Danish operator Maersk Oil, together with its project partners BP and JX Nippon, is developing one of the largest gas discoveries of recent years—the HP/HT Culzean Field on the U.K. Continental Shelf.

There are currently less than 100 HP/HT wells producing around the world, with Culzean to eventually feature six such production wells. Drilling of the first well was underway in September 2016.

Faced with engineering complexity and cost challenges associated with such large independent jacket-based operations, there is a growing trend toward small- to medium- sized developments that can produce to an existing facility or dedicated floating facility.

This is especially important in regions that do not support HP/ HT projects that would otherwise be uneconomic without government incentives, similar to that used on Culzean, which is due onstream in 2019.

In today’s cost-constrained climate there is now an increasing move toward marrying complementary systems that can combine field-proven technologies to reduce cost and time-to-field implementation.

Changing mindset

Aquaterra Energy and Plexus Holdings have developed a lightweight dual-barrier HP/HT riser system that can be deployed by a jackup rig and is a viable and cost-efficient alternative to semisubmersible installation for HP/HT well operations. Though the dual-barrier system has not yet been deployed in the field, all of its component parts already are well-established.

The jackup-deployable system is suitable for water depths of up to 150 m (492 ft) and harnesses the attributes of the companies’ respective subsea technologies by combining Aquaterra’s HP/HT 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.

Jackup advantages

Semisubmersible units have traditionally been used to perform drilling, completion, intervention and abandonment services on subsea wells.

However, in comparison to semisubmersible units, heavy-duty jackup drilling units rated for up to 150 m water depth 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 subsea tree.

As top-tensioned risers and lower jackup rig offsets impart lesser loads onto 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 semisubmersible units, the use of a jackup rig removes the need to disconnect from the well in extreme weather conditions, eliminating unnecessary nonproductive time.

As new-generation jackups are now capable of operating in increasing water depths, the range of subsea wells that are a viable option for a jackup-deployed HPR also continues to increase. This alternative means of deployment has the potential to access about 60% of the total number of subsea wells worldwide.

It’s a ‘keeper’

The most significant benefit to the operator is the ability to convert exploration or appraisal wells into a “keeper” or production well. The U.K. government, in particular, has identified this as a key issue, and the jackup-deployable dual-barrier HP/HT riser system directly addresses this challenge.

As current mudline suspension hanger technology does not provide a means of safely suspending HP/HT wells, it is not possible to reenter or tie back exploration or appraisal wells to existing platform-based or FPSO infrastructure.

However, when combined with an HPR and a jackup, using a subsea wellhead allows such wells to be converted to a keeper, and the cost base during the exploration phase, compared to a semisubmersible unit, is considerably reduced. Deploying a dual-barrier riser in such wells allows them to be reentered or put into production, potentially saving an operator millions of dollars.

Dual-barrier capabilities

The challenge of expanding incumbent technologies into HP/HT environments is simply increasing the pressure and temperature rating of fullbore risers, which is outside current manufacturing limits.

According to research, at pressures of about 10,000 psi and elevated temperatures in excess of 121 C (250 F), the required material chemistry, available manufacturing techniques and capital cost of large-bore dynamic HP/HT steel pipe could become prohibitive. It also can increase the cost of HPRs by more than 400% from that of a standard 5,000-psi system. This, in turn, has limited the advance in HPR deployments to projects with lower temperatures and pressures due to the technical and commercial restrictions and the trend toward thicker walled riser joints.

This issue is of particular importance for a subsea HPR as the riser is exposed to tension, environmental loading via wave action and—during well control events—wellbore pressure. Systems subsequently have much heavier walls to withstand this combined loading, and typical subsea HPR codes, such as American Petroleum Institute RP 2RD and International Organization for Standardization (ISO) 13628-7, account for design and qualification at 1.5 times the working pressure of the riser.

The dual-bore system addresses this by using a standard outer HPR for lower pressure/temperature zones in the well. Once HP/HT zones are about to be encountered, a protected inner HP/HT 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.

Incremental step forward

The joint system developed by Aquaterra and Plexus represents an incremental step forward in subsea capabilities as it facilitates safe and effective drilling operations in HP/HT conditions and provides a structurally sound pressure-retaining conduit between the subsea wellhead or 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 HP/HT capability from the subsea wellhead all the way to the surface BOP. The latter 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 HP/ HT riser string to be terminated inside. Subsea at the wellhead, the HP/HT riser string is connected to the POS-GRIP, creating an HP/HT conduit.

The dual-barrier HP/HT 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 in.), this negates challenges with setting the inner liner space out between two points.

Field-proven technology

Over the last 20 years POS-GRIP has been used on more than 300 wells drilled by jackup rigs, with full metal sealing capability and pressure ratings up to 20,000 psi at 190.5 C (375 F).

The dual-barrier HP/HT riser system is based on field-proven technology using all metal-to-metal gastight 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 drilling, completion, intervention and abandonment modes.

Amid the ever-increasing industry focus upon HP/HT operations, this methodology represents an innovative 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 of more than 5,000 psi and for those more than 10,000 psi, the dual-barrier HPR system can offer significant financial savings and safety benefits over single-barrier systems.