Allan Magee

Model Test Experience on Vortex Induced Vibrations of Truss Spars

In order to evaluate the Vortex Induced Vibration (VIV) response of truss Spars and to optimize their strake configuration several model test programs have been carried out at MARIN. The results show that it is possible to optimize the strake design of Spars to obtain minimum VIV-response. The results of the model tests also suggest that modeling details, such as appendages, can have an influence on the Vortex Induced Vibrations. In order to reliably predict the fullscale VIV-behavior of the prototype Spar these details must therefore be accurately represented on the model. Furthermore, damping of attached structures such as the truss on a truss Spar can significantly contribute to the reduction of VIV. Loads on such structures have been measured in the model tests. An important aspect that needs consideration in VIV model testing is effect of model scale on the Reynolds number. Roughness can be added to the hard tank of the Spar to minimize scale effects. The paper discusses possible scale effects and the effect of hull roughness on model test results. The repeatability of VIV model tests and reliability of these tests in representing the full-scale situation is discussed. The effect of Spar heading with respect to the current direction as well as current speed will be discussed. Introduction Since 1996 Spars have been used as production platforms in the Gulf of Mexico. Vortex Induced Vibrations (VIV) of the Spar are an important consideration in mooring system design. The Vortex Induced Vibrations of Spars are typically reduced by adding helical strakes to the Spar hull. The effectiveness of the strakes must be verified in the design stage of the Spar. At present numerical tools are not capable of accurately predicting VIV-behavior of Spars. Model tests are therefore currently the most practical method to verify and optimize the strake design. A new development in Spar design is the so-called truss-Spar (Refs 1 & 2). In order to evaluate the VIV-behavior of this type of Spars dedicated model tests have been conducted on several truss Spars. Vortex Induced Vibrations A blunt structure placed in a flow (either air or water) will experience an oscillating force due to the shedding of vortices. This phenomenon is studied and discussed extensively (e.g. Ref. 3). If this structure is able to move in the flow Vortex Induced Vibrations (VIV) can occur. The predominant direction of these motions is transverse to the direction of the flow. Large steady state type oscillations occur when the vortex shedding frequency coincides with a natural frequency of the structure. This is known as ‘lock-in’. For offshore structures these vortex induced vibrations could add to the fatigue damage of mooring and risers, shortening the total fatigue life and also increase the overall drag on the structure. Experience has shown that in offshore structures cylindrical objects such as risers, calm buoys and Spars are most susceptible to VIV, but also other shapes can exhibit VIV-behavior. The vortex induced vibrations for moored Spars are characterized by a number of dimensionless numbers. These are defined as: 1) Reynolds number : Re = UC⋅D/υ 2) Strouhal number : St = fVIV⋅D/UC 3) Reduced velocity : UR = UC⋅TSWAY/D 4) Dimensionless amplitude : A/D = (Amax-Amin)/(2⋅D) Where: UC is the free stream current velocity D is the diameter of the Spar υ is the kinematic viscosity fVIV is the vortex shedding frequency TSWAY is the natural period for sway of the Spar A is the single sway amplitude As the natural period for sway may vary with offset in the mooring system, the actual measured sway period of the Spar during each test is used. The Strouhal number ‘St’ is more or less constant and typically in the range of 0.18 to 0.2 for cylinders, see Ref. 3. The reduced velocity is referred to as ‘true’ reduced velocity, based on mean current velocity in line with the tow direction. The A/D-value is based on sway excursions perpendicular to the direction of incident current. OTC 15242 Model Test Experience on Vortex Induced Vibrations of Truss Spars Radboud van Dijk, Maritime Research Institute Netherlands, Allan Magee, Technip Offshore, Inc., Steve Perryman, BP Americas, Inc., Joe Gebara, Technip Offshore, Inc.

