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FLOATING PRODUCTION: Risers, mooring configurations top Petrobras' priorities for deepwater


Location of FPSOs and FSOs on the North-West European continental shelf.
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Petrobras is leaning toward ship-shaped floating production, storage, and off-loading (FPSO) vessels for its upcoming offshore field developments. Meanwhile, North Sea FPSO units may have to be re-designed to avoid damage from waves (green water) on the deck. These were two of the stand-out themes addressed at the 15th FPSO confer-ence, staged by IBC Global Conferences in London.

The Petrobras paper was co-authored by Roberto de Oliveira Goulart (Basic Design Coordinator on the Barracuda project) and Deborah Martinez de Mattos and Paulo Mauricio Videiro of Petrobras' R&D Center. Currently, the company has 29 floating production systems operating in Brazilian waters (all but one - Caravela - in the Campos Basin), with four more due to enter service by 2003, following conversion programs.

Semisubmersibles dominate, with 18 in operation, and a 19th - P-40 - undergoing installation in Marlim Sul. Production depths range from 100 meters of water for the P-12 on Linguado to 1,340 meters water depth for the P-36 on Roncador. The latter also has the highest oil processing capacity of Petrobras' semisubmersibles, at 180,000 b/d.

The main driver behind the selection of semisubmersibles has been the large numbers of flexible risers required on the host fields. However, finding suitable hulls for conversion to large production semis, in line with future development plans, is becoming more problematic. Petrobras' sole experience to date with newbuild semis is the Marlim P-18, which despite being a production unit, is based on a drilling platform design with four columns and two pontoons connected by horizontal braces. Petrobras has also engineered some designs of its own, optimized for Brazilian conditions, which follow the normal pattern of four columns with closed ring pontoons.

Ultra-deep mooring

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Production in deeper waters has been made possible by switching from chain-wire, rope-chain moorings to taut-leg systems with polyester rope. These are lighter than steel ropes and have a shorter mooring radius, which reduces interference with subsea equipment. Petrobras expects the new generation of polyester taut-leg lines to be suitable for duty in 3,000 meters water depth. The group has less confidence in riser systems at those depths.

The latest addition to the FPSO ranks is the P-37, which has been producing on the Marlim Field since July in a water depth of 905 meters, with 43 flexible risers and oil production capacity of 150,000 b/d. Petrobras has taken a stricter line on the more recent tanker conversions, the authors said.

Initially, it tolerated repair of some existing equipment, such as pumps and pipes. However, closer study has revealed that some items were either inadequate or too old, and sourcing spare parts during the FPSOs' 20-year lifespan could prove difficult. Petrobras now insists that all equipment and piping is changed during the conversion process, with only the tanker hull retained.

The Campos Basin is prone to a multi-directional environment. Misaligned wind and current with long period swells can induce beam seas and in consequence, high roll motions on FPSOs. This can impact adversely cargo handling and oil-water processing. For new FPSOs, Petrobras is looking to improve loading and offloading, and a bilge keel is being specified to reduce motions, so that processing can be maintained for longer periods.

Shuttle tankers are expected to offtake 60% of all Brazilian oil production through FPSOs and floating storage and offloading (FSO) vessels by 2004. To ease inspection of the offloading hose, Petrobras specifies a chute-type system whereby the hose is stored in a long cradle alongside the deck. This, it claims, provides an improved stern layout compared with hydraulic reel systems. However, problems were being encountered when handling the hose in the cradle and in connecting the hose end to the rigid pipeline at the FPSO's stern. As a solution, a back-up 8-in. floating hose has been employed, which has helped extend offloading periods, but also increased operating costs. Chute suppliers are currently evaluating adjustments to the main offload system.

SCRs for FPSO units

Procap 3000 is Petrobras' in-house R&D program for deepwater fields of the future. The key issues under review are risers, mooring systems, and new concept floating production systems.

Flexible risers with maximum 10-in. diameter have been deployed for semis in water up to 1,500 meters deep. Petrobras is now examining their potential with FPSOs, which have higher motion amplitudes. Steel catenary risers (SCRs) have been adopted successfully on the newly installed Marlim and Roncador Field semis (in the latter case, one for oil and one for gas export). However, extending this concept to FPSOs is not yet practical, the authors said, due to the higher static offset and more severe heave motions. But Petrobras is evaluating a tethered buoy riser that could lead eventually to use of SCRs with FPSOs.

For mooring systems in water up to 3,000 meters deep, the company is considering spread catenary taut-leg systems and the differentiated compliance anchoring system (DICAS), which has a Petrobras patent. The main focus in each case is on materials, design procedure and criteria, installation, and maintenance. Taut leg systems have been adopted for Petrobras semis since October 1997, and for the FPSO II on Marlim Sul, installed in late 1999 using a vertical loaded anchor.

