bluuebunny/crossref_metadata_2025 huggingface dataset:
New Advances in Flexible Riser Monitoring Techniques Using Optical Fiber Sensors
Authors: Morikawa Sérgio R. K. , Braga Arthur M. B. , Camerini Claudio S. , Kato Carla C. , Llerena Roberth A. , Camerini Murilo G. , Simões Tiago B.
Published: August 2013

Petrobras oil and gas production in the deep and ultra deepwater fields in Campos Basin and other provinces off the Brazilian coast heavily relies on flexible pipes. Maximizing the availability and reliability of an extensive offshore pipeline network poses innumerous challenges to the Company, which is steadily moving towards a condition based approach to maintenance of their flexible risers. In this context, Petrobras, in cooperation with its academic partners, has launched a comprehensive R&D program named MONFLEX, focusing on novel techniques for structural monitoring of flexible risers.

Years of field experience have demonstrated that one of the most frequent failure modes of flexible pipes is the sequential rupture of wires in their tensile armor layers [1]. The MONFLEX Program has explored a range of different technologies in order to timely detect and monitor the growth of this class of progressive structural damage. Some of the proposed approaches have relied on video cameras pointed towards fixedly mounted targets on the riser outer sheath, vibration and acoustic methods, these in a wide frequency range, and techniques based on fiber optic strain sensors. All three have been experimentally deployed in the field and are currently being evaluated. Among those, fiber optic monitoring is the one that has shown the better promise of becoming the chosen method for detecting wire ruptures in the riser’s armor layers.

The fiber optic based monitoring system developed in the MONFLEX R&D Program has been named MODA, which, in Portuguese, stands for Direct Wire Optical Monitoring. The MODA system consists in instrumenting all the wires of the riser’s external tensile armor layer with fiber Bragg grating strain sensors. In flexible risers already in operation, a window in the polymeric outer sheath of the pipe is temporarily opened in order to allow the sensors installation, and then repaired with a protective, anticorrosive layer. Even though in MODA the strain sensors are installed in the external armor layer, full scale laboratory tests have demonstrated that the algorithm employed to treat and analyze the real time data provided by the system is capable of instantaneously detecting ruptures of wires either in the external or internal layers of the tensile armor. The proposed contribution will report the later results of extensive laboratory tests and field trials performed with the MODA system.
Fatigue Analysis of Tether Chain in Hybrid Risers
Authors: Cerkovnik Mark , Chang S. , Griffin Cary
Published: August 2013

Hybrid risers represent an excellent way to isolate the riser from most of the host vessel motions and thereby limit riser fatigue. A common arrangement features the riser supported by a buoyancy can via a tether chain. The tether chain is a cheap simple way to make the connection while providing flexibility for installation. However, in service the tether is under very high tension, and the chain is not really flexible in the face of small amplitude fatigue loads. The friction effectively “welds” the chain together. Moment and torque input to the system by first order vessel motions and vortex induced vibrations are carried through the chain and induce fatigue loading in the links.

Analysis of the chain can be problematic because the determination of the detailed stress in the chain requires a refined FEA model with contact element between the links. From the global sense the analysis may require running hundreds of sea-state realizations in the time domain and the vortex induced vibration (VIV) assessment of thousands of current profiles. In this paper an efficient numerical method is described to rigorously determine fatigue damage at locations throughout the chain.
Hydrate Plug Movement by One-Sided Depressurization
Authors: Gonçalves Marcelo , Camargo Ricardo , Nieckele Angela O. , Faraco Rafael , Barreto Claudio Veloso , Pires Luis Fernando G.
Published: August 2013

A new model that accurately predicts hydrate plug displacement during a one-sided depressurization is presented. This model is both simple to handle and rigorous in the physical representation of the phenomenon. It was implemented as a finite volume transient simulator capable of determining the flow field coupled with the plug displacement dynamics after its detachment. It takes into consideration velocity, pressure and temperature profiles across chambers upstream and downstream the plug at each instant of time, as well as pipe deformation due to pressure variations inside the chambers. Typical cases for deep offshore production are analyzed. The influence on the plug displacement of the gas composition, the temperature variation due to the heat loss to the environment and high pressure variation is addressed. Results show that, depending on the conditions, and after performing a careful risk evaluation, it may be safe to remediate hydrate plug by one-sided depressurization in a number of typical situations in offshore production scenario.
Sharp Phase Change in Shape Memory Alloy Thermal Actuators for Subsea Flow Control
Authors: Gilbertson Eric , Hover Franz , Freeman Bryan
Published: August 2013

