bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact of Soil Modeling on Fatigue Design of Lazy Wave Riser Systems
Authors: Ghosh Rupak , Arslan Haydar
Published: November 2019

The Liza risers comprise production risers, water injection risers and gas injection risers, and a lazy wave configuration is selected considering FPSO motion, reservoir fluid and overall project execution requirements. During operation, the risers are expected to move cyclically with small vertical displacement amplitudes (e.g. 0.1% to 1% of the riser diameter), and a key design issue is the fatigue life of these risers at critical locations including the touch-down zone which will be governed by the seabed stiffness. The role of soil response on fatigue life of riser with buoyancy has been investigated through nonlinear finite element and comprehensive lab and field testing program. Published methodologies for determining seabed stiffness values for risers concentrate more on larger amplitude motions based on the design requirements of steel catenary risers.

The paper presents the sensitivity of the fatigue life at TDP to various soil model and provides insight in the results. Also included is the importance of site specific soil investigation in the context of design of riser.
A New Fully-Detailed Finite Element Model of Spiral Strand Wire Ropes for Fatigue Life Estimate of a Mooring Line
Authors: Bussolati Federico , Guiton Martin , Guidault Pierre-Alain , Poirette Yann , Martinez Michael , Allix Olivier
Published: November 2019

Spiral strand wire ropes are commonly used in the mooring system of offshore structures. When dealing with the fatigue limit state, engineers have to consider many different load cases, according to the variability of the environmental state. This usually prevents the use of any detailed numerical model of the mooring lines. In this paper, we propose a new method to evaluate with an affordable computational cost the detailed mechanical stress state in different parts of the wire ropes used for mooring a floating offshore wind turbine. We first compute tension and bending history in the mooring, with the hydrodynamic software Deeplines™, assuming for simplification stationary aerodynamic loads on the floater. These time series are then accounted for in a novel Finite Element Model of the spiral strand, with small sliding among the wires. The obtained kinematics and stress state of the wires can then feed a fatigue law based on fretting fatigue, which has been experimentally evidenced to condition the fatigue life of spiral strand wire ropes. The potential of this method is illustrated with an application to a cylinder-like shape floater equipped with 3 pairs of catenary mooring lines. It is shown that bending and tension histories do not significantly depend on the wire rope bending stiffness.
Ship As a Wave Buoy: Estimating Relative Wave Direction From In-Service Ship Motion Measurements Using Machine Learning
Authors: Mak Bart , Düz Bülent
Published: November 2019

For operations at sea it is important to have a good estimate of the current local sea state. Often, sea state information comes from wave buoys or weather forecasts. Sometimes wave radars are used. These sources are not always available or reliable. Being able to reliably use ship motions to estimate sea state characteristics reduces the dependency on external and/or expensive sources.

In this paper, we present a method to estimate sea state characteristics from time series of 6-DOF ship motions using machine learning. The available data consists of ship motion and wave scanning radar measurements recorded for a period of two years on a frigate type vessel. The research focused on estimating the relative wave direction, since this is most difficult to estimate using traditional methods. Time series are well suited as input, since the phase differences between motion signals hold the information relevant for this case. This type of input data requires machine learning algorithms that can capture both the relation between the input channels and the time dependence. To this end, convolutional neural networks (CNN) and recurrent neural networks (RNN) are adopted in this study for multivariate time series regression.

The results show that the estimation of the relative wave direction is acceptable, assuming that the data set is large enough and covers enough sea states. Investigating the chronological properties of the data set, it turned out that this is not yet the case. The paper will include discussions on how to interpret the results and how to treat temporal data in a more general sense.
Analysis of the Coupled Dynamics of an Offshore Floating Multi-Purpose Platform: Part B — Hydro-Elastic Analysis With Flexible Support Platform
Authors: Ruzzo C. , Failla G. , Arena F. , Collu M. , Li L. , Mariotti A.
Published: November 2019

A multi-purpose platform (MPP) is an offshore system designed to serve the purposes of more than one offshore industry. Indeed, over the past decades, a number of industries have expanded, or are expanding, from onshore to offshore locations (renewables, aquaculture, tourism, mineral extractions, etc), and the research on these type of platform is increasing. In the present work, a MPP able to accommodate wind turbines, wave energy converters, and aquaculture systems is considered. For an overview of the MPP platform considered and its research context, please refer to the EU H2020 project overview (OMAE 2019-96104). This work presents the second part (Part B) of the analyses of the dynamics of the floating support structure for this MPP, focusing on the hydro-elastic analysis, while its complementary rigid-body hydrodynamic analysis is presented in Part A (OMAE2019-96212).

