Steel Lazy Wave Riser (SLWR) configuration has recently becoming an attractive riser solution for deep water developments. However, SLWR technology is still new, and has limited test data and design experience on the critical riser location that is fitted with buoyancy modules. A comprehensive test on straked riser with buoyancy modules has been planned, designed, and executed in a wave basin. The objective of the model test is to obtain hydrodynamic coefficients for the straked pipe and to better understand the global dynamic behavior of pipe with distributed buoyancy modules due to vessel induced motion and vortex-induced-vibration (VIV)., and thus advance its design. The scopes of the model test are to measure the response of the riser under floater motions; and measure any VIV. In total, five (5) different configurations were tested.
The paper presents the challenges and experiences during the design and execution of this complex model test campaign. The SLWR pipe was carefully scaled and designed to meet the test objectives. The model test setup includes a fully instrumented pipe, a planar motion mechanism (PMM) which simulates the vessel motion. The riser is instrumented with Fiber Bragg Grating (FBG) strain gauges along the pipe length and circumference. Both in-plane and out-of-plane signals were recorded. The drag and inertia coefficients for the straked pipe are calculated using the modal reconstruction method. The response for different pipe and buoyancy modules are also discussed.
Several Steel Lazy Wave Risers (SLWRs) were recently installed and are becoming a popular alternative due to their improved performance [Hoffman, 2010; Lahey, 2013]. However, there are still challenges among the design, optimization, and have very little field experience and data. In 2015, DeepStar® did a model test qualification program on the steel lazy wave riser concept for harsh environment and high vessel motions [Constantinides, 2016]. The test data analysis and validation identified the gaps for SLWR design. One practical issue for the riser design is how to choose the reasonable hydrodynamic coefficients for the straked pipe, as most of risers are installed with strakes to prevent vortex induced vibration (VIV). As demonstrated in the simulation [Cheng, 2016], the standard hydrodynamic analysis coefficients do not produce results that match in-plane response measurements. Both industry and academics have done experiments on the strake to understand the VIV suppression over the years. Among of them, Chung [1994] measured the drag and lift coefficient for different strakes in its ocean mining riser test. Hidetaka [2017] measured the hydrodynamic coefficients for strake using the towing test. Still, there is the need to obtain the hydrodynamic coefficients of strake, particular for the dependence of Keulegan-Carpenter (KC) number. In addition, the SLWR has a middle buoyancy region consists of distributed buoyancy modules, that have a different response from uniform pipe. It's difficult to predict the accurate response for the pipe with buoyancy modules using current riser design software (FLEXCOM, ORCAFLEX), even with advanced CFD tools.
