From subsea to topside – Statoil’s approach

Published December 13th, 2000 - 02:00 GMT

Big platforms are off the menu for Statoil as it embraces advanced seabed installations. But existing infrastructure still has a key role to play in a subsea future. 

 

Most of Norway's big offshore fields are at plateau production or in decline. No major developments are in the offing, and the remaining small and medium-sized discoveries cannot be brought on stream profitably with traditional platforms at today's oil prices.  

 

"We're at a crossroads," comments chief engineer Ove Kallestad in Statoil's sub sea technology unit. "Our challenges are to develop the next generation of underwater solutions and to find users for them.  

 

The whole industry must review its experience and get new projects off the ground." 

 

He is an enthusiast for the looming underwater era off Norway, where developing and installing sub sea wells will be part of tomorrow's solutions together with new surface units. And he points to the investment already made and the infrastructure now in place. 

 

A good example of the way existing developments can be utilized and extended is Statoil's Gullfaks field in the North Sea. This currently gives the group about 350 000 barrels of entitlement crude per day. 

 

Produced from three massive concrete platforms, that output is twice the amount of oil consumed daily by Norway. In addition comes 3.5 million cubic meters of rich gas per day. 

 

Recoverable reserves originally available in the main field are put at more than two billion barrels of oil and 36 billion cubic meters of gas. 

 

In addition, Statoil has brought a series of satellite fields on stream. These add some 400 000 barrels to the combined reserves, and will boost rich gas production tenfold within two years. 

 

Discovered in 1978, Gullfaks was developed at a time when the licensees expected an oil price of USD 40 per barrel. Its platforms were given high processing capacity and lots of topside space for future developments, reports planning manager Aage Holstad. 

 

The Gullfaks South satellite was in Statoil's mind even when the platforms were being designed and built. As much as 20 years ago, it was clear that great flexibility would be an advantage. 

 

Gullfaks A came on stream in 1986, followed by the B installation in 1988 and Gullfaks C in 1989. After years of plateau production, the main field is now in decline. 

 

It peaked in 1994 with an annual production of more 188 million barrels of crude, and is currently flowing some 125 million barrels per year. 

 

In addition comes about 31 million barrels of oil from the nearby Tordis field operated by Saga Petroleum, which was developed in the early 1990s with sub sea wells tied back to Gullfaks C. 

 

And production from three satellites has been flowing to Gullfaks A since last October. Gullveig came on stream first, followed by Rimfaks and Gullfaks South in February and March respectively. 

 

This trio currently produces around 40 000 barrels per day, with a daily plateau of 130 000 barrels due to be reached in the course of the year. 

 

The Gullfaks satellites project ranks as the second largest sub sea development off Norway, outdone only by Statoil's إsgard field in the Norwegian Sea. 

 

Nothing of these fields is visible on the surface when flying to Gullfaks by helicopter, but they are certainly there - and doing a good job on the bed of the North Sea. 

 

Many kilometers of flow lines, control lines and cables link their wells to Gullfaks A, which stands 175 kilometers west of the Sogne Fjord. 

 

Ranked as one of the world's biggest concrete structures, the platform is back to a normal routine after a long and hectic construction period. 

 

Lasting 18 months while production continued uninterruptedly, this work modified the topside facilities to accept and process crude and condensate (light oil) from the satellites. 

 

At peak, 650 people were at work on the platform compared with a normal staffing of 180. No other producing installation off Norway has been through such an extensive conversion. 

 

But the modifications are a success, says platform manager Annas Bollmann. He is proud to report that the work was completed without affecting health, environmental and safety results or production regularity. 

 

"You could well say that bringing the satellites on stream marked the start of a new era for us," he comments. "While output from the main field is declining, satellite production will be rising." 

 

He heads operations on a platform with complex processing technology, where gas and various types of crude flow in from wells on the main and satellite fields as well as on the nearby Vigdis and Visund development operated by Saga and Norsk Hydro respectively. 

 

Gullfaks A has been fitted with three major new modules, cutting down on free space and making movement around the decks more complicated. 

 

The limited size of the pipe deck on top of the M35 gas treatment module, which measures 40 by 12 meters and stands 15 meters high, poses problems for drilling and well operations. 

 

Cargo handling during and after the conversion process has also presented challenges. 

 

"We really notice that there's less room on deck and reduced storage space since the satellites came on stream," says crane driver Henning Andersen. "We've got to juggle things around when the supply ships arrive." 

 

He glances down at M35 from his little cabin high above, and explains that placing pipes up to 15 meters long on a deck just 13 meters wide is no easy task. 