The aim here is to assess if the support platform structural elasticity has a substantial impact on the dynamic response of the platform. A beam model and a 3D solid model of the support structure have been developed, and the inertial forces, hydrodynamic added mass forces, hydrostatic and mooring restoring forces have been considered in the hydro-elastic analyses performed. The results show that the dynamic response to the wave loads is not substantially influenced by the elasticity of the support structure, and that, at first approximation, a rigid-body approach is acceptable.
Numerical and Mechanistic Modelling of Two-Phase Liquid-Gas Flow’s Pressure Drop Across Sharp-Edged Orifices
Authors: Khan Zurwa , Tafreshi Reza , Franchek Matthew , Grigoriadis Karolos
Published: November 2019

Pressure drop estimation across orifices for two-phase liquid-gas flow is essential to size valves and pipelines and decrease the probability of unsafe consequences or high costs in petroleum, chemical, and nuclear industries. While numerically modeling flow across orifices is a complex task, it can assess the effect of numerous orifice designs and operation parameters. In this paper, two-phase flow across orifices has been numerically modeled to investigate the effect of different fluid combinations and orifice geometries on pressure drop. The orifice is assumed to be located in a pipe with fully-developed upstream and downstream flow. Two liquid-gas fluid combinations, namely water-air, and gasoil liquid-gas mixture were investigated for different orifice to pipe area ratios ranging from 0.01 to 1 for the superficial velocity of 10 m/s. Volume of Fluid multiphase flow model along with k-epsilon turbulence model were used to estimate the pressure distribution of liquid-gas mixture along the pipe. The numerical model was validated for water-air with mean relative error less than 10.5%. As expected, a decrease in orifice to pipe area ratio resulted in larger pressure drops due to an increase in the contraction coefficients of the orifice assembly. It was also found that water-air had larger pressure drops relative to gasoil mixture due to larger vortex formation downstream of orifices. In parallel, a mechanistic model to directly estimate the local two-phase pressure drop across orifices was developed. The gas void fraction was predicted using a correlation by Woldesemayat and Ghajar, and applied to separated two-phase flow undergoing contraction and expansion due to an orifice. The model results were validated for different orifices and velocities, with the overall relative error of less than 40%, which is acceptable due to the uncertainties associated with measuring experimental pressure drop. Comparison of the developed numerical and mechanistic model showed that the numerical model is able to achieve a higher accuracy, while the mechanistic model requires minimal computation.
Operational Behaviour of Supersonic Separators for Real Gas Mixtures of Methane and Carbon Dioxide, From the Homogeneous Nucleation Point of View
Authors: Restrepo Julián , Simões-Moreira Jose R.
Published: November 2019

One of the several challenges of the oil and natural gas offshore extraction in pre-salt reserves in Brazil, is the high content of carbon dioxide inside the wells, reaching values close to 80% in a molar fraction. This issue has a big impact on the crude extraction, due to the necessity of further expensive equipment occupying considerable deck space in platforms.

Therefore, it is necessary to research and find new technologies, which allow separating high contents of carbon dioxide at low energy consumption and low maintenance. The supersonic separation concept fulfils all these requirements due to the absence of moving parts and simple operation, because its working principle is based on the strong temperature drop of gas mixtures at supersonic expansion as it occurs in de Laval nozzles. Eventually, at the right operating conditions, the mixture starts to nucleate and, therefore, it will drive the phase change.

Nevertheless, the physical phenomena involved in this device is complex, because it involves phase change at supersonic speeds, which comprise compressible, multispecies, and multiphase flow. This paper analyses the threshold of the phase change at supersonic speeds.

The objective of this work is to calculate the influence of operation conditions (pressure, temperature and composition) in the phase change performance in supersonic separators.

The phase change was modelled using the classical theory of homogeneous nucleation. In this approach allows to estimate the phase change inside the supersonic flow for different operating conditions and allows to establish operating strategies to ensure low Mach numbers (M < 1.5) in the phase change region, allowing a further pressure recovery.
Development of Harsh Environment Field With Ice Loadings Using Concrete Spar: Variability of Options
Authors: Sablok Anil , Hovland Erlend , Stromme Svein , Blundon Andrew
Published: November 2019

A floating platform in deep water East Canada is required to withstand iceberg loads and/or be disconnected and towed away in the event of very large approaching icebergs, leaving the mooring lines and risers in-place, support large topsides and in most cases, provide large quantities of oil storage in the hull. Concepts considered for deep water application in the area include ship shaped self-propelled disconnectable FPSO and disconnectable and permanently connected deep draft floaters. This paper presents the details of Concrete Spar platforms that have been configured to satisfy all the above requirements. Several variations in the functionality and configurations of the platforms were studied to understand the impact on the overall cost and risks. The case variations primarily included:

• Disconnectable options for hull, mooring and risers

• Non-disconnectable option able to handle the full iceberg load

• With and without crude oil storage in hull

• Sacrificial or non-sacrificial mooring or risers

• Steel riser configurations (freely hanging catenary, SCR vs. lazy wave, SLWR)

• Hull with and without riser support Buoy

• Location of mooring fairleads (on Upper hull or Buoy)

• Riser support configuration (Pull Tubes or Flex Joints)

The iceberg loads experienced by the hull depended on the disconnection philosophy. The Concrete Spar hull is a 6-cell structure with the risers located inside the open centerwell. The paper describes several key features of the hull, mooring and riser systems for each case that are specifically designed to withstand iceberg loads and other environment loads while maintaining the characteristic low motion response for all the options considered. Additionally, the system has been designed to minimize the disconnection and reconnection time for the disconnectable cases.

