Minimized subsea production system offers fast-track route to first oil

By Jeremy Beckman, Editor, Europe

A minimum-facility integrated subsea production and mooring concept is undergoing assessment for various fields in different offshore settings. The technology was originally developed as a low-capex, re-deployable solution for small or marginal fields in water depths of up to 500 m. This has now been extended to 1,500 m following recent validation by DNV, based on a full-scale prototype system test.

Developer Pivotree was formed in Perth, Australia, in August 2018 by Inventor and Managing Director Chris Merrick and Executive Director Marcus Christian. Following an initial period of largely self-funded activity, the board has completed a series of capital raisings from private investors, Christian said.

“We have funded operations from this capital, R&D tax refunds and income from customer studies,” he added.

Merrick, who devised the Pivotree concept, also leads subsea engineering operations at Infinity Offshore. His prior experience included project engineer at Santos, supporting decommissioning programs offshore Australia; subsea engineering/consulting roles with DORIS Group, Blakemere and PTTEP; and earlier leadership positions with Oceaneering, one of Pivotree’s preferred equipment providers for its future projects. Over the course of his career, Merrick worked on multiple small oil fields and associated operations/repairs of the subsea systems.

During Pivotree’s startup phase, he examined the lessons learned from various projects deemed to be successes or failures. His main focus was on design shortcomings, cost inefficiencies and operational rigidities apparent in traditional offshore developments.

Recurring factors identified included:

High opex arising from design choices;
Overly complex surface facilities and process systems;
Limited flexibility in the subsea field equipment, constraining intervention and expansion; and
Unnecessarily costly equipment due to a “gold-plated engineering mindset” leading to oversized processes and operations.

These insights, he added, shaped his thinking around how offshore developments should work—“leaner, more modular, more flexible and with simplified operations from day one."

"And that directly inspired the formation of the Pivotree concept," he said. "Pivotree is a reimagined approach to small field development, which offers modularity, disconnection capability, smarter design choices and better lifecycle economics. This allows for reduced opex, improved maintainability and operability without adding legacy burdens in decommissioning.”

Xmas tree electrical and hydraulic fittings.

The system in its basic form comprises a single subsea production tree, a conductor-based mooring assembly and a flexible riser-swivel interface. The latter enables connection to a small FPSO, typically with a deadweight of less than 50,000 tons, without the need for a turret mooring or spread-mooring. A converted tanker would be able to maintain station using the vessel’s existing bow mooring equipment, the company claims. Once the well has been drilled and completed, the FPSO could be used to install Pivotree’s flexible riser and swivel assembly, with no need for further equipment installed by heavy-lift or crane vessels before production starts.

Merrick and his team have identified more than 5.4 Bboe of potential opportunities in economically stranded discoveries globally, based on analysis by Rystad Energy. And that figure rises by a further 3 Bboe when adding stranded resources in water depths of between 500 m and 1,500 m. In all cases, the discoveries are situated more than 10 km from export infrastructure or offtake, with an API gravity >20° and with at least one well drilled. The identified fields are spread out among most of the main producing regions including the North Sea, West Africa, US Gulf of Mexico, eastern Canada, Brazil, India, Australia and Southeast Asia.

Testing timeline

Pivotree’s focus is on fast-track production from known, drilled accumulations previously considered unviable, without the need to prove up further resources through time-consuming seismic surveys. Other considered applications include early production to support appraisal of complex reservoirs and extending the life of mature fields to defer platform decommissioning costs.

“During the early phases of R&D and engineering,” Merrick said, “we started with establishing the technical feasibility of the overall concept. That involved assessing the holding capacity of the well conductor through static load studies performed by Fugro Marine and dynamic load studies undertaken by 2H Offshore. The key challenge was to demonstrate that the conductor could be laterally restrained in seabeds with poor soil strength. With the addition of our QuadPod with outrigger piles, which is akin to a permanent guide base [PGB], we were able to achieve this design goal. That enabled DNV to award a Statement of Technology Feasibility in 2021."

A multi-disciplinary team then conducted an Failure Modes & Criticality Analysis, identifying a comprehensive list of risks and failure modes, he explained. That led to a risk mitigation and testing exercise that was signed off by DNV in 2023 called the Statement of Endorsement of Qualification Plan.

