The plug and abandonment (P&A) operation is the final stage in the life cycle of oil and gas wells. The aims of the P&A operation are to seal the well and permanently maintain the well-integrity similar to that of the original natural caprock. Using this approach, it is possible to isolate fluids movement between different strata, to prevent environmental disasters. Generally, the main steps in the P&A operation are (i) accessing the annulus section of the well, (ii) cleaning the defected area, and (iii) placing the cement plug barriers into the target area. To prepare the target area and avoid cement plug contamination, a cleaning process is required inside and outside the casing. The jet cleaning process is one of the effective methods for the cleaning step, in which a liquid of a higher density is usually injected into the target area to displace a lighter ambient fluid. During the jet cleaning process, several forces affect the cleaning efficiency, including inertial, viscous and buoyancy forces. In this work, we analyze a fundamental component of the jet cleaning process in the P&A operation, via experimentally studying the characteristics of a miscible positively buoyant jet. In our work, a heavy fluid is injected downwards into a large rectangular tank filled with a light ambient fluid. Due to the large dimensions of the experimental tank, the wall effects on the flow are neglected, i.e. we consider a free jet. We investigate some of the parameters affecting the behaviour of our positively buoyant jets, such as the injection velocity, the nozzle diameter, and the ratio between the viscosity of the jet fluid to that of the ambient fluid. In the parameter ranges of our interest, the jet flow exhibits certain critical flow features, such as the laminar length (i.e. the initial stable part of the jet where the injection fluid remains laminar and does not mix with the ambient fluid) and the spread angle (i.e. the area occupied by the jet). Our results show that both the laminar length and the spread angle decrease by increasing the injection velocity. In addition, increasing the viscosity ratio results in increasing the maximum laminar length and decreasing the spread angle. These results can help to better design an efficient cleaning in the P&A operation of oil and gas wells.

The degradation of equipment used in production platforms operating in a maritime environment occurs, in most cases, by corrosion defects. High to low pressure separators remain operational but suffer from a progressive reduction of wall thickness by external corrosion. Repair techniques for tubes and pressure vessels using laminate composite material are widely known and already defined by technical standards. However, there are doubts regarding the effectiveness of these repairs when performed near openings, nozzles or other stress concentrators. The objective of the present work is to verify the efficiency of the composite repair and the influence of the defect thickness. To meet these objectives, four vessels were tested until burst. The results showed that composite repair was efficient since the burst pressure of the repaired vessel was considerably higher than its limit pressure without repair.

This study was built upon previous works conducted by the authors in a series of numerical studies on submarine hydrodynamics and is aimed at enhancing the accuracy of computational fluid dynamics (CFD) application processes, which estimate the hydrodynamic performance of underwater vehicles for steady translation conditions in the horizontal and vertical planes. In an earlier work, the computed straight-ahead resistance of a submarine agreed with those of experiments within a comparison error of 2%. However, a maximum comparison error of approximately 20% was obtained for sway force under a steady translation condition. The Defense Advanced Research Projects Agency (DARPA) Suboff submarine model was adopted as a benchmark, and the computational modeling was based on the Reynolds-averaged Navier–Stokes (RANS) turbulence model for steady simulations. The curvature correction approach was tested to improve the computation of circumferential flow around the cylindrical hull, in particular. The dominant maneuvering coefficients were calculated using the computed forces and moments as a function of the yaw and pitch angles along with simplified equations of motion by fitting a curve to the plots. The hydrodynamic forces and moments exerted on the stern plane were individually computed using a locally refined mesh around the tail section. It was confirmed that the curvature correction approach improved the computational accuracy for the steady translation conditions, and general trends were captured over the tested yaw and pitch angles. However, some data points had notable comparison errors. Some of the estimated maneuvering coefficients agreed well between the CFD simulation and the experiments, whereas others had considerable comparison errors. The individually computed forces and moments exerted on the stern plane that had attack angles were inconsistent with those obtained in experiments. Those comparison errors may have been amplified by the complexity of configuration and arisen from differences in the experiments, such as the presence of a free surface and supporting strut to mount the hull to a carriage, and, perhaps, the geometrical differences owing to machining accuracy. The investigation of flow field at the propeller plane revealed that the wake distributions inside the nozzle were significantly affected by the angled stern planes; the reduced velocity area was expanded and shifted. Furthermore, harmonic analysis of the wake fraction was conducted, and several primary n th harmonics were observed, which were associated with the struts and stern planes. This result suggests the risk of higher noise levels associated with the number of blades.