The platform dimensions and weights have been compared for the various options considered. The cases without oil storage in the hull have significantly smaller and lighter hull relative to similar cases with oil storage. The cases without hull disconnection option had similar hull dimensions but marginally higher concrete weight relative to the hull disconnection option. However, the cost of Buoy, required for the disconnection option, compensates for the concrete cost difference. The SCRs work for the inplace extreme environment and iceberg load cases. Depending on the hull offset required to avoid icebergs for the non-disconnection cases or the risers lowering depth for disconnection cases, SLWR may be needed.

Recommendations are made for the preferred option for field development.
Hydraulic Behavior in Cased and Open Hole Sections in Highly Deviated Wellbores
Authors: Ytrehus Jan David , Lund Bjørnar , Taghipour Ali , Kosberg Birgitte Ruud , Carazza Luca , Gyland Knud Richard , Saasen Arild
Published: November 2019

In this paper we present results from flow loop experiments with an oil-based drilling fluid with micronized barite as weight materials. The use of micronized barite allows using lower viscosity drilling fluid, providing non-laminar flow, which is advantageous for particle transport in near-horizontal sections. While transition to turbulence and turbulent flow of non-Newtonian fluids has been well studied both theoretically and experimentally, there are very few published results on the effect of wellbore wall properties on flow regime transition and turbulence. This is relevant because horizontal sections are often open-hole with less well-defined surfaces than a steel casing surface. We have conducted a series of flow experiments with and without cuttings size particles in a 10 m long annular test section using steel and concrete material to represent the wellbore wall of a cased and open hole section. In both cases the annulus was formed by a freely rotating steel pipe of 2” outer diameter inside a 4” diameter wellbore. Experiments were conducted at 48°, 60° and 90° wellbore inclination from vertical. The two materials result in different hydraulic behaviour without particles with stronger turbulence when using concrete wellbore wall material than when using steel casing. While there is negligible difference at low flow rates, at 0.8 m/s and below, there is an increasing difference as the flow rate increases and becomes transitional to turbulence. Hole cleaning is found to differ dependent on the wall material. However, the effect on hole cleaning is less clear than for the pressure loss.
Assessment of Weld Overlays in Cladded Piping Systems With Varied Thicknesses
Authors: Kogo Bridget , Wang Bin , Wrobel Luiz , Chizari Mahmoud
Published: November 2019

This paper continues the research previously done by authors on computer simulation of the dissimilar welded joints with varying clad thicknesses using numerical methods. For different cladding thicknesses comprising of stainless steel and mild steel, stress curves have been generated. The welding of the two dissimilar materials has been carried out in-house with the aid of a tungsten arc weld with dynamic measurement of the temperature profile in vicinity of the welding track using high temperature thermocouples. Comparison of the experimentally measured stresses from literature versus the simulation results shows close agreement.
Gap Resonance of Fixed Floating Multi Caissons
Authors: Chen Limin , He Guanghua , Bingham Harry B. , Shao Yanlin
Published: November 2019

Generally, numerous marine and offshore structures are composed of a number of modules which introduce narrow gaps between the multi-modules arranged side by side. The interaction between water waves and floating structures excites complex wave runup in the gaps and wave forces on the adjacent modules. In this study, free surface oscillations in twin narrow gaps between identical floating rectangular boxes are investigated by establishing a 2D viscous flow numerical wave tank based on a Constrained Interpolation Profile (CIP) method. The Tangent of Hyperbola for INterface Capturing (THINC) method is employed to capture the free surface. The rigid floating bodies are treated by a Virtual Particle Method (VPM). The incident waves are generated by an internal wave maker. For the fixed module cases, the computational results of wave height in narrow gaps are found in good coincidence with the available experimental measurements, especially for the resonant frequencies. The wave forces on the floating bodies are calculated numerically. The characteristic response of wave forces on the leading and rear bodies are consistent with the free surface elevations in the corresponding narrow gaps. With shallow draft, the gap resonance occurs at higher wave number.
Multiphase Flow Induced Forces on Bend Structures
Authors: Belfroid S. P. C. , Gonzalez-Diez Nestor , Pereboom H. , Tümer C.
Published: November 2019