Gas-lifted oil wells are susceptible to failure through malfunction of gas lift valves (GLV). One failure mode occurs when the GLV check valve fails and product passes into the well annulus, potentially reaching the wellhead. This is a growing concern as offshore wells are drilled thousands of meters below the ocean floor in extreme temperature and pressure conditions and repair and monitoring become difficult. The authors have previously developed a thermally-actuated safety valve to prevent product backflow into the annulus in the event of check valve failure. The safety valve uses shape memory alloy (SMA) wires to translate a temperature change into a displacement and, based on commercially available SMA wire material properties, requires a 6°C temperature change to fully actuate. In some wells, however, check valve failure may result in less than 6°C temperature change. In this paper a new concept is developed to sharpen the austenitic phase change in SMA actuators. The concept has broad practical implications because it will allow thermally-activated devices, such as fluid control valves, to become much more precise, i.e., translating a small temperature change into a large displacement. The new concept uses the fact that SMA transition temperatures are stress dependent. By specifically controlling stress in the wire, the temperature difference required for austenitic transition can be decreased. This is achieved with a negative-differential spring — a spring that exerts a decreasing amount of force as it is displaced. The concept is tested experimentally by conductively and electrically heating SMA wires connected to a negative-differential spring. Results show a 2.9°C-5°C reductions, respectively, in the temperature difference required for austenitic transition.
MPSO Design: Part 1 — Wave Excitation Forces and Moments
Authors: Vieira Daniel P. , Malta Edgard B. , Watai Rafael A. , Gonçalves Rodolfo T. , Fujarra André L. C. , Nishimoto Kazuo , Oliveira Allan C.
Published: August 2013

The MPSO is characterized by the use of hydrodynamics appendages, such as moonpool, beach and skirts, which improve the hydrodynamic behavior of the unit in waves. This type of platform may be designed for different offshore scenarios as, for example: the possibility of oil and gas storage, dry tree completion system and the use of steel catenary risers (SCR). An optimization procedure to choose the geometric dimensions of the MPSO becomes important in order to achieve the optimum hydrodynamic behavior to operate in harsh environmental conditions for each scenario. The optimization procedure might be useful in the preliminary design phases to reduce the verification time of the solution evaluated with model tests; for that reason it is necessary to create a database with experimental results to make the optimization procedure possible. The main idea of the study is to carry out an extensive experimental model test aimed at obtaining the parameters not well predicted using numerical codes. With this intent, the work is subdivided into three parts: Part 1 – Wave Excitation Forces and Moments; Part 2 – Damping and Added Mass Forces and Part 3 – Optimization Process. Results will be presented in different papers. The first one presents the experimental results for captive tests, the second one the experimental results for forced oscillation tests and the last one the methodology to use the experimental results as input in an optimization tool. The first paper presents the methodology in which nondimensional variables based on MPSO geometric characteristics were defined. These variables were related to a fixed moonpool diameter and they were determined in terms of four geometric dimensions: external diameter; height and diameter of the beach and platform draft. As a consequence, 21 different MPSO model geometries could be defined and experimentally tested in order to obtain the wave excitation forces and moments in 6 DOF. The experiments included transient waves so as to better understand the hydrodynamic behavior of the hull, such as, the response amplitude operator (RAO), cancelation points, the beach/bottom/moonpool effects for the different dimensions. The wave forces and moments obtained experimentally were compared to the results of a numerical code based on potential wave theory.
Shifting the Paradigm: The Effective Way to Use Real-Time Data for Improved and Safer Oilwell Operations
Authors: Pritchard David M. , Roye Jesse , Cunha J. C.
Published: August 2013

When analyzing root causes for minor or major problems occurring in oilwell drilling operations, investigators almost always can track past events, step by step, using recorded data that was produced when the operation occurred. In recent catastrophic blow-outs, investigators were able not only to determine the causes of the accidents but also to indicate mitigating actions, which could have prevented the accident if they were taken when the operation actually took place. This is a strong indicator that, even though the industry has valuable real-time information available, it is not using it as a tool to avoid harmful events and improve performance. Real-time data is not about well control, it is about well control avoidance. Recent catastrophic events have underscored the value of having the right kind of experience to understand and interpret well data in real time, taking the necessary corrective actions before it escalates to more serious problems. What is the well telling us? How do we use real time data to ensure a stable wellbore? Real-time monitoring, integrated with rigorous total well control analysis, is required to embrace and achieve continuous improvements — and ensure the safest possible environment. Next generation monitoring requires a step change that includes hazards avoidance as a precursor to drilling optimization. Real-time data can be used effectively in operations to avoid, minimize, and better manage operational events associated with drilling and completion. Real-time data can also provide the foundational support to improve training in the industry as well as develop hands-on simulators for hazards avoidance.
Influence of Iceberg Geometry and Scour Depth on Drag Force in Sand
Authors: Ivanović Ana , Banimahd Meysam , Oliphant John
Published: August 2013

The seabed disturbance in Arctic regions comes predominently through horizontal cutting mechanisms induced by natural processes such as iceberg scour. The zone of soil being disturbed by the iceberg and the soil resistance to the object movement is of interest to the geotechnical engineer. The influence of the iceberg on potential movement of the pipeline buried in the seabed has been of concern recently with recovery of the oil and gas assets situated in Arctic region.