As the underwater structures of offshore Jacket platforms are always immersed in seawater, the marine growth, which include various forms of algae, slime, and seaweed, barnacles, mussels and other species of adhesive shellfish, will attach to steel-pipe surface and will accelerate structural corrosion and impair structural safety. Currently, routine cleaning and inspection task is undertaken by divers using cleaning jets in normally every 3∼5 years. The cleaning duration for one single platform will take up more than two months, even up to half a year, due to the constraints of weather windows and limited working hours of divers. It is a risky job for divers not only because of huge pressures that water-jets produce, but also the harsh working conditions of poor visibility, unexpected vortex and waves around platform, and etc. Underwater robots are being developed for various applications in offshore oil industry ranging from inspection to maintenance and cleaning of submerged surfaces and constructions. This paper introduces a novel underwater robot specializing in cleaning marine growth for offshore Jackets. Since the diameter of steel-pipes varies from about 600mm to 2000mm, a self-adapted mechanism is designed. The self-adaption mechanism makes the robot travel on pipes in different directions with high mobility and clean continuous region of underwater pipes’ surface at the same time. Two key issues have been studied in this paper. The magnetic adhesion method is adopted in the robot. A sensitivity study on the distance between steel-pipes and thickness of steel pipes with the adhesion force are conducted both using finite element method and experiments. Besides, the flushing capability for various nozzles has been simulated using computational fluid dynamics method. The proposed underwater robot is needed in the inspection and maintenance of offshore Jacket platforms. Compared with traditional maintenance by divers, it is more efficient, economic and safe.

The Bozi block of Tarim Oilfield is a typical tight sand condensate gas reservoir with serious wax plugging issue. It is necessary to study the phase behavior and the wax deposition pattern of well fluids in Bozi gas reservoir, which provides the phase enveloping diagram of the condensate gas and can help to control the paraffin deposition. In this study, the PVT tester and self-designed microscopic solid deposition tester are used to get the phase enveloping curve of the condensate gas, which is combined with software simulation results and production data of Bozi 104 well to analyze wax deposition from formation to wellbore, wellbore to ground and surface pipeline. The main conclusions and suggestions are as follows: with different temperature and pressure, condensate gas produces different phase regions such as gas phase, gas-liquid phase, gas-solid phase, gas-liquid-solid phase, etc. Above the dew point pressure, the WAT is less impacted from pressure while the WAT gradually decreases due to the dissolution effect below the dew point pressure. The experimental results are used to revise the software simulation results and then to obtain the phases of different temperature and pressure regions. The state envelope is used to predict the liquid separation and wax production situation at different production stages; the liquid condensation and waxing area of the Bozi 104 well lies in the surface pipeline behind the nozzle, where appropriate wax prevention measures are needed.

Energy saving devices (ESD) such as propeller ducts, pre-swirl stators, pre-nozzles, etc have been explored as a more economic and reliable approach to reduce energy consumption for both in-operation and newly design ships over the past decades. Those energy saving devices work in the principle of reducing ship resistance and improving propulsion efficiency as well as hull-propeller interactions. Potential saving from various types of ESD have been reported in literature from the range of 3–9% [1] for propulsion efficiency dependent on different measures. Deployment of those devices on actual full-scale ships has been limited over the past years. One of the key obstacles in application of ESD is the lack of confidence in measuring its efficiency on full-scale ships in actual operational conditions. Advances in computational fluid dynamics (CFD) has provided an alternative approach from model scale test to better understand uncertainties in prediction of ESD efficiency in full-scale ship operations [Shin et al, 2013]. In this work a high fidelity CFD model is presented for investigation effects of pre-nozzles on propulsion efficiency and ship resistance. The model is based on the Reynolds Average Navier-Stokes (RANS) solver with different turbulent models including a hybrid detached eddy simulation (DES) approach for predictions of complex near body flow features as well as in the wake regions from hull and propeller. The model is validated with model test for both towing and self-propulsion conditions. Finally a study of pre-nozzle effects on propeller efficiency as well as hull-propeller interaction is presented and compared with available experimental data (Tokyo 2015 Workshop). The current work constitutes a fundamental approach towards designing more efficient ESD for a specific hull form and propeller.

