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Operational experience has shown that flexible risers producing different combinations of oil, gas and water can be subjected to increased dynamic motions due to slugging — a cyclic accumulation of finite volumes of liquids at a low point of the riser (e.g. sag point of a lazy-S riser) until sufficient pressure is built up behind the slug to push the liquids up through the riser. It has been observed that the slug induced dynamic riser motions can cause riser displacements larger than those generated by moderate and some extreme waves in the absence of slugging. A major impact of the slug induced riser motions is the increased fatigue damage of the tensile wires — the cross-sectional component that most frequently defines the fatigue resistance of flexible riser systems. While international standards like ISO 13628-2 & -11 require and recommend that the effects of slug flow on riser response are considered, they provide no guidance on how to practically incorporate potential slugging effects in pipe design or analysis. A methodology has been developed to determine the remnant fatigue life of a riser subjected to slug induced motions combined with the normally considered vessel motions and wave loading. The methodology is based on using commercially available global and local riser analysis tools. The global analysis tool is used to determine the riser response induced by continuous and regular slug loading combined with loading from different irregular waves, vessel offsets and motions. The slug loading parameters are determined through an iterative process calibrating riser displacements and frequencies with those observed in the field. The local analysis tool is used to determine wire stress transfer functions, which in turn are used to derive wire stress time series from the riser tension and curvature time histories. Stress ranges are identified through rain-flow counting applied on all the calculated stress time series and fatigue performance is estimated using the Palmgren Miner summation of damage using an appropriate wire S-N curve. In a case study, the combined slug and first order wave induced fatigue damage increased by a factor of approximately two compared to the wave induced damage alone. This methodology can be used for: a) riser fitness for service assessments by bounding the impact of slug-induced riser motions observed in the field, and b) new riser design when slugging parameters are adequately bounded by flow assurance calculations.
The effects of slowly varying wave drift forces on the nonlinear dynamics of mooring systems have been studied extensively in the past 30 years. It has been concluded that slowly varying wave drift may resonate with mooring system natural frequencies. In recent work, we have shown that this resonance phenomenon is only one of several possible nonlinear dynamic responses of mooring systems to slowly varying wave drift excitation. We were able to reveal new phenomena based on the design methodology developed at the University of Michigan for autonomous mooring systems and treating slowly varying drift as an external time-varying force. In this paper, the U of M methodology is used systematically to reveal seven phenomena induced by mean and slowly varying drift forces; one of those is resonance. Conceptually, numerous qualitatively different behaviors may be induced. The next step toward the comprehensive identification of such phenomena is taken by introducing the method of harmonic balance to study nonautonomous mooring systems.
An empirical time-domain (TD) vortex-induced vibration (VIV) prediction model has been implemented in a software called VIVANA-TD based on its earlier development by Thorsen at NTNU. It models the synchronization of VIV loads and structural responses with a set of empirical parameters generalized from model tests. Combining this time domain hydrodynamic load model with a non-linear finite element structural model makes it possible to account for structural non-linearities and time-varying flow. A joint industry project (JIP), i.e., Lazy Wave Riser JIP has been organized to improve the design basis for SLWRs. This JIP is executed by SINTEF Ocean with support from NTNU. The industry participants are Equinor, BP, Subsea7, Kongsberg Maritime and Aker Solutions. The overall objective of this JIP is to systematically validate VIVANA-TD, in order to establish it as an industrial tool for VIV prediction. It is also aimed to improve the empirical basis and methods for calculation of VIV of deep-water steel lazy wave risers (SLWRs). In the present paper, the validation study is presented for selected model tests in constant flow conditions with uniform and sheared profiles. The test model includes bare pipe, pipe with partial strake coverage and riser model with staggered buoyancy elements. The empirical parameters have been generalized based on extensive model test data. Limitations and improvement of the model have been also been explored. The results show that the present TD model can represent reasonably the VIV loads and that the prediction has good agreement with measurements in general.
Offshore drilling risers, top-tensioned risers and many production risers are top tensioned, connecting the vessel and seabed via joints. External loads such as currents, waves and vessel motions introduce cyclic loads and motions on riser sections, which may shorten the service life due to accumulated fatigue damage. Dynamic responses under combined currents and waves are more complicated than vortex-induced vibrations (VIV) due to pure currents, and it is not fully understood. Several model test campaigns on top-tensioned riser (TTR) have been carried out at SINTEF Ocean (former MARINTEK) during the past decades. Currents, waves and vessel motions were modelled, and the riser model responses were measured. In this study, selected cases from such model tests are analysed, and used to validate a semi-empirical time domain VIV prediction tool — VIVANA-TD. A better understanding of the dynamic responses of TTR under combined currents and waves has been achieved. By comparing the results from numerical simulation using VIVANA-TD and model test measurements, validity and limitation of the time domain tool have been investigated. Important features that need to be considered are discussed. The experience gained from the present study establishes a good basis for VIV and wave load prediction of full-scale TTRs under combined currents and waves where the uncertainty of VIV prediction is further reduced.
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