Casing wear poses a significant safety hazard during drilling and production of hydrocarbons. Failure to maintain integrity due to inaccurate casing wear estimation can cause severe accidents or preclude prospective operations. Current industry practice is to estimate casing wear during the planning phase of the well and subsequently use assumed operational parameters with inherent uncertainties. This paper aims to study how to utilize real-time data to improve the industry standard methodology and evaluate the benefit of the modification. The research was conducted by applying the model on data from a well on the Norwegian continental shelf. There were two main objectives of the research. Firstly, the industry standard approach to casing wear estimation was expanded to include real-time data. Application of real-time data to the industry standard method for estimating casing wear caused a significant difference in results. The approach using real-time data resulted in an estimate of more casing wear compared to the standard approach. Secondly, an algorithm for continuous prediction of casing wear at the end of operation was developed. The predictive algorithm resulted in consistently more accurate estimates in relation to the final value throughout the operation. With variations in input parameters and consecutive casing wear of this magnitude, well integrity cannot be ensured during operation without application of real-time data. The failure to maintain well integrity demonstrates the necessity of the proposed approach.

Wear factor is an important parameter for estimating casing wear, yet the industry lacks a sufficient data-driven wear-factor prediction model based on previous data. Inversion technique is a data-driven method for evaluating model parameters for a setting wherein the input and output values for the physical model/equation are known. For this case, the physical equation to calculate wear volume has wear factor, side force, RPM, tool-joint diameter, and time for a particular operation (i.e., rotating on bottom, rotating off bottom, sliding, back reaming, etc.) as inputs. Except for wear factor, these values are either available or can be calculated using another physical model (wear-volume output is available from the drilling log). Wear factor is considered the model parameter and is estimated using the inversion technique method. The preceding analysis was performed using soft-string and stiff-string models for side-force calculations and by considering linear and nonlinear wear-factor models. An iterative approach was necessary for the nonlinear wear-factor model because of its complexity. Log data provide the remaining thickness of the casing, which was converted into wear volume using standard geometric calculations. A paper [1] was presented in OMC 2019 discussing a method for bridging the gap. A study was conducted in this paper for a real well based on the new method, and successful results were discussed. The current paper extends that study to another real well casing wear prediction with this novel approach. Some methods discussed are already included in the mentioned paper.

In modern oil/gas producing industry, vertical, 2D and 3D directed, and multilateral (branched) boreholes are drilled. Their trajectories are designed depending on the petroleum deposit depth and structure, properties of mining rocks, their hardness, heterogeneity, fracturing anisotropy, permeability, and so on. Therefore, the borehole cost and its productivity are determined by the length, smoothness, and configuration of its trajectory. To enhance efficiency of a borehole and to reduce cost of its drivage, to enlarge rate and volume of the reservoir depletion, it is proposed to use methods of optimal control for the best tracking of its trajectory. Through application of the differential geometry correlations, the mathematic model of the borehole outline in the form of nonlinear ordinary differential equations system is elaborated. Different objective functions, representing total integral curvature of the borehole axis line, its length, and cost of its drivage, are selected; additional constraints, separating allowed and forbidden zones of passing, are chosen. The functions of the trajectory curvature and torsion are used as controlling variables. The continuous correlations of the model are discretized and, further, the techniques of nonlinear programming and optimal control are employed. On the basis of the method of objective function gradient (antigradient) projection on the linearized constraint planes, the step-by-step algorithm of approaching to the optimal trajectory is elaborated. To correct the spoilt constraints, at every step of calculations, the Newton method is used. The elaborated approach is applied to optimization of deep curvilinear borehole outlines. The results of numerical analysis are discussed. It is shown that smoothing the hole trajectory permits also to diminish the contact and frictional interaction between the drill string and bore-hole wall and, by this, to decrease the resistance forces acting on the string during tripping in/out operations performing and to diminish energy expenditures for these operations fulfillment; to decrease the rate of the drill string tube wear; and to reduce the drill string sticking occurrence probability.

For reducing the maintenance cost of floating offshore wind turbine structures, it is necessary to establish a quantitative wear estimation method for the mooring chains. In this paper, attempts have been made to improve the accuracy of the estimation method in terms of the mooring chain model. These investigations were performed about a spar-type floater moored with three catenary mooring lines at Goto, Nagasaki prefecture, Japan. Up to now, the mass-spring model had been used for the mooring chain in response analysis and the relative angle between two spring lines was considered as only a sliding angle without friction. However, there is also rolling in the motion between mooring links, which should cause less wear than by sliding. In this study, the detailed motion of the link in response analysis is calculated and applied to the wear estimation by using a 3-D model in MSC. Adams. This enables the wear estimation considering link motion closer to a real phenomenon. A Contact analysis between the 3-D chain model requires some contact properties (e.g. contact stiffness and friction). In this paper, these properties are calculated based on the Hertzian contact method and FEM analysis. As a result, the wear amounts overestimated by using the mass-spring model in the previous investigation, especially at the point located clump weight and touchdown point, decrease getting closer to the measurements. In addition, by tracking the contact points it is found that the major motion caused between links is the rolling. For future works, there remains a need for further validation and the consideration of elasticity between mooring links, impressions caused by proof load test and the effect of corrosion.

