Offshore operators are turning to Bureau Veritas for analysis of structural issues associated with FPSOs. "We have worked on a lot of new floater-based developments these past three years," says Franck Legerstee of BV's Marine division, Offshore Project Department, "and on some of these projects, we have been involved continuously for two years."
BV's two major new contracts in this field involve providing classification and verification services for the Plutonio and Dalia FPSOs, operated respectively by BP and Total in Angolan deepwater blocks 18 and 17. Both are newbuildings, with 2 MMbbl storage capacity and an anticipated service life of around 25 years. Other installations classed recently by BV include Cnooc's Panyu FPSO, and the ExxonMobil-owned Chad FSO stationed offshore Cameroon. The group also works with tanker conversion specialists such as Bluewater and SBM, and has participated in several LNG FPSO research programs.
One of its main design tools is VeriSTAR Offshore, an expanding suite of software modules applicable to all types of fixed and floating production and storage structures. If required, the modules can be integrated with each other to varying degrees, combining finite element, hydrodynamic and mooring analysis with risk-based inspection planning, rules checking and database management.
Among the modules most relevant for structural analysis of floating structures are:
- Ariane-3Dynamic, providing frequency/ time domain simulation analysis for all types of shallow and deepwater mooring systems, including SPM, CALM, and spars
- HydroSTAR, a 3D diffraction and radiation program for evaluating complex first and second order wave load impact on the platform hull, and the associated interaction with mooring systems or vessels moored alongside, such as shuttle tankers
- VeriSTAR Hull and New Strudl Offshore (NSO), both used to investigate global and local strength, buckling, and fatigue susceptibility. A database can subsequently be assembled, allowing the operator to perform continuous assessments of the hull's condition during field life, checking these against appropriate strength/fatigue design criteria
- VeriSTAR RBI, which systematically identifies inspection requirements as they arise, recommending action to minimize both associated risks, and the cost of remedial action. This program is suitable for both new and converted FPSOs or FSOs.
VeriSTAR Offshore can also be applied to conceptual engineering studies. This capability was illustrated in two presentations at last November's Deep Offshore Technology Conference in Marseille. One project, in association with ChevronTexaco EPTC, concerned evaluation of tandem berthing availability of a 1.5-MMbbl storage FPSO stationed in 1,000 m of water off Brazil, offloading to Suezmax-type export tankers without DP capability, and assisted by tugs.
The study evaluated two mooring configurations for the FPSO, spread-moored and single-point. For the spread mooring, the vessel's bow was on a northeasterly heading, into the prevailing wind direction, while the single-point mooring was modeled for an FPSO without thrusters. In both cases, the mooring lines were polyester ropes with chain segments at top and bottom. The 80-m-long hawser connecting the vessel with the export was made from superline nylon with chain at either end.
Artist's impression of the Plutonio FPSO.
A response-based approach was employed, involving application of a continuous one-year series of wind, wave, and current. This equated to 2,920 three-hour combinations of conditions. The study also pre-defined safe positions of the tanker relative to the FPSO during approach, offloading, and disconnection.
BV evaluated the two mooring configurations via time domain simulations of the behavior of the FPSO and tanker in tandem under time-varying environmental loads. Hydrodynamic analyses of both vessel's hulls were performed using HydroSTAR Offshore, to derive second-order wave drift load characteristics, first-order wave motion characteristics and the added mass matrix of each vessel. Mooring analyses were conducted using Ariane-3Dynamic to evaluate the low frequency response of the FPSO by numerical resolution in the time domain. Once each time-step of this numerical integration was completed, wave frequency motions were added. Instantaneous tensions were then derived from the tension-offset curves for the mooring system and hawser.
The authors concluded that offloading operations in this case would on the whole be more efficient using a single-point mooring, adding that "simulations of this type need to consider vessel responses on more than a static equivalent basis, for various loading conditions, but still on a time-efficient and accurate basis. Operational simulations that do not rigorously account for vessel motions will give erroneous estimates of system availability."
BV has also worked with Total on a metocean data acquisition program in Angolan offshore block 17, with the aim of transferring the findings to current and future FPSO projects such as Dalia and Rosa. Based on the data – acquired through instrumentation applied to the Girassol FPSO and mooring system – BV is now formulating directional criteria for mooring analysis, in which the combinations of metocean parameters such as swell, wind and currents are defined in terms of both the intensity and relative direction.
Based on this wide-ranging experience, BV is now preparing to issue a new set of rules both for FPSOs and associated offloading buoys.
