions. In order to ensure robust VIV suppression, DRBM HG was utilised in combination with other VIV suppression solutions ensuring functionality under any challenging metocean conditions. Our investment in this technology ensures drag-reduction and VIV suppression capabilities to be readily available for our further operations in the harsh conditions of South Africa as well as future endeavours.” Thomas Borsholm, Principal Engineer Rig Design, Odfjell Drilling

The trend of expanding drilling and production operations in high current environments is set to continue throughout the 2020s. The challenges these environments present are significant. However, by employing the correct equipment, technologies and designs, specifically tailored for such conditions, as well as know-how, experience and expertise, these challenges also present significant opportunity to those willing to step up and overcome them.

The most influential equipment design advancements are those that deliver improved performance, while minimising the changes they introduce into holistic system design and original standard operating procedures. DRBM HG is specifically manufactured to integrate into offshore riser handling, running, operation, retrieval, and storage, without necessitating changes compared to traditional buoyancy designs used in the industry for decades.

Passive manipulation of the fluid structure interaction eliminates the possibility of mechanical fouling and ensures valuable motion suppression properties are available on each riser deployment. Rigorously tested, with an expansive track record in some of the most challenging offshore conditions, the unique CRP Subsea (formerly Trelleborg Offshore) technology has proven itself viable, expanding the operability windows for offshore drilling rigs, and facilitating the continued exploration of high current regions around the globe.