Surface rust layers of corroded steel of coastal structures is removed by sea ice action in winter, leaving bare mild steel. This phenomenon is repeated every winter, promoting wear due to increasing corrosion. Serious damage to training levees comprising steel-sheet-pile seawalls on the Sea of Okhotsk, JAPAN occurred earlier than expected, since large wear rates were not considered in their design. This paper proposes a simple method for estimating wear amount (thickness) per year based on wear rate in a steady (mild) wear region obtained from sliding wear tests. This wear thickness can thus be taken as an additional thickness for the expected lifetime of a structure, as a rational countermeasure to corrosion as a sacrificial layer.

Floating wind turbine facilities installed in deep sea areas play an essential role in the promotion of green energy. One of the problems associated with the commercialization of facilities installed in the deep sea is the maintenance cost of mooring chains, because they are expensive and wear between links leads to chain breakage. Therefore, it is necessary to establish a quantitative wear evaluation method for mooring chains. An experimental facility to reproduce the wear caused by sliding between links at the scale of an actual floating wind turbine was developed to investigate the wear performance in seawater conditions, and wear tests were conducted. Substitute ocean water was applied to the experiment instead of seawater. In addition, a procedure for nonlinear finite element analysis was improved to estimate the behaviour of wear between links. Measured stress versus strain relations of the links was considered in the finite element analysis. The experiments and numerical analysis confirmed that the amount of wear in the substitute ocean water was less than that obtained in dry air and that the tensile force between links is an important factor for the degree of wear between links.

Dynamic umbilicals are widely used in the wet tree development of deep-water field. In the application of dynamic umbilicals, fatigue is a key failure mode which may cause severe economic consequences. The fatigue behavior of dynamic umbilical is complicated by the mechanical properties of the material, the cross-section design and the interaction of different components. Published studies have shown that the interaction friction stress of steel wires is critical for fatigue analysis. Coefficient of friction were used as a constant during the fatigue life analysis. However, the friction stress and the coefficient of friction may change during the wear of the steel wire. This paper presents a new experiment method and device to evaluate the change of the coefficient of friction of steel wire. Different cycles of reciprocating wear were carried out on samples to simulate interaction of the steel wires during the process of fatigue. The samples are pairs of steel wires which were cut on a dynamic umbilical. The interaction stress and friction stress were measured during the test. Micro morphology on the surface of steel wire at the contract zone were measured to investigate the wear of the sample. The test result shows that the coefficient of friction and stress changes during the test. Worn marks were found on the contact zone of the steel wires. The change of the coefficient of friction may affect the accuracy of fatigue life analysis of dynamic umbilical. Conclusions were also presented on the coefficient of friction test and theoretical calculation method to approach a more accurate fatigue life for dynamic umbilical design.

Japan Oil, Gas and Metals National Corporation (JOGMEC) has been conducting R&D of production system for Seafloor Massive Sulfides (SMS) mining. In order to reach commercialization stage in SMS development, the pipe wear due to slurry flow is one of many technical issues to be solved. In the present paper, the authors conducted the large scale experiment using full scale pipes to obtain the various data on pipe wear, and investigated the pipe wear amount in commercial operation. In the experiment, SUS 304 and UHPE were used for the inner materials of test pieces. In addition, the test pieces were installed in the vertical, horizontal and inclined section. Then the authors estimated the thickness reduction of pipe due to slurry flow based on the experimental results.

For reducing maintenance cost of floating offshore wind turbine structures, it is necessary to establish quantitative wear estimation method for the mooring chains. In this paper, attempts have been made to improve the accuracy and applicability of the estimation method in the following steps. By using the wear analysis method between the links of mooring chain that Gotoh et al. investigated with the finite element analysis software (MSC. Marc), the expression of the wear amount was obtained, which was a proportional functional form associated with sliding angle and tension. The relative sliding angle and tension between links were analyzed by using a multibody dynamics software (MSC. Adams). A spar-type floater moored with three catenary mooring lines at Goto, Nagasaki prefecture, Japan was analyzed. Here, the floating body was modelled as a rigid body and mooring chains were modelled by mass-spring (lumped mass) model. From these results, the wear amounts calculated by using the estimation formula and relative sliding angle and tension between links were compared with the measured wear amounts for mooring chain of the floater which was deployed for about one-year at Goto. The cases with only waves and those with wind and waves were analyzed. From the comparison between the simulation results and the measured ones, it was found that the proposed method can fairly predict the wear amount of mooring chains. However, it was also found that the proposed method has a tendency to overestimate the measured results. These reasons were discussed in the paper.

