100-km subsea tiebacks projected for Atlantic Margin

Conoco's successful appraisal this summer of an extension to BP's Suilven oil discovery west of the Shetlands was a morale booster to the region's exploration effort. It may also add impetus to a project to harness deepwater marginal reserves in the Atlantic Margin. Production systems west of Shetland are limited to BP's large Foinaven and Schiehallion FPSOs. Elsewhere in these waters, numerous small oil and some substantial gas fields have been discovered, but none merit a


Marginal fields a strong inducement

Artist's impression of the C-Fast raw water injection system installed on a seabed.

Conoco's successful appraisal this summer of an extension to BP's Suilven oil discovery west of the Shetlands

was a morale booster to the region's exploration effort. It may also add impetus to a project to harness deepwater marginal reserves in the Atlantic Margin.

Production systems west of Shetland are limited to BP's large Foinaven and Schiehallion FPSOs. Elsewhere in these waters, numerous small oil and some substantial gas fields have been discovered, but none merit a standalone development based on current technology.

For some time, Shetland area op erators have been performing gas gathering studies under the heading of the Aurora project. That effort was augmented this April by the launching of a "zero-surface facilities" study, backed by the Atlantic Margin operators and the UK Depart ment of Trade and Industry, and coordinated by the Centre for Marine Petroleum Technology (CMPT), based in Aberdeen. Their aim is to install subsea tiebacks of more than 100 km, which would be more than double the limit of existing technology.

At the time of the launch, Shell UK's Ian Ball (before moving to New Orleans) commented: "In the past, industry thinking has been constrained by focusing on the individual components involved in subsea production. This study breaks ground by examining the bigger picture and adopting a system-based approach. The initiative evolved to address the challenges of subsea development in deep Atlantic waters, but we hope this will also extend our production capabilities throughout the UK continental shelf."

According to Dick Winchester, CMPT's Dir ector of Technology Programs, the results could also be transferred to other deepwater regions in need of innovative subsea solutions such as the Gulf of Mexico, West Africa, and Brazil. Petrobras has expressed keenness to be involved in some of the component issues being tackled.

Winchester also elaborated on Ball's other comment. "Most subsea projects in the past have been product-type developments - a separator, a booster pump, and so on. But to pull things together properly, you have to treat subsea production in system terms. That means, for instance, dealing with the hydraulics of the reservoir and with hydrate fluid flow, just as you would on an FPSO or a jacket-based project.

"With subsea production, though, the system approach is more important. Unlike platforms or FPSOs, you can't make changes once it's in place. So as an initial phase, we are undertaking a scoping study." The aims of this study are to ascertain the current capabilities of individual subsea technologies and to identify any gaps and perhaps to revive old shelved concepts which remain conceptually strong, through the introduction of newer technologies.

If longer distance subsea tiebacks could be achieved, Winchester said, "we would be pushing the economic threshold for field developments well below 100 million bbl in deepwater. That's not an easy task - however, we're not aiming to outdo what manufacturers and contractors are already doing, but rather to pull the whole thing together, thereby creating confidence in new types of subsea production systems."

As part of the scoping study, 20 companies have been asked for vision statements on how they see subsea production evolving. They have a certain licence to be imaginative, Winchester said, but within the confines of CMPT's fixed budget.

Assuming this study is completed on schedule by the end of this year, the next phase will be to mount a series of joint industry projects to ad dress current technology gaps. Basic aims would be to achieve a fully integrated system design a complete system prototype installation and trials.

Key system components

CMPT's list of building block technologies towards ultra remote marginal field development include:

  • Power generation: Low energy, remote reservoirs may demand new types of high power, submersible downhole pumps.
  • Pipelines: "The cost of laying flowlines in deepwater is likely to become prohibitive," said Winchester, "and maintaining and inspecting those lines will also be very costly. In addition, deeper water implies colder fluids. There is a question over whether current pipeline design is adequate. One solution could be a bi-stable composite pipe currently being developed by a UK company, which would be welded ultrasonically."
  • Control and monitoring downhole: "If you put more intelligent sensors down a well to understand it better, you need to use that data sensibly. That requires smarter control systems to react quicker, in order to close down vales, shut down sections, or inject chemicals. Current systems are mostly dumb, they just re-enter data. Little work is being done on this currently - we're trying to impose a culture change."
  • Downhole compression for gas reinjection: "As part of this effort, we want to do away with hydraulics, and instead introduce all-electric systems. A good example in the UK is Newcastle University's work on developing silicon carbide-based switching systems. This is a material suited to high voltage and high temperature switches, for operating electronics in temperatures up to 400°C. If you can do that, you can then control a motor driving a pump in a very hot, deep well."
  • High rate acoustic telemetry: "We have a project in progress here looking at 35 kbit/sec transmission rates."
  • ROVs: "So-called deepwater operations with ROVs are currently simplistic - picking things up, turning a valve. When it comes to replacing actuators or changing out a separator, we need to think how to do these things in 2,000 meters of water."