Multiphase conditions can lead to high vibration in flexible pipe systems due to high dynamic flow induced forces. To better model the dynamic forces, large scale 6” air-water, near-atmospheric experiments have been done with different bend radii and upstream configurations. This paper gives an overview of the obtained results with respect to the overall dynamic force and force power spectral density description parameters. Scaling rules based on these experiments are given.
Field Trial of Vortex-Induced Vibration Suppression Technology for Drilling Riser Buoyancy
Authors: Kurts Phillip P. , Marcollo Hayden , Kilner Andrew A. , Johnstone Daniel , Potts Andrew E. , Pezet Peter , Hill Tricia
Published: November 2019

Instrumented field trials of Longitudinally Grooved Suppression (LGS) VIV suppression buoyancy modules have been completed on deep water drilling risers in the Gulf of Mexico. The field trials were used to validate the performance of the technology, which had previously been evaluated using prototype scale model tests. The measured riser responses over two drilling campaigns spanning more than six months were compared with each other and the outputs of computational riser modeling to validate the hydrodynamic parameter set derived through scale model tests and provide validated assessments of the suppression technology performance.

The measured response of drilling risers equipped with LGS buoyancy has been compared with a publicly available dataset for the VIV response of a conventionally buoyed riser, showing reduced VIV response in agreement with model test results. Measured flex joint angles, current profiles and riser accelerations were used to validate the hydrodynamic parameters used in numerical riser analysis. Using the validated hydrodynamic parameters, the VIV and drag suppression performance was demonstrated by comparison with the model predictions for risers equipped with conventional buoyancy modules. Eddy current occurrence statistics for a location in the Gulf of Mexico were used to calculate the expected annual operability performance for both configurations.

For the base case parameters, 12 days of annual operability improvement was predicted when using LGS buoyancy modules. A sensitivity study determined the effect of varying analysis assumptions on the predicted operability improvements. Measured current data from 2014 was also used to determine the operability benefits which could be realized within a year in which severe eddy current activity occurred.

The analysis performed serves to validate the previous laboratory tests as well as answer questions about the applicability of high Reynolds Number test results to VIV suppression devices in the field. The use of previously validated testing and analysis methods is shown to provide reliable estimates of suppression technology performance which are borne out by testing in the field. This paper presents the first published field trial of shaped buoyancy type VIV suppression, a group of technologies which have until now only been demonstrated using scale model tests and Computational Fluid Dynamics simulations.
Wave Propagation in CFD-Based Numerical Wave Tank
Authors: Kim Jang , Baquet Aldric , Jang Hyunchul
Published: November 2019

A heuristic approach to investigate the characteristics of the numerical waves propagating in various CFD-based NWT set ups is proposed to develop an optimal grid system for a numerical wave tank. Linear dispersion relations of the waves in a Cartesian grid system are derived analytically. The analytic results lead to an optimal grid aspect ratio for the best dispersion approximation. Extensions of the analytic approach to determine an optimal set up of the grid system of CFD-based NWT are discussed. A roadmap to develop CFD modeling practices based on these heuristic approaches and further numerical verification is proposed for the on-going industry efforts to develop the guidelines for numerical wave tanks, such as the Joint Industry Project on “Reproducible CFD Modeling Practices for Offshore Applications”.
Study on the Multi-Body Dynamic Characteristics of FPSO Soft Yoke Mooring System Based on Symplectic Algorithm
Authors: Wu Wenhua , Lyu Baicheng , Yao Ji , Yue Qianjin , Yantao Zhang , Guo Xinglin
Published: November 2019

The soft yoke single-point mooring (SYMS) system is the main mooring approach for the floating production storage and offloading (FPSO) unit. As a typical multi-rigid-body system, a SYMS consists of the single-point turret, yoke, mooring legs, and mooring support. It releases the rotational degrees of freedom of an FPSO through the combined effects of multiple joint structures, so as to deliver the weather-vane effect of the FPSO. In this paper, a multi-body dynamics model of the soft yoke mooring system was established. To deal with the difficult integration in the process of solving differential-algebraic equations, a symplectic numerical integration method was proposed on the basis of the Zu Chongzhi method. The proposed solution format had simple symplectic property automatically satisfying the Hamilton system, as well as a high accuracy in solving nonlinear systems. The measured data of the FPSO’s six degrees of freedom (6DoF) under two different sea conditions were selected, and the mooring restoring force of the SYMS was calculated. The calculated results showed that the symplectic solution method could the actual stress state of the structures with more obvious dynamic characteristics. Furthermore, the displacement and stress state of the single-body structures, such as the mooring legs and yoke, and the analysis result could comprehensively evaluate the overall working state of the SYMS.
Tension Based Heading Control Strategy of the Arctic FPSO With DP Assisted Mooring System
Authors: Lee Jaeyong , Choi Sol-Mi , Lee Seung Jae , Jung Kwang Hyo
Published: November 2019

