Because Floating Production, Storage, and Offloading (FPSO) modules experience significant deflections from wave motion as well as hog/ sag, on board piping must be analyzed to assure that it is suitably designed for high cycle fatigue. This is done by keeping accumulated damage to a value less than 1.0 using the Palmgren-Miner rule. In order to simplify the acceptance criteria, a method must be developed to convert allowable accumulated damage into an allowable stress range that pipe stress engineers are accustomed to evaluating. This is done by combining methods from PD5500, DVN publications and the Fatigue Handbook: Offshore Steel Structures Probabilistic Fracture Mechanics; Tapir 1985.
To consider the effects of deck bending and module sway, displacements must be calculated from the naval architects hull data for every restraint in the pipe stress analysis model. Multiple loading cases require this process to be repeated for each loading case being considered.
Most engineering companies have developed a technique for automating the computation of module and/or deck displacements. These values once computed must be entered into the pipe stress analysis software. This task normally requires 8 to 24 hours per calculation depending on size and complexity of the piping system being analyzed. Since the data has been manually entered it must also be checked, which requires another 4 to 12 hours.
If a method can be found to calculate displacements and then automatically load them into the pipe stress analysis software, significant cost savings can be realized through reduced engineering work hours. On a project requiring 100 calculations, the potential savings using a reduction of 25 hours percalculation will be 2500 hours. This will result in a cost savings of $225,000 using $90/ hour as the cost basis. Savings could range as high as $700,000 on large FPSO's. By using the Caesar II neutral file writer to import/ export input data, it is possible to automate this process. An engineer, after entering the piping geometry into Caesar II and assigning restraints at the applicable nodes, can export a neutral file which can be read into displacement generating software (this is a company proprietary tool) where the displacements and rotations are applied to each restraint node. This enhanced neutral file can be re-imported into Caesar II ready for analysis complete with displacements and rotations. The most complex calculations can be processed in less than 30 minutes. By automating this process it may be possible to reduce FPSO pipe stress analysis time by as much as 40%.
No comments:
Post a Comment