Aquaterra Energy and Plexus Holdings have devised a high-pressure riser, enabling safer HPHT developments using jack-up rigs
As costs continue to be pressured, operators have attempted to do more with less. That might be achieving greater production with smaller budgets, or use older equipment to tackle new problems. With regards to the latter, the humble jack-up rig may be next in line for an upgrade.
In an interesting technical collaboration, offshore engineering firm Aquaterra Energy and wellhead systems designer Plexus have developed a new light weight, dual barrier, high-pressure/high-temperature (HPHT) riser system for deployment by jack-up rigs. The technology enables an inner liner to be installed inside a conventional high-pressure riser, up to 20,000 psi. Their hope is that the system will radically change the risk and economics of performing HPHT operations with such rigs.
At present, the industry’s default position is to opt for semi-submersible rigs when installing wells and infrastructure. In part, this issue may be based on experience; many operators assign subsea engineers to these programmes and their backgrounds are based on the use of semi-subs, and the extra effort needed to examine and achieve the technical and commercial benefits of jack-up use is rarely considered.
This advance should enable the use of heavy duty jack-ups for HPHT operations in depths of up to 150m – allowing access to around 60% of the world’s subsea developments.
With global rates for semi-submersible rigs averaging around US$250,000 per day in 2016, jack-ups in Europe and Asia remain considerably cheaper. Most have remained well below US$100,000 per day, dropping to around US$60,000 over the summer, in many cases making them as much as 75% cheaper.
That can have a major impact on well costs. According to Aquaterra, anecdotal customer evidence suggests that the average jack-up HPHT well costs GBP50 million (US$61 million), but will be far higher when using a semi-sub.
The high-pressure riser system combines two existing technologies: Aquaterra’s HPHT riser system and Plexus’ POS-GRIP wellhead technology. The latter uses friction to flex and grip pipes and connectors, replacing the conventional load shoulder or slips to create a high-load hanger support mechanism. This mechanism is adjustable, full-bore, fully elastic and provides instant lockdown.
This combination system allows an inner casing liner to be temporarily installed, meaning full HPHT capacity between a dry surface BOP and a wet subsea tree. A surface blow-out preventer is connected to an Aquaterra riser via a POS-GRIP surface wellhead system. Subsea, this riser can connect to another optional POS-GRIP spool before reaching the subsea wellhead.
The combination system provides 20,000-psi capability and uses all metal-to-metal gas tight “HG” seals on the internal riser string. The system also eliminates the perceived risk of elastomeric seals, particularly those located between the mudline and surface, many of which have not been rated or proven in real-world subsea developments.
Using top-tensioned risers and lower jack-up rig offsets also puts reduced load onto the wellhead and other subsea infrastructure. The system should therefore result in less fatigue damage, extending the life of equipment and increasing safety.
Safer, more secure
In addition to the reduced costs of HPHT well development, the wider use of jack-ups also carries inherent safety and risk benefits. First, jack-ups are not constrained by the same weather and heavy loading restrictions as semi-submersibles, meaning safer operations and wider weather windows. Moreover, high-pressure risers are designed to withstand 50-year storm conditions under a well control situation; using a jack-up rig removes the need to disconnect from the well in heavy weather, meaning substantially reduced downtime.
Aquaterra product line manager for riser and conductor solutions Ben Cannell commented: “The innovation carries with it the potential to change the industry’s default position of selecting semi-submersible rigs for developments in water depths up to 150 metres. It renders the use of jack-up rigs in such scenarios both feasible and attractive in both a technical and commercial context.”
Plexus technical director Craig Hendrie also added: “This is a new, technology-led drilling proposition to the international market and is one that directly addresses the industry’s cost-reduction agenda. Amid the ever-increasing industry focus on HPHT operations, this methodology represents an innovative and cost-effective alternative while maintaining safety, integrity and operational performance.”
Aquaterra also referenced some specific applications in which the technology would aid operators. A representative explained to InnovOil: “The biggest benefit to the operator is the ability to convert exploration or appraisal well into a keeper. The UK government has identified this as a key issue and in AE’s view this is one that is directly addressed by this solution.”
Many operators such as Maersk Oil Culzean and Total Solaris have opted for jack-up based exploration with a dry tree. “As current mudline suspension hanger technology does not provide a means of safely suspending HPHT wells, all these wells to date cannot be re-entered or tied back to existing platform-based/FPSO developments. Using a subsea wellhead allows such wells to be converted to keepers, when combined with HPR and a jack-up, the cost base during the exploration phase – when compared with a semi-sub – can be reduced,” the company added.