For ultra-deepwater subsea wells, a riser system is required to conduct completion, intervention/workover and end of life activities. For ultra-deepwater riser systems with high temperature and pressure requirements, the intervention riser system often requires vessel interface optimization to achieve acceptable design response. The upper riser can be configured in several different ways, each with its own benefit from a safety, risk and performance perspective. This paper compares the riser response for various vessel interfaces for ultra-deepwater applications. As discussed above, intervention riser structural response is sensitive to the riser configuration at the vessel interface. For a typical intervention riser, due to ultra-deepwater and high tension requirements, the functional tension load may utilize up to 40% of yield strength thus decreasing the capacity available to accommodate bending and pressure loads. Vessel operators have options to modify the system configuration to improve the strength and fatigue response of the riser. The different vessel interface options include the tension lift frame (TLF) to vessel interface, the top tension application method and the use or otherwise of a surface tree dolly. Upper riser assembly (URA) loads may be optimized by use of rotary wear bushings, a cased wear joint assembly or flexjoints as a part of the stack-up. The various riser-vessel interface options are evaluated and compared in this paper. This paper highlights the riser design challenges for ultra-deepwater applications.

There are currently 365 FPSOs in service around the world. These vessels all use mooring lines to maintain position and provide stability, keeping the vessel and cargo safe. However, more than 21 failures have occurred between 2001 and 2011 and approximately 50% of the reported failures occurred in the first 3 years of 20-year design life. Each mooring line failure represents the potential for serious environmental and economic consequences. Based on industry surveys, the most common failure mode is fatigue failure. In the current offshore standards, the surface degradation due to wear and corrosion is modelled as a diameter loss at a standards rate. To assess whether the uniform reduction in chain diameter suggested in the offshore standards is able to explain the early chain failures seen in service, this paper incorporates two wear rates into a fatigue life calculation; one wear rate is taken from DNV-OS-E301 and is compared against one taken from NORSOK M-001. Three fatigue life estimation approaches: tension, nominal stress and hotspot, are used to compare the differences in fatigue method. The stress in the chain is calculated using an analytical model, which is verified against an FE model. The effect of wear degradation on the ultimate strength of the chain is calculated based on the minimum breaking load. The results show that the diameter loss rates suggested in the offshore standards are not able to explain the early mooring chain failures seen in the past and that the reduction of diameter cannot solely explain the early failures seen in service. The hotspot approach, not often used in mooring line predictions, is best able to predict these shorter lives, as it offers more accurate fatigue predictions by considering high peak stresses compared to standard methods such as tension and nominal stress approaches.

The last decade Equinor has taken a significant amount of mooring chains out of service. Typically, one mooring line for an offshore installation has been replaced. Purpose has been to assess whether current chain condition (corrosion and wear), make the remaining chains fit for further use. The chains have been subject to thorough inspection, break load testing and fatigue testing. Depending on results of the inspection and tests, the remaining mooring chains have either been replaced or found fit for further use. From one installation, smaller cracks on the inside of the crown of chain links have been observed. These cracks do not coincide with known fatigue hot spots or fatigue breakages from fatigue testing of chains. Many cracks are visible with the bare eye, and NDT techniques are not necessary for detection. Chains with such cracks have been subject to both break load testing and fatigue testing. Overload breakage and fatigue cracks have not occurred or developed from the cracks. Chain links with cracks have been dissected to investigate crack depth. Dissection has also aimed to assess the age of the cracks. Both chain links that have and have not been subject to full scale fatigue testing have been dissected. This paper presents example of the observed cracks, including details on link location in the mooring line, crack positions in links, and measured depth and length of cracks. The paper also discusses the different possible causes of the cracks, and whether the cracks are critical for the endurance of the chains.

Side-by-side operation of multiple floaters in the ocean environment is very challenging and the operators always prefer a maximum operable weather window, in order to minimise the cost incurred from the downtime. The safety of the gangway connecting the floaters is very crucial and its dynamic response in the ocean environment raises concerns during operations. Therefore, an efficient dynamic positioning system is essential to maintain the floater and ultimately, the gangway response within the desired limits. In this work, a novel dynamic positioning system for floater operating aside another vessel is presented. The system includes an adaptive controller combined an optimised thruster allocation law and with a sea state detector. The adaptive control is achieved by using the barrier Lyapunov function and a predictor-based method in combination with the neural network scheme. The limitations include the saturation of inputs and the forbidden zones due to thruster-thruster or thruster-hull interaction. An optimised allocation for lower fuel consumption, wear and tear of the thruster equipment and to ensure the resultant command in the respective direction of the azimuth thrusters is designed. The optimisation here is a non-convex problem and a locally convex reformulation of second order is implemented. The presence of unknown shielding effect due to nearby vessel in a side-by-side configuration and input time delay is also considered in the development of this thruster allocation law. In order to overcome these effects, a novel sea state detector is designed. The sea state detector can effectively monitor the variation of drift wave-induced force on the vessel and activate the neural network compensator in the controller when a large wave drift force is identified. Simulation studies are conducted to verify the efficiency of this dynamic position system and a demonstration of flotel in side-by-side configuration with a turret moored FPSO is presented for the non-collinear ocean environment.

