Flexible pipe with staggered buoyancy elements such as lazy wave riser and drilling riser has been widely used in ocean engineering. Under the influence of sea current, both of the buoyancy elements and the riser may experience vortex induced vibrations (VIV). However, when VIV occurs, hydrodynamic characteristics of the buoyancy elements and its influence on hydrodynamic force of the bare pipe still need investigation. The purpose of this paper is to reveal the hydrodynamic characteristics of flexible pipe with staggered buoyancy elements undergoing VIV. The crossflow (CF) hydrodynamic coefficients of the flexible pipe with 25%, 50%, and 100% coverage of staggered buoyancy are obtained from model tests, using hydrodynamic forces and coefficients identification method. Then, the characteristics of added mass coefficients and excitation coefficients in CF direction are analyzed. The results show that the added-mass coefficients of bare pipe are relatively larger than those of buoyancy module, while the total mass per unit length (sum of structural mass and added mass) is consistent along the pipe. Similarly, the range of excitation coefficient on the buoyancy elements is smaller than that on the bare pipe, and their ratio is equal to the reciprocal of diameter ratio 2.5.
Abstract Vortex induced vibrations (VIV) have been subjected to extensive studies, but most of the research focuses on VIV in a constant flow. However, slender marine structures are often experiencing complex environmental conditions. As an example, the drilling riser may be subjected to vessel motions at the top end of the riser, wave loads, in addition to ocean currents. The consequence of the top motions is that the relative flow speed field is changed especially close to the top end of the riser. VIV responses can be significantly affected if the relative flow speed induced by top motions is in the same order as the current speed, which are further affected by the direction of the top motion relative to the current direction. The influence of top motions on VIV responses is less understood. Such effects are also neglected in the present design practice based on frequency domain prediction tools, which can not combine two simultaneously acting load processes. This has been investigated in the present work based on the analysis of laboratory test data and numerical case studies with a time domain VIV prediction tool. The results showed the importance of including top motions in a time domain VIV analysis.
The motivation for this study is the fast development of floating offshore wind energy and the immature methodology and engineering practice related to predictions of vortex-induced motions (VIM). Benefiting from the oil and gas industry, in the past several decades, extensive knowledge and experience on vortex-induced vibrations (VIV) on slender marine structures has been gained. As the learnings from these efforts should be transferred and adapted to the renewable energy industry, a state-of-the-art review on influential VIM research has been carried out in this paper, focusing on: (1) engineering practice, (2) model tests, (3) numerical calculation, and (4) field measurement. Engineering gaps and potential research topics are identified as future work.
When predicting slowly varying resonant vessel motions, a realistic estimate of the motion damping is crucial. Mooring line damping, which is mainly induced by the drag force on line, can dominate the total damping of catenary moored systems and methods for predicting mooring line damping are therefore required. Based on a simplified dynamic model of mooring line tension, an approach to estimate the corresponding damping is presented in this paper. Short-term time domain simulations of dynamic line tension are carried out to verify the accuracy of the simplified frequency domain approach. Compared with the simulation results, the practical simplified method proposed herein gives a maximum 30% lower prediction of the damping coefficient of each mooring line and an about 20% smaller estimate of the total line damping and therefore yields conservative estimates of the low frequency vessel motions.
Steel Lazy Wave Riser (SLWR) is an attractive deep water riser concept. When subjected to vortex induced vibrations (VIV), the vortex shedding process of the buoyancy element and the bare riser section will be different due to the difference in diameter. VIV responses can be strongly influenced by the dimension of the buoyancy element and its arrangement. Empirical VIV prediction programs, such as VIVANA, SHEAR7 and VIVA, are widely used by the industry for design against VIV loads. However, there is lack of hydrodynamic data to be used in these programs when buoyancy elements are present. Experiment to obtain hydrodynamic data for riser with staggered buoyancy elements was carried out in the towing tank in SINTEF Ocean. A rigid cylinder section with three staggered buoyancy elements was subjected to harmonic forced cross-flow (CF) motions. Hydrodynamic forces on one of the buoyancy elements were directly measured in addition to the measured forces at both ends of the test section. Two buoyancy element configurations were tested and the corresponding hydrodynamic data are compared with that of a bare cylinder. The obtained hydrodynamic data was also used in VIV prediction software and good prediction against existing flexible cylinder staggered buoyancy element VIV test data was achieved. A roadmap to achieve an optimal SLWR design by combining different experimental and numerical methods is suggested.