Kinetic turbines harnessing tidal and ocean currents make use, in some designs, of nozzles and/or diffusers. Nozzles come at a cost, but they can help from the structural, hydrodynamic or positioning points of view. In those cases, they might make sense as long as they drive the LCoE (Levelized Cost of Energy) down, which is the ultimate objective of energy-harnessing devices. The design must then optimize the combined performance of both blades and nozzle. However, the interaction between turbine blades and nozzle is not always fully clear, and even less its optimization. A relevant amount of efficiency can be lost if the design spiral is not appropriate. The authors have suggested in [1] an approach for the optimization of turbines within nozzles. This approach was followed in [2] and validated with model tests. In the approach, the turbine is initially substituted by an actuator disc that applies a radially constant pressure drop. But in these references, the optimum pressure drop in the actuator disc was the same as if there was no nozzle at all, i.e., 4/9 ρv 2 . This is equivalent to considering the nozzle coefficient does not depend on the pressure drop, and thus, on the induced velocity field. Hence it is a somewhat arbitrary assumption. This paper describes, using actuator disc theory, how nozzles affect the disc optimum pressure drop in uniform flow conditions. The effect of a hub is also analyzed. Then, using a viscous FVM CFD code, the variation of the pressure drop is quantified for two different acceleration nozzles, one suffering flow separation and the other one not. As the pressure drop increases, so does the flow expansion downstream. This rises the average radial component of velocity at the nozzle, increasing the thrust and nozzle coefficient. Therefore the optimum pressure drop goes up compared to that without nozzle. The increment in efficiency that can be obtained with this approach is quantified for the studied nozzles. Finally, the integration of this effect into the blade design is discussed.

In this paper CFD results are presented for the thruster-hull interaction effects for a drillship with 6 azimuthing thrusters. The results using different approaches to model or simulate the propeller are compared and their advantages and disadvantages are discussed. The approaches investigated are the so-called Frozen Rotor approach, where the propeller rotation is modeled, the Actuator Disk approach with prescribed body forces and the unsteady Sliding Interface approach where the motion of the propeller is simulated in time. First, open-water calculations for a tilted thruster are carried out using the Frozen Rotor approach. The open-water calculations are repeated using the Actuator Disk prescribing the propeller thrust and torque distribution obtained from the Frozen Rotor calculations. The results with Actuator Disk are very similar for the unit thrust and nozzle thrust compared to the results using the Frozen Rotor approach. Furthermore, the results using the Frozen Rotor or the Actuator Disk are very close to the experimental results for the nozzle thrust. The thruster-hull interaction of one active thruster under the drillship is investigated using the Actuator Disk approach, the Frozen Rotor Approach and the Sliding Interface approach. A comparison to experimental results is presented for the thruster-hull interaction coefficients. Using the Actuator Disk a good agreement with the experiments is obtained. The results using the Actuator Disk and Sliding Interface are very similar to each other, but the computational costs for the Sliding Interface method are at least a factor 20 higher. The results using the Frozen Rotor deviate due to an unphysical wake behind the thruster. Based on the results presented in this paper we conclude that, using the steady-state approach with the Actuator Disk, CFD can be a cost-efficient and accurate method to determine the thruster-hull interaction effects at bollard pull conditions for a typical offshore vessel.

The flow interaction between a dynamic positioning (DP) thruster and a floating structure (semi-submersible) hull attracted quite a lot of attention in recent years. In this study, the Spalart-Allmaras RANS model has been evaluated to simulate single thruster rotated in the open water with OpenFOAM. The actual thruster geometry has been meshed with structured grid, and the gap between the blade tip and nozzle is carefully treated. The Moving Reference Frame (MRF) method is used for steady-state simulation, and the arbitrary mesh interface (AMI) method is applied to simulate the rotating blade for transient dynamic mesh simulation. The numerical results are compared with available experimental and numerical data, especially in the wake flow. Good agreement is shown in this study.