For offshore plants operating in the Arctic region, it is important to reflect the motion characteristics of the ice in the design stage. When the ice load is applied, the platform should rotate in the direction to reduce the tension acting on the mooring system using the DP system. However, it is very difficult to conduct the simulation by predicting the size and direction of the ice. This paper presents a heading control strategy to reduce the ice load acting on the arctic production platforms equipped with DP (Dynamic Positioning) Assisted Mooring System. In order to save computation time, grids are created in the operating area and the size of the tension acting on each grid is calculated considering the current position, the current tension, and the rate of change of tension. Based on the tension information, target heading angle is assigned and the platform is rotated with DP module. Developed process is validated through the simulation.
On the Estimation and Application of Directional Design Criteria
Authors: Feld Graham , Jonathan Philip , Randell David
Published: November 2019

This article discusses the estimation of directional metocean design criteria for engineering applications. We provide a summary of current code recommendations relating to directional design criteria and illustrate conceptually and mathematically some of the difficulties of their derivation. We also discuss the application of directional criteria for the specific examples of Code Check and Pushover analyses for fixed structures and jack-up rigs.
Mechanical Friction in Well Construction and Laboratory Testing of Friction Coefficients
Authors: Ghaedi Parisa , Khalifeh Mahmoud , Saasen Arild , Hodne Helge , Omland Tor H. , Shoghl Farzad
Published: November 2019

A new device for measuring the lubricity of drilling fluids for controlling the mechanical friction is developed. This is able to measure mechanical friction at temperatures between room temperature and 200°C and at pressures in the range of atmospheric to 20 000 psi. The effects of lubricants on mechanical friction is outlined. Different types of available test instruments are reviewed. These instruments include both commercial equipment and special designed equipment. In addition, the effect of pressure and temperature as well as their combined effect on the friction coefficient of deionized water, mineral oil and oil-based mud (OBM) have been investigated as initial test of the developed instrument. Our initial results show that friction coefficient increases with pressure increase for the mineral oil and OBM, at 25°C. However, when deionized water was used as lubricant, the friction coefficient decreased when pressure was increased.
Assessment of the Carcass Role in Designing Deepwater Riser Connectors
Authors: Saneian Mohsen , Bai Yong
Published: November 2019

Technically, in deep water, the most challenging task is the end-fitting design of pipes. This connector has two main functions: It ensures sealing of the pipe fluid and transfers the loads from the pipe to the connecting system unit. High pressure and harsh environment condition raise the pressure on the connector; therefore, this pressure should be endured by a segment which is the carcass. Carcass holder and installation rings are components of the connectors in termination of the carcass layer. This paper assesses the role of the carcass in a sealing system of riser connectors. The sealing method is based on dividing the termination of risers into two devices; inner liner that is the mechanical restraint on the polymer barrier, and the seal system that is applied to seal the fluid of riser. This seal system includes many ring joint gaskets separated by metallic rings, and they should be placed on the carcass.
Experimental and Numerical Study of Motion of Rotating Drill Pipe Owing to Magnus Effect
Authors: Inoue Tomoya , Suzuki Hiroyoshi , Katsui Tokihiro , Tsuchiya Keita , Notani Yusuke
Published: November 2019

During riserless drilling operations conducted in some scientific drillings and the initial stages of all oil and gas drilling operations, drill pipe motions such as vortex induced vibration, whirl motion, and motion due to the Magnus effect are generated. The last motion represents an interesting and important phenomenon that generates a lift force in addition to a drag force due to the ocean current and the rotation of the drill pipe. Accordingly, this study focuses on the drill pipe motions owing to the Magnus effect.

An analytical model of a drill pipe was established by applying an absolute nodal coordinate formulation (ANCF) that can capture the behavior of a relatively flexible and long pipe, such as a drill pipe. The lifting and drag forces are calculated using computational fluid dynamics (CFD), and the lift and drag coefficients are calculated for several different drill pipe rotational velocities and ocean current velocities.

A series of model experiments were conducted in a towing tank, with changing water flow velocities and rotational speed of the drill pipe model to observe the corresponding changes in the Magnus effect and to measure the resulting drill pipe motions. Additionally, the resulting drag and lift forces were measured. It was observed from the experiments that the motions in the cross-flow direction increased as the rotational speed of the drill pipe model increased, and that the lifting force increased as the rotational speed increased.

The drill pipe motions were then simulated using a previously established analytical model and the results of the CFD simulations. The results of the simulations were evaluated against the results of the experiments, and reasons for observed discrepancies are discussed.
Integrated Acoustic Communication and Positioning System Between an Autonomous Surface Vehicle and Autonomous Underwater Vehicles
Authors: Watanabe Yoshitaka , Meguro Koji , Deguchi Mitsuyasu , Kida Yukihiro , Shimura Takuya
Published: November 2019

In underwater observation using an autonomous underwater vehicle (AUV), a support vessel typically monitors the AUV to support the observation. In order to make the AUV operation more efficient, an autonomous surface vehicle (ASV) and an acoustic multi-access communication and positioning system have developed.