Despite the significant developments in recent years in numerical modelling the ice gouging process still remains a challenge and further work is required to obtain a fully calibrated and validated numerical model. This paper therefore aims at developing a validated numerical model of the interaction between the iceberg and seabed through a series of laboratory tests undertaken at the University of Aberdeen.

This paper reports on a preliminary study focusing primarily on the comparison between laboratory and finite element (FE) modelling of the interaction between an iceberg and the seabed in terms of drag force and disturbance of the sediment. The FE model and experimental rig are described. The effect of the shape of the iceberg and the depth at which it penetrates has been investigated. Outputs of drag force from FE calculations showed a good agreement with the results obtained from the laboratory model giving the confidence to move to the next stage where both the inclusion of a pipeline segment and sand with different densities will be considered.
Transient Hydroelastic Analysis of Pump Performance of an SES-Waterjet System
Authors: Motley Michael R. , Young Yin L. , Savander Brant R.
Published: August 2013

Surface Effect Ships (SES) have great potential to increase the speed and cargo capacity of modern surface vessels. Waterjets are typically used for propulsion of SES because of the geometric constraints and its ability to absorb high thrust and power requirements. The objective of this paper is to investigate the transient hydroelastic performance of waterjet propulsion systems. The goal is to advance the fundamental understanding of the transient hydrodynamic and structural dynamic performance of waterjet propulsion systems with consideration for non-uniform inflow and transient fluid cavitation at heavily loaded operating conditions. Special attention is given to investigating the effects of flow non-uniformity and fluid cavitation on the system performance, as well as the structural dynamic performance of the rotor and stator blades.
Vortex Shedding From a Bilge Keel in a Transient Turbulent Flow
Authors: Piehl Henry , el Moctar Bettar Ould
Published: August 2013

The hydrodynamic damping of a bilge keel during the roll motion of a ship is fairly well understood and its basic principle can be summarized as follows: The larger the bilge keel attached to the hull, the stronger is its roll damping effect.

The geometric limitations regarding the size of the bilge keel are set by class regulations mainly dependent on the ship’s length, breadth and draft. Most bilge keel shapes are simple flat bars attached to the center of the bilge radius. While the added resistance of the bilge keel has to be kept as low as possible, the effective area for the cross flow generated by the roll motion should be maximized. Therefore the bilge keel’s cross section has to be kept small and its camber line parallel to the streamlines. In more sophisticated designs L-shaped bilge keels are applied in order to increase the damping effect on the roll motion. The aspects above need to be considered when defining the geometric limits of a bilge keel, however to further optimize the design of bilge keels numerical simulations are needed.

Even with today’s computing power, the costs of simulating a full ship hull with a sufficiently high mesh resolution to capture viscous vortex shedding effects would be prohibitive. To address and overcome this restriction a numerical test setup was developed that simulates the flow only in the near vicinity of the bilge keel. By further neglecting the influence of the free surface, it was possible to use a standard single-phase, incompressible, turbulent, transient solver. The open source FVM code OpenFOAM was used for all three stages of the simulation: mesh generation, solution process and post-processing.

With this simplified simulation model a systematic investigation of the turbulence model, the temporal and spacial discretization, as well as the principles of vortex shedding was carried out. The damping efficiency of the bilge keels was evaluated on basis of the mechanical work — by moving the hull through a viscous fluid — and the kinetic energy transported within the vortices. The findings from these flow simulations provide insights into the principles of bilge keel vortex shedding and their interaction with the hull and enable the development of bilge keel design guidelines.
A Real Sea Test of Wave Powered Generation Buoys Using Dielectric Elastomers
Authors: Masuda Koichi , Ikoma Tomoki , Nakatsugawa Kazushi , Waki Mikio , Chiba Seiki
Published: August 2013

For the practical application of wave powered generation, it is necessary that an epoch-making technology or performance have to be developed. This paper describes an actual sea test for application of the dielectric elastomer to wave powered generation. The power generation device using the dielectric elastomer of a floating buoy is installed to the fishery harbor of Suzaki in the IZU peninsula [1]. In the actual sea test, accelerations in three degree of freedom and output energy due to motion of the powered buoy are measured. Incident waves are measured using an ultrasonic wave meter. Expected acquirable power of the powered buoy with the dielectric elastomer is from 25 mW to 56 mW in observation time which is one minutes. Relationship between motion of the buoy transferred from the accelerations and the output power is investigated from the results. From the results, it is proven that a small wave powered generation buoy system is available with the dielectric elastomer.
Probabilistic Risk Analysis of a LNG Carrier Loading Pipeline
Authors: de Souza Gilberto Francisco Martha , Hidalgo Erick Miguel Portugal , Silva Dennis Wilfredo Roldán , Martins Marcelo Ramos
Published: August 2013