In the paper, the integrated structure of a dual cylindrical caisson embodying an OWC device is designed. The seaside hemi-toroidal column of the dual cylinder caisson is an air chamber for water column oscillation while the leeside hemi-toroidal column of the dual cylinder is filled with sand for caisson stability. Experimental investigations were carried out on the performance of the OWC at different water depths, wave heights and wave periods, as well as on the layout of the caissons and the nozzle area ratio. Experimental results show that the water depth has significant effect on the wave energy conversion efficiency of dual cylindrical caisson breakwaters embodying an OWC. The new structure proposed as a breakwater or revetment structure especially arranged in a one-third convex arc has advantages in collecting more energy and creating a fine landscape environment.

Sustained Casing Pressure (SCP) is a well integrity problem and its removal is required. Techniques that involve replacing the fluid inside the annulus with a heavier fluid (kill fluid, KF) to stop gas migration have so far failed due to issues resulting from fluid incompatibility. This study aims to develop an intervention fluid compatible with water-based annular fluids. Based on the theory of buoyant slippage, brominated organic fluids have been produced and tested to assess compatibility and performance with multiple physical models. Results showed that the KF was able to settle down in water-based fluids, build up and exert pressure at the bottom. Experiments also exposed the formation of a mixture zone just above the building-up KF column. Lower injection rates and/or larger nozzle sizes decrease KF dispersion, prevent mixture zone formation and increase KF recovery. Intervention fluids developed in this study may revive the defunct bleed-and-lube (B&L) technique that would dramatically reduce the cost of SCP removal or may be used in an alternative process of continuous displacement that would significantly reduce the time of well intervention. Presented in the paper is also a road map for testing the SCP removal process that would lead to development of this technology.

The objective of this study is to investigate flow behavior and survival ratio of different fluids with Hollow Glass Microspheres (HGM) as they pass through jet nozzles under various flow conditions. Three types of HGM with different compressive strengths (5,000 – 19,000 psi) and specific gravities (0.38 – 0.46) for 10% and 30% volumetric concentrations were tested under 196 – 345 ft/s average fluid velocities from 0.5 – 1.5 inch standoff distances during impact tests using a new High Shear Rate Facility (HSRF). Density measurements before and after each circulation cycle were used for calculating survival ratio of the HGM. Particle size analysis was carried out to see the change in the size distribution of HGM after impact experiments. A scanning electron microscope (SEM) was used to view samples of HGM from before and after impact tests to define breakage behavior. The Hertz Impact Law was used to build a mechanistic model to estimate survival ratio under several assumptions. An empirical equation was developed and compared with experimental results. Breakage type is the result of cyclic fatigue because breakage does not occur in one circulation. Standoff distance, concentration of HGM and velocity of the fluid are strong parameters of the survival ratio function. Size distribution analysis and SEM microphotographs show that larger HGM break first.

Combined with the anticorrosion character and requirement of the ocean engineering, the painting growth model sprayed by the coating spray robot is studied in this article. A special experiment platform is set up to carry out the test orthogonal experiment design method (OED), and enough data are recorded. Experimental data are used to build back propagation (BP) neural network as a sample set, while validated by the genetic algorithm (GA). Then the dry film thickness (DFT) growth model is established to analyze the relevance between dry film thickness and influence factors such as distance between the spray gun and surface, spray gun nozzle, velocity of the spray gun and so on. The model can provide important theoretical direction for the use of spray robot in the ocean engineering.

The viability of offshore wind turbines is presently affected by a number of technical issues pertaining to the gearbox and power electronic components. Current work is considering the possibility of replacing the generator, gearbox and electrical transmission with a hydraulic system. Efficiency of the hydraulic transmission is around 90% for the selected geometries, which is comparable to the 94% expected for conventional wind turbines. A rotor-driven pump pressurises seawater that is transmitted across a large pipeline to a centralised generator platform. Hydroelectric energy conversion takes place in Pelton turbine. However, unlike conventional hydro-energy plants, the head available at the nozzle entry is highly unsteady. Adequate active control at the nozzle is therefore crucial in maintaining a fixed line pressure and an optimum Pelton turbine operation at synchronous speed. This paper presents a novel control scheme that is based on the combination of proportional feedback control and feed forward compensation on a variable area nozzle. Transient domain simulation results are presented for a Pelton wheel supplied by sea water from an offshore wind turbine-driven pump across a 10 km pipeline.

