bluuebunny/crossref_metadata_2025 huggingface dataset:
A Generic Study on Lowering Analysis of Massive Jumper Structures
Authors: Sun Guomin , Wang Facheng , Zhang Yigong , Tang Ke , Wang Jun
Published: October 2015

Lifting activity has been usually employed to deploy subsea equipment, especially for huge structures. In one target field of China South Sea, there was one 3D “M” shaped, long and massive jumper being lowered to 200 meter water depth. The jumper was nearly 90 meters long, 22 inches in diameter, deployed by using a spreader frame. The total weight of jumper and frame has been up to nearly 400 Te in air. The whole lifting system mainly includes spreader frame, jumper, two connectors, wire rope and serial slings, etc. Two connectors were welded on both ends of the jumper, and jumper was lifted under spreader by several slings. The deployment operation was proposed by deepwater pipelay crane vessel Hai Yang Shi You (HYSY) 201. To ensure a successful installation, COTEC Offshore Solutions, together with its mother company, China Offshore Oil Engineering Company, have developed an advanced analysis by considering practical offshore field procedure and accurate modeling technology.

In this paper, firstly, wet lift capacity of wire rope and natural period of the hosting system have been obtained for certain circumstances including axial resonance caused by hostile environment. In addition, lowering though the splash zone operation procedure was simulated by OrcaFlex 9.8a. Allowable Max crane tip tension force, Dynamic Amplification Factor (DAF), Min wire rope force, Max Cranemaster® stroke motion and heave velocity at jumper were obtained during the numerical analysis. Also, allowable values of these key factors are further studied.

According to the above analysis, COTEC provides a series of environmental parameters such as the optimum wave direction and period for the offshore lifting operation.
Wave Slamming Model Test Data and Analysis for a Spar Hull Design
Authors: Koo Bonjun , Lim Ho-Joon , Sablok Anil , Lambrakos Kostas , Dalane Oddgeir
Published: October 2015

This paper presents wave slamming loads on the Aasta Hansteen Spar from model test data, and it discusses analysis methodologies for extreme loads in order to derive slamming design pressures for the local and global design of the Spar hull.

A 3×3 array of slamming load transducer panels was employed to measure the horizontal wave impact loads for the 100-year and 10,000-year storms at the Aasta Hansteen field in the Norwegian Sea. The wave height and period for these storms were varied to investigate wave steepness effects on slamming loads. Three-hour simulations and as many as 20 realizations per sea state were used to capture the statistics for the slamming loads.

Gumbel distribution was used to derive extreme pressures for the local and global design at the measured direction by using various panel combinations. The short term target percentiles used in the Gumbel distribution were determined by long term analysis. The detailed long term analysis results are the subject of another paper [1].

The main objective of this study is to establish the extreme pressure distribution along the length of the Spar above the mean water level (MWL). The linear correlation was found between the slamming pressure coefficient and incident wave steepness and it was used to obtain the extreme pressure profile for other wave directions around the Spar hull.

The methodology presented in this paper can also be applied to slamming pressure of other platforms/floaters.
Validation of Computational Fluid-Structure Interaction Analysis Methods to Determine Hydrodynamic Coefficients of a BOP Stack
Authors: Venuturumilli Raj , Tognarelli Michael , Khanna Samir , Triantafyllou Michael
Published: October 2015

Drilling riser systems are subjected to hydrodynamic loads from vessel motions, waves, steady currents and vortex-induced motions. This necessitates a proper structural analysis during the design phase using techniques such as finite element analysis (FEA). Common approaches within the FEA packages approximate the individual components including BOP/LMRP (Blow-Out Preventer/Lower Marine Riser Package), subsea tree and wellhead using 2D or 3D beam/pipe elements with approximated effective mass and damping coefficients. Predicted system response can be very sensitive to the mass, hydrodynamic added mass and drag of the large LMRP/BOP/Tree components above the wellhead. In the past, gross conservative estimates on the hydrodynamic coefficients were made and despite this, design criteria were generally met. With the advent of large sixth-generation BOP stacks with the possibility of additional capping stacks, such approximations are no longer acceptable. Therefore, the possibility of relying on the more detailed capability of computational fluid-structure interaction (FSI) analysis for a better calculation of these coefficients is investigated. In this paper, we describe a detailed model developed for a 38:1 scaled down BOP and discuss the subsequent predictions of the hydrodynamic coefficients. The model output is compared against the data from the concurrent tests conducted in an experimental tow tank. The comparison demonstrates that computational FSI can be an effective and accurate tool for calculating the hydrodynamic coefficients of complex structures like BOPs.
Numerical Analysis of Experimental Data of Subsea Jumper Vortex Induced Vibrations
Authors: Zheng Haining , Slocum Scott T. , Huang Jerry Z. , Srivastava Vikas , Lee Sungho , Wang Howard H.
Published: October 2015

