Poised on the crest of a Cascade-Chinook wave

Scheduled to arrive in US waters in January, the BW Pioneer will become the first FPSO to operate in the US Gulf of Mexico when it goes into service on Cascade-Chinook, Petrobras America’s first development in the region. It will also extend the FPSO operational water depth record to 8200ft. Meg Chesshyre talks to Norwegian naval architect Inocean about what sets this vessel apart from earlier designs.

Engineers from Oslo-based Inocean have worked on the BW Pioneer for almost two years, on assignment from the vessel owners BW Offshore. The converted tanker, scheduled to sail away from Singapore’s Keppel Shipyard in December in anticipation of a 1Q 2010 start-up at the fields’ Walker Ridge location, is rated by Inocean managing director John Namork as ‘the largest and most demanding of the 15 FPSO projects we have executed to date’.

‘The main engineering challenges have had to do with the ocean area where the vessel will operate the next five to eight years,’ he explains.

‘Generally the weather window in the Gulf of Mexico is quite benign; 98% of the waves have a significant wave height (HS) of 3m or less. But the American Petroleum Institute (API) has categorized Cascade-Chinook as a high risk area when it comes to hurricanes. In addition, BW Pioneer is the first FPSO that the US government and its various authorities have approved for operations in the Gulf of Mexico. The result is that the rules for this project have been more complex than for a typical conversion job,’ adds Namork.

‘US Coastguard rules as well as those issued by the Minerals Management Service were about to be upgraded. The same applied to the rules from API and MARPOL’ (the international convention for the prevention of pollution from ships).

With its combination of hurricanes and loop currents, the Gulf of Mexico’s environmental conditions lead to multiple design events. In the event of a hurricane warning for the area, the FPSO will take no longer than 24 hours to disconnect from its submerged turret production (STP) buoy – supplied by BW Offshore subsidiary APL – and sail out of harm’s way under its own power. To cut down the risk of collision between hull and buoy, the STP buoy with its mooring and riser system will be disconnected from the FPSO before a given weather threshold is reached. Additional design scenarios that might subsequently come into play, such as not being able to escape the hurricane or loss of power and steering capability, were subject to risk analysis and incorporated into the design as accidental load cases.

Model testing at Marintek indicated that during a 100-year winter storm green water on deck events were likely to occur. The tests showed that with the Cascade-Chinook area’s combination of wind, waves and eddy currents the FPSO may be subjected to a larger variation in heading relative to the waves than, for example, a turret-moored FPSO in the North Sea. Consequently, when the vessel was loaded to maximum draught, water on deck was observed along its full length.

To mitigate this, an extended bulwark was designed to protect the area from the bow and past the turret. Structural details that might be subjected to green water on deck were designed to withstand green sea loads. The model test data were used to tune numerical models and these were later employed to determine the extent to which green water loading would apply.

When it came to hull fatigue assessment Inocean had to predict the used fatigue life as a sailing tanker.

‘The ship log showed that the ship had been in trade for 15 years,’ notes Namork. ‘The normal design life for a ship in trade is usually 20 years. At the same time the FPSO is designed for a 15 year life time.’

The hull was inspected to establish its exact condition in terms of corrosion rate and number of cracks after 15 years’ tanker service, and all class authority ship documentation was checked thoroughly.

The Det Norsk Veritas software Nauticus Hull Fatigue was used to quantify the usage of ship fatigue life.

‘The model basin tests and the technical specification given in the FPSO Design Basis gave clear requirements in how to combine wind, current and waves,’ explains Namork. ‘This meant that a full stochastic fatigue calculation for the future life as an FPSO was required.’

For a full stochastic fatigue analysis, stress response amplitude operators (RAO) for each hotspot had to be established, requiring a global model of the ship to be modeled using DNV’s SESAM software to determine the global and local loads in addition to structural response.

‘The amount of data was huge – 2.5 terrabytes were analyzed during the engineering period of one year,’ Namork. reports. ‘The Design Fatigue Factor required by the US Coastguard was fulfilled with a minor repair scope in the waterline region.’