CFD Simulation for Vortex Induced Motions of a Multi-Column Floating Platform

The potential of vortex induced motion (VIM) in multi-column floating platforms such as semi-submersibles and tension leg platforms (TLPs) is well-acknowledged although the industry guidelines for design for VIM are not comprehensive and more research effort is required. Significant VIM in multi-column floating platforms will affect the fatigue life of the steel catenary risers and must be quantified and sometimes reduced. Industry-standard design tools used for drag estimation based on model tests of fixed structures may not accurately reflect the effects of drag augmentation due to VIM. Model tests and Computational Fluid Dynamics (CFD) analysis are feasible methods to investigate VIM, with the latter being more resource-efficient, provided sufficient benchmarking has been carried out to ensure reliable results.Subsequent to the model tests and preliminary Computational Fluid Dynamics (CFD) simulations done for a multi-column floating platform [1, 2], further CFD analyses for the VIM of the floating platform have been carried out using improved simulation techniques with a commercial software. Good agreement between model test results and CFD calculations for VIM of a multi-column floating platform is observed. Sensitivity of CFD results to the modeling assumptions such as mesh size and density, time-step size and different turbulence models is presented.Copyright

CFD Simulation of Flow-Induced Motions of a Multi-Column Floating Platform

Recent improvements in capabilities of both hardware and software allow solving the coupled rigid body motions for the floating platform together with the fluid transport equations. This makes CFD a possible alternative or complement to model tests for predicting VIM performance. In addition, CFD allows simulating certain factors which cannot be addressed in scale model tests, and the two methods can ideally serve as cross-validation tools to bound the remaining uncertainties. Previous applications of CFD to Spar VIM predictions have shown promising results. Building on this, flow-induced motion simulations of multi-column floating platforms are being carried out using CFD as part of the R&D effort within Technip. The purpose of this paper is to present the results of two separate preliminary simulations applied to the prediction of vortex-induced motions of a TLP design and compare to the model test results.Copyright

Analysis of Ringing Response of a Gravity Based Structure in Extreme Sea States

In designing fixed offshore platforms located in regions of severe wave conditions, the potential resonant response of the hull structure due to wave loads must be checked. Since the natural frequency of vibration of the hull structure is typically much higher than the dominant design wave frequency, conventional wave load analysis based on linear wave theory does not show dynamic amplification. However, it is known that steep waves are nonlinear and may contain significant energy at higher harmonics of the fundamental frequency. When the forcing frequency of the higher-harmonic wave load is close to the natural frequency of the structural vibration, a resonance i.e. ringing will occur and the structural dynamic response will be significantly amplified.This paper describes an analysis procedure to estimate high-frequency dynamic load on a Gravity Based Structure (GBS) exposed to severe sea states using Computational Fluid Dynamics (CFD) analysis and modal analysis. To fill the statistical gap between the extreme values from short-duration CFD-modal analysis and that from 3-hour design sea states, an approximation method has been developed to estimate the global dynamic load from the measured quasi-static load in earlier model test and to obtain a calibration factor for the CFD-modal analysis results.Copyright

Tandem riser VIV suppression fairing model test

In deepwater development areas of Southeast Asia, the current is strong and relatively more persistent compared to other deepwater regions. Top tensioned risers (TTR) are critical submerged components of offshore platforms, constantly exposed to currents. These currents cause unsteady flow patterns around the risers i.e. vortex shedding. When the vortex shedding frequency is near the risers natural frequency, undesirable resonant vibration of the riser also known as Vortex Induced Vibration (VIV) occurs. Several types of VIV suppression devices are used in the offshore industry. Among them, the U-shaped fairing claims to have the capabilities of reducing VIV effectively as well as lowering drag loads. This study investigates the effectiveness of a U-shaped fairing in suppressing riser VIV. The model test was successfully performed in a towing tank facility located at Universiti Teknologi Malaysia (UTM), Johor Bahru, Malaysia. This study is a significant collaboration between a local academic institution and the offshore oil and gas industry, aligned with the industrys initiative of increasing local capabilities for research and development. In this study, the VIV of two risers in tandem is simulated using scaled test models. The current flow is simulated by towing the vertically submerged test models with a moving carriage. The riser with fairing models are attached to a pair of custom-designed test rigs which are able to measure the forces and also allow movement of the test model during towing tests. The two test rigs are attached to a steel structure under the carriage which accommodates different tandem riser configurations and spacings. Two different sizes of risers and fairings are tested to check for Reynolds number effects. For each tandem riser configuration, three different riser conditions are tested, i.e. (a) bare risers without fairings; (b) risers with weathervaning fairings, and (c) upstream riser with fairing stuck at different orientations and downstream riser with weathervaning fairing. The test results show significant reduction in drag and VIV for the risers with weathervaning fairings in different tandem configurations. Interesting motion characteristics are shown in some of the stuck fairing cases highlighting the adverse effects should the fairings fail to perform normally in the field. Effective mitigation of VIV in risers using fairing suppression devices could lead to improved riser fatigue life and overall a more economical platform design. These benefits are highly applicable to local deepwater developments for the oil and gas industry.