Vortex Induced Vibrations (VIV) may cause fatigue damage to subsea jumpers that are exposed to bottom currents. ExxonMobil Upstream Research Company (URC) has been conducting research on VIV of subsea jumpers since 2011. Model tests conducted on an “M” shaped jumper in 2012 showed that VIV can occur in subsea jumpers over a wide range of bottom current speeds and directions [1]. Presently, there is no well-established industry practice for assessing subsea jumper VIV and determining the need for its suppression. In this paper, we present two different methods for characterizing jumper VIV response based on the model test data described in [1]. Specifically, these methods consist of spectral analysis of local response and modal scalar analysis of global response. These methods are used to analyze measured response over a wide range of towing speeds and towing directions. A brief summary of the findings are provided and some general conclusions are drawn.
The Effects of Mixing Helical Strakes and Fairings on Marine Tubulars and Arrays
Authors: Allen Don W. , Lee Li , Henning Dean , Liapis Stergios
Published: October 2015

Most deepwater tubulars experiencing high currents frequently require vortex-induced vibration (VIV) suppression to maintain an acceptable fatigue life. Helical strakes and fairings are the most popular types of VIV suppression devices in use today.

It is quite common to use only one type of device (helical strakes or fairings) on a single tubular and, in fact, to use a single device type on an entire tubular array. The use of both styles of suppression devices on a single tubular has grown in popularity, but mixing them within an array is a relatively new concept. It is sometimes desirable to use one suppression device on one tubular and another suppression device on an adjacent or tandem tubular.

This paper utilizes results from two different types of VIV experiments. The first consists of a long tubular at high Reynolds numbers with VIV suppression on the outer end where current speeds are the highest. The use of only fairings, only strakes, or a mixture of the two devices is examined.

The second VIV experiment examines the use of helical strakes on one tubular and fairings on a tandem tubular. Results are compared to experiments with either helical strakes on both tubulars or fairings on both tubulars. This paper is intended to provide some direction, and in many cases assurance, for mixing helical strakes and fairings on deepwater tubulars.
Shallow Water Current Loads on a LNG Carrier Using CFD
Authors: Koop Arjen
Published: October 2015

To determine shallow water effects on current loads for an LNG Carrier, CFD calculations with MARIN’s CFD code ReFRESCO have been carried out. The CFD results are compared to model tests carried out in MARIN’s Shallow Water Basin for the HAWAI JIP. The loads have been determined for three different water depth to draft ratio’s, from relatively deep water to shallow water. For all water depths the difference in CY coefficient between the CFD results and experiments is 5–10%.

Furthermore, the blockage effects from basin side walls are investigated. For shallow water it was found that the blockage effects are significant, i.e. 30–50%, and that they vary for different current headings. For deeper water the blockage effects are smaller than 5%.

The influence of “towing” versus a “current” flow situation is also studied. In the experiments the model is towed through the basin leading to a relative velocity between the model and the basin floor. It is found that for shallow water the results for the “tow” situation are 10% lower than for the “current” situation.

Lastly, preliminary results for full scale are presented. At full scale the current coefficients are found to be lower than at model scale. However, more investigations should be carried out at full scale to be able to draw definite conclusions.

From the results presented in this paper it is concluded that shallow water effects on current loads can be accurately obtained with CFD. Furthermore, blockage effects and the influence of “towing” versus a “current” situation have been quantified and a first study into scale effects has been presented.
Numerical Study of Three Dimensional Effects of Wave Impact on an Oscillating Wave Surge Converter
Authors: Wei Yanji , Dias Frederic
Published: October 2015

A CFD model to study the three-dimensional (3D) effects of wave impact on an Oscillating Wave Surge Converter (OWSC) has been demonstrated. Considering the excessive computational cost of CFD models, a relative small computational domain is used here. The velocity at the outside boundary is prescribed based on the classical non-linear wave theory. A relaxation zone is applied to absorb the diffraction/reflection waves by the device to avoid the re-reflection from the outer boundary. This zone is implemented by adding momentum source terms in the N–S equations to blend the near field flow into the far-field wave environment. The simulation of wave interaction with a fixed flap is performed to demonstrate the validity of the relaxation zone. Simulations of wave interaction with an OWSC by various wave conditions (different wave heights and incident angles) are then carried out to understand the 3D effects of the wave impact. The water elevation in the simulation is in agreement with the observations in the experiment. The variation of the pressure distribution indicates that the wave impact is enhanced at the centre of the flap, due to the water re-entry from the sides of the flap into the centre in 3D tests.
Ice Loading Decrease at Life Cycle Main Stages for Fixed Offshore Structures
Authors: Shaposhnikov Valery M. , Aleksandrov Anatolii V. , Litonov Oleg E. , Platonov Viktor V.
Published: October 2015

At the present time design values of ice loads on fixed offshore structures are rather conservative. Conservatism of design ice loads consists in assuming the most unfavorable ice action direction and the worst ice drift speed; the most unfavorable combination of the consolidated layer thickness, ridge keel depth and ice strength; as well as supposing the ice ultimate strength value constant along the whole ice–structure contact area perimeter.