 

There has been more to do since the satellites came on stream, he notes, but finds that no problem. His only concern is that work could grind to a halt sooner or later because the platform has become so over-filled that nowhere can be found to put equipment arriving for drilling and well operations. 

 

Mr. Bollmann explains that opportunities for making adjustments on deck are kept under constant review. 

 

The control room has also seen its workload increase since oil and gas began flowing from the satellites. Plans call for the latter to be operated with no increase in Gullfaks A staffing. 

 

Liv Judith Ferkingstad and Hهvard Aga can have busy watches when they must keep an eye on all the wells. Production from the satellites mean they have a few extra screens to monitor closely. 

 

Seeing that downhole pressure and temperature stay within acceptable limits is largely straightforward, but crude from the satellites is more viscous than main-field production. 

 

"Chemicals to inhibit wax formation in the well stream are being added continuously until we've gained more experience of this flow," Ms Ferkingstad explains. 

 

"If the wax separates out, it could create blockages when flowlines are shut in or brought back on stream." 

 

She had the honor last October of pressing the button to start Gullveig flowing from sub sea installations which have cost roughly NOK 2.5 billion for all three satellites. 

 

Because crude from these fields has a complicated composition, technically complex flowlines are needed to transfer production from wellhead to platform. 

 

The Gullfaks satellites represent the only North Sea development with such sophisticated pipelines on the seabed, says Mr Bollmann. 

 

Wellstreams from Rimfaks and Gullveig travel through eight flowlines, each 10-18 kilometers long. Waxier crude from Gullfaks South is carried in a specially-developed flowline bundle. 

 

In addition to pipes carrying the well stream, this bundle also has "central heating" to prevent wax solidifying and blocking the production lines during shutdowns. Hot water provided by waste heat on Gullfaks A circulates through narrow tubes. 

 

In addition, the bundle incorporates control cables, chemical and service lines and piping to take injection gas back to the field. 

 

The Gullfaks satellites project is one of the first in the world to use flowlines made from a specially-adapted stainless steel which costs only half the price of standard corrosion-resistant materials. 

 

This technology has also been adopted on إsgard, reports Tor Willgohs Knudsen, who heads Statoil's sub sea operations support department in Bergen. 

 

He notes that the sub sea systems installed on the Gullfaks satellites incorporate new technology from the wellhead and up to the A platform. 

 

Together with Norway's Kongsberg Offshore, Statoil, Mobil, Elf and Shell have joined forces to develop a new generation of sub sea production systems. 

 

The result is the hinge-over sub sea template (Host), which can be installed in water depths down to 2 500 meters and has been adopted for several Norwegian and foreign fields. Gullfaks was the pilot project. 

 

Using the Host solution has cut template, Xmas tree and control system costs on the Gullfaks satellites by 50 per cent compared with Statoil's Statfjord satellites project in the early 1990s. 

 

But not everything has gone smoothly on the Gullfaks satellites, notes Mr. Holstad. The same point is made by operations vice president طyvind Reinertsen. 

 

"Although drilling sub sea wells from floating rigs can be simpler than from a fixed platform, the latter permits easier and cheaper down hole operations and maintenance," Mr. Reinertsen observes. 

 

"Having a large number of sub sea wells to maintain from floating rigs represents a challenge for Statoil." 

 

Mr. Holstad adds that finding efficient well maintenance solutions will be important, given that the group is due to be operating up to 150 sub sea wells off Norway within a couple of years. 

 

He says work is under way to find effective forms of cooperation in this area, including the pooling of rigs between several offshore licenses. 

 

And the planning manager underlines the importance of qualifying technology to maintain wells from dynamically positioned vessels in relatively shallow waters such as the Gullfaks area. 

 

Such units, which keep on station by engine power alone without the need for anchors, must not be confined to deepwater fields such as إsgard, he says. 

 

Gullfaks had five satellite wells on stream in April, and this number will increase at the rate of one a month during the spring and summer. 

 

Phase I of the satellites development covers a total of 24 wells in four templates on Gullfaks South, three on Rimfaks and one on Gullveig. 

 

The second phase involves installing two additional templates and drilling another nine wells, bringing the planned total to 33. 

 

Statoil and its partners have thereby secured a steady flow of oil revenue, and jobs on Gullfaks are safeguarded until well into the next millennium. 

 

Without sub sea production from the satellites, preparations for shutting in the Gullfaks installations would have had to start as early as 2003. 

 

Berit Bryne 

Source:Statoil.com 

© 2000 Mena Report (www.menareport.com)

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