Free fall lifeboats have been developed for emergency evacuation from offshore installations, when conventional means of transportation cannot be applied. As a consequence, the ability of the lifeboats to perform safe drop- and sail away under all circumstances has to be demonstrated. This paper focuses on efficient and robust numerical simulation of these operations. To predict the lifeboat behavior under a large variety of stochastic conditions, such as irregular waves, in combination with wind and current, calls for many individual simulations of the sailaway performance. The present paper presents a brand new mathematical model that is able to predict the behavior of a free fall lifeboat during drop-phase, the submerged phase, the surfacing phase and the “sailing in waves” phase (combined, not gluing time traces from different predictions together). The proposed mathematical model is completely non-linear in nature and considers instantaneous submergence and attitude of the lifeboat. All forces and moments in 6 degrees of freedom are calculated instantaneously. Consequently, accelerations, velocities and displacements, are calculated based on this. The paper describes the build-up of the mathematical model. This is based on a summation of forces due to impact forces, cross flow drag forces, generated lift, centrifugal forces, buoyancy forces, propulsion (propeller and nozzle), steering forces due to steering action and resistance. Obviously, also the instantaneous added masses play an important role. This results in a mathematical model for rigid body motions in 6 degrees of freedom, which can be used for predicting the motion response during drop- and sail away. This means a validity range in very extreme weather and in calm water. The paper will show basic validation and several applications.

Extracting the spudcan footings of mobile jack-up rigs from the seabed at the end of their operations is challenging when the capacity of the rig to pull is low compared to the extraction resistance of the spudcans. This is particularly the case when the spudcans are deeply embedded in soft clay and subjected to long periods of operation that place load on the foundations and allow for consolidation to occur in the soil. A technical solution used by the offshore industry to overcome spudcan extraction resistance is to use a water jetting system that ejects pressurised water through nozzles on the spudcan face. The aim of using water jetting with nozzles located at the top of the spudcan is to reduce extraction resistance through fracturing and softening of the upper soil. However, the efficiencies of such systems are not known to offshore jack-up operators. Top jetting efficiency is therefore addressed in this paper, which reports a series of physical experiments on jetted spudcan extraction conducted in a geotechnical beam centrifuge. The efficiency of water jetting is reported for extraction from depths of up to three diameters in normally consolidated clay, for different jetting flow rates. The excess pore pressure and maximum breakout force measured reveal insights into the extraction process with top jetting. The maximum extraction resistance is shown to be unaffected by top jetting but relates to the suction developed at the spudcan base, which can be reduced by jetting at the spudcan base (Bienen et al. 2009; Gaudin et al. 2011). Top jetting can, however, reduce the extraction resistance post breakout as indicated by the experimental results of this study.

The ability of a DP-vessel to keep its position depends highly on the performance of the DP system. The thrust efficiency of the DP-system depends on the efficiency of the individual thrusters, but also on the interaction of the thruster wake and the hull of the vessel. This thruster-hull interaction becomes even more important when the vessel is a semi-submersible vessel; the thruster wake of the thruster on the upstream pontoon might impinge on the downstream pontoon resulting in high losses in efficiency and reduced DP-capability. Heerema Marine Contractors has two DP-semi-submersible crane vessels; the Thialf and Balder. An assessment of the thrust efficiency of the DP thrusters of these vessels has been made by comparing CFD computations with dedicated model tests. In previous benchmark studies CFD is used to assess the current loads as well as thruster-hull interaction without current on a semi-submersible vessel. The logical next step is to perform a numerical study on a thruster-hull interaction with current. Similar as the previous benchmark studies the numerical data are validated with a series of dedicated model tests. The model test data include the global forces, the forces on each individual pontoon and the forces of each individual thruster, including the nozzle thrust and propeller thrust. The comparison between the CFD and model test data shows that CFD is able to predict the relevant force components within a sufficient accuracy for engineering purposes. At present not much is known about the extrapolation of model scale DP-thrust efficiency to full scale DP-thrust efficiency, neither for model test results, nor for CFD results. Scaling CFD from model scale to full scale is not trivial; it involves a significant change in Reynolds number, a different description of boundary layer and poses challenges to meshing and grid. Therefore, validation is required. A first validation study is performed based on data acquired during a transit of SSCV Thialf in Q4 2011. In preparation, CFD simulations are performed for different thrust combinations. These results are compared to full scale observations and, where possible, improvements to the numerical modeling are assessed. The paper addresses lessons learnt to improve the CFD computations as well as practical aspects and limitations of thrust efficiency modeling, including all interaction effects, using CFD from an engineering perspective.