With accumulation of the knowledge on ice formation failure under interaction with ice-resistant fixed platforms, the requirements contained in Rules of classification societies are reduced. For example, for the last forty years the lowering of requirements to design ice load values was equal to about four times [1].

For the last time specialists of Krylov State Research Centre have performed design and experimental studies where further tendency to decreasing design values of ice loads is traced. Ice monitoring is one of the main elements for justification of design ice load values’ decrease.

Modern monitoring systems permit to warn about occurrence of a state close to a limit one, as well as to record actual ice loads. Ice load monitoring is a necessary part of accident prevention during ice-resistant structures operation. Monitoring of ice loads is a necessary part for providing safe operation of ice-resistance structures, and systematic accumulation of monitoring data for several years gives a positive effect in the form of justified decrease of static and dynamic design ice loads.
Evaluation of the CSR-H Sloshing Criterion Using Direct Fluid Structure Calculation
Authors: Wang Po-Wen , Lee Chi-Fang , Quéméner Yann , Huang Chien-Hua
Published: October 2015

The objective of this study was to clarify the theoretical basis of sloshing loads and required plate thickness formulations in the harmonized common structural rules. This study used computational fluid dynamic (CFD) to calculate sloshing loads and used finite element analyses (FEA) to evaluate structural response. The sensitivity of the CFD predictions to the time step and grid size was also investigated. Cargo oil tanks were then selected in a handy size oil tanker and a very large crude carrier to evaluate the longitudinal and transverse sloshing loads on the tank boundaries. The results showed that the sloshing pressures computed at four filling levels were mostly consistent with CSR-H. Afterward, the sloshing pressure produced by CFD was applied to the finite element model by using a fluid-structure interaction technique to obtain the dynamic response of the structure. The dynamic responses were investigated to validate the quasistatic approach for sloshing assessment.
A Parametric Sensitivity Study on TLP Hydrodynamic Performance in South China Sea
Authors: Yang Xiaolong , Shen Hui , Li Hui , Qi Xiaoliang , Lv Guosen , Su Zhiyong
Published: October 2015

As a typical floating platform, TLP (Tension leg platform) is widely used around the world of the water depth from 140m to 1400m due to its significant advantages over other deep-water production systems. This paper describes a conventional TLP type application for relatively shallow water depth of 410m, it was studied for a potential development plan in China south sea. Considering the safety and efficiency in the construction, installation and operation, a comprehensive sensitivity study of the simulation results against various analysis/environment parameters (including column form, wind coefficients, tendon pretension, etc.) was carried out and the role of each critical parameter was analyzed deeply. The numerical simulations were conducted for 1000-yr Hurricane condition with collinear wind, wave, and current directions. The results based on this study show that the change of the column pattern (from round column to square column) does not have much difference on the TLP hydrodynamic performance without other conditions changed; the variation of pre-tension has limited influence to the TLP hydrodynamic performance as this relatively shallow water.
Modeling Turbulent Flow of Non-Newtonian Fluids Using Generalized Newtonian Models
Authors: Bizhani Majid , Kuru Ergun
Published: October 2015

Computational Fluid Dynamic (CFD) is used to model turbulent flow of non-Newtonian polymeric fluids in concentric annulus. The so called Generalized Newtonian Fluid (GNF) approach is used. Four turbulence models are tested. Applicability of each model in predicting turbulent flow of non-Newtonian fluids in annulus is assessed by comparing results of pressure loss and or velocity profiles with experimental data.

The first tested model is a modified version of Lam-Bremhorst k–ε turbulence model. The modification was originally developed to model flow of power law fluids in smooth circular pipes. Results of simulation study showed that this model significantly overestimates the pressure losses.

Two k–ε closure type turbulence models, one developed to model turbulent flow of Herschel-Buckley and the other for power law fluids, are shown to fail in predicting turbulent flow of polymer solutions. One of the models contains a damping function which is analyzed to show its inadequacy in damping the eddy viscosity.

The last tested model is a one layer turbulence model developed for predicting turbulent flow in annular passages. The model has an adjustable parameter, which is shown to control the slope of velocity profiles in the logarithmic region. It is demonstrated that if the model constant is selected carefully, the model accurately predicts pressure loss and velocity profiles.
Experimental Study on Hydrodynamics of a FPSO in Deepwater Gulf of Mexico
Authors: Lopez Jaime Torres , Tao Longbin , Xiao Longfei , Hu Zhiqiang
Published: October 2015