Signs of upturn
Following the new FPSO’s recent naming ceremony in Singapore (pictured right), Namork remarked that the event had coincided with early signs of an improving FPSO market. ‘After a year with many challenges, we now see significantly more activity in the market,’ he said. ‘This bodes well for 2010.’

‘An upturn in activity was evident from the summer, after a relatively quiet period in the beginning of the year,’ explains Namork today. ‘We started to get more positive responses to our sales and marketing efforts as well as increasing customer requests for our services. These were obviously the first signs of recovery from the more gloomy days behind us and we succeeded in settling one major and some smaller contracts, but we still have capacity to take on more work ahead.

‘As was evident for all in this business, the financial crisis lead to a near halt to all development plans,’ he adds. ‘Several companies in need of contracts and financing to complete their newbuilds and conversion went bankrupt. However, history has shown us that this business has its ups and downs.

‘When looking at the potential for the future FPSO market, I would say the prospects are very good – and stronger for other types of production units too. The industry is moving into ever deeper waters and often remote locations where there is lack of nearby infrastructure, and there is a need to store and offload the produced hydrocarbons at site.’ The US Gulf Cascade-Chinook development (2500m water depth) and the Tupi (2400m) and Papa Terra (1200m) fields offshore Brazil provide ample evidence, he believes, that the FPSO remains the floater of choice for these types of developments. ‘Its suitability will not diminish’.

Challenges ahead?
Looking at the challenges ahead for this sector, Namork says that as is natural with any country with a growing oil and gas industry and production level, there will be a desire to increase their local content of the developments taking place. At the same time, financial, economic and production capability aspects will often dictate that significant parts of the supply and work will be done elsewhere. Typically today the drive is to build or convert a hull in the Far East, fabricate the topsides elsewhere, assemble the FPSO locally, and do the associated engineering at three or four different locations worldwide.

‘The challenge and importance here is then to establish well-defined interfaces between the parties involved and make sure that the information sharing, decisions and actions are handled professionally throughout the project,’ he says. ‘From participating in more than 15 FPSO conversions we have gained a lot of experience from such projects and learned to respect the fact that each one throws up its own specific challenges that must be handled.

‘On the technology side, finding feasible solutions for the handling of produced or associated gas is probably one of the biggest challenges when moving into deepwater and remote field locations,’ Namork concludes. ‘Gas reinjection or pipeline export pose their own challenges which naturally leaves the option of LNG a desirable one given the right circumstances. Inocean has therefore for some time been involved in technology development for onboard storage and subsequent shuttle export of LNG.’ OE

Cascade-Chinook
The Cascade-Chinook development is situated in Walker Ridge blocks 206 and 469 in the deepwater Gulf of Mexico. It will feature the first FPSO in US waters, and only the second in GoM waters after the Yuum K’ak’ Naab FPSO (commissioned by Mexico’s Pemex in 2007 on the Ku-Maloob- Zaap field in the Bay of Campeche).

According to Offshore Engineer’s latest ‘Deepwater Development Review’, the BW Pioneer will have oil storage and processing capacities of 600,000 barrels and 80,000b/d, respectively, plus 16mmcf/d gas export (OE April 2009).

It is being leased from Norway’s BW Offshore for five years plus an option for a further three and will initially serve at least two subsea wells at the Cascade field and one at Chinook. Operator Petrobras will decide whether to commit to a full-field development based on the initial productivity of this pair of fields.

Production will be through five freestanding, hybrid riser towers linking the vessel via jumpers to its disconnectable turret. As well as the FPSO and the Technipsupplied and installed riser towers, other ‘firsts’ Petrobras is bringing to the GoM with this development include an FMC-supplied, ESP-based subsea boosting system and ‘torpedo pile’ vertical loaded anchors.

Other major contractors and vendors associated with the project include: KBR Granherne (the owner’s engineer); FMC (subsea trees and boosting systems); Aker Solutions (power cables and carbon fiber rod umbilicals); Technip (gas export pipeline and infield flowlines) and Heerema and Subsea 7 (installation work).

Petrobras America operates Cascade on behalf of 50:50 partner Devon and Chinook with 66.7% on behalf of partner Total.

Issue: December 2009
By: Meg Chesshyre