FPSO designs adapt to harsh northerly/sub-tropical conditions

May 1, 2002
Each floating production, storage, and offloading (FPSO) project is a learning process, prone to pitfalls. This was a recurring theme at the International Conference on Floating Production Systems in London last February, organized by the Institute of Petroleum.

By Jeremy Beckman
Editor, Europe

Each floating production, storage, and offloading (FPSO) project is a learning process, prone to pitfalls. This was a recurring theme at the International Conference on Floating Production Sys-tems in London last February, organized by the Institute of Petroleum. One operator speaker making a presentations was concerned at the seemingly remorseless delays to his schedule. Another was preparing for typhoon encounters. A third was drawing up contingency safety plans, having lost a semi-submersible, the P-36, in mild conditions.

The Nan Hai Fen Jin FPSO will shortly enter service on CNOOC's Wenchang development in the South China Sea.
Click here to enlarge image

The most positive paper came from Li Da-Quan, Deputy Production Manager for the Wenchang project in the South China Sea. Two oilfields, Wenchang 13-1 and 13-2, are being developed by operator China National Offshore Oil Corp. (CNOOC) and its partner Husky Energy in 117 m of water 132 km offshore Hainan Island. Wenchang is named after a town on the east side of the island.

Having analyzed several production concepts, the partners settled on a scheme involving a wellhead platform on each field, both linked to an FPSO via subsea pipelines, risers, a power cable, and an umbilical. According to Da-Quan, this is the world's first new-build FPSO with an internal turret/single point mooring system to be permanently moored, i.e., non-disconnectable, in a typhoon region. It is also the first time that CNOOC has attempted such a project. As seems to be the norm with floaters, the schedule is tight and the budget constrained, which calls for creative thinking at all times from all parties. In preparation, said Da-Quan, CNOOC's project management team gleaned what they could by studying presentations by other FPSO operators, and by consulting experienced contractors in this field, such as SBM and Norway's Advanced Produc-tion Loading (APL).

CNOOC had an option to issue the entire construction program to one contractor on a turnkey basis, or to appoint several contractors. The turnkey approach looked more expensive, although from CNOOC's point of view, the interfaces would be simpler to manage, lowering the associated risks. But cost control was paramount, so the project was split into four main packages:

  • Detailed design of the vessel and the process modules were awarded to Chinese design companies Maric and China Offshore Oil Engineering Corp. (COOEC), respectively
  • Dalian New Shipyard was awarded the contract for the FPSO. It was also subcontracted by COOEC to fabricate the process modules, to ensure that all construction work would be done in the same place by the same contractor
  • Long-term main equipment items would be procured by CNOOC PMT, which was also appointed general engineering, procurement, construction, and installation (EPCI) contractor for the FPSO
  • APL was given the EPCI contract for the single-point mooring, based on its submerged turret production (STP) concept.

Additionally, COOEC was designated engineering contractor for the two platforms, the subsea power cable and the sea lines, to safeguard overall integration of the surface/subsea facilities.

The vessel was completed in January, following a 22-month construction program. It has since undergone commissioning, in readiness for the production start, probably in June. Da-Quan claimed that the final installed cost would be 25-30% less than the international norm for such vessels.

The FPSO is 262.2 m long, with a beam of 46.4 m and a depth of 24.6 m. The topsides arrangement includes crude oil processing, heating, and gas turbine power generation, in addition to a flare tower, living quarters, and a stern discharge system. Process capacity is 75,000 b/d of oil, with storage capacity of 1 MMbbl. The vessel is designed to withstand a 100-year return period typhoon condition, maximum wave heights of 20 m, and maximum wind speeds of 52 m/sec.

The Chinese engineering institutes lacked experience working with FPSOs, and were to an extent feeling their way in this project. CNOOC learned that the still water bending moment and shear force used for the hull structural design must be based on the results of 3D hydrodynamic analysis and model tests. Also, the topsides would need to be arranged to avoid large levels of distortion stresses on the hull.

