As marine riser strings are used in seawater under high varying loads and the fatigue strengths of high tensile strength steels are remarkably lower in seawater than that in air, the most principal performance properties of a marine riser with threaded connection are the seal integrity and the fatigue strength. The optimum designs for the seal integrity against seawater and the fatigue strength corresponding to the required service life are presented by Finite Element Method(FEM) and reliability analysis. As a result of the optimum design, it was proved that the newly developed marine riser connection had the expected high performances. Furthermore, performance evaluations under design loads are discussed by FEM and experiments. The optimum designs of 9 5/8 inch pipe were actually applied to a marine riser connection which was installed in the Gulf of Mexico.
Recently, as offshore oil field developments are moving forward to deep seas. The Jacket-type Platform and Tension Leg Platform(TLP) have been developed as the deep sea offshore production systems. For all the offshore systems, marine riser installed between the upper structure and and the marine riser strings are used varying loads as a consequence of vertical wave motions. Therefore, the most important performance property of a threaded connection is the high fatigue strength. Furthermore, the seal integrity of a connection against seawater is required because the fatigue strength of high tensile strength steels in seawater becomes remarkably lower than that in air. Authors developed a new integral threaded connection which had the high fatigue strength and the stable seal integrity derived from the following optimum design methods for the two performances. The seal integrity of the connection is guaranteed by actual size test specimens and the fatigue strength of the connection is evaluated by a reliability analysis based on the Miner" s rule.