The scaled model testing of a FPSO with its mooring lines and risers for deepwater and ultra-deepwater installation sites is considered to be the most reliable methodology to study the complex hydrodynamic behaviour of the complete system, since it can represent the nearest real environmental conditions and the comprehensive hydrodynamic interactions of the waves, current and wind with the total floating production system. The best technical option, at the present time, is a combination of an appropriate scale model of the FPSO and a suitable level of truncation obtained using a hybrid passive truncated experimental methodology for the mooring lines and risers. This is in order to minimize the various uncertainties in model behaviour and to study the hydrodynamic behaviour of the complete floating system and thus to validate numerical design of prototype systems for installation in deepwater and ultra-deepwater locations. This paper investigates the global response of a specific FPSO to prevailing environmental conditions, based on a hybrid passive truncated experimental methodology for the mooring lines and risers in a specific deepwater location with a water depth of 1000m in the GOM. The main objective of the experiment is to examine the nonlinear effects of the FPSO vessel and its mooring lines and to provide reliable experimental data for subsequent extrapolation to a full scale prototype design. Several case studies were carried out. The FPSO global responses for Full load and Ballast Load conditions with static and dynamic load tension components of the truncated mooring lines were studied for both collinear and non-collinear extreme storm environmental conditions. The experiments revealed that the main horizontal plane motion response of the FPSO (surge) under non-collinear loading condition is almost two-times that of the collinear loading condition.
A Numerical Study on Motion Responses of a Mat-Support Jack-Up During Positioning
Authors: Zhang Zhaode , Wang Yuhong
Published: October 2015

The motion response of a mat-support jack-up during positioning is studied in this paper using numerical analysis software SESAM. In the process of jack-up positioning, the square bottom mat is gradually lowered and the floating jack-up, secured by anchor chains, moves in six degrees of freedom in response to the dynamic loading of wave, current and wind combined. Numerical simulations are carried out to solve motion responses of the floating structure with mat at different depths. The sensitivity of motion responses to wave periods and directions are analyzed. The maximum motion amplitudes under the design environmental conditions and the risk of the mat crashing with the seafloor are evaluated.
Maneuvering Prediction of a VLCC Model Based on CFD Simulation for PMM Tests by Using a Circulating Water Channel
Authors: Liu Han , Ma Ning , Gu Xiechong
Published: October 2015

Since ship maneuverability is a vitally important characteristic of ship design, several experimental techniques to determine ship maneuverability are recommended, among which using the planar motion mechanism (PMM) test for captive model in a circulating water channel (CWC) has become a new and effective way for captive model tests. This paper uses the numerical method to study the viscous hydrodynamic forces acting on a KVLCC2 model. The viscous flow around the model and its hydrodynamic forces in the oblique towing, pure sway and pure yaw test are simulated by CFD, in which the steady and unsteady RANS equations in conjunction with a RNG k-ε turbulence model are solved. By applying the dynamic mesh technique, the motion of pure sway is simulated. As a key technique for realization of pure yaw motion a new method combining the layering and local remeshing that treats restricted water region problem like in a CWC is developed. Accuracy of the proposed numerical method is confirmed by comparing the calculated hydrodynamic forces with the measured one. Then hydrodynamic derivatives of ship maneuvering movements are analyzed, and used in maneuvering prediction which is based on MMG model for turning test. Results show that the hybrid dynamic mesh technique is a practically efficient way to simulation of the pure yaw motion in CWC, which balances computational accuracy and efficiency. It also demonstrates that the present numerical model gives satisfactory results on PMM tests and maneuvering motions in terms of accuracy and it can be an economical method for engineering practices.
Generating and Absorbing Boundary Conditions for Free-Surface Flow Simulations in Offshore Applications
Authors: Düz Bülent , Huijsmans René H. M. , Wellens Peter R. , Borsboom Mart J. A. , Veldman Arthur E. P. , Luppes Roel
Published: October 2015

Numerical simulations of wave phenomena necessarily have to be carried out in a limited computational domain. This implies that incoming waves should be prescribed properly, and the outgoing waves should leave the domain without causing reflections. In this paper we will present an enhanced type of such generating and absorbing boundary conditions (GABC). The new approach is applied in studies of extreme hydrodynamic wave impact on rigid and floating structures in offshore and coastal engineering, for which the VOF-based CFD simulation tool ComFLOW has been developed.
A Mathematical Model for Roll Damping Prediction
Authors: Piehl Henry , el Moctar Ould
Published: October 2015

A ship in seaway is always prone to roll motion. For the safety of personnel, ship and cargo it is essential to optimize the roll damping properties of the hull shape in order to prevent exceeding roll angles. Therefore, a tool for the prediction of roll damping is an important requirement during the design phase of ship hulls.