This paper present results from numerical simulations and laboratory experiments investigating the flow around a riser with fairings at Reynolds number of 5000. We present fully resolved direct numerical simulations (DNS), large eddy simulations (LES) and compare the results with flowfields obtained from particle image velocimetry (PIV) experiments in a circulating water tunnel. The DNS and LES results do agree very well on surface integral quantities such as drag and lift force, but there are discrepancies in first order statistical flow parameters such as recirculation length. This indicate that a comparison of force coefficients is not sufficient to validate this type of bluff body wake flows. Comparing the simulation data with the experimental PIV data, also reveals significant discrepancies in the mean flow field, although the Strouhal number agrees between DNS and PIV results.
For deep and ultra-deepwater applications, synthetic fibre ropes are considered an enabling technology due to their higher strength-to-weight ratio as compared to steel wire ropes and chains and due to their superior station-keeping performance. The advantages of synthetic fibre rope mooring systems include: • A higher floater payload and reduction in structural costs due to lower vertical load from mooring lines. • A reduction in vessel offsets and associated riser loads due to taut mooring system. • A potential reduction in installation costs due to lighter installation and handling equipment. • Superior endurance under cyclic loading compared to steel moorings. Synthetic fibre ropes have visco-elastic stiffness and stretch characteristics. The change-in-length response of a fibre rope is non-linear, load-path dependent (different unload-reload stiffness), and the length varies with the rate and duration of loading (due to elongation and contraction). The commonly accepted analysis approach is a simplification where a lower-bound and an upper-bound stiffness is used. This practice is primarily based on two factors: 1. The industry at large does not at present have a common, well-defined understanding of fibre-rope change-in-length performance. 2. There is a lack of commercially available mooring analysis programs with the capability to simulate the non-linear change-in-length response of the synthetic fibre rope. Individual designers may however have more advanced analysis procedures, but these are not commonly accepted yet. This paper presents results from the Syrope pilot study, Ref. /5/ and /6/, which has used rope testing to determine the characteristics of the elements in the spring-dashpot model. On this basis a strategy for software implementation in the frequency-domain has been proposed. A case study was performed for a semi-submersible production unit in deep water and harsh environment. The paper focuses on the differences between a commonly accepted, hereafter called traditional analysis approach and the proposed new frequency domain approach. The results show that there are large differences in extreme tensions and offsets as well as fatigue results. Hence, the new approach is considered to represent a significant improvement.
Abstract Hydrodynamic force coefficients are important parameters in the design and assessment of marine risers. The hydrodynamic coefficients are widely used for assessing marine riser responses due to floater motion excitation and vortex-induced vibrations (VIV). Traditionally, the hydrodynamic coefficients have been obtained from physical model tests on short rigid riser sections. Recently, the offshore industry has started to use Computational Fluid Dynamics (CFD) analysis for predicting the hydrodynamic coefficients, due to the recent advancement of CFD software and high-performance computing capabilities. However, a reliable CFD modeling practice is required for CFD analysis to be a more widely accepted prediction tool in the industry. A joint industry effort has been initiated for developing and verifying a reliable CFD modeling practice through a working group of the Reproducible Offshore CFD JIP. Within the working group, a CFD modeling practice document was written based on existing practices already validated with model test data and verified by blind validations with three CFD practitioners. The first year work focused on a bare riser with circular cross-section and has been published in OMAE 2021. This paper presents the working group’s second-year verification activities for a staggered buoyancy module and a straked riser. The verification work covers three numerical test problems: 1) stationary riser in steady current, 2) riser under forced-oscillation in calm water, 3) riser under forced-oscillation in steady current. In the stationary riser simulation, drag coefficient and lift coefficient from verifiers are compared. In the forced-oscillation simulation in calm water, the fully-submerged riser section oscillates with a sinusoidal motion, and damping and added-mass coefficients are compared. In the forced-oscillation simulation in steady current, where the riser oscillates in either inline or perpendicular direction to the steady current, lift coefficient and added mass coefficient are compared. By following the modeling practice, the CFD predictions are consistent with each other and close to the model test data for the majority of the test cases.
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