HPHT 2016: Dual barrier HPHT riser system
In the UKCS, Maersk Oil, together with BP and JX Nippon, is developing one of the largest gas discoveries of recent years: the landmark HPHT Culzean field. Faced with engineering complexity and the cost challenges associated with such large, independent, jacket-based operations, there is a growing trend towards small to medium-sized developments, which can produce to an existing facility or to a dedicated floating facility. This is especially important in regions that do not support HPHT projects that would otherwise be uneconomic without government incentives, similar to that utilized on Culzean field.
The Dual-Barrier HPHT Riser System provides a means of accessing HPHT wells with a subsea riser system deployed from a jackup.
Handling HPHT operations safely, economically, and with environmental sensitivity, calls for highly specialized equipment and technology to exploit future reserves. Aquaterra Energy and Plexus Holdings have developed a light-weight, dual-barrier HPHT riser system which can be deployed by a jackup and is a viable and cost-efficient alternative to semisubmersible installation for HPHT well operations.
The companies will be presenting the system to delegates attending the 11th World Oil HPHT Drilling and Completions Conference, taking place in Houston from Sept. 21-22.
The jackup deployable system is suitable for shallow water depths up to 150 m and harnesses the attributes of the company’s respective subsea technologies by combining Aquaterra’s HPHT riser system and Plexus’ POS-GRIP wellhead engineering technology. This enables an inner riser string to be installed inside a conventional high pressure riser (HPR) to provide full 20,000 psi capability, without compromising safety, integrity and operational performance.
Semisubmersible mobile units have traditionally been used to perform drilling, completion, intervention and abandonment services on subsea wells. However, in comparison to semisubmersibles, heavy duty jackup drilling units rated to 150-m water depths, can now undertake such activities at reduced cost and risk when the jackup deployed HPR is used to span the gap between a dry surface BOP and a wet subsea tree.
As top tensioned risers and lower jackup rig offsets impart less load into the wellhead and other subsea components, an additional benefit is the possibility for improved wellhead/low pressure housing/subsea tree loading performance. This results in less fatigue damage thereby extending the service life of equipment, increasing safety margins, and improving operational envelopes. Moreover, typical HPR systems are designed to withstand 50-year storm conditions under a well-control situation. Unlike semisubmersibles, the use of a jackup rig removes the need to disconnect from the well in extreme weather conditions, eliminating unnecessary non-productive time.
As new generation jackups are now capable of operating in increasing water depths, the scope of subsea wells that are a viable option for a jackup deployed HPR also continues to increase. This alternative means of deployment now has the potential to access around 60% of the total number of subsea wells worldwide.
Dual-barrier riser system capabilities
According to research, at pressures of around 10,000 psi and elevated temperatures in excess of 121°C, the required material chemistry, available manufacturing techniques and capital cost of large bore HPHT steel pipe could become prohibitive. This has in turn limited the advance in HPR deployments to lower temperature and pressure projects due to the technical and commercial restrictions the trend towards thicker walled riser joints commands.
This issue is of particular importance for subsea HPR as the riser is exposed to tension, environmental loading via wave action and, during well control events, well bore pressure. Systems subsequently have much heavier walls to withstand this combined loading and typical subsea HPR codes, such as API RP 2RD and ISO 13628-7, account for design and qualification at 1.5 times the working pressure of the riser. The dual bore solution addresses this by utilizing a standard outer HPR for lower pressure/temperature zones in the well. Once HPHT zones are about to be encountered, a protected inner HPHT riser string is run inside the outer HPR between the surface BOP and the subsea wellhead. This philosophy allows the inner riser string to have a thinner wall due to the protection provided by the outer string and its qualification to casing codes such as ISO 13679 CAL IV.
The joint solution developed by Aquaterra and Plexus, represents an incremental step forward in subsea capabilities as it facilitates safe and effective drilling operations in HPHT conditions and provides a structurally sound, pressure retaining conduit between the subsea wellhead/tree and the rig’s surface BOP.
Rather than increasing the pressure rating of the outer riser, POS-GRIP allows an inner riser string to be temporarily installed, allowing full HPHT capability from the subsea wellhead all the way to the surface BOP. The BOP can then remain in place for the entire project once nippled up. The pressure retaining well control inner riser string will see a reduced environmental load, with the main riser supporting the majority of the bending. At surface, the POS-GRIP surface housing allows the inner HPHT riser string to be terminated inside. Subsea at the wellhead, the HPHT riser string is connected to the POS-GRIP creating a HPHT conduit.
The dual barrier HPHT riser system eliminates the issues associated with surface wellhead developments that contain elastomeric seals, particularly those located between the mudline and surface. Due to the flexible placement of the POS-GRIP to surface and subsea systems (+/-4”), this negates challenges with setting the inner liner space out between two points.
Over the last 20 years, POS-GRIP has been used on more than 300 jackup wells, with full metal sealing capability and pressure ratings up to 20,000 psi at 375°F.
The dual barrier HPHT riser system is based on field-proven HPHT technology utilizing all metal-to-metal gas tight seals on both the external and internal riser strings. It is capable of withstanding environmental and operational conditions expected during the HPR service life and can be used in both the drilling, completion, intervention, and abandoment modes. Amid the ever-increasing industry focus upon HPHT operations, this methodology represents an industry-leading and cost-effective alternative while maintaining safety, integrity and operational performance.
Aquaterra’s engineers developed the first HPR systems deployed in the North Sea in the 1990s, which were typically used on lower pressure wells. With increased well pressure requirements over 5,000 psi and for those over and above 10,000 psi, a dual barrier HPR system, which has taken a year to develop, can offer significant financial savings and safety benefits over single barrier HPR systems.
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