Riser Dynamic Response Research Articles

Dynamic behavior of risers under nonlinear oceanic environmental loading

The riser system acts as the vital link between the subsea blow-out preventer and the drilling platform. Affected by factors like top tension and marine environmental forces, the riser undergoes deformation and wear, carrying the risk of environmental pollution and financial losses upon failure. Hence, this study examines the riser's dynamic response to marine environmental loading. Initially, the motion differential equation for the riser system under the influence of nonlinear oceanic load is deduced using the principle of minimum potential energy and the variational method for extremum seeking. Subsequently, a nonlinear wave-current load model based on the Morrison equation is established, and the resulting equation is discretized into a finite element model using third-order Hermite interpolation function and the Galerkin weighted residual method. Finally, the dynamic response of the riser is scrutinized employing the Newmark numerical integration method. The study also investigates the impact of both oceanic environmental parameters and drilling parameters on the riser’s dynamic behavior. Comparative analysis of the numerical results reveals that the maximum displacement of the riser occurs at the middle section, whereas the maximum deflection angle is observed at the end of the riser. The periodicity of the deflection angle response is influenced by the position of the riser, showing a trend of decreasing and then increasing from the middle section towards the ends. Notably, the top tension and the velocity of the surface tidal current significantly affect the dynamic behavior of the riser. The findings of this study provide a theoretical foundation for the assessment of riser reliability and the determination of operational parameters.

Vortex-induced vibration of risers with staggered buoyancy modules of small aspect ratio

Vortex-induced vibration (VIV) is one of the critical design considerations for the offshore riser and pipeline system. Significant research effort has been devoted to better understanding and predicting riser VIV. Compared to the bare uniform riser configuration, the attached staggered buoyancy modules of a small aspect ratio will extensively alter riser hydrodynamic characters, and hence the riser dynamic response. In the current study, experiments were performed on forced inline (IL)-crossflow (CF) combined vibration of a rigid cylinder with staggered modules (module to bare cylinder diameter and length ratio of 2.5 and 1.0.) as well as self-induced vibration of two flexible models with 100% and 50% buoyancy module coverage ratios. From the rigid model experiment, positive regions for lift coefficient in the phase of velocity (Clv) for both riser and buoyancy module induced vibration are observed. The result indicates potential power-in excitation from both riser and buoyancy modules, which is later confirmed by the bi-frequency and bi-modal vibration from the flexible cylinder experiment. Furthermore, several other phenomena have been reported, including low reduced frequency for buoyancy module induced motion, the phase between CF and IL trajectory, and buoyancy module VIV suppression ability, emphasizing the connection between the rigid and flexible cylinder experiment.

Nonlinear riser-seabed interaction response among touchdown zone of a steel catenary riser in consideration of vortex-induced vibration

Steel catenary riser (SCR) is a preferred solution for deepwater oil and gas exploitation under harsh marine environment. Its riser-seabed interaction (RSI) among touchdown zone (TDZ) in consideration of the vortex-induced vibration (VIV) of sag bend is at present a research frontier in ocean engineering. This paper integrates Randolph-Quiggin model into a published global numerical riser model, to investigate the nonlinear RSI response among TDZ of an SCR with VIV. The adopted numerical model is validated against some published experimental measurements for VIV and RSI respectively. Three combined cases with top-end platform heave motion and VIV are simulated, and the trench development as well as the SCR's response characteristics among TDZ is analyzed. VIV makes the trench develop more smoothly, and the trench length as well as depth turns smaller. The VIV effect on RSI is relatively significant with violent heave excitation, under which the P-z curves present clearer interaction mode transitions. VIV enlarges the maximum bending moment visibly, and makes the sag bend of SCR dominated by higher-order modes, which are both detrimental to the structural safety. The coupling effect between RSI and VIV on the riser's dynamic response needs to be considered for the design of full-scale SCRs.