Vortex Induced Motion of TLP With Consideration of Appurtenances

Offshore floating platform configurations often consist of geometrically simple and symmetrical shapes which are made complicated by the presence of appurtenances such as helical strakes, tendon porches, steel catenary riser (SCR) porches, pipes, chains, fairleads and anodes on the surface of the hull. Previous studies mainly on spars show that these hull external features affect the Vortex Induced Motion (VIM) performance of the platform significantly. This is to be expected since VIM is controlled by the flow separation on the hull surface and the resulting vortex shedding patterns. Scale effects may also play a role in model tests for bare cylinders or hulls with bare cylindrical columns, whereas previous studies have shown less Reynolds dependence when appurtenances are modelled.This study investigates the effect of hull appurtenances on VIM of a multi-column floating platform, i.e. a Tension Leg Platform (TLP) designed for Southeast Asian environment. Significant difference in VIM behaviors is expected between spars and TLPs since the column aspect ratios are very different and TLPs do not have helical strakes that are commonly fitted on spars. Model testing and Computational Fluid Dynamics (CFD) simulation are used in this VIM study, with the former being the emphasis of this paper. Descriptions of the respective experimental and numerical methodologies are presented and the comparison of the results is made. Further work required to improve the model test set-up and the CFD simulation are suggested. From this study, it is shown that the effect of appurtenances on TLP VIM simulation is important and must be taken into account to obtain realistic results.© 2014 ASME

Prediction of Motions and Loads for Floatover Installation of Spar Topsides

There are several substantial advantages to installing integrated topside onto a Spar using floatover method, particularly for large topsides which exceed the single lift capacity of the available heavy lift derrick barge fleet. These advantages include schedule and cost savings for the integration and commissioning of modules on land rather than at sea. Uncoupling the deck fabrication schedules from the availability of heavy lift vessels is another advantage. The performance of a successful floatover installation requires adequate design and analysis of each phase of the floatover installation, and a sufficient weather window in which to perform each phase. Design of floatover installation includes: a) Global motions / mooring analysis to determine motions and loads on mooring lines, fenders, and structural members, b) Structural design including structural integration of the topsides with the barges and shock cell design on the Spar and barges, and c) Operational procedures for mating and barge separation. Validated analysis tools are essential to ensure adequacy in the design of all stages in the floatover operation. This paper presents data from floatover installation model tests, performed at OTRC (Texas A&M University, College Station, Texas, USA), and results from numerical analysis tools for motion and load predictions. The scale of the model tests was 1:60, and the simulated topside was approximately 18,000Te. The simulated environmental conditions included expected upper limit operational sea states for the Gulf of Mexico. The details of the model tests are described in Ref [1]. The analytical challenges related to floatover installation simulations are several and include multi-body hydrodynamics, and prediction of relative motions and interface loads during the mating operations. Available numerical analysis tools include the time domain multi-body proprietary code MLTSIM, and WAMIT, a frequency domain potential code that is widely available in the industry. The validation of MLTSIM involves viscous damping, multi-body hydrodynamic interaction, and simulation of impact forces. This paper presents the results from the validation on the basis of full scale, and quantifies the accuracy of predictions by comparing the measured and predicted motions and loads.Copyright