The objective of this study is to use regression analysis and numerical simulation of roll motion to develop an analytic expression for the determination of roll damping. The development procedure starts with a variation of several hull shape parameter that influence the roll damping. For each of the parameter variants, a numerical roll simulation is conducted and the according roll damping coefficients are determined by time series analysis. Finally, regression analysis is applied to the computed results in order to derive a mathematical model that allows to determine the roll damping coefficient depending on the hull shape parameter.
Enhanced Active Heave Compensation Control Design for New Riser Hybrid Tensioning System in Deepwater Drilling
Authors: Wu Yin
Published: October 2015

This research paper presents the active heave compensation control design using a novel hybrid riser tensioning system by integrating an electrically powered riser tensioning system into existing hydro-pneumatic tensioners. This is a further control design development paper following the previous framework introduction paper of riser hybrid tensioning system. The realization of the active heave compensation operation mode using this novel hybrid tensioning system is analyzed. A mathematical model for describing this operation mode is built integrating electrical and hydro-pneumatic tensioning systems. As a multi-input multi-output system, one Linear Quadratic Gaussian control design is proposed and implemented in both Matlab simulation and hardware realization. The simulation results and the experimental study have confirmed that the riser hybrid tensioning system is a feasible concept with better control accuracy and improves riser reliability compared to the traditional hydro-pneumatic system. The main benefit of the electrical tensioning system is its ability to deliver dynamically changed tension within milliseconds. The actualization of this principle control mode gives us confidence to carry on for further riser dynamic control using this riser hybrid tensioning system.
A POD-Based Method for Low-Dimensional Components of Free/Moving Surface Flows
Authors: Zhang Yi , Yim Solomon C.
Published: October 2015

To study the intrinsic physics and improve computation efficiency of free–surface flow problems, in this study we propose a proper orthogonal decomposition (POD) technique that couples the velocity flow field and the level–set function field for free–surface flows. In this method, the snapshots data from numerical or experimental results are used to assemble a low–dimensional basis so that the flow characteristics can be retrieved with a priori knowledge of equal distribution of the total variance between velocity and level–set function data. Through numerical examples of a sloshing problem and a water entry problem, we show that the low–dimensional components obtained provide an efficient and accurate approximation of the flow field Moreover, we show that the velocity contour and orbits projected on the space of the reduced basis greatly facilitate understanding of the intrinsic dynamics of the flow systems.
Global Wave Persistence Study for Offshore Operation and Planning
Authors: Jeong Chan Kwon , Valsaraj Alokraj , Velazquez Harold
Published: October 2015

This paper describes the guidelines for preliminary workability analysis for offshore operation and planning using global wave persistence study. In offshore engineering, the weather information has to be studied to select the appropriate path for the project during the concept design, the detail design and the operation plan. The workability analysis is part of the weather study for various offshore operations and this study is used as a guide for the operation schedule, the selection of the operation method, the design of the equipment and the supporting structures. The detailed workability analysis is based on response based simulations which are only available after or during detail system design. On the other hand, preliminary workability can be calculated based on hindcast data with statistical methods before the detail design and this study can lead the project on the appropriate path at the beginning stages. The wave persistence analysis with thresholds on time and wave heights is used for the preliminary workability calculation and can also be used for route planning during offshore transportation. The objective of this paper is to provide global wave persistence analysis data to guide offshore operation and planning and makes the transition to workability with the required working wave period easier. For the global wave persistence analysis, the National Center for Environmental Prediction (NCEP) Climate Forecast System Reanalysis Reforecast (CFSRR) system data was used which has 35-year numerical wave hindcast. Data resolution is 0.5°×0.5° grid size with a temporal resolution of 3 hours for the period from 1979 to 2013, and this database covers most offshore locations except the Caspian Sea, and the North and South Pole areas. The persistence analysis was calculated with 0.5m, 1.0m, 1.5m and 2m wave thresholds, and each wave threshold is categorized as 24 hours, 48 hours and 72 hours persistency.
Experimental Studies of Hydrodynamic Properties and Screening of Riser Fairing Concepts for Deep Water Applications
Authors: Baarholm Rolf , Skaugset Kjetil , Lie Halvor , Braaten Henning
Published: October 2015

The VIV oscillations of marine risers are known to increase drag, and lead to structural fatigue. One proven method of suppressing this vibration is the use of fairings and strakes. These coverings essentially modify the flow along the cylinder, tripping the production of Karman vortices so that they act less coherently or far enough downstream so they interact less with the body.

The Norwegian Deepwater Programme (NDP) has conducted a project with the objective to develop and qualify effective low drag fairing concepts with respect to VIV mitigation and galloping. Furthermore, emphasis is put on easy handling and installation.

This paper describes the work and findings in an early phase of the development. This includes small scale model test campaigns. In addition to the bare riser for reference, the behaviour and performance of a total of 10 different fairing concepts are evaluated. Free oscillation tests are performed in a towing tank, where 2D fairings were tested in a pendulum set-up. The set-up enables free vibrations in up to 3 DOF (in-line and cross-flow vibrations and yaw). Fix tests with the purpose of establishing hydrodynamic coefficients for the various fairings have been performed in a large cavitation tunnel.