Pivotree prototype stack-up with swivel assembly

"Under the qualification plan, various tests and analyses took place including FEA [finite element analysis] by Blakemere Engineering of the structural elements of the design under full load conditions; conductor bending analysis by 2H to demonstrate well integrity; production-scale testing of a full-scale (44-mt) prototype of the mooring element and subsea tree; and finally fatigue analysis under operating conditions by 2H to demonstrate sufficient fatigue life of the system," Merrick said. "In the Pivotree setup, the riser is connected to a swivel stack at the top of the tree with the hawser connected to the mooring (PGB), which is welded to the 36-inch conductor. The FPSO would not require full-time dynamic positioning capability, but a thruster may be needed to provide a minimum level of tension on the hawser in calm weather."

The prototype tree was modified and tested in Perth, with the modification works managed by Blakemere. The swivel function was tested at full pressure.

"For the mooring PGB, the safe working load [SWL], based on a safety factor of 2.0, is 100 mt; the prototype, however, was tested at 150 mt during its load test," he explained. "Accelerated burn-in testing was also conducted to simulate 1,000 hours of operation in the field, which was achieved by pulling the maximum SWL (100 mt) on the mooring and rotating the cylinder continuously. Inspection of the bearing surfaces after the test revealed no damage or deformation, demonstrating that the system is fit for purpose."

He added, "Throughout the campaign, all the testing and analyses were handled by independent providers, and the results were vetted and signed off by DNV. With the qualification plan successfully completed, DNV issued their Technology Qualified certificate in June this year, under the internationally recognized DNV-RP-A203 standard.”

Targeted fields

Over the past few decades, numerous engineering groups have devised allegedly game-changing solutions for marginal fields including small Spar-type production buoys, mini-TLPs and mini-FPSOs, but few if any have featured in actual developments.

“Our view on this is that both the reticence of the industry to try and test new technologies in the field and structural market issues are at play,” Christian said. “We have engaged with over 50 companies globally, and while all are interested in using Pivotree where they have suitable fields, most wish to proceed once we can demonstrate the technology has worked in a production setting. From a market perspective, novel production systems are often caught between the focus of the large players on high-impact discoveries and the risk to smaller players that failing to succeed on a given project can dictate the fate of the company. We focus on mid-sized players for these reasons."

Currently about 20 E&P companies are considering the technology for their offshore projects, with a wide variety of potential applications.

“Our target field profile is 5-25 MMbbl,” Merrick explained, “which tends to fall under the minimum economic field size for a conventional development (depending on a range of factors, but predominantly the oil price). This has been validated by the size of the fields held by the clients we are engaging with. The exception is where the larger reservoirs are compartmentalized, requiring multiple, widely spaced wells for effective drainage."

The company has one such project where they are planning multiple phases of Pivotree deployments to produce the field, which Merrick said would incur about one-third of the capex costs compared with a conventional development concept.

“Water depths for small field developments tend to be no greater than 100-120 m, with minimum facilities platforms often perceived as the go-to solution for small subsea tiebacks in these depths," he continued. "However, where a tieback is not economical based on the size of the reserves, or where there is a need for subsea pumping or boosting, Pivotree provides a more economic route to market. In most cases, now that we can accommodate two wells, clients are keen to capture the increased production that would bring.”

Pivotree Two Well Configuration — The schematic of a dual well, tandem mooring system shows offtake from the FPSO by a separate shuttle tanker.

Pivotree has formed partnerships with various big-hitters in the offshore industry, all of which would be available to support any future development contracts awarded, depending on the clients’ preferences. The list includes Blakemere, DNV, DORIS Engineering, FES, HWC, Moog/Focal, Proserv and 2H Engineering.

36-in. conductor section with drive wheel

“These are market leading and highly reputable suppliers of both services and key components through the development of the technology,” Merrick said. “For the tree itself, Pivotree’s design can be adapted onto any of the major OEMs' trees, allowing the customer to select their preferred vendor. The completion system can be handled by the tree supplier or by the other OEMs who provide downhole equipment. With the exception of the last joint of 36-inch conductor, the implementation of a Pivotree system allows the client full flexibility in design of the casing and completion system.

“Pivotree’s own scope of supply would be limited to the subsea elements, less the production riser, mooring hawser and any subsea flowlines. We have functional specifications written for all the subsea components to assist the customers with procurement. Our system is also designed to accommodate the manufacturing capability of all of the major riser OEMs.”

