A literature review of experimental and computational hydrodynamic research is presented with the objective of highlighting the parameters that affect riser added mass coefficient (Ca) and drag coefficient (Cd). Typical riser analysis is conducted assuming constant values of hydrodynamic coefficients. However, depending on the flow regime, the hydrodynamic coefficients can vary significantly, and hence using the most conservative values in all analyses can result in overly conservative riser responses. For example, the appropriate coefficients for a steady current on the straked portion of a riser near the surface may be very different from those for small oscillatory motions of the riser near the Touch Down Point (TDP). Several opportunities of reducing conservatism are presented based on using variable hydrodynamic coefficient-based analysis. Hydrodynamic coefficients are dependent on surface roughness, nature of the flow (steady vs. unsteady) and presence of Vortex Induced Vibration (VIV) suppression devices (strakes or fairings), among other factors. Relevant and important bodies of work as documented in papers, test reports or doctoral theses in the public domain are used to determine sensitivity of Cd and Ca values to Reynold's number (Re) and the Keulegan Carpenter (Kc) number. Steady and unsteady flows are treated separately, as are unstraked (bare) and straked pipes. The practice of using constant values of hydrodynamic coefficients in riser analysis is simplistic and often overly conservative. A common practice is to use Cd of 0.7 in bare pipe fatigue analysis as a lower bound. Several tests have demonstrated that Cd can in fact lie in the 1 – 2 range depending on Kc number and Re number. Higher Cd is associated with higher hydrodynamic damping and considerable improvement in TDP fatigue life prediction. Similarly, strake Cd's in low Kc flow have also been observed to be higher than what is used in typical riser analysis, sometimes by a factor of 3 or more. On the other hand, higher Ca values can lead to higher fatigue damage prediction. The correct input of Ca is thus essential to avoid being under conservative. Ca variations with respect to Kc number for both bare and straked pipes are provided in this paper. A refined approach to application of hydrodynamic coefficients in riser analysis is necessary to avoid over or under conservatism. This is especially important in life extension studies where there is little room for additional conservatism. The riser design codes typically do not provide an in-depth review of hydrodynamic coefficients based on factors such as roughness, nature of flow or pipe cross section. This review paper of fundamental experimental and computational work on hydrodynamic parameters is a useful reference to be used in design and life extension assessments.
The objective of the STREAM (Steel Riser Enhanced Analytics using Measurements) JIP is to provide a measurement based foundation for SCR and lazy wave riser modelling to ensure that the fatigue response is assessed with adequate but not overly conservative parameters. To achieve this objective, the JIP utilizes field measurements from 4 in-service SCRs and 1 SLWR in water depths from 3,000 ft to 5,000 ft. The field measurements correspond to a range of environments including hurricanes and loop currents, riser functions, sizes, VIV suppression coverages and host vessels. The processing commences with data QA, error assessment and data filtration. Riser response is categorized into wave dominated events, VIV events and others such as MIV events. As-built finite element riser models are developed and simulations are conducted using measured motions. The resulting analytical responses are compared with the measured motion and strain data to determine the level of conservatism or otherwise in typical riser wave fatigue analysis. SHEAR7 models driven by measured current profiles are used to compare predicted VIV response to observed VIV amplitudes and frequencies. Analysis results indicate that industry standard fatigue assessment is indeed conservative. Sensitivities are conducted and presented on key design parameters that are known to be conservatively used in design such as hydrodynamic coefficients and SHEAR7 inputs. A set of parameters is derived that not only reduces fatigue damage bias but also improves the reliability in predictions. Recommendations are made with regards to further refinement of analysis parameters and understanding of atypical riser responses. Measured riser response that does not conform to typical wave and VIV spectra are presented and discussed. The combined assessment of full scale field data from multiple catenary risers is an industry first. The results from this JIP offer valuable insight into riser response characterization with potential applications for SCR/SLWR life extension and more efficient new designs. Note that in this paper, the term SCR is often used a generic term to describe both the regular SCR as well as the lazy wave implementation of it, the SLWR.
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