The loading and unloading operation of a LNG carrier is dependent on the perfect performance of the loading/unloading equipment, such as valves, tanks and pipes. The rupture of a ship pipeline (part of the manifold or other secondary pipes) used for LNG loading or unloading can cause the leakage of the fluid causing not only the complete stop of the loading or unloading operation but also exposing the ship and other terminal facilities to a risk associated with LNG leakage. The paper applies structural reliability concepts to evaluate the probability of failure of a pipeline due to the presence of a crack in the pipe wall. The analysis considers the probability of occurrence of brittle fracture associated with a through thickness crack propagation. The limit state function as for brittle fracture analysis is presented and as for reliability analysis three random variables are considered: material fracture toughness, crack size and pipe thickness. The Monte Carlo simulation method is used to calculate the probability of failure. Based on those results, the paper proposes the use of the cause-consequence diagram to evaluate the accident scenarios associated with the pipe rupture, which failure probability was previously calculated. The events that appear in the diagram are associated with alarm and control systems that are used as monitoring system for loading and unloading operations. Those failure probabilities can be calculated using reliability database. The consequences of each scenario can be defined based on literature review. The main analysis result will be the risk profile associated with a pipe brittle fracture. The method is used for the analysis of a LNG carrier operating in a Brazilian harbor.
A Method for the Prediction of Design Space for a Bend Stiffener Cone
Authors: Drobyshevski Y.
Published: August 2013

Bend stiffeners are commonly used to protect flexible flow-lines and dynamic risers from over-bending, especially in the areas where they interface with fixed or floating structures. Sizing of bend stiffeners and confirmation of their feasibility is an important step in the design of the complete riser system.

The objective of this paper is to develop a method for the prediction of the design space for the bend stiffener cone, which is defined in terms of its length and diameter at the base. The analysis method is based on the analytical formulation for an elastic cone consisting of the main pipe and the hollow bend stiffener. A “marching” technique is developed which is based on combining linear solutions for several conical segments. The analysis method requires a small number of segments, and is efficient in dealing with large deflections. It is shown that even a linear solution alone for the bend stiffener cone provides good accuracy for moderate deviation angles.

Criteria for setting the design space boundaries for the bend stiffener cone are discussed. The selection of load cases from the tension-angle curve is discussed and its effect on the shape of the design space is demonstrated. The method is illustrated by developing a design space for the bend stiffener of a typical flexible riser connected to the turret of an FPSO.
Modeling Oscillatory Behavior of ESP Wells Under Two-Phase Flow Conditions
Authors: Vieira Rinaldo Antonio de Melo , Prado Mauricio Gargaglione
Published: August 2013

The effect of free gas on the Electrical Submersible Pump (ESP) performance is well known. At a constant rotational speed and constant liquid flowrate, small amount of gas causes a mild head reduction when compared to the single phase liquid head. However, at higher gas rates, a drastic reduction in the head is observed. This critical condition, known as surging point, is a combination of liquid and gas flow rates that cause a maximum in the head performance curve. The first derivative of the head with respect to the liquid flow rate change sign as the liquid flow rate crosses the surging point. In several works on ESP two-phase flow performance, production conditions to the left of the surging region are described or reported as unstable operational conditions.

This paper reviews basic concepts on stability of dynamical systems and shows through simulation that ESP oscillatory behavior may result from two-phase flow conditions. A specific drift flux computation code was developed to simulate the dynamic behavior of ESP wells producing without packer.
WindFloat Contraption: From Conception to Reproduction
Authors: Aubault Alexia , Cermelli Christian , Lahijanian Alireza , Lum Alan , Peiffer Antoine , Roddier Dominique
Published: August 2013

This paper will discuss the very serious topic of the design of the WindFloat, a full-scale floating wind turbine. The importance of the fundamentals of hydrodynamics in achieving the desired performance cannot be overstressed. These will be discussed in this paper, together with some of the key considerations that entered into the design process. At the time of writing of this manuscript, a full-scale WindFloat prototype has been spinning for a few months, and the electricity it generated powered all the Christmas lights of Pòvoa de Varzim, a small town in the north of Portugal — whose inhabitants are not seeing a reduction in their electricity bill.

