Multi-Tube Model Application in Inner Riser Centralizer Analysis

For top-tensioned riser analyses, the multi-tube modeling scheme is increasingly being used due to the growing application of the concentric dual casing riser systems. One of the major advantages of using the multi-tube modeling scheme over the conventional composite modeling scheme is the multi-tube model’s capability to determine the size and locations of the inner riser centralizers, which are used to prevent the inner riser from buckling when the riser is in production, protect the inner riser and its couplings from rubbing the outer riser, and minimize the contact load between the outer and inner risers. This paper explains the methodology used to perform this unique analysis and introduces the concept of the stress increase factor that should be included in the riser strength check and fatigue life analyses. This paper also demonstrates that misplacing centralizers or improper spacing will increase the contact load at the two most critical regions of the riser system, the stress joint and keel joint, rather than decrease the contact load. Based on the analysis results, guidelines for designing and analyzing centralizers are developed.