The HGSS wellhead will incorporate an 18 ¾-in. full-bore system, rated to 15,000 psi and 350°F, and have 4 million lbs of instant casing hanger lockdown capacity. It also will have the ability to open and reseal the casing annulus to enable remedial cement jobs.
By Katie Mazerov, contributing editor
A subsea wellhead being designed under a joint industry project (JIP) that includes Shell, Maersk Oil, Eni, Tullow, Wintershall and TOTAL will be able to be assembled without removing the blowout preventer (BOP). The HGSS subsea wellhead, an alternative to conventional mandrel hanger subsea systems, uses a proprietary POS-GRIP technology that engages a casing hanger in the wellhead with a gripping mechanism that creates enough friction to carry the casing load, lock the system down and activate the seals as soon as well cementing is complete. The technology has been used in surface wellheads on more than 300 wells worldwide.
“We are developing a subsea system with remotely activated gripping power that can employ our POS-GRIP technology to simplify and improve the internals of subsea wellheads, delivering a system that uses metal-to-metal sealing for very high-pressure gas applications,” said Ben Van Bilderbeek, CEO of Plexus Ocean Systems, which is designing the HGSS. Field testing will start in the second half of 2014.
“Recent well control incidents around the world have highlighted the need for high-performance subsea wellheads in oil and gas operations, particularly in extreme and hostile environments,” he said. Plexus is already providing the technology for surface wellheads in high-pressure, high-temperature (HPHT) areas of the North Sea and wider global market for applications at 15,000 psi or higher.
The HGSS project came about after Macondo, amid regulatory concerns around technical issues and requirements for subsea wellheads, Mr Van Bilderbeek explained. “We were asked by operators to develop a more dependable design for ‘through-the-BOP,’ metal-to-metal sealing technology in subsea applications. Our friction-grip method is completely rigid and offers instant, automatic lockdown of casing hangers without having to lift the BOP,” he said. The technology, used in surface exploration wells, eventually became a necessity for jackup drilling operations, where customers and regulators insisted that BOPs could no longer be removed to assemble the wellhead and energize seals.
Targeting HPHT market
Traditionally, most surface wellheads are of the conventional spool-type design, requiring BOPs to be removed every time casing is set in the well, he explained. Mandrel wellhead technology eliminates that requirement but is limited in its ability to deliver high-pressure, metal-to-metal sealing. As industry ventured into higher pressures and temperatures, operators drilling in surface applications reverted to the spool design, which delivers power generated by flange connections between the spool sections to lock in casing hangers and energize seal devices.
In subsea applications, spool designs cannot be used because the BOPs must stay in place, forcing reliance on rings that expand in grooves to lock down the casing hangers. “But, over the last 40 years, that system proved to be so problematic, companies sometimes decided to not use the lock rings altogether,” he said. “After Macondo, the US Department of the Interior issued a directive that casing hangers be locked down at the time casing is installed.”
POS-GRIP technology squeezes the wellhead housing and grips the hanger, locking it up and down and in rotation, while energizing multiple seals in one run. “It eliminates the option of leaving the casing hanger unlocked,” Mr Van Bilderbeek said. Plexus believes the system is safe and compliant, cost-effective, quicker to install and simpler because it contains fewer components than conventional systems. “The seals are effective over a substantial area of contact between the hangers and the wellbore.”
The system is also more resistant to temperature variations, Mr Van Bilderbeek said. “Conventional systems use components that slide together, and thus have an assembly tolerance, meaning they are loose when impacted by forces induced by pressure and heat in HPHT environments,” he added. “We see huge expansion and contraction of components in pressure and temperature cycles, as well as shortening and lengthening of pipe below the casing hanger, causing degradation of seals over time.”
The HGSS wellhead will incorporate an 18 ¾-in. full-bore system, rated to 15,000 psi and 350°F and upgradeable to 20,000 psi and 450°F; 4 million lbs of instant casing hanger lockdown capacity; annulus monitoring and bleed-off capability to address sustained casing pressure, with diagnostic and remedial capability; and the ability to open and reseal the casing annulus to enable remedial cement jobs.
POS-GRIP is a registered term of Plexus Ocean Systems; HGSS is a trademarked term of Plexus Ocean Systems Ltd.