ssc - 419 supplemental commercial design guidance for fatigue ship ...

Operational EnvironmentsTable 3.1 Wave Height Probabilities for Different Operating Areas (Glen et al., 1999)NATOSea StateNorth AtlanticCalifornia to Alaska(TAPS Trade)Eastbound Westbound Northbound Southbound1 0.0148 0.1005 0.0484 0.11202 0.0620 0.1128 0.1258 0.15373 0.1906 0.1803 0.1928 0.18804 0.1804 0.1525 0.1644 0.14725 0.2526 0.2667 0.1999 0.18866 0.2996 0.1818 0.2679 0.20707 0.0000 0.0054 0.0013 0.0035In special cases where a ship is to operate only in benign conditions, reduced loading canbe used for fatigue assessments. In most cases, the fatigue assessment performed during designrepresents an owner’s requirement so that maintenance costs can be reduced. A fatigueassessment is used to increase scantlings and fatigue classifications of structural details above theminimum rule requirements, and the extent of such increases is often a decision made by theowner, not the classification society. One area where the classification societies reduce theloading requirements is in the number of fatigue cycles assumed during the lifetime of the ship.For fatigue analysis, ABS assumes that tankers operate 100 percent of the time during a 20-yearlifetime. For bulk carriers and containerships, a 70 to 80 percent operability over a 20-year lifeis assumed. ABS also assumes that the ship will take headings relative to waves of equalprobability, and that the ship will be operated at 75 percent of maximum ship speed.The actual operating conditions of ships may vary from the assumptions made byclassification societies or military design authorities. In a study made for the Ship StructureCommittee (Glen et al., 1999), information was gathered from commercial ship owners and theU.S. Coast Guard on actual operational conditions that ships encountered over a period of time.This information is somewhat limited in that the commercial ship data was limited to 3 shipsoperating over an average of 1.7 years for a total of 5 ship years of operation. It should thereforenot be considered as typical for all commercial ships, only indicative of what operational profilesmight actually be. The report includes data from a high-speed container ship operating on aregular route in the North Atlantic, a tanker operating between California and Alaska, a trampbulk carrier, and a U.S. Coast Guard cutter. In all cases, it was shown that an assumption ofrandom speeds and headings relative to the direction of waves in different sea states is not valid.However, the relationship between speed, heading, and sea state varied depending on the shipsize and type. The report provides such probabilities for the ships analyzed, but does notgeneralize the results for use with other ships. Most importantly, the report did not assess thedifference in fatigue life prediction that results from the use of specific operational profilescompared to random operational profiles.The percent of time at sea of the four ships studied is shown in Table 3.2. Also shown inTable 3.2 is data on the operation of 86 combatant ships of the U.S. Navy. This data was taken3-2