The wake flow behind a ducted azimuthing thruster was investigated. The thruster wake is an important factor in thruster interaction effects. Model tests were carried out for 3 different configurations; a thruster in open water conditions, a thruster under a flat plate and a thruster built into a barge. Two different thrusters were considered, a ‘normal’ thruster with a horizontal propeller axis and a ‘tilted’ thruster with a propeller axis and nozzle oriented 7 deg down-wards. In the tests the propeller thrust and torque were recorded, as well as the nozzle thrust and unit thrust. The velocities in the wake of the thruster were measured using a PIV (particle image velocimetry) system, for down-stream locations up to x/D = 19. The influence of the thruster tilt, the plate above the thruster and bilge radius on the thruster wake flow were investigated. Detailed PIV measurements were carried out on the wake flow behind the thruster in open water conditions. The PIV system used can measure 3D velocities in large set of points in a 2D plane, which is illuminated by a laser light beam. The flow velocities were measured in a large number of cross sections at different distances from the thruster. The PIV measurements provide a detailed image of the flow velocities in the thruster wake, showing the axial velocities, as well as the rotation and divergence of the wake. Subsequently, PIV measurements were carried out for the thruster under a flat plate and the thruster under a barge. The measurement results show a thruster wake that is deformed by the presence of the plate and the barge. The plate and the bottom of the barge form a flat plane above the thruster, clearly flattening the cross section of the thruster wake. Furthermore, the wake flow at the side of the barge, near the bilge radius, results in a low pressure region, causing the wake flow to diverge up as it flows from under the barge into the open water. This phenomenon is known as the Coanda effect and is strongly dependent on the bilge radius and the distance between the thruster and the side of the barge. The effect of both these parameters was confirmed in the model test results presented. The typical flow patterns observed as a result of the Coanda effect are illustrated in Figure 1 below. The results of the present model test research are used to further improve the understanding of the physics of thruster interaction effects. Furthermore, the results will serve as validation material for CFD calculations.

This paper describes performance improvement of wave power absorption by using a new concept. Basic system proposed is an oscillating water column (OWC) type. An artificial harbor surrounded by projecting walls is installed. The type is called as PW-OWC in this paper. Standing waves occur in the artificial harbor, the absorbing device consequently has a resonance frequency differing from that of OWC. From the effect, the system is able to absorb wave power in very wide range of the wave frequency. From the experimental results, PW-OWC types are very good performance of wave power absorption comparing with conventional OWC types. In addition, the performance of the PW-OWC type is insensibility to the nozzle ratio of an orifice. The performance can be easily improved by installing the harbor part even if the performance of a base OWC device is not good. Finally, we investigate the expected values of acquirable wave power in not only a year but also every season. The expected values of PW-OWC types are greater than that of conventional OWC ones in seas around the Japanese islands.

This paper provides an overview of the development of abrasive-slurry-jet methods for completion and stimulation applications. Abrasive particles added to the fluid dramatically reduced the system pressure requirements. The paper discusses the technical capabilities of cutting through various materials and formations and also discusses improvements and proven applications. Abrasive fluid-jet systems are capable of cutting through rocks of all types, and with greater location control that is not susceptible to the geologically induced deviations encountered with mechanical methods. Abrasive fluid-jets drill rock through the erosion induced by very small particles which individually remove only small fragments but are in such numbers that the drilling rate is at or above that of conventional tools. The particles are powered by the velocity of the supporting fluid, generated in turn by pumps on the surface. The cutting occurs ahead of the nozzle body allowing the nozzle assembly to be fed in the tunnel to create drain-holes. Fluid-jet methods have the potential of improving completion and stimulation efficiency in heterogeneous formations such as fractured and/or vuggy carbonate reservoirs. Application for completion, stimulation and, even, well/platform abandonment have been successful.