The authors have chosen to disassociate the presentation — a progress report — from this manuscript, which should discuss a topic more appropriate to the permanent literature.
Hydrodynamic Forces on a Pipeline With Uneven Embedment
Authors: An Hongwei , Cheng Liang , Zhao Ming
Published: August 2013

Hydrodynamic forces on a pipeline with uneven embedment on either side, subject to oscillatory flow, are investigated numerically. Two-dimensional Reynolds-Averaged Navier-Stokes equations with a k-ω turbulent model are solved to simulate the flow in the fluid. It is assumed the seepage flow in the seabed is governed by Darcy’s law and Laplace equation is solved to calculate the pore pressure under the assumption of isotropic and homogenous seabed. The effects of embedment depths and KC numbers on the hydrodynamic force are investigated. The flow structure and pressure distribution around the pipeline are discussed. The inline force and lift exerting on the pipeline are presented in the form of peak values and Fourier coefficients. It is found that flow structures around the pipeline are asymmetric due to the difference of seabed levels on the two sides of the pipeline. The degree of asymmetry increases with the increase of |e1-e2|/D. Obvious difference exists between the hydrodynamic forces experienced by the pipeline in two succeeding halves of a period due to the asymmetric flow structure around the pipeline. The peak values of inline force and lift reduce as e2/D increase for all values of e1 examined in this study. The maximum error of the inline force and lift predicted by using sixth order Fourier series is about 4%.
Oil Transfer Between DP Shuttle and Conventional Tankers in Tandem Configuration: A Numerical Evaluation of Alternatives
Authors: Tannuri Eduardo A. , Morishita Helio M. , Miyazaki Michel R. , Moratelli Lazaro , Madureira Rafael M. L.
Published: August 2013

The recent Pre-Salt oil fields discovered in Brazil are imposing some new logistical challenges concerning distribution in the internal and external markets. In order to reduce the demand in the on-shore terminals, the export oil could be directly transferred from the platforms to the export tankers. However, internal regulations define that the offloading operation must be carried out by DP Shuttle Tankers (ST). Due to this definition, since 1994, the DP-ST fleet has been increased, and the safety and uptime of the offloading operations are improving. The utilization of a DP-ST to navigate across the oceans for exportation is not economically advantageous, since the day-rates of those vessels are significantly higher than conventional (non-DP) STs. Hence, a possible solution is to transfer the oil from the DP-ST to the conventional ST in the offshore waters. Several alternatives were proposed, considering stationary tandem configuration, ship-to-ship or tandem with advance speed.

This paper is focused on the stationary tandem configurations, considering the aid of offshore tugboats and/or monobuoys. The alternatives comprehend the conventional ST connected to the DP-ST or vice-versa, with the position being held by the DP system, a tugboat or a monobuoy. The numerical time-domain simulations were used for predicting the safety under typical environmental conditions, with crossed-bimodal sea states and sudden wave-wind changes. The relative motions of the vessels, hawser force, DP or tugboat utilization and loads on the vessels mooring equipment were verified in order to define the limiting environmental window for a safe operation, for each one of the alternatives. This comparative analysis was used to support the decision about this topic, since the advantages and problems of each solution could be identified.
Microbiologically Influenced Corrosion of Gulf of Mexico Mooring Chain at 6,000 Feet Depths
Authors: Miller Jonathon D. , Warren Brett J. , Chabot Luc G.
Published: August 2013

During a post-installation inspection of a polyester and chain mooring system in water depths of approximately 6,000 ft, evidence of microbiologically influenced corrosion (MIC) was found in the form of rust tubercles known as rusticles. These porous concretions commonly form on submerged steel shipwrecks and provide evidence that subsea corrosion occurs in a hypoxic environment. Iron and sulfate-reducing bacteria cause corrosion in marine environments.

This paper will discuss one form of MIC found on submerged steel structures, analyze the ambient conditions required for MIC to occur, and compare rusticles found during the mooring inspection to those found on other subsea shipwrecks such as the RMS Titanic. An analysis of the type of iron used in mooring chains and the rate of rusticle formation will be presented. Possible remedies to prevent rusticle growth on mooring chains will be summarized.
Comparison of DP Performance Prediction Techniques for Scaled Models
Authors: Very Stephen P. , Millan James P. , Colbourne D. Bruce , Walker Dan L.
Published: August 2013

Utilization of Dynamic Positioning (DP) systems for offshore exploration and production of hydrocarbons is increasing due to the need to exploit deeper water depths, where mooring becomes less feasible. In conducting analysis or predictions for DP System performance, there are two common techniques used: Either an experimental investigation at reduced scale using a simplified mooring system without thrusters; or, a similarly scaled experiment using active DP thrusters. This paper identifies differences in the DP system performance estimates for each method by assessing the same system in identical wind–wave environments. Experiments were completed using a 1:40 scale model of a typical 99,000t monohull drillship equipped with an active DP system consisting of six azimuthing thrusters. These experiments were repeated with the vessel unpowered on two mooring systems with different stiffnesses. A comparison of system performance predictions provided by each method is presented.
Aero-Hydro-Elastic Simulation of a Semi-Submersible Floating Wind Turbine
Authors: Philippe Maxime , Babarit Aurélien , Ferrant Pierre
Published: August 2013

This paper presents an aero-hydro-elastic model of a semi-submersible floating wind turbine. A specific attention is drawn to hydrodynamic modelling options and its effect on the dynamic response of the platform.

