Keeping cool, cutting costs

Scope reduction has been the key focus on Statoil’s Johan Castberg floating production project. Elaine Maslin reports.

An illustration of the Johan Castberg field layout. Image from Statoil.

Statoil’s Johan Castberg development is set to become one of Norway’s northern-most oilfields, sitting in the Barents Sea some 240km northwest of Hammerfest.

It’s at a latitude north of Alaska, far from existing infrastructure in 370m water depth. The Johan Castberg development comprises three oil fields, Skrugard, Havis and Drivis, discovered in 2011, 2012 and 2014, respectively, in 380-400m water depth in PL532.

To help make this 460-650 MMboe project economically viable, operator Statoil has selected a floating production development with a simplified subsea infrastructure for the field, as set out in our August issue (De-engineering, OE: August 2016). This, together with reducing the number of wells on the field from 40 to 31 and the number of templates from 15 to 10, plus two satellites, has helped slash about 50% off the cost of the development, from US$11.3 billion to about $6 billion.

Aker Solutions has been charged with coming up with a lean concept for the floating production, storage and offloading (FPSO) facility for the project. Aker Solutions’ project director Nils Olav Solheim gave an update on progress on the concept at ONS in Stavanger, Norway, in late August.

He said that Aker Solutions’ study has gone through every function on the FPSO, looking for simplification and taking off equipment not needed to slim down the cost of the unit.

The unit is currently 295m-long and 54m-wide. It will have 1.1 MMbbl storage capacity and 30,000 cm/d oil processing capacity and 8.3 MMcm/d gas capacity. The turret, positioned forward of the center of the vessel, for weathervaning, will weigh 7000-tonne, and the topsides will weigh 18,000-tonne.

The original storage capacity was 1.3 MMbbl, on a 320m-long vessel, but it was reduced 1 MMbbl as the vessel was slimmed down, then increased slightly without increasing size of boat due to a better solution of oil export pumps, Solheim says. The living quarters has been slimmed down from 140 to 120 cabins, with 20 of those having turnable beds to accommodate short-term increased capacity for maintenance.

One of the key considerations for the vessel has been around being able to operate in the harsh Arctic weather conditions. A laydown area has been set out from where equipment can be moved to a heated area. Life boats are protected, covered on all sides, and could be heated with an infrared heating system to ensure release mechanisms will work. Process heat will be used to heat water to clear the helideck and there will also be covered walkways, to protect staff. Ice and snow could also impact production. If there is a lot of snow fall and ice, production may have to stop, so that the unit doesn’t become too heavy with produced oil, as well as the ice and snow. However, Solheim doesn’t see this happening more than once in 10 years.

A more likely issue is not being able to export due to bad weather. But still, this is no more of an issue than elsewhere on the Norwegian Continental Shelf, he says. The design temperature here is five degrees lower than on the Haltenbanken, but, the weather conditions are in fact worse on the Haltenbanken.

Thanks to the reduced wells scope, meaning fewer templates and so risers, the turret was able to be slimmed down, from the original 11,000-tonne to a far leaner 7000-tonne.

On the process side, initially the design included three separation stages. One, the second stage separator has been removed – “we don’t get more oil with all three,” Solheim says. By taking away the unnecessary separator, piping and controls are also reduced, he adds.

The FPSO will have a chemical injection plant, including triethylene glycol, or TEG, to prevent hydrates. Gas injection, using a gas compressor, and water injection pumps, both up to 200 bar, are also included in the project for pressure support and for improved lift. Produced water, plus treated seawater, will also be reinjected, post-treatment in a sulphur removal module.

The vessel also has a fiscal metering package. Offloading will be with a shuttle tanker every 4-5 days at the start and for some period thereafter. Extra deck space has been created for future tie-ins and there are also spare riser slots.

How the field will be powered has yet to be decided. Solheim says there are several studies, but it will be up to Statoil. Power from shore has been a hot topic in Norway, with a number of projects already using renewable power from the mainland to power process facilities, including Valhall, Gjøa and Goliat, which is the only producing oilfield in the Barents, and which had a recent power outage issue. The longest power from shore cable to date is to Goliat, at just over 100km. A longer cable is planned for the Martin Linge field, at 163m-long, but this is some way short of the 240km for Johan Castberg, which could require new cable technology.

The next milestone for Johan Castberg will be a decision to go into front-end engineering in design later this year. Then, next Autumn, an investment decision is due to be made. Detailed design would then likely be over 1.5 years, with first production potentially around 2022.