CFD as a Design Tool for Hydrodynamic Loading on Offshore Structures

Time-domain numerical integration of the rigid body equations of motion is a popular choice for analyzing the global motions of a single or multi-module floating platform. Potential flow theory cannot accurately account for all the hydrodynamic forces on certain components of the platform. However, for practical analysis, these members can be modeled as Morison members in the time-domain simulations. Computational Fluid Dynamics (CFD) can be used to calculate Morison coefficients for the given flow conditions on the exact geometry of the member. This paper presents the results from CFD simulations performed on several individual components of a floating platform (like heave plates, truss members etc.,) in realistic environment conditions. The procedure used for extracting the linear and non-linear coefficients from the total calculated hydrodynamic force is also explained. Results from CFD are compared to existing published experimental results. Differences between full-scale and model-scale results will be emphasized where important. Some of the advantages of using CFD as opposed to model tests are highlighted.Copyright

Model Tests for Floatover Installation of Spar Topsides

There are several substantial advantages to installing an integrated deck on a Spar using floatover installation, particularly for large topsides which exceed the single lift capacity of the available heavy lift derrick barge fleet. These advantages include schedule and cost savings for the integration and commissioning of modules on land rather than at sea. Uncoupling the deck fabrication schedules from the availability of heavy lift vessels is another advantage. The purpose of the model tests described in this paper was to generate data on motions and loads for the operational sea states in the Gulf of Mexico, and to define and validate different approaches of transferring the topsides to the Spar, using a catamaran configuration. The data are intended to (1) demonstrate the feasibility of the installation method for the GOM, and (2) validate Technip’s analysis tools. The model tests were performed at OTRC (Texas A&M) with a model set-up corresponding to a 1:60 model scale. The simulated topsides was about 18,000Te, and Jones Act compliant barges were modeled for the catamaran configuration. The paper will describe the catamaran and spar models, and the instrumentation to measure motions and loads for transportation and installation. It will also describe the shock cell configuration used for the mating operation, and several alternative methods for performing the mating. The environmental conditions tested included several random sea states, harmonic waves, and three headings (beam, head, and quartering seas). Selected data will be shown to demonstrate the range of motions and loads associated with the floatover installation in the GOM. Estimates of limiting sea states for the GOM will be discussed. The validation of the analysis tools is the subject of another paper in Ref [1].© 2010 ASME

Centerwell Water Motions and Hydrodynamic Loading Using Viscous Flow Calculations

Most Spar platforms have a wet centerwell which provides a termination point for umbilicals and risers. The column of water in the centerwell is a dynamic system which can be excited by the wave action around the Spar as well as the platform’s own motions. When the exciting frequencies are close to the natural frequency of the water column, the vertical motion of the water in the centerwell can become large in large seastates. This might damage structures within the centerwell. A natural response to this problem is to restrict the fluid flow at the bottom of the centerwell by adding a plated structure to partially close the opening. The remaining open area in the centerwell determines the amount of damping as well as the loads on the plating which can be quite large in heavy seas. The problem addressed in this paper is the determination of the appropriate open area in the centerwell plate that will control the fluid vertical motion without requiring expensive reinforcements to the plating beyond the riser guide structure already present. Traditional design tools based on potential flow models appear to perform poorly for this problem because they do not model the viscous damping in the flow correctly. In this paper we use a Navier-Stokes solver to study the centerwell motions and centerwell plate loads for three centerwell plate geometries. It is found that the Spar motions and the free surface waves need to be included in these simulations. The centerwell water motions and centerwell plate loads are compared with those measured in a scale model experiment. Full-scale calculations are also carried out to determine the corresponding centerwell plate loads and centerwell water motions to assess scale effects. NOMENCLATURE a = centerwell water motion amplitude (m) h = centerwell water motion height (peak to trough) (m) g = acceleration of gravity A= centerwell cross section area (m 2 ) H = incident wave height, regular waves (m) Hs = significant wave height, random waves (m) L = centerwell height z= centerwell depth from mean waterline (m)