It has been proofed that 149.2 mm landing string used in ultra-deepwater drilling and completion operations has an advantage over 168.3 mm string in operability and space saving. This paper presents the comparisons between these two landing strings in mechanical behavior. Under the ultra-deepwater conditions, the slip crushing capacity is one of the critical limitations for landing string to run longer and heavier casing to deeper depth. Bending stress is also an important consideration in landing string design especially under the condition of non-riser casing running operations since the landing string will sustain the current force and drilling ship offset. Dynamic load induced by vessel heave may be serious because the self-vibration frequencies will become smaller as the water and well get deeper. Through the comparisons between 149.2 mm string and 168.3 mm string with the same tension strength in their loads and stress it will be helpful to choose a more suitable landing string under ultra-deepwater condition.

The results show that 149.2 mm string has a larger slip crushing load than 168.3 mm string because of its smaller outer diameter. There is no obvious difference between these two strings under the condition of vessel offset. But 149.2 mm string has smaller bending stress than that of 168.3 mm string by the action of current. The axial vibration characteristic of pipe string is related to its elastic modulus, density of material and cross section area, so the self-vibration frequencies and dynamic axial load caused by vessel heave of these two strings are similar. On the whole, 149.2 mm string is more suitable used in ultra-deepwater operations in comparison with 168.3 mm string considering its mechanical behavior.

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