Subsea jumpers are susceptible to in-line and/or cross-flow vortex induced vibration (VIV) fatigue damage due to sea bottom currents. However, there is no proven industry standard design analysis methodology currently available specifically for assessing subsea jumper VIV response. In 2012, ExxonMobil conducted a jumper VIV model test to assess the validity of potential jumper VIV prediction approaches. A towing test rig was used to expose a small scale jumper model to flow conditions simulating uniform bottom currents. The jumper model was instrumented to acquire acceleration, bending strain and end connection load data. Several accelerometers and strain gauges were installed to enable reconstruction of static and dynamic deformations and bending deflections along the jumper model. Towing tests at different orientations and tow speeds were performed on both a bare pipe model and a straked pipe model. The data were analyzed to examine the frequencies and amplitudes of the jumper vibration. The data from these experiments provide a benchmark for validating jumper VIV prediction approaches. In this paper, the model test program is presented including model testing philosophy, jumper design and fabrication, and high level model test results.
ExxonMobil Canada Properties and its co-venturers are building a gravity based structure (GBS) in Newfoundland and Labrador to be installed on the Hebron Field offshore Eastern Canada. This area is characterized by harsh storms with large waves and high winds. The geometry of the Hebron GBS has an effect on the behavior of the incident waves with regards to their likelihood of breaking onto the shaft. Model tests of the structure in storm waves were executed to provide local wave impact load data on the shaft of the GBS. These tests required significant planning and design of the model, environment, and instrumentation in order to properly satisfy the test objectives. The results of the test showed that the measured wave impact loads on the structure were highly variable, requiring a long-term, response based method to quantify the design loads on an annual exceedance basis. In this paper, we discuss the salient aspects of the model testing effort and the long-term analysis approach which was utilized to define the wave impact loads that were incorporated into the Hebron GBS structural design.
Platform Vortex Induced Motion (VIM) is an important cause of fatigue damage on risers and mooring lines connected to deep-draft semi-submersible floating platforms. The VIM design criteria have been typically obtained from towing tank model testing. Recently, computational fluid dynamics (CFD) analysis has been used to assess the VIM response and to augment the understanding of physical model test results. A joint industry effort has been conducted for developing and verifying a CFD modeling practice for the semi-submersible VIM through a working group of the Reproducible Offshore CFD JIP. The objectives of the working group are to write a CFD modeling practice document based on existing practices validated for model test data, and to verify the written practice by blind calculations with five CFD practitioners acting as verifiers. This paper presents the working group’s verification process, consisting of two stages. In the initial verification stage, the verifiers independently performed free-decay tests for 3-DOF motions (surge, sway, yaw) to check if the mechanical system in the CFD model is the same as in the benchmark test. Additionally, VIM simulations were conducted at two current headings with a reduced velocity within the lock-in range, where large sway motion responses are expected,. In the final verification stage, the verifiers performed a complete set of test cases with small revisions of their CFD models based on the results from the initial verification. The VIM responses from these blind calculations are presented, showing close agreement with the model test data.
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