Zafiro field expansion entails new drilling, hookup methods

The floating flare buoy offers the Zafiro Producer additional flare capacity while avoiding the added noise and heat of installing an additional flare boom on the FPSO.

The rapid expansion of the Zafiro oil field off Equatorial Guinea in West Africa relied on a number of recent technological advances to optimize production from a very prolific field.

In the wake of success with Zafiro Phases I and II, Mobil feels it is more than prepared for the challenges of deepwater. By applying a myriad of new and emerging technologies to this fast-track field, the company has received a dividend of experience in just the sort of applications it is likely to encounter in deepwater fields.

In March of 1995, Mobil commissioned the conversion of the Zafiro Producer floating production, storage, and offloading (FPSO) vessel as a major component to the development of the Zafiro oil field 42 miles offshore of Equatorial Guinea. With no onshore infrastructure, the FPSO allowed Mobil to bring the first eight wells of this development on line only 18 months after the initial discovery, an industry record according to Mobil.

While the fast track was in progress, additional exploration drilling was ongoing. By the time Zafiro Phase I was underway, it became obvious that an expansion would be inevitable and discussion of Zafiro Phase II began. This would involve bringing another eight wells on line increasing the capacity of the Zafiro Producer and bringing up daily production from the 40,000 b/d of Phase I to 90,000 b/d. Ultimately, with the addition of water injection wells, gas lift, and a fixed platform, this number is expected to rise to 120,000 b/d.

FPSO expansion

There is nothing revolutionary about expanding the capacity of an FPSO, but the Zafiro Producer underwent a major refit while on station and producing oil. Ten new modules were installed on the Zafiro Producer, including a 900-ton gas turbine module. The modules were loaded onto a heavy lift barge, which was positioned next to the FPSO and acted as a construction vessel.

The original FPSO equipment had been installed on the port side of the vessel deck, leaving an ideal space on the starboard for the new equipment. Production was cut back during the heavy lift process so that those performing the lift could communicate without the noise inherent in oil production.

Fire watches and a complete safety system were maintained during "hot work" associated with the expansion, and on-site inspections were conducted while the vessel was still producing oil. With the expansion complete the field actually hit a peak production rate of 100,000 b/d.

Floating flare

One of the limitations of the original Zafiro equipment was the capacity of the flare boom. The boom could flare gas from the original eight wells, but the expansion called for a doubling of production, which would exceed the boom's capacity.

In choosing a second flare, Mobil took the novel approach of installing a floating flare buoy. This flare buoy is connected to a 400 ton gravity base and tied back to the FPSO by 330 m. catenary flexible pipes.

By flaring the gas at a distance from the platform Mobil spares the crew the noise and heat associated with the process. According to Chuck Alexander, Subsea Manager for Mobil, this is the first floating flare attached to a floating system.

HOST system installed

When it came time to choose a subsea system for the Zafiro, Mobil already had a product in mind. The company had worked with FMC on the design of the hinge over subsea template. According to Clair Menning, Venture Manager for Mobil, the HOST is a modular template system that adapts to the specific field configurations of an operator.

The operator can have standard components in stock to develop new fields or expand an existing template. The new wells will be tied back to the HOST template reducing the number of flow lines on the seabed. The diverless "remote" system will be tied back to the HOST where the oil will be commingled and sent to the FPSO.

The installation of the four HOST templates and components was achieved using a drilling vessel for installation. The vessel drilled in a center pile, then ran the hingeover guide structure, on drillpipe, through the rig's moonpool, down guidelines to connect with the center pile. This installation process saves the time and expense of securing a separate construction vessel.

The template is flexible enough to allow these wells to be predrilled, drilled through after the template is in place, or satellite wells could be tied back to the template after it is in place. All three of these methods were used on Zafiro. A piping manifold on the center section will commingle production from the tied-back wells.

Additional wells were also drilled after the flowlines had been connected to the manifold. The tiebacks are achieved using the Universal Tie In System (UTIS). This self-contained, diverless, flowline pull-in and connection system was used on 18 flowline pull-ins and connections on the Zafiro field. The UTIS contains a winch that can grab onto a flowline and reel it into a receptacle, which lines it up with a manifold hub. These are the first completely diverless flowline installations Mobil has completed.

The four HOSTs will be used to bring an additional 12 wells online. One of the HOST will be the home of the electrical subsea multiphase pump. This pump will be installed the first part of next year as part of an enhanced recovery program designed to increase production to 120,000 b/d. The pump will be installed to replace a pigging loop already on the template. The pump will create pressure to lift the production to the FPSO from that HOST.

All of the new wells are drilled in 500-2,000 ft of water. In addition to identifying additional reserves, the on-going exploration drilling has helped Mobil better understand the structures and formations it is dealing with in Zafiro.

This has enabled the use of horizontal wells to be gravel-packed for the entire volume of the hole between the pipe and formation throughout the horizontal pay zone. The longest of these was 6,000 ft. All of the Phase II wells also include pressure and temperature gauges down hole. The Phase II wells also are choked at the seafloor rather than the surface giving the operators better control over the new system.

Additional drilling

A program to drill five additional wells to provide water flood for the field is currently underway. Three of the five wells have been drilled. These will tie back to Zafiro Producer. A 40-slot, fixed platform, the Jade, is currently under construction.

Current plans are to produce through 28 of these slots and use the others for waterflood and gas reinjection. Based on the experience gained thus far in Zafiro, the engineers are confident that they can hit all of the objectives stepping out from this fixed platform. It is these 28 wells and water injection/gas lift program that will allow Zafiro to hit its goal of 120,000 b/d by 2003.

Now that it is clear the Zafiro Producer will be on station for at least 15 years, engineers are busy designing and installing a single-point mooring system for the vessel. The waters off Equatorial Guinea are relatively calm, and the vessel is currently held in place by a spread-mooring system (SPM) and is able to offload crude to tankers in a bow-to-bow scenario. The SPM will offer additional safety and efficiency in the offloading process and will feature a 40-in. pipe extending two miles from the FPSO.

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