The developed acoustic system achieves multi-access with frequency division multiple access (FDMA) method, and the ASV can monitor up to three AUVs simultaneously. Positioning is performed with super short baseline (SSBL) method. The acoustic device has operation mode in which positioning and communication functions are integrated to achieve efficient uplink and accurate downlink simultaneously.

Two observation operations were conducted successfully. In one of those, the ASV communicated with two types of AUVs during observation in 1250m water depth, then multiple access were achieved. Even nadir angle for one AUV became almost 40 degrees, the acoustic communication was performed. In another observation, two cruising AUVs were operated with a vessel and the ASV in 1500m water depth. The ASV monitored one AUV. Condition in case the device is equipped on small body of the ASV was evaluated. The communication was performed in this depth in severe condition. Furthermore integrated sequence of positioning and communication was successfully performed. Requirement in next phase, in which operation depth and number of multiple access are increased, is discussed.
A New Method for Deriving Soliton Design Criteria
Authors: Jeans Gus , Jones Oliver , Zhang Michael , Jackson Christopher R. , Stashchuk Nataliya , Osborne Alfred R. , Petersen Ole Svenstrup , da Silva José
Published: November 2019

A new method for deriving extreme soliton current criteria for offshore engineering applications is described. The primary data source was site specific measurement close to the continental shelf break where metocean criteria were required. A dedicated oceanographic mooring was designed to quantify solitons, with rapidly sampled measurement of seawater temperature and velocities through the vertical.

As described in two previous OMAE papers, quantification of soliton velocity profiles was achieved via temperature measurement and theory, with measured velocities playing a secondary role in critical validation. The previous methodology was extended in the present study, with separate contributions quantified from variations in soliton amplitude and water column density structure.

The nonlinear Fourier techniques first described in OMAE 2017 were again used to reduce uncertainty in estimates of extreme soliton amplitude. In a new development, the long-term distribution of the density structure contribution was quantified using a calibrated hindcast of seawater temperature.

Extreme conditions were defined at the boundary of a MITgcm model domain. This sophisticated model was then used to estimate extreme soliton velocities, through the water column and a few metres above the seabed, at a wide range of shallower target locations.
A Hybrid Methodology for Maritime Accident Analysis: The Case of Ship Collision
Authors: Bowo Ludfi Pratiwi , Prilana Ramdhani Eka , Furusho Masao
Published: November 2019

Human error is recognized as the most common factor that causes maritime accidents. Human Error Assessment and Reduction Technique (HEART) as a Human Reliability Assessment (HRA) has been widely applied in various industries. However, in the maritime industry, machine, media, and management are also considered as factors that can strongly affect human behavior, judgment, and other human elements. These factors are particularly relevant for bridge resource management that perform the task of maintaining a proper look-out. Therefore, this study considers the effect of those factors integrated with 4M (man, machine, media, management) factors. The study was conducted with 37 collisions accident reports from 2007–2017, using data from the NTSB and TSB-Canada. There are 229 Error Producing Condition (EPC) found in this study. The classification of the EPC to the 4M conducted in this study.
Random Combination Factors for Still Water and Wave Bending Moments
Authors: Huang Wenbo
Published: November 2019

Based on the extreme value of the primary loads of ship hull girder instead of characteristic values, the more reasonable load combination factors are defined. In order to evaluate the random variation of newly defined load combination factors, based on Ferry-Berges & Castanheta (FBC) and Poisson square wave models, the still water bending moments (SWBM), vertical wave bending moments (VWBM) and their combined processes are simulated to get the random realizations of load combination factors. The statistical analysis results show that the load combination factors take the value of 1 with the highest probability and can be well fitted by the Weibull distribution. Such information should be incorporated appropriately in the reliability analysis of ship hull girder.
Understanding the Phenomenon of Dissolved Gas Migration of Gas in Riser During Drilling Operations
Authors: Nahri Syed Y. , Chen Yuanhang , Williams Wesley , Santos Otto , Thibodeaux Louis , Zhu Jianjun
Published: November 2019

The prevention and control of gas kicks is a major concern in the petroleum industry during deepwater drilling operations. The problem is further aggravated when dealing with synthetic and oil-based muds (SOBM and OBM) that can dissolve a gas influx entering the wellbore. Due to the solubility of formation gases in drilling fluids, the gas cut mud resulting from gas absorption has a density lower than that of overlaying unsaturated drilling fluid. Lab scale experimental tests were conducted in order to understand whether buoyancy-induced convection and diffusion attribute to mass transfer of a dissolved influx. Experiments were performed on a low-pressure mass transfer apparatus using carbon-dioxide (CO2) and mineral oil to study the extent of mass transfer due to buoyancy induced convective flow and diffusion. Measurements were made on the axial distance travelled by the dissolved carbon dioxide and gas concentration over the length of a pipe by measuring the mass of gas accumulated in different test sections of the experimental apparatus. This arises due to a concentration gradient developed when contaminated fluid comes in contact with a fresh column of drilling fluid. Experimentally obtained measurements made on the mass transfer coefficient are used to tune simulations carried out using a computational fluid dynamic (CFD) software — ANSYS Fluent. This enables us to replicate field scenarios to study the extent of well control issues that could arise when a gas influx enters the wellbore, even when circulation has ceased. Results obtained here can be used as a base case to understand a similar phenomenon occurring when extended to other fluid systems such as that of a natural gas influx in synthetic oil-based drilling fluids.
Ultimate Compressive Strength of Eccentrically Loaded Stiffened Panels in Ship Structures: A Computational Study
Authors: Anyfantis Konstantinos
Published: November 2019