The NREL 5MW reference wind turbine mounted on the historical concept of semi-submersible platform Dutch Tri-floater is considered. A specific hydrodynamic model of loads on semi-submersible platform is used within the wind turbine design code FAST from NREL. This hydrodynamic model includes non linear hydrostatic and Froude-Krylov forces, diffraction/radiation forces obtained from linear potential theory, and Morison forces to take into account viscous effects on the braces and damping plates. The effect of the different hydrodynamic modelling options is investigated. As one could have expected, it is found that the effect of viscous drag and non linear Froude-Krylov loads, becomes larger with increasing wave height. Simulations are run with directional wave spectrum, it is found that wave directionality induces larger transverse motions.
Theoretical Prediction of Tension-Stroke Relationship of Hydro-Pneumatic Tension Systems
Authors: Zhang Hanqing , Song Ruxin
Published: August 2013

Hydro-pneumatic tension systems have been widely used to support production and drilling risers on floating production systems (such as Spars and TLPs). A hydro-pneumatic tension system utilizes several hydro-pneumatic piston-cylinders (typically four) filled with a working gas (such as nitrogen) to provide the riser it supports with a tension. When the riser experiences strokes with respect to its supporting vessel caused by vessel motion (e.g. due to environmental change), the tensioner will accommodate most of the strokes by changing the volume and pressure of the cylinders. The riser tension will accordingly fluctuate with the pressure change about its design value. Such tension variation directly affects riser design and the vessel’s performance. Therefore, in the design of a hydro-pneumatic tension system, the stiffness of the tensioner, which describes the tension and stroke relationship of the tensioner, is one of the key design parameters to be concerned of. Since it is difficult to perform full-scale tests to determine the relationship for a tensioner, a theoretical model, which considers the tension’s stroke motion as a polytropic process, has been used to simulate the tension-stroke behavior of the tensioners. A polytropic process is well defined by a single parameter, the exponent. For nitrogen as the working gas, an exponent of 1.1 to 1.3 has been adopted in the offshore industry without theoretical or experimental verification.

The objective of this study is to theoretically predict the pressure-stroke relationship of the hydro-pneumatic tension systems and to determine proper values of the exponent for the polytropic process used in the offshore industry. The study uses the first law of thermodynamics and the knowledge of heat transfer to predict status change of the working gas with piston stroke and then to calculate the pressure-stroke relationship of the tensioner. The status of the gas is determined from the work exchange induced by piston strokes and heat transfer through the outer surfaces of piston rods and cylinder barrels. As a numerical example, a tension system similar to those used in the Gulf of Mexico is analyzed for 100-year hurricane environments. The predicted pressure time traces are compared with those given by a polytropic process with a series of exponents (or gas constants). The comparisons show that an exponent of 1.3 or 1.4 is a proper value for the polytropic process.
Extraction of Harmonic Components From Intermittent and Aperiodic Wave Elevation Measurements
Authors: Yang Wenlong , Hu Sau-Lon James , Li Huajun
Published: August 2013

Sea waves never repeat so any measurement of irregular wave elevation time history should be treated as an aperiodic function. Furthermore, wave elevation measurements might also be intermittent. The objective of this study is to extract harmonic components from an aperiodic and intermittent wave elevation measurement. A mathematically sound method, which is completely different from the traditional Fourier method, is developed for extracting all harmonic components from an aperiodic and intermittent wave elevation measurement. The method involves two main steps. In the first step, the number of harmonic components in the wave elevation measurement and the frequency of each component are estimated. After knowing the frequencies, the amplitude and phase angle of each harmonic component are computed at the second step using a least squares method. The superiority of the newly developed method over the traditional DFT analysis on extracting harmonic components is demonstrated using simulated aperiodic wave elevation signals. Whereas using the proposed method can nicely recover all target harmonic components, the Fourier analysis fails to decompose the signal into the target harmonic components due to its periodicity assumption imposed on the aperiodic wave elevation signal. In addition, the new method also performs well with a simulated aperiodic/intermittent wave elevation signal, and can accurately recover the missing part of the aperiodic/intermittent signal.
Effects of Second-Order Extreme Waves on the Dynamics of a Non-Linear Floating Body
Authors: Nava Vincenzo , Arena Felice
Published: August 2013

Nowadays technical and scientific communities are increasingly interested in the development of technologies for floating devices which can serve different purposes both in coastal and offshore environment. Thus, strong effort is required in the development of correct and efficient algorithms for studying the behavior of such structures under the action of sea wave loadings. At this purpose, in the past few years several approaches were investigated, both in frequency and in time domains, using linear and non-linear structural models and linear and non-linear wave theories.