Many of the identified fields are in the Southeast Asia/Australasia region, and according to Merrick, the system has been designed for a swift disconnect in the event of a typhoon approaching.

“This is a key feature that was built in as a result of the lessons learned from previous small field developments," he said. "Pivotree is designed as a robust anchor point that is capable of full 360° rotation (i.e., weather-vaning) and facilitates quick connection and disconnection. The mooring PGB has the capacity for the FPSO to remain on station in a one in 100-year storm (e.g., a typhoon), with several layers of fail-safe, emergency disconnection equipment to ensure the safety of the crew and the integrity of the well.”

Pivotree prototype with production spool

The technology is ready to deploy today. The company has completed testing of the fabricated prototypes at full load and full pressure, and DNV has qualified the technology.

"Together, this substantiates that the technology will achieve its intended performance and reliability across a range of deployment scenarios," Merrick said. "The development timeline for a Pivotree-enabled oil field would be around 18 months or less. Where key equipment is available, such as trees in stock and availability of a suitable FPSO rather than the need for a tanker conversion, the time to first oil could be as little as six months.

“There are dozens of tankers that are available for conversion to FPSOs, and there are several FPSOs/FSOs that could be redeployed onto new fields using the Pivotree concept. The beauty of Pivotree is the mooring uses the standard bow mooring equipment on the tanker, and no structural modification of the hull is required to accommodate a turret. Only minor works for the addition of a riser porch, including a launch and recovery winch, are needed on the host vessel.”

Case studies

In June, Pivotree completed a concept feasibility study for the Block 2A Gas project offshore South Africa’s west coast. Sunbird has a 76% operated interest in the block, with the remaining 24% held by SANPC (ex-PetroSA). Soekor drilled the initial two gas discoveries in the 1980s via wells A-K1 and A-G1, with A-K1 successfully tested. During the early 2000s, Forest Oil drilled a further eight wells, of which six were gas discoveries, with three flowing gas in tests. The Block 2A field has 2C reserves independently certified at 540 Bcf, with a prospective resource of up to 7.3 Tcf.

According to Merrick, the main issues that have hindered a development until now have been high capex, uncertain market conditions and inflexibility concerning the gas market. He said, “We were referred to Sunbird by PetroSA, which we had gained an audience with via our Africa agent, Paul Howlett.”

The paid study, Merrick added, confirmed the strong commercial potential of a Pivotree subsea production system deployed in tandem with two compressed natural gas FPSOs, both transporting 200 MMcf to 300 MMcf of gas per cargo to the selected port (one of Saldanha Bay, Cape Town or Mossel Bay). “Sunbird (via major shareholder EverSeed) is now in the process of raising the capital needed to advance to FID, pending regulatory approvals,” he added.

Elsewhere, Pivotree has signed memorandums of understanding (MoUs) with IPB Petroleum for the Gwydion oil and gas discovery offshore Australia, and with Harvester Energy for the Curlew-A and Phoenix fields in the central UK North Sea in Blocks 29/7b and 22/12b, respectively.

“Both MoUs are purely based on using Pivotree as the enabler of the development concept and are binding, based on the projects reaching FID," Merrick said. "Gwydion is an interesting project that requires a specially designed topside process that can handle both oil and gas.”

Gwydion, discovered in 1985, is in the Yampi Shelf area of the Browse Basin offshore Western Australia’s northwest coast. Harvester’s fields are both liquid oil accumulations, with plans to produce Phoenix after Curlew-A is fully depleted. Shell discovered Phoenix in Forties oil sandstone; according to Harvester’s website, a Pivotree system could produce the oil via a single or multi-well development from the same borehole. A single well would likely suffice for Curlew-A.

Another application under investigation for the Pivotree system is offshore CO₂ geo-sequestration, being marketed under the brand-name Sea-Quester Offshore. Pivotree is assessing a potential deployment for injection of CO₂ emitted by companies in Perth’s Kwinana industrial area into the offshore Perth reservoir.

According to Christian, “The idea for Sea-Quester initially came about from a conversation within our board, from Chairman Gavin Ryan. From there Chris investigated the potential to use the Pivotree to inject carbon dioxide into depleted reservoirs and saline aquifers. Apart from material selection, no other modifications are required to the Pivotree technology.”

The collected CO₂ would first be sent to a Sea-Quester Carbon Harvester floating facility, a Handysize vessel modified to store CO₂, before being transferred to the Pivotree well for injection.