Large-scale deployment of multiple structures within a Floating Offshore Wind Farm (FOWF) will place many challenges on both the approach to effective integrity management and the demand to reduce through-life operating costs. Additionally, wind farm designers and operators will need to consider the issues related to design robustness in the design of their systems, whereby rational robustness criteria are applied to address possible accidental conditions that are not explicitly addressed in code-specified design basis conditions. This paper will review the significant advances in the approach to integrity management that have been recently made in the mature offshore oil industry and relate them to the nascent offshore wind industry. In particular, the requirement for consideration across the full lifecycle of the potential for threat introduction and the application of controls to prevent or mitigate the evolution of those threats will be described. This includes threats that may be inadvertently introduced during the design, manufacturing and installation phases, in addition to the more traditional rate-based degradation mechanisms such as fatigue, corrosion and wear that occur once the facility is in operation. The challenges that commercial-scale wind farm developments will face relate particularly to integrity issues arising in the design, manufacturing and installation phases, where the focus needs to ensure that degradation threats are not being introduced into the deployment of multiple repeat-copy units. The issues of common cause/common mode degradation threats will have much higher significance for a commercial-scale wind farm, where rectification of a common issue across a large number of floating units could have significant impacts in terms of reliability, operating costs, insurance premiums and power purchase agreements. FOWF mooring system designs are also likely to be more optimized than that for a typical one-off offshore oil facility. This will require the wind farm designer to have a deep understanding of the fundamental dynamic behavior of the overall system and the local dynamic behaviors of the components within the mooring system in order to be able to fully identify the types of threats that may be present. This may also involve robustness considerations where there may be step changes in the dynamic behavior of the system, often termed as cliff-edge effects. The paper will outline issues that wind farm designers will need to consider in building integrity considerations into the design and execution phases of a development, as well as the opportunities that risk-based integrity management processes offer in terms of through-life condition verification and inspection optimization.

Generally, equipment or mechanical parts are worn out by abrasion, impact, erosion, and environmental corrosion. To reduce these losses, the welding that wears the surface of the material with abrasion or corrosion resistance is called overlay welding. Corrosion and high temperature oxidation are very important factors in overlay welding material selection because they have a great effect on wear rate. Typical equipment is made of carbon steel, stainless steel or a material that is not wear resistant. Therefore, overlay welding is applied to parts where the parts themselves are subject to severe wear or corrosion. The diffusion of chlorine and alkaline ash during the combustion process causes foiling, corrosion, and erosion in the water wall panel & tube which is a heat transfer device. In order to protect the water wall panel & tube from various chemical and physical phenomena occurring during the combustion process, heat-resistant alloy steel such as Inconel is overlay welded on the outer surface of carbon steel pipe. However, since the length of the water wall panel & tube is more than 7 meters, the deformation after overlay welding is very large and the straightening work requires a lot of time and money. Also, since cooling water flows through the pipe during welding, the temperature of the cooling water also affects the welding deformation. In this study, welding process sequence and coolant temperature are optimized to minimize welding deformation during overlay welding.

Condition monitoring technique has been widely applied in Maritime to ensure safe operation and minimise unscheduled downtime. However, in practice, ship operators need to assure that a failure mode is indeed monitored by the sensor intended for it, and the sensor has sufficient accuracy and precision for its purpose. Additionally, for a reliable condition monitoring technique, issues such as sensors degradation or drift that will reduce the data quality over time must be addressed. All these require that ship owners to select a monitoring system with the best suitable sensors technology while is economically viable. In this paper, tunnel thruster was used as a case study to demonstrate the basic approach to develop a reliable condition monitoring technique through Failure Mode, Effects and Criticality Analysis (FMECA). Based on failure modes, four types of condition monitoring techniques were identified including Vibration Monitoring, Acoustic Emission Monitoring, Wear Debris /Water in Oil Monitoring, and Thermal Monitoring, where vibration monitoring is discussed in detail as an example for defining the sensor specification. For a reliable condition monitoring technique, prediction of sensor reliability will be especially useful in the situation where sensors systems can degrade over time in service. Using temperature sensors as an example, a Bayesian network (BN