In this note, the effects of nonlinearities in the wave model on the dynamics of a non-linear floating rigid body model are calculated using the second-order Quasi-Determinism (QD) theory (see [1], [2]; [3]) under the action of extremely high waves. Structural nonlinearities consist essentially in non linear damping and nonlinear stiffness due to mooring lines, following the model shown in Nava & Arena, [4]. Numerical nonlinear simulations were performed by means of an algorithm based on the approach showed in Zheng et al. [5], and the results compared to those provided by the nonlinear QD theory.

The purpose of this note is to show not only the effects of nonlinearities in the behavior of a floating body and to compare them with those obtained from a linear approach, but also to estimate them through non linear QD theory under the occurrence of a large wave in order to evaluate the reliability of the proposed approach.
Optimized Operations and Integrity Management Through Instrumentation
Authors: Jenkins Peter , Pytte Trond , Holden Harald , Marre Ignacio , Rønningen Jo Espen , Johnstone Les , Berg Ole Johan
Published: August 2013

During drilling and well intervention (DWI) operations today operating limits are normally given as limiting wave height, and sometimes wave periods. The resulting diagrams are often not directly comparable with weather information received on the rig and the final decisions are often based on subjective assessment of wave height and period.

The paper will present how BP, on the newly developed Skarv field in the Norwegian Sea, through thorough planning in the engineering phase has implemented a system where operating limits are specified based on directly measurable parameters such as rig heave and upper and lower flexjoint angles. How weather forecasting can be translated to give the rig crew direct forecasting of the limiting vessel or riser responses (e.g. flexjoint angles or heave), will also be presented. It will be shown how this allows for improved operational planning and support from onshore.

Over the last years requirements for oil companies to be able to document the structural integrity of their subsea assets, including wells, has increased. On the Norwegian Continental Shelf (NCS) there has been a particular focus on fatigue loading in the wellhead structure, including the upper sections of casing and conductor, due to loads induced by the riser and BOP during DWI operations. There have been cases where the design fatigue life of a wellhead system limits the number of days one can perform operations with a rig on a given well. This in term affects future oil recovery rates as the well fatigue life may not be sufficient to allow for side step drilling or intervention work required to maintain an optimal production from the well.

The paper continues to present how BP on the Skarv field, stores and utilizes the measured lower flexjoint response to track and document well integrity. It will be demonstrated how the return on investment of a drilled well can be improved by documenting actual fatigue loading from each operation on a well compared to conservative design calculations.

BP has addressed the above issues in a way that is likely to set a new standard for drilling and intervention operations in the North Sea in the future.

4Subsea AS has provided the engineering and instrumentation services that formed the basis for this paper.
Modeling the Effect of Reduced Liquid-Wetting on Spontaneous Imbibition for Condensate Blocking Applications
Authors: El Cheikh Ali Nour , Abouseida Mahmoud , Fahes Mashhad
Published: August 2013

In this paper, we present our interpretation for some of the unexpected experimental results that we obtained during 3D spontaneous imbibition experiments. We treated carbonate rocks with flourochemical polymers where we altered the wettability towards reduced liquid-wetting. The expected result is a reduced imbibition rate as a result of reduced capillary forces. Although the early imbibition rate decreased, we observed an increase in rate at late imbibition time resulting in a higher liquid saturation in the altered rock. We used numerical modeling to interpret the result and show that this observation actually represents the target wettability state we should be seeking in some applications.
Ringing Analysis of a Vertical Cylinder by Euler Overlay Method
Authors: Kim Jang , O’Sullivan Jim , Read Alex
Published: August 2013

A Computation Fluid Dynamic (CFD) analysis procedure is demonstrated for estimating the ringing loads on an offshore structure modeled as a simple vertical cylinder. The procedure is based on the Euler Overlay Method, where the computational domain of CFD solution is minimized to the immediate vicinity of the structure by overlaying Euler solution defined in the outer domain. The simulated ringing load shows good agreement with experimental data. Dynamic amplification factor due to the simulated ringing load is investigated for different damping ratio and natural frequency of the structure. Ringing response due to breaking wave shows significance of ringing load when evaluating dynamic amplification of wave load.
PAZFLOR Flexible Riser Diverless Bend Stiffener Fixation: Successful Innovation From Concept to Project Implementation
Authors: Decoret Laurent , Bastard Laurent , de Kerdanet Tegwen
Published: August 2013

For the TOTAL E&P Angola (TEPA) PAZFLOR field development, the decision was taken at the tender stage to propose an innovative diverless Bend Stiffener Fixation system (BSF) to connect the flexible risers to the FPSO. Avoiding diving intervention was driven primarily in order to minimize weather downtime during this critical offshore installation phase.

