Vortex Induced Vibrations of Marine Risers

SummaryThis Master s Thesis goal is to present fundamental physical aspects of Vortex Induced Vibrations (VIV) of marine risers, and outline methods for suppression of VIV. Main emphasis has been given to the use of strakes, and relevant theories connected to strakes influence on excitation of riser, riser response and drag is presented. Variation of outer diameter of the riser is also studied.The theory has been put to test through case studies of two Aker Solutions in-marine workover systems of 321 and 1300 m water depth. The computer program VIVANA has been used. Several analyses have been performed for various different riser configurations. For the 321 m water system, the following configurations have been studied:- Base configuration, i.e. no use of strakes- Staggered configuration, staggered bare and buoyant joints- Bottom strakes configuration, bottom section of riser covered with strakes- Middle strakes configuration, middle section of riser covered with strakes- Top strakes configuration, top section of riser covered with strakesFor the 1300 m water depth system, the following configurations have been evaluated:- Base configuration, i.e. no use of strakes- Staggered configuration, staggered bare and buoyant joints- Middle strakes configuration, middle section of riser covered with strakes- Top 50_150 strakes configuration, top 150 m of riser bare, then coverage of strakes- Top stakes configuration, top section of riser covered with strakesFor each water depth a total of 4 different current profiles have been applied. The current profiles include both measured current profiles from the relevant oil fields, as well as several other more theoretical current models.The main findings from the 321 m analyses were:- Staggered configuration gives generally lower VIV amplitudes of the dominating frequency compared to base configuration.- Staggered configuration gives generally lower maximum stress amplitudes compared to base configuration.- No clear trends when comparing fatigue life of staggered and base configuration are found, however significantly better fatigue results found for the staggered configuration in measured current.- Maximum accumulated damage is located at the WH/XMT interface.- Top and middle strakes configurations give best VIV suppression results.- Applying strakes to the top section of the riser gives significantly lower VIV amplitudes, stress amplitudes and higher fatigue lives across all current profiles.- Top strakes configuration supress VIV completely for the sheared current profile.- Very small riser deflections and corresponding low flex joint angles are found; thus no operational consequences for the 321 m water depth. The main findings from the 1300 m analyses were:- A significant increase in active response frequencies compared to the 321 m water depth, more than 30 active frequencies calculated.- No clear trends in VIV amplitudes of the dominating frequency when comparing staggered and base configuration.- Highest stress amplitudes found for the base configuration in all current profiles.- No clear trends in calculated fatigue life when comparing the staggered and base configurations. Maximum accumulated damage found in the WH/BOP interface.- Top 50_150 strakes configuration the most efficient in suppressing VIV.- Significantly lower VIV amplitudes of the dominating response frequency for the top 50_150 strakes configuration. Same result found for the maximum stress amplitudes.- Compared to the base configuration significantly better fatigue lives found for the top 50_150 strakes configuration, however for the measured current profile an increase of only 1 decade was found.- Staggered configuration gives lowest static riser deflection for all current profiles, also after drag amplification from VIV.- Percentage increase in riser deflection from VIV reduced by roughly 80% when comparing the top 50_150 base configuration to the base configuration.- LFJ angles exceeded lower limits for certain drilling and workover operations, however applying the top 50_150 strakes configuration will generally give a larger operational window compared to the base configuration.Some of the results from the 1300 m analyses revealed certain discrepancies linked to the dominating frequencies and frequencies inducing maximum stress amplitudes. These inconsistencies are probably related to the convergence limit given as input in the VIVANA module