Another patented Petrobras solution is Free Fall Piles, which have been deployed for installing flowlines in the Campos Basin and are now undergoing tests for use as fixed points for FPS mooring systems. One will be installed in the mooring system of the P-36, which is a semi.

Petrobras is also looking at new generation combination concepts, such as floating, prod-uction, drilling, storage, and offloading units. It has its own concept called the FPWU-3000 (floating production and workover unit for 3,000 meters water depth). This is a production semi with workover facilities and wet completed wells situated beneath the semi, making completions and workover operations achievable from the platform. The wells are connected to subsea manifolds, in turn linked to the FPWU through rigid risers.

Green wave impact

A paper by W. Morris, a naval architect with Bomel in the UK, B. Buchner, principal consultant offshore at test institute Marin (Maritime Research Institute Netherlands), and J.Millar of the UK Health & Safety Executive, examined the impact of green water on North Sea FPSOs and also outlined a new project to strengthen floaters against wave impact.

Green water is unbroken waves overtopping the bow, side, or stern of an FPSO or FSO hull. In harsh environments, like the North Sea, the problem is more common - weathervaning turret-moored systems are particularly exposed at the bow, leading to potential damage to control valves, emergency shutdown equipment, fire detection systems, and cable trays. In cases where accommodation is placed at the bow, the same risk applies, extending to the helideck. Resultant repairs cause production downtime. On some converted tankers without a poop deck, green water has also been observed at the stern.

The first joint industry project on FPSO green water loading was undertaken in 1997 by Marin. One result was a new software package called GreenLab, which can be integrated into designs of monohull-shaped floaters, allowing green water counter-measures to be assessed interactively. The software's methodology for predicting freeboard exceedence (water on deck) is based on a combination of linear diffraction analysis and non-linear corrections derived from the joint project model tests.

GreenLab predicts green water exceedence at the vessel's bow, side and stern. Water height on deck is determined at three locations - at the bow, five along the side, and one position at the aft end. When maximum freeboard exceedence is established, water heights, impact loads, and so on, can be calculated. Relative motions between the waves and the floater form the basis of these calculations - when they exceed the freeboard level, green water will flow onto the deck.

Freeboard exceedence

At the most recent count, 16 FPSOs were operating on the UK continental shelf. Most are based in the Central North Sea, with two west of Shetland and one in Liverpool Bay off northwest England. In Norway, five floaters are on duty. Total operational experience is around 88.5 vessel years.

The joint industry project categorized low levels of freeboard exceedence as a water height of less than three meters. Medium susceptibility was defined as 3-6 meters, while above six meters counted as "high." This definition applies to areas of the deck that are more prone to shipping green water. Based on analysis of 15 UKCS floaters and the five Norwegian ones, the team drew the following conclusions:

  • Bow area: Almost half UK floaters may be exposed here to high green water susceptibility. Head seas tend to generate higher green water freeboard exceedence.
  • Side area: Most FPSOs and FSUs susceptible. Side green water is highest mainly aft of midships. Vessel heading is a major influence - most would be greatly exposed to waves for a high incident angle of 30 degrees relative to head seas.
  • Stern area: None of the installations at high risk here.

The largest waves tend to show most green water. Maximum freeboard exceedence occurs at conditions less than the design maximum condition. This suggests that frequency of green water at the bow may be greater than previously imagined, as the wave height and period (showing maximum response) could be closer to one-year return period values than the design (100-year) return period. In other words, some floaters could be at risk for an annual incident.

Incidents

Since 1995, there have been 17 reported incidents of green water affecting UK/Norwegian floaters and two wave slam incidents. Most tend to occur in January and February, however, there may be a substantial number of unreported minor wave incidents. Seven cases affected the bow area. Fewest incidents were reported in the stern area.

The authors suggest that for the earlier FPSOs/FSUs, green water may not have been accorded sufficient attention by either topside or hull designers, and model tests do not always include green water critical wave conditions. However, growing awareness in recent years has led to remedial measures being applied including installation of bow protection structures and side protection walls and re-location of certain other structures.

Operational measures taken include restriction of draft, particularly in winter, and improved heading control to reduce wave incidence angles. However, these and other actions may not be appropriate or possible, depending on the floater configuration. In some cases, increasing bulwarks could adversely affect environmental loads transferred to the mooring system.

Following these findings, a new JIP (joint industry project) has been established named Flow (floater landing by waves), also coordinated by Marin. The aim is to develop guidance, calculation methods, and risk assessment procedures for green water and wave impact loadings suited to each stage of the floater project (concept development, detail design, operation).

The chief deliverable will be a methodology for evaluating wave impact and green water effects on FPSO design. The results should also be of use to oil companies, shipyards, vessel operators, and classification societies. There are 26 participants in Flow, and the project is to be completed in 2002.

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