Technip decided to develop its own system to retain full control of the design process and ensure it fulfilled all the installation constraint requirements. The patented system consists of a 3 part clamp activated by a hydraulic motor controlled from topside. An innovative disconnection device is used to release the riser pulling head once the bend stiffener is clamped to the I-tube. The development of the BSF was handled by an integrated team as a “project within the project”. From the start a robust design gate review process was applied to rapidly freeze the design of the system main components and enable the initial order placement of the long lead items. This was critical to ensure on time delivery to the FPSO Contractor. Extensive full scale testing was performed to ensure good reliability of all the system components. This testing enabled further optimization of parts of the system, particularly the items mounted on the risers to provide the maximum operability. Finally the involvement of the installation team from the onset ensured that robust installation procedures and operations were developed. The hook-up operations for the 21 risers was conducted safely with no weather downtime and with a riser connection cycle faster than planned.

This proves that implementation of innovative design solutions can be successfully managed from initial concept to final installation within a project environment, ultimately enabling faster hand over for first oil.
A New Technique to Visualize Metocean Parameters
Authors: Parente Carlos E.
Published: August 2013

This paper presents a technique (PLEDS) to plot the evolution of a metocean parameter against time, distance or other independent variable, allowing a much clear understanding of the underlying process. The technique has been applied to waves, winds, currents, tides and underwater acoustics.
CFD Modeling for the TOTAL E&P Angola PAZFLOR Subsea Field Cold Spot Management Plan
Authors: Parenteau Thomas , Guindeuil Geoffrey , Decoret Laurent
Published: August 2013

The PAZFLOR field is located in Block 17, deep offshore Angola, and is one of the largest offshore/subsea development operated by TOTAL E&P Angola (TEPA). As part of the SURF package scope of work, flow assurance analyses had to be performed to verify the global thermal integrity and operability of the subsea architecture. In particular, a cold spot management study was carried out to ensure that adequate insulation was implemented on all the SURF equipment to achieve the project specified thermal performances. The purpose of the cold spot management plan is to complete the general Flow Assurance study mainly using OLGA software by performing a detailed flow and thermal analyses, using dedicated CFD simulations, for each subsea singularities identified as potentially creating a local cold or hot spot. Hot spots could lead to an accelerated ageing of some of the equipment materials, whereas cold spot could lead to a quicker cooldown time than required creating a local risk of wax or hydrate plug formation. The Pazflor SURF cold spot management plan has combined detailed CFD modeling and full scale thermal testing to validate the equipment insulation design and check via a validated simulation the exact thermal behavior under subsea conditions. An extensive amount of subsea elements have been studied such as Rigid Pipe-in-Pipe (PIP) waterstops, flexible IPB riser, pipeline In Line Tee structure piping and valves, Field Joint Coating with anode pad, flexible end-fittings and thermal insulation covers amongst other things. This paper will detail the CFD simulations performed, present the comparison between the simulation results and the full scale thermal tests and draw conclusions on the benefit of such cold spot analyses for future projects.
Selecting the Best Heading for FPSOs in Santos Basin Considering the Wave Induced Motions
Authors: de Oliveira Mauro Costa
Published: August 2013

The selection of the best heading considering the wave induced motion behavior is a design decision that should be taken in the early stages of the field development. Particularly for FPSOs with spread mooring pattern, that are not weathervane, this can bring the reduction of angular motions and accelerations, leading to better operational conditions, reduced wave induced loading acting on the FPSO deck structures and over the risers. The procedure adopted to select an optimized heading, is based in the analysis of several cases covering all the wave environment of Santos Basin, Offshore Brazil, and assessing parameters like roll motion and riser connection accelerations in order to select the optimum heading range. This problem has several input sea states, once the wave climate is defined by the extreme contour curves for the 100 year data. Another aspect is related to the combination of the FPSO loading conditions, which add more variables to the problem. A search algorithm have been implemented to enable the comparison of a large number of statistical results and to determine an adequate heading for the FPSOs.
Fiber Optic Monitoring of Subsea Equipment
Authors: Brower David , Hedengren John D. , Loegering Cory , Brower Alexis , Witherow Karl , Winter Karl
Published: August 2013

Bass Lite deepwater field in the Gulf of Mexico, at water depths of approximately 2,050 m (6,750 feet), commenced operation in February 2008. Natural gas is produced from Bass Lite via a 90-km (56-mile) subsea tieback to the Devils Tower Spar. This project involved several innovations, one of which was the incorporation of a fiber optic sensing system that measures real-time temperature, pressure and strain along the pipeline length. This is a first of its kind innovation that is in actual operation.