Structural configurations and dynamic performances of flexible riser with distributed buoyancy modules based on FEM simulations

Flexible risers are usually used as conveying systems to bring ocean resources from sea bed up to onshore. Under ocean environments, risers need to bear complex loads and it is crucial to comprehensively examine riser's configurations and to analyze structural dynamic performances under excitation of bottom vehicle motions, to guarantee structural safe operation and required service lives. In this study, considering a saddle-shaped riser, the influences of some important design parameters, including installation position of buoyancy modules, buoyancy ratio and motion of mining vehicle, on riser's configuration and response are carefully examined. Through our FEM simulations, the spatial distributions of structural tensions and curvatures along of riser length, under different configurations, are compared. Then, the impacts of mining vehicle motion on riser dynamic response are discussed, and structural tolerance performance is assessed. The results show that modules installation position and buoyancy ratio have significant impacts on riser configurations. And, an appropriate riser configuration is obtained through comprehensive analysis on the modules positions and buoyancy ratios. Under this proposed configuration, the structural tension and curvature could moderately change with buoyancy modules and bottom-end conditions, in other words, the proposed saddle-shaped riser has a good tolerance performance to various load excitations.

Non-parametric prediction of the long-term fatigue damage for an instrumented top-tensioned riser

Marine risers are susceptible to sustained vortex-induced vibration (VIV) because of their slenderness and light damping. Commonly used tools for analyzing VIV and the associated fatigue damage are based on the finite element method and rely on simplifying assumptions on the riser's physical model, the flow conditions, and characteristics of the response. In order to assess the influence of VIV and to ensure the integrity of the riser, field monitoring campaigns are often undertaken wherein data loggers such as strain sensors and/or accelerometers are installed on such risers. Given the recorded riser's dynamic response, empirical techniques can be used in VIV-related fatigue estimation. These empirical techniques make direct use of the measurements and are intrinsically dependent on the actual current profiles. Damage estimation can be undertaken for the different current profiles encountered and can account explicitly even for complex riser response characteristics. With a significant amount of data, “short-term” fatigue damage probability distributions, conditional on current, can be established. If the relative frequency of different current types is known from a separate metocean study, the short-term fatigue damage distributions can be combined with the current distributions to yield an integrated “long-term” fatigue damage model, which then can be used to predict the long-term cumulative fatigue damage for the instrumented riser. Non-parametric statistical techniques (that do not assume a specific function for the underlying distribution as parametric techniques do) are employed to describe the short-term fatigue damage data. In this study, data from the Norwegian Deepwater Programme (NDP) model riser experiments are used to demonstrate the effectiveness of empirical procedures and non-parametric statistics applied to field measurements to predict long-term fatigue damage, life, and probability of fatigue failure.

A nonlinear dynamic model for 2D deepwater steel lazy-wave riser subjected to top–end imposed excitations

ABSTRACTDeepwater steel lazy-wave riser (SLWR) has won more and more applications in deepwater oil and gas resources development. The dynamic response induced by the wave, ocean current, ship motion, internal flow and other factors in operation phase has been highly concerned by researchers and engineers. A 2D nonlinear dynamic model for SLWR based on Euler–Bernoulli beam theory with initial conditions and boundary conditions under top excitations considering the effect of ocean current and internal flow is proposed. Keller Box finite difference scheme is adopted to solve the partial differential equations. Numerical analysis is conducted to investigate the influences of tangential and normal excitations on the riser's dynamic response. The proposed model can be a basic reference for the design and analysis for deepwater SLWR.

Experimental Investigation on Vortex-Induced Vibration of a Free-Hanging Riser Under Vessel Motion and Uniform Current

A model test of a free-hanging riser under vessel motion and uniform current is performed in the ocean basin at Shanghai Jiao Tong University to address four topics: (1) confirm whether vortex-induced vibration (VIV) can happen due to pure vessel motion; (2) to investigate the equivalent current velocity and Keulegan–Carpenter (KC) number effect on the VIV responses; (3) to obtain the correlations for free-hanging riser VIV under vessel motion with VIV for other compliant risers; and (4) to study the similarities and differences with VIV under uniform current. The top end of the riser is forced to oscillate or move, in order to simulate vessel motion or ocean current effects. Fiber Bragg Grating (FBG) strain sensors are used to measure the riser dynamic responses. Experimental results confirm that the free-hanging riser will experience significant out-of-plane VIV under vessel motion. Meanwhile, vessel motion-induced VIV responses in terms of response amplitude, response frequency, and cross section trajectories under different test cases are further discussed and compared to those under ocean uniform current. Most importantly, the correlation among VIV response frequency, vortex shedding pairs, and maximum KC number KCmax is revealed. The presented work is supposed to provide useful references for gaining a better understanding on VIV of a free-hanging riser and for the development of future prediction models.