Raw water injection

Another technology being evaluated for inclusion in the zero-surface facilities project is raw water injection, which involves minimum seawater treatment for pressure maintenance and sweep. It is designed to replace conventional topsides water treatment inventory and subsea flowlines with simpler and cheaper subsea systems powered via a subsea electrical power and control cable.

Studies have been ongoing since 1987, led by McDermott Marine Construction in the UK, to increase understanding of the reservoir and technical issues associated with injecting "raw" minimally treated seawater, to the extent that the com mercial benefits can now be clearly quantified. Credibility advanced further recently when the program's C-Fast subsea water injection module, developed by UK corrosion specialists Cap cis, came through wet dock trials successfully.

In 1994, BP commissioned a study to assess a seabed minimal treatment raw water injection facility for a six-well, single drill center tied back 3 km to a floating facility in 500 meters of water. Each well had an injection re quirement of 20-30 million bbl, with flow pressures of 1,000-2,000 psi. Filtration for the injected water was set at 50-150 microns.

This study generated a conceptual design that used sand-bed filtration technology and was ROV-assisted in operation. But subsequent evaluation suggested a Capex saving of 40% would have to be realized for such a system to be attractive to operators.

Around the same time, Capcis was working on a combined filtration/chlorination membrane through which water would be drawn by the pump into the adjacent injection tree. The entire system was named C-Fast (combined filtration and seawater treatment system).

McDermott and Capcis then funded research in 1995 which evaluated seawater conditions and injection water quality more critically, identifying two types of particulate - hard, dense materials such as sand and clay particles, and neutrally buoyant organisms and other marine life. This work led to development of a new filtration technology called the tube settler, designed to knock out the hard particulates.

Further development of this system eventually superseded the original membrane design. Through using the tube settler in combination with a Bernoulli strainer as the neutral buoyant particulate filter, the two parties believed they were on the way to providing a complete filtration system.

Under another UK joint industry study, called Rawwater Engineering, various filtration systems were then evaluated in the search for a configuration that would achieve the coveted 40 % cost savings. Two cost drivers were drawn up.

  • Sand-bed filter systems were larger, heavy and costly, and the associated control equipment created concerns over reliability.
  • Most operators specified filtration quality in absolute terms, without specifying the types of particulates that might be present in the seawater.
Following evaluation, the tube settler design was considered simpler than the other systems assessed, largely because there would be no risk of filter erosion, due to the low velocity of the water passing through it. This, in combination with a Bernoulli strainer already proven in fish farming for removing neutrally buoyant matter, appeared to offer solutions to both technical problems. In tandem, the two devices appeared to allow raw intake seawater to be conditioned to a quality suited for injection into most reservoirs likely to be encountered offshore.

This summer, a combined filtration and seawater treatment C-Fast subsea water injection module was proven in dirty estuarine dockwater trials in terms of component reliability and cleaned water quality. The module therefore provides a suitable model for engineering into a fully marinized subsea raw water injection system, claims Capcis.

The C-Fast, trialed at the Euro Seas subsea test facility in Blyth, UK, consists of a submerged skid-mounted unit supporting a tube settler, for fine sands removal, and a downstream pump operating at 12,800 b/d. Further downstream components were situated on the dockside, consisting of a Bernoulli strainer and an electrochlorination system for fouling control. System operation was monitored via a modem link back to the sponsors' offices.

System trials

Trials can be divided retrospectively into three phases. The first was the troubleshooting phase when problems associated with the pump, strainer and water sampling technique were solved. The second was a period when an unresolved sampling problem was finally corrected, and the reliability of the C-Fast hardware was demonstrated. The final "foul water trial" phase provided proof of the unit's filtration performance under dirty operating conditions. Throughout, inlet water quality was maintained at typically 0 - 8 mg/l suspended solids.

The tube settler consisting of around 600 inclined tubes feeding into a header box. There was no anti-fouling protection of the tubes and heavy marine fouling developed over the course of the trials. The arrangement re moved sands to within 22 microns, and the unit ran without fault for the full 170-day trial duration.

The dockside mounted electrochlorination system operated without hardware failure. In the foul water trial, an accidentally crimp ed chlorinated water hose prevented adequate water treatment. However, the chlorination loss did not disrupt the trials since the high volumetric flow rate through the system prevented fouling downstream of the tube settler header box. The C-Fast module is remaining at Euro Seas on permanent duty and will be available periodically for inspection.

Capcis has also just launched Phase 2 of its own raw water program which will involve 15 months of tests to evaluate materials used in construction of critical subsea valves, fittings, christmas tree components, and other non-tubular goods for raw water service. Recent projects have investigated effects on materials of increasing seawater content up to 170 ppm by pressurized injection and down to 10 ppb using a purpose-built de-aeration system based on counter-flow nitrogen stripping. Raw water 2 will use this de-aeration technique and expose samples to seawater temperatures up to 75°C. It will also examined effects of commingling produced water with natural seawater as well as seawater alone.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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