Clear differences in performance have been noticed; particular for drag and galloping responses. Based on the results from the 2D tests, a screening of the fairing designs has been performed and the findings have set the course for further development of the most promising candidates for real life applications.
Effect of Padeye Depth on the Behavior of Offshore Anchor Piles Under Mooring Forces
Authors: Ramadan Mohamed I. , Butt Stephen D. , Popescu Radu
Published: October 2015

Offshore anchor piles are usually loaded at a padeye on pile surface. The padeye depth can be at the seabed or below it. Using a padeye below the seabed is widely used in case of suction caissons. However, anchor piles are more flexible and the mode of failure will be different from that for suction caissons. In the current parametric study, the effect of padeye depth on the behavior of offshore anchor pile subjected to mooring forces in dense sand was studied. Finite Element Model (FEM) had been established. The model had been calibrated based on the centrifuge tests that were carried out by the authors. Three piles of different soil-pile rigidity covering a wide range of pile flexibility were used in the study. The piles were pulled out at an angle of 15° to horizontal. In all cases the padeye depth was changed from at the ground surface to a depth of four times the pile diameter. From this parametric study, it was found that pulling out an offshore anchor pile at a level below the seabed has some advantages of increasing the ultimate capacity of the pile, decreasing pile deflection, and decreasing bending moment. An optimum depth of padeye was recommended.
Utilization of Surface Drilling Data for Estimation of Sediment Properties in Scientific Drilling of Tohoku Earthquake Zone
Authors: Inoue Tomoya , Katsui Tokihiro , Ujiie Kohtaro , Ishiwata Junya
Published: October 2015

Japan Agency for Marine-Earth Science and Technology (JAMSTEC) operates the scientific deep-sea drilling vessel Chikyu, which undertook the drilling project under Japan Trench Fast Drilling Program (JFAST) to determine the mechanism of the catastrophic Tohoku Earthquake and the resulting huge tsunami. As targeted drilling sites were located about 1,000 m below the seafloor, at water depths of 7,000 m, JFAST was a challenging deep riserless drilling.

Drilling operation can also provide information on the properties of the drilling layer. This is important as a main aim of the scientific drilling is to obtain core samples from sediment layers under the seabed for evaluating sediment properties. Even a rough estimate of sediment properties could potentially provide an important notice for conducting coring operations.

Thus, we attempted to estimate properties of sediment layers using surface drilling data, such as drilling torque acquired from JFAST, and compare with properties of the core samples. As shear stress of sediment is a key parameter, it was estimated from surface drilling data. It was then compared with shear stress data determined from laboratory tests using core samples, and data from temperature sensors set in borehole.
Stability of Artificial Subaqueous Slopes in Sandy Soils Under Wave Loads
Authors: Bubel Julian , Pick Marc-André , Grabe Jürgen
Published: October 2015

Shallow foundation structures in marine environments can rarely be placed on top of the sea floor. Weak soils usually need to be excavated to place the structure on more stable ground. Steep but stable slopes of the resulting pit meet both economic and ecologic aims as they minimise material movement and sediment disturbance. This paper focuses changes of geometry of submarine slopes in non-cohesive soils (erosion, sedimentation, breach failure, liquefaction failure) due to surface waves.

After Terzaghi the angle between slope and the horizontal of the ground surface of cohesionless soil is at most equal to the critical state friction angle, as obviously true for dry soil. However, it can be observed that natural submarine slopes of sandy soils are always mildly sloped. During the construction of artificial submarine pits under offshore conditions it should be considered that the long-term slope-inclination is less than onshore due to hydrodynamic actions (e. g. flow, waves, earthquakes). Large surface waves cause excess pore water pressures within the soil body, leading to a reduction of effective stresses and in case of submarine slopes to changes of the slope geometry depending on wave length L, wave height H, water depth h and soil properties (permeability k, relative density Dr).

During our preliminary work we investigated such processes based on the coupling of linear wave theory and linear quasistatic consolidation theory (e.g. [1]). With the help of numerical modelling we solved corresponding equations considering also materially nonlinear consolidation. However, deformations were always limited by used Lagrangian-FEM. Recent developments at our Institute enable the use of an Eulerian-FEM approach with an u-p-Formulation for fully saturated soil [2]. This allows larger deformations of the subaqueous slope to be numerically investigated.
High Froude Number Viscous Flow Past a Circular Cylinder
Authors: Bouscasse Benjamin , Colagrossi Andrea , Marrone Salvatore , Souto-Iglesias Antonio
Published: October 2015