The team considered two options for the STP mooring arrangement. The first included eight legs with equi-distant spacing. The second was three sets of three legs fanning out at 120 degrees, with in-between angles of 5 degrees. The team decided on the latter for safety reasons. The selected design maximized space for the riser configuration and reduced the potential discrepancy in performance of the mooring system between intact and damaged cases.

Finally, reliability of the umbilical was a sensitive issue. Design and manufacture were awarded to DUCO in the UK, as such expertise was not available in China. The vessel was classified by Bureau Veritas.

Terra Nova overruns

Gordon Carrick, Petro-Canada's Terra Nova Asset Manager, outlined some of his experiences offshore Newfoundland. The project began four years ago, with a budget of $2 billion. When completed in January, it was a year behind schedule and $800 million over budget.

"We were always behind our plan," he said. "The topsides and hook-up costs finished up 200% over budget. In the end, we devoted 16-17 million man hours to this project. He said the experience points out how important it is to set realistic expectations. Comparing the Terra Nova experience and North Sea FPSOs indicates most of those also ended up behind schedule, with cost overruns.

"We went for a field development alliance, including the wells, due to the success of some of the alliances in the North Sea, and because Petro-Canada, being small, needed engineering back-up. It was done on a risk-reward basis, with a production incentive. Contractors would all receive payments based on early production.

"We've learned as an operator that you need to make sure that every contractor is competent, and can deliver as expected. Contractors are not operators; therefore, they need an operator's guidance in making decisions." In retrospect, Carrick said, Petro-Canada would have opted for an arrangement involving fewer contractor interfaces.

The vessel is 292 m long, and 45 m wide, with an operating draft of 13-18 m. It is ice reinforced at the front with 3,000 tons of extra steel, and a very high prow to cope with significant seas in this area. At peak, it will process 150,000 b/d of oil, with 60 MMcf/d of gas flared and some water reinjected. "We were at peak production within nine days," says Carrick, "but we had to scale back after flaring more than the permitted level. So we're now in the process of adding compression."

The vessel features the world's first fully automated, quick-disconnect turret, which also happens to be the largest turret ever built. This turret was also the single item that caused most problems during commissioning, Carrick said. In an emergency, the turret can disconnect in 20 seconds. This is necessary because there is a 60% probability of iceberg encounters in winter. However, Petro-Canada plans to avoid disconnection where possible, using tugs to tow away menacing ice parcels.

The FPSO's topsides are small compared to some counterparts in the South Atlantic, Carrick added. A blast wall protects the accommodation area. Two turbines positioned toward the stern generate more power - 95 MW - than Newfoundland's capital St. John, he claimed, which is a city of 175,000 people. There are three thrusters at the vessel's fore and two aft.

Marlim build-up

Luis Bastos, Petrobras' Marlim Sul Produc-tion Asset Manager, touched briefly on the sinking of the P-36 semi last year. Loss of control/ stability was the problems, he said, and Petrobras had now implemented six projects to avoid replication of this incident in the future. These include drawing up special plans for the drainage lines, which he said were the basic cause of this incident, and studies of compartments on double-bottom decks in the area above the columns. "These could also be a weak point in terms of stability," he said. Petrobras is also looking at the possible relocation of some double-block valves to avoid any connection between gas and drainage systems.

Marlim Sul Module 1 is the most recent development onstream in the Campos Basin, using the P-40 FPSO. The field was discovered in 1987, in 1,000 m water depth, but at that time the technology was not available for development, Bastos said. Total reserves are estimated at 3 BBOE. "The reservoir spread makes the use of (tension leg platforms) TLPs difficult, likewise the shallowness of the reservoirs." Between 2005 and 2008, Petrobras plans to add three more production systems on different parts of the field, exploited by 95 wells, including 45 injectors. For Module 1, one large-bore, extended reach well is producing at 37,000 b/d, which is a record for Petrobras, according to Bastos. It features a 7-in. production string and a 6 5/8-in. sand-control screen. "It took 166 days to drill and cost $27 million. I wish we could have more of these wells - that's the key for us to expand our production in the future," he said.