Ship scantlings are obtained through a linear elastic design framework. In relatively long ships, the mid-ship’s ultimate strength capacity must be checked whether it is below a certain level (given factor of safety). Smith’s method is based on an incremental-iterative approach where constitutive equations of idealized representative components are used to model the different involved buckling modes. The literature body neglects the local bending effect applied at the stiffener with the attached plating (considered here as the representative element). This work studies the eccentrically loaded representative element from its compressive resistance point of view. The eccentricity sources from the linear varying strain field produced as a result of pure bending of the ship’s mid-section and is relative to the location of the stiffened panel from the ship’s section Neutral Axis. Therefore, the representative element has been considered as a part of the ship’s structure and the inherent eccentricity is analytically calculated in terms of geometric and material properties. A computational study has been further performed in a non-linear finite element environment in order to figure out the effect of the eccentricity level to the ultimate compressive strength of the representative element. Geometric distortions have been also included in the FE models in order to allow for accurate strength calculations. Numerical results have shown that, in realistic proportions of large ships, the further the representative element is located from the mid-ship’s NA the smaller is the deviation from pure compressive load strength analysis results. However, for the element laying near the ship’s NA, idealized models overestimate the strength by 10% compared to realistic analytical / numerical results for the typical scantling case considered.
An Application of Fault Tree Analysis for Decommissioning of Subsea Flexible Pipeline in Brazil
Authors: Ramos Rafaela , Pasqualino Ilson , Souza Marcelo Igor L. , Nicolosi Eduardo Ribeiro
Published: November 2019

The decommissioning of underwater structures has expanded in recent years, as the number of facilities at the end of their useful lives is increasing and legislation becomes more severe. Unlike the production units which already have their decommissioning plans consolidated, subsea pipelines represent the greatest technological, environmental and financial challenge of this stage. Currently, there are few studies to evaluate failures in the decommissioning of flexible pipelines. In addition, these studies generally use simplified analyses. Therefore, this study aims to elaborate a failure analysis for a Reverse Reeling option, in order to contribute to the decision-making process and reduction of failures. For this, a Fault Tree Analysis will be developed followed by the subsequent elaboration of its minimum cut sets. The main results present a total of 13 first-order minimal cut set, 5 second-order minimal cut set and 7 third-order minimal cut set.
Hydrodynamic Model Tests With a Large Floating Hydrocarbon Storage Facility
Authors: Fonseca Nuno , Chi Zhang , Rodrigues José Miguel , Ren Nianxin , Hellan Øyvind , Magee Allan R.
Published: November 2019

The paper presents model tests with a floating hydrocarbon storage facility performed in SINTEF Ocean’s basin. The system may be considered a very large floating structure with a footprint of 300m × 310m. Hydrocarbons are stored into independent tanks, which are soft moored to a modular floating frame which is kept in station by dolphins. The system is composed of 14 tanks and 21 frame barge modules, therefore 35 connected floating bodies.

The objective of the model tests was twofold. First, to verify the feasibility of the global system, namely in terms of tank and frame motions, connection loads, station keeping loads and relative wave elevations. Second, to acquire data for tuning and validation of numerical models, i.e. to investigate effects that cannot be reliably established by simulations alone.

The model tests were performed in wave, current and wind conditions corresponding to 1 year and to 100 years return periods, in addition to a set a simplified wave conditions. The paper describes the model test setup, the experimental program, the procedures for analysis of test data, it presents representative results and discusses the main observations.
A Study on Motions and Connector Loads for a New Type of Two-Module Semi-Submersible
Authors: Ding Jun , Tian Chao , Miao Yuji , Zhang Zhengwei , Zhao Zhanhua , Ni Xinyun
Published: November 2019

The hydrodynamic model and three-dimensional finite element model of a new type of two-module marine platform were built for the subject of research. The motions and connector loads were predicted based on the COMPASS THAFTS software and the results were in agreement with the model tests. It shows that the new type platform has a good sea keeping performance. On the other hand, the short-term forecast results indicate that the connector loads, Fz, is larger than other two directions, approaching 200t. The connector loads, Fx and Fy, are usually in the order of magnitude of tens tons. Some useful conclusions obtained in this paper can provide an important reference for the optimal design and construction of the new type platform and its connector system.
Comparative Evaluation of Elastomer Seal Energization in Conventional and Expandable Hanger Assembly
Authors: Patel Harshkumar , Salehi Saeed
Published: November 2019

Elastomer seals are extensively used in various wellhead and casing/liner hanger equipment as barriers for isolating fluids. Seal assemblies have been identified as one of the major cause of well control incidents. Majority of hangers utilize conventional weight- or mechanical-set slip-and-seal assembly. The objective of this paper is to conduct detailed investigation of seal energization in conventional and relatively newer expandable type hanger seal assembly.

