Complexities of Annulus Line Interaction for HPHT Open Water Intervention Risers

Abstract Open water intervention riser systems (OWIRS) are used for performing intervention/workover activities on subsea wells during production or for end of life activities. The OWIRS production bore access typically consists of a main riser tubular with an annulus line tubular clamped to the main riser pipe. Typically, the top and bottom connections are rigid clamps, with strategically placed polymer clamps along the length. The clamps prevent the annulus line from buckling and also manage the span on the annulus to prevent vortex induced vibrations. The objective of this study is to present the challenges and complexities of specifying the annulus line clamp placement. This paper outlines the approach and methodology used for defining annulus line clamp placement. The assessment encompasses load distribution calculations and load sharing between the two pipes for both installation and connected scenarios. A step by step approach is presented with an emphasis on load calculations from self-weight friction from clamp preload, temperature and pressure effects, bending loads from global vessel offsets and environmental loading. Load calculation and buckling assessment results from this study will be presented. The findings from this study will demonstrate how the installation stage can be more critical for the annulus line compared to the in place connected scenario. Furthermore, a sensitivity analysis is conducted on the friction effects of the clamp and the load sharing attributed from the clamp torque to illustrate its contribution. This paper will present a comprehensive methodology and load case matrix for establishing clamp placement and clamp design requirements for a high pressure high temperature intervention riser system. The risk of improper design will also be discussed and the importance for HPHT applications highlighted. While the methods presented here are applicable to all types of intervention risers with annulus lines, this study will be especially useful for HPHT systems which are more sensitive to pressure and temperature effects.