Two-dimensional monophasic flow past a circular cylinder intersecting or close to a free surface at Reynolds 180 is numerically investigated using the Smoothed Particle Hydrodynamics (SPH) method. The wake behaviour for a range of conditions with Froude numbers between 0.3 and 2.0, a gap ratio of 0.55 and for a half-submerged cylinder are studied. The SPH technique allows for a non-diffusive computation of the free surface evolution, even while breaking and fragmentation may occur. Vorticity shed by the cylinder, vortex generation due to free surface breaking, mixing processes, and drag and lift coefficients behaviour are discussed. It has been found that, for certain Froude numbers and for the analysed submergence ratio, a vorticity layer remains spatially localized between the cylinder and the free surface and a large recirculating wake area develops, which eventually gets detached after several shedding cycles, being advected downstream, a previously unreported form of wake instability, to the authors’ knowledge. This behaviour is blocked when the cylinder is intersecting the free surface.
Forced Oscillation Model Tests for Determination of Hydrodynamic Coefficients of Large Subsea Blowout Preventers
Authors: Arino Xavier , de Wilde Jaap , Russo Massimiliano , Grytøyr Guttorm , Tognarelli Michael
Published: October 2015

Large scale model tests have been conducted in a towing tank facility for the determination of the hydrodynamic coefficients of subsea blowout preventers. A subsea blowout preventer (BOP) is a large, complex device 10–15 [m] tall, weighing 200–450 [ton]. The BOP stack consists of two assemblies, the ‘lower marine riser package’ (LMRP) connected to the riser string and the BOP itself, connected to the wellhead. Together they represent a large lumped mass, which directly influences the natural frequencies and vibration modes of the riser system, particularly those of the BOP-wellhead-casing assembly.

Large uncertainties in the estimates of the hydrodynamic coefficients (added mass, lift and drag or damping) result in large uncertainties in the fatigue damage predictions of the riser and wellhead system. The trend toward larger and heavier BOPs, which could place BOP-wellhead-casing oscillation frequencies in the range of wave frequencies, has motivated Statoil and BP to start a new research project on this subject. The project involves a large scale model test for experimental determination of hydrodynamic coefficients.

Two different BOP designs were tested in a towing tank at model scale 1:12. The models weighed about 50 [kg] in air and were about 1.2–1.5 [m] tall. A six-degree-of-freedom oscillator was mounted under the carriage of the towing tank for oscillation of the models in different directions. Static tow tests and forced oscillation tests with and in the absence of steady current were carried out. Keulegan-Carpenter (KC) numbers ranged between 0.2 and 2.0, while the Sarpkaya frequency parameter β was in the range from 4,000 to 50,000. The Reynolds numbers of the static tow tests ranged between 50,000 and 150,000. This paper focuses particularly on tests in the surge direction with and in the absence of a steady current. Results indicate that the hydrodynamic coefficients for BOP stacks are quite different from those of simpler geometries like a circular cylinder. In addition, they provide new insight for analytical modeling of global hydrodynamic forces on BOPs in many configurations and scenarios.
Experimental Investigation of the Stick-Slip Phenomena of the Drill Pipe
Authors: Katsui Tokihiro , Mogi Yoshitomo , Inoue Tomoya , Rheem Chang-Kyu , Matsuo Miki Y.
Published: October 2015

The stick-slip is one of the critical problems for the scientific drilling, because it causes a crushing of the sampled layer. The present study investigates the characteristics of stick-slip phenomena of the drill pipe with the model experiments and numerical methods. The model experiments are carried out using a 1m length drill pipe model made with the Teflon. The angular velocity at the top and the bottom of the pipe are measured with the gyro sensor on some conditions of rotating speed at the pipe top and the weight on bit (load at the pipe bottom). The numerical simulations are also carried out to reproduce the stick-slip phenomena of the model experiments. The stick-slip is a kind of torsional vibration which is governed by the convection equation. By considering the boundary condition at the top and bottom of the pipe, we can obtain a neutral delayed differential equation (NDDE). The solutions of the NDDE is depend on not the initial value but the initial history of the solution, because NDDE contains a delayed function term. Therefore, it should be solved carefully to avoid the numerical error. The NDDE is solved with the 4th order Runge-Kutta scheme with very small time increment until the truncation error could be neglected. And also, we have found out that the effect of the initial history on the solution become to be very small after a certain period of time. The experimental results are compared with the numerical results under the same rotating condition. The experimental results of the stick-slip suggest that the period of the slip is mainly depend on the rotation speed at the pipe top and the magnitude of the slip is mainly depend on the weight on bit. Those characteristics of the stick-slip such as the period or the magnitude of slip are also obtained with the numerical calculations. However, in order to obtain an acceptable numerical results of NDDE, we have to adjust the frictional torque acting on the drill bit. Though, the frictional torque model was determined by reference to the measured torque at the top of the drill pipe model in the present study, it is desired to be improved. Therefore, the physical model of the frictional torque on the drill bit should be evaluated much carefully for the precise estimation of the stick slip in the future.
A Fast Method for the Evaluation of the Dynamic Positioning Capability in Time Domain in the Early Design Stage
Authors: Luebcke Adele , Augener Philip , Falkenhorst Arne
Published: October 2015