Norway's record

Statoil Technical Director Jonas Odland detailed operating experiences of floaters in the Norwegian sector. Fifteen were operating on the shelf early this year, including two TLPs, seven semisubmersibles, and six FPSOs. There also are two permanently moored storage tankers, and various shuttle tankers are in operation. Typically, the fields served are high production, high regularity, with commercial exploitation of oil and gas, in harsh environments, and in up to 400 m water depths.

While the TLPs and two of the semis have production and drilling facilities, the remainder are only equipped for production. Conventional tankers and semis are not robust enough, Odland claimed in his paper, for continuous offshore operation in Norway over a typical full-field life of 20 years. One TLP and one semi are concrete units, which aids durability/motion characteristics. But the current trend is to use steel, mainly for commercial reasons.

Mostly, Norwegian floaters feature layouts derived from principles used for the large fixed platforms of the 1980s. Their living quarters are located forward with the flare tower aft for safety reasons. In calm conditions, the FPSOs are oriented against the weather with the wind coming in at the port side of bow to optimize ventilation. Offshore loading is weather-dependent, but production regularity is generally unaffected. However, during prolonged periods of rough weather, production has been shut down due to insufficient storage capacity.

In the Haltenbanken area off mid-Norway, conditions are harsher, with temperatures dipping in winter to -10°C. The attendant wind chill can make working in the open process areas difficult, and some utility systems are prone to freezing. In certain cases, walls have been installed to provide protection against the wind. Extreme wave conditions are no worse than those in the northern North Sea, but swells coming in from a different direction can increase roll motions. Use of thrusters is therefore necessary to maintain heading at times, but even this can be difficult in very rough seas. In certain cases, roll motions in FPSOs and storage tankers have exceeded model test predictions, necessitating strengthening of some module support structures.

Last November, the floaters in the Haltenbanken endured one of the area's worst storms in memory, with significant wave heights touching 14 m. The Norne FPSO was worst hit, being stationed in a more northerly latitude. It had just finished offloading before the storm hit, and was therefore in a light loading condition. Its largest recorded pitch amplitude was around 9 degrees, with the bow moving downward at one point almost 40 m over a period of eight seconds. The Heidrun TLP was subjected to shaking, but the measured effect on tether tension was found to be well within design values. Generally, the Norwegian FPSOs have been subject to wave-slamming damage caused by a low freeboard. However, this situation can be rectified through altering draught and trim.

All the FPSOs incorporate a turret/swivel arrangement. Some mechanical problems have been reported, mainly related to the turret bearings and turning system, but repairs have generally proved straightforward. Flexible risers are used on all the floating units, except the TLPs. Again, few problems have been reported, probably due to the thorough qualification programs for these critical pieces of equipment. It is considered advantageous for FPSOs to have the risers hanging from a turret located roughly 1/3 of the vessel's length from the forward perpendicular, rather than from the bow, where combined heave/pitch motions are more severe.

Finally, tanks used in the hull for oil storage are normally fitted with an inert gas system, which is designed to maintain a non-explosive atmosphere in the cargo tanks' vapor spaces. However, chemical reactions can lead to the formation of inert gas/hydrocarbon gas mixes known as volatile organic compounds (VOCs), which can be released to the atmosphere. Sloshing of tanks exacerbates the volume of VOCs produced. In response, the Norne and Åsgard FPSOs have been designed with three parallel cargo tanks.

Åsgard A also has been equipped with a produced gas blanketing system, whereby produced gases are used to raise the concentration in the vapor spaces of cargo tanks above the explosive/flammable limits. During produc-tion, when oil replaces gas, the gas is routed back to the topsides process facilities in a closed system. An inert gas system is fitted as back-up and for gas-freeing operations. The experience has been so good, Odland claimed in his paper, that a similar system may now be installed on Norne.