Time Domain Simulation of the 3D Bending Hysteresis Behavior of an Unbonded Flexible Riser

This paper presents a “state-of-the-art” development in time domain dynamic simulation of 3D bending hysteresis behavior of a flexible riser under offshore environment loading. The main technical challenge is to understand and model the riser tensile armor behavior under continuous changes in both the magnitude and direction of bending, and its subsequent impact on the riser’s bending hysteresis characteristics. On account of this technical obstacle, the current industry practice is to model the riser as a linear structure, with certain conservatism enforced, and then to extract the global dynamic loads to a detailed local model for stress and life assessment. Two 3D flexible riser bending hysteresis models developed by Wellstream and Orcina are introduced in this paper, with their calibrations against the bending hysteresis loops measured in full scale tests. Both models are implemented using the analysis program ORCAFLEX. The Wellstream model is a detailed model that calculates both the total bending moment and the stresses in the tensile armor, whereas the Orcina model is a simpler model that only calculates the total bending moment. The study presented illustrates the difference in riser dynamic responses with and without consideration of the bending hysteresis behavior and assesses the difference in dynamic responses between the Wellstream and Orcina 3D bending hysteresis models. This development permits the modeling of more realistic riser structural properties in the dynamic simulation and reports detailed time history stress or strain results for strength components of the riser, and so expands the current practice of riser fatigue analysis, which uses the regular wave approach only, to using an irregular wave approach employing the rainflow counting method.

Deepwater-Riser-VIV Assessment Using Time-Domain Simulation

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18769, "Deepwater-Riser- VIV Assessment Using a Time-Domain- Simulation Approach," by Kevin Huang, Hamn-Ching Chen, and Chia-Rong Chen, Texas A&M University, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May. A computational-fluid-dynamics (CFD) approach was used to assess deepwater-riser vortex-induced vibration (VIV). The studied riser was a top-tensioned riser in 3,000-ft water depth. The proposed CFD approach is feasible for practical riser-VIV assessment. It also is an effective tool to disclose riser-VIV details, provide fundamental understanding and insight to VIV phenomena, predict riser VIV for complex current conditions, evaluate sensitivities including vessel motion coupling and transient effect, and verify riser-VIV design and analysis. Introduction Riser VIV is a design challenge for deepwater applications. Many software tools have been developed to perform riser-VIV analysis. However, most of the tools are based on empirical formulas and rely heavily on model-test data. This approach could provide satisfactory VIV predictions for shallow-water risers with a length/diameter (L/D) ratio that is relatively small, and model tests could be carried out easily to provide input data and/or verification. Deepwater risers are likely to have high-order-mode vibration in strong current. Under such conditions, model tests in a wave tank are difficult, limited either by tank size or model scale, and field experiments are feasible but costly. Further, important characteristics associated with deepwater-riser VIV are not yet studied or understood. Deepwater risers tend to experience multimode vibration; therefore, it would be overly conservative to assume single-mode lock-in. Also, the excited modes in deepwater-riser VIV could be very high; however, higher modes are more sensitive to damping and, hence, demonstrate strong nonlinear behavior. The CFD approach uses an overset-grid technique to handle the data-grid movement caused by riser deflections and vibrations. A typical single-casing top-tensioned riser was sized for 3,000-ft water depth. The riser's dynamic response under different currents was simulated in 3D, and the effects of the riser/fluid interaction were included through instantaneous drag and lift forces. The riser in-line and crossflow responses (including modal shapes and frequencies) and VIV-induced stresses were studied in detail.

Interaction of Riser and Floating Structure

In this paper, special attention is given to the basic assumption frequently used in riser dynamic analysis. This is that motions of a floating vessel can be calculated independently of long line structure dynamics and then motions of a floating vessel can be used in the riser dynamic analysis.Wave actions on a floating vessel are estimated based on three dimensional potential theory and motions of a floating vessel are predicted. Riser dynamic response is theoretically calculated using finite element techniques where wave loads are estimated from Morisons' type equation. Loads from surface vessel displacements are also considered.Finally, comparison is made between experimental results and theoretical calculations and effects of the low frequency motions on riser dynamics are verified. Results in two directional irregular waves are discussed.