Vessels equipped with a Dynamic Positioning (DP) system for station keeping have become quite common on the offshore market. The propulsion system of such ships has the capability to compensate the counteracting environmental forces caused by wind, waves and current. Since the DP capability is an important part of the specification, it is necessary to consider this aspect in the early ship design stage. For this purpose a procedure of a fast calculation method is developed by the authors to predict the limiting environmental conditions and the maximum deviation of the position and the course angle for a pre-established propulsion system.
Comparison of Fatigue Design Codes With Focus on Offshore Structures
Authors: Micone Nahuel , De Waele Wim
Published: October 2015

Different codes are used in the industry for the fatigue design of offshore structures. Because the fatigue phenomenon is very sensitive to different design conditions, standards select those with higher impact and try to cover all other unpredictable factors making use of safety factors. To the authors’ knowledge there is not any comparison of fatigue codes with focus on analyzing the strength and limitations of the design approaches and safety factors included in each one available. This paper has the aim of comparing a number of recognized fatigue design codes for this purpose. Based on this review BS (British Standard) presents the most complete information for offshore design applying the fracture mechanics approach while DNV (Det Norske Veritas) offers the most updated and complete information for fatigue design applying the endurance approach. The safety factors contemplated in the codes are not always comparable and consequently it is not possible to confirm whether one standard is more or less conservative than the others.
Optimization of RANS Solver Simulation Setup for Propeller Open Water Performance Prediction
Authors: Islam Mohammed , Jahra Fatima , Doucet Michael
Published: October 2015

Mesh and domain optimization strategies for a RANS solver to accurately estimate the open water propulsive characteristics of fixed pitch propellers are proposed based on examining the effect of different mesh and computation domain parameters. The optimized mesh and domain size parameters were selected using Design of Experiments (DoE) methods enabling simulations to be carried out in a limited memory environment, and in a timely manner; without compromising the accuracy of results. A Reynolds-Averaged Navier Stokes solver is used to predict the propulsive performance of a fixed pitch propeller. The predicted thrust and torque for the propeller were compared to the corresponding measurements.

A total of six meshing parameters were selected that could affect the computational results of propeller open water performance. A two-level fractional factorial design was used to screen out parameters that do not significantly contribute to explaining the dependent parameters: namely simulation time, propeller thrust and propeller torque. A total of 32 simulations were carried out only to find out that the selected six meshing parameters were significant in defining the response parameters. Optimum values of each of the input parameters were obtained for the DOE technique and additional simulations were run with those parameters. The simulation results were validated using open water experimental results of the same propeller.

It was found that with the optimized meshing arrangement, the propeller opens simulation time was reduced by at least a factor of 6 as compared to the generally popular meshing arrangement. Also, the accuracy of propulsive characteristics was improved by up to 50% as compared to published simulation results. The methodologies presented in this paper can be similarly applied to other simulations such as calm water ship resistance, ship propulsion to systematically derive the optimized meshing arrangement for simulations with minimal simulation time and maximum accuracy. This investigation was carried out using STAR-CCM+, a commercial CFD package; however the findings can be applied to any RANS solver.
An Empirical Heave Induced VIV Prediction Model
Authors: Wu Jie , Lie Halvor , Larsen Carl M. , Baarholm Rolf J.
Published: October 2015

Steel Catenary Risers (SCRs) are widely used in the development of deepwater oil and gas fields due to their advantages of low manufacturing cost, resistance of high temperature and high pressure, etc. The vertical platform heave motions can cause significant vertical oscillations and hence relative fluid velocities close to the sag-bend area of an SCR. Vortex induced vibrations (VIV) can occur subjected to such periodic variation of relative flow speed.

In order to understand the mechanism of heave induced VIV, Statoil has carried out two model tests in the Ocean Basin in Shanghai Jiaotong University in 2012. A straight flexible beam was tested in an oscillatory flow in one of the tests to study the fundamental behaviour of VIV. Heave induced VIV was investigated in the other test, a truncated SCR model was tested with calculated motions at the top end. Recent studies on these experiments have revealed unique features of VIV in an oscillatory flow. The vortex shedding process is more complicated than that of constant flow. One of the reasons for this is that the pipe will interact with the recent shed vortices when it reverses its motion direction, which may amplify the response. VIV response is not only influenced by the reduced velocity, but also Keulegan–Carpenter (KC) number. In addition, different eigen-frequencies of the pipe can be excited and the mode transition is observed due to the variation of the flow speed during one oscillation period.

There is still lack of prediction tools to evaluate heave induced VIV. It can be desirable to modify the present empirical VIV prediction tools by including additional effects from oscillatory flow. In the present paper, the vortex shedding process during one oscillation period is approximated by several time spans with constant flows. VIV response at each time span is considered to be controlled by a representative flow speed. The final response will be the sum of the responses at different time spans. A riser system analysis program is used to determine the relative flow speeds along an SCR due to heave motions. Then, an empirical VIV prediction program is used to calculate VIV response at different time spans over one top motion period. The prediction results from this integrated approach are compared with experimental results.