FRANCE: Comprehensive structural analysis for ice-prone Terra Nova hull
Petro-Canada's Terra Nova floating prod-uction, storage, and offloading (FPSO) vessel is the first designed specifically for an ice-laden environment. In addi-tion to its quick-disconnect capability to avoid collisions with icebergs roaming the Grand Banks region, special attention was paid to the hull's long-term integrity.
VeriSTAR Hull, Bureau Veritas' design and inspection management software tool, was used to build a model of the vessel, initially to summarize and analyze the "hot spot" areas that needed to be checked. Now that the field is in production, the condition of the hull and its various components has been stored in a database and will be updated continually, in readiness for future inspection programs. Marie-Francoise Renard, Offshore Product Manager at Bureau Veritas, described work to date at IBC's annual Floating Production Systems conference in London last December.
Terra Nova is located in 94 m water depth in the Jeanne d'Arc Basin, 350 km southeast of St. John's, Newfoundland. Crude is being gathered into the vessel through subsea production wells, and offloaded to a shuttle tanker positioned at the FPSO's stern. The facility can operate in a minimum sea temperature of -1.7°C and a minimum air temperature of -17.3°C. Accordingly, the topsides process equipment has been insulated to avoid freezing or wax deposition.
The combination of low temperatures and wind contribute to superstructure ice accumulation on the FPSO. It has therefore been designed to accommodate a deckload of up to 2,000 tons of superstructure icing. An additional 3,300 tons of grade DE steel allows the vessel to withstand impact from an iceberg of up to 100,000 tons, moving at 0.5 m/s. The hull has also been strengthened to permit operations in sea ice. Most of this strengthening is located at the vessel's forward end, due to the system of weathervaning around the geostationary turret. Wing and double-bottom ballast tanks are also reinforced for collision protection. The FPSO is moored by nine chains and is designed for a 20-year production span.
VeriSTAR Hull
The Terra Nova team chose VeriSTAR Hull for the following capabilities:
- A 3D coarse mesh finite element analysis of the entire FPSO hull, and the flare tower
- Fine mesh analysis of critical areas identified by the coarse mesh results, as requested by the team
- Comparison of the hull/flare tower analysis results against design standards for strength and fatigue
- Critical areas highlighted could be add-ressed in the inspection program.
The FPSO's global model covers the entire hull structure from keel to main deck, bow to stern, and includes the main elements of the forecastle and poop structures. Supports for the topsides modules, the flare tower, turret, and other topsides supports have also been modeled to reflect load distribution at the hull attachment.
Local critical areas of the hull modeled in detail include the flare tower connections and supporting structure, the moonpool and turret reaction points, thruster openings and attachments, and crane pedestals.
The coarse mesh model is intended to represent the behavior of the primary structure under cargo and environmental loading conditions. It was built up using an orthotropic shell element, including secondary stiffeners. Global hull cases were agreed between Bureau Veritas and the Terra Nova team. These reflect the operating cycle and lightship, ballast, and scantling draught conditions, and were combined with the mean operating (one-year storm) condition and 100-year storm design conditions.
Around 20 load cases were analyzed, and these were in turn combined with various external loads, factoring in site-specific conditions. Global load cases included static loading of the modules, cranes, thrusters, turret, and flare tower. The flare tower model included member stresses, peak stresses below the flare tower, and stress ranges for the flare tower structure and the hull's flare tower base.
Fine mesh locations were chosen on the basis of the coarse mesh results. The project team opted to perform fine mesh analysis on the flare tower base, moonpool structure, transverse section, and a ring frame amidships. The flare tower base fine mesh was built up in order to check the connection between the flare tower and the hull for various acceleration configurations.
The turret area was also addressed. Due to structural transverse symmetry considerations, only the starboard side was modeled. Main holes and intermediate brackets were modeled with great care. Loads acting on this part of the structure are sea pressure/cargo loads. There are also concentrated loads exerted by the topsides and the forces applied on the upper and lower bearings. The turret's most heavily loaded areas are mainly on the centerline bulkhead, where there are significant shear stresses.
Buckling was identified in a frame of the turret area, along with significant stresses on the second stringer of the moonpool. Stringers were checked thoroughly in several locations - here intermediate brackets and main holes have been refined to check the potential stress concentrations. The fine mesh results pinpointed where high stresses would occur and confirmed the detail structural arrangement would be capable of withstanding the loads. Finally, one complete typical transverse frame was refined to check for potential stress concentrations and local effects under the stool supports. Although stresses increased at the stools, the structure appears strong enough to support the modules.
For each of the tanks, a hot spot map was drawn up showing the most heavily loaded areas that needed to be checked during the inspection campaign. For the flare tower area, high stresses were identified at the connection point of the flare legs on the deck. Also, for some of the stringers, high stresses were identified at the connection with the centerline and the side longitudinal bulkheads.
Since the database has been passed to the Terra Nova team, baseline and inspection data, reports, and photos have been stored within VeriSTAR. A graphical view of each compartment/module can be called up, and the data can be referenced at any time for annual reporting/budgeting purposes. The database will be updated during each inspection campaigns, with calculations re-run on the structural model to analyze the hull's actual situation and to focus inspections on the relevant areas.