To achieve the objective, finite element modeling approach was employed. Three dimensional computer models consisting of concentric casings and annular elastomer seal element were constructed. Seal energization process was modelled by manipulating boundary conditions. Conventional seal energization was mimicked by applying rigid support at the bottom of elastomer element and compressing it from the top. Expandable hanger type seal energization was modelled by radially displacing the inner pipe to compress annular seal element. Seal quality was evaluated in terms of contact stress values and profile along the seal-pipe interface.

Different amount of seal energization were simulated. Both types of seal energization processes yielded different contact stress profiles. For the same amount of seal volumetric compression, contact stress profiles were compared. In case of conventional seal energization, contact stress profile decreases from the compression side towards support side. The seal in expandable hanger generates contact stress profile that peaks at the center of contact interface and reduces towards the ends. Convectional seal assembly has more moving parts, making it more prone to failure or under-energization. FEA Models were validated using analytical equations. Good match was obtained.

Majority of research related to elastomer seal is focused on material properties evaluation. Limited information is available in public domain on functional design and assessment of seal assembly. This paper adds novel information by providing detailed assessment of advantages and limitations of two different seal energization process. This opens doors for further research in functional failure modes in seal assembly.
Development of Offshore Structural Analysis Software X-SEA Coupled With FAST
Authors: Kim Ki-Du , Vachirapanyakun Sorrasak , Plodpradit Pasin , Dinh Van Nguyen , Park Jin-Ho
Published: November 2019

Coupled analysis of offshore structures is currently challenging. The 3D finite element analysis software X-SEA coupled with FAST 8 program is therefore developed and discussed in this paper. The current version of X-SEA includes the results of extensive research and development based on finite element program XFINAS, which was originally developed in Imperial College London. The solution of the X-SEA ranges from the simple static to highly advanced dynamic analysis applied to the offshore structures. GID is used as pre- and post processor of X-SEA. The brief theoretical background of X-SEA software is summarized. Numerical examples of offshore monopile, wind turbine jackets, pile super element and fatigue analysis verified with SACS software in terms of reactions, displacements and member forces are investigated.
“Three Sisters” Measured As a Triple Rogue Wave Group
Authors: Magnusson Anne Karin , Trulsen Karsten , Aarnes Ole Johan , Bitner-Gregersen Elzbieta M. , Malila Mika P.
Published: November 2019

On November 30, 2018, our attention was caught when analyzing wave profile time series measured by a platform mounted wave sensor (a SAAB REX radar) at Ekofisk, central North Sea. The 20-minute time series had not only one, but three consecutive waves with individual heights that all were more than twice the significant wave height, the two last of them being almost equally high with a factor 2.35 to the significant wave height of 4m (from 4σ(η), over 20 minutes).

Counting three rogue waves in one sequence seems to be very rare. In this study we analyze how the shape is evolving in space and time using linear and non-linear propagation methods developed by Mark Donelan [1,2] and Karsten Trulsen [3,4]. Weather conditions and characteristics of the sea state with the ‘Three Sisters’ (named the “Justine Three Sisters”) are presented. It is found that the Three Sisters occurred in a crossing sea condition, with wind sea and swell coming from directions 60 degrees apart, with about same frequency, but very different energy.
Prediction of Liquid and Gas Leak Rates in Packed Stuffing Boxes
Authors: Salah Omar Aweimer Ali , Bouzid Abdel-Hakim
Published: November 2019

The prediction of gas and liquid leak rate through packed stuffing boxes subjected to gas flow is a subject of very few studies in the literature. For better prediction of leakage, the change of porosity with length due to the non-uniform axial stress must be accounted for. There are few theoretical models on the prediction of leak rates in packing rings with capillary models. However, a model that incorporates the change of the capillary area with stress gives a better prediction.

In this paper, the first slip flow model is used to predict gas and liquid flow considering the straight capillaries and capillaries having an area dependent on the axial stress in the packing rings. An approach that uses an analytical-computational methodology based on the number and the size of pores obtained experimentally is adopted to predict gas and liquid leak rates in uniform and non-uniform compressed yarned packings. The Navier-Stokes equations associated with slip boundary condition at the wall are used to predict leakage. Experimental tests with helium, argon, nitrogen and air for gazes and water and kerosene for liquids will be used to validate the models. The porosity parameters characterization will be conducted experimentally with helium at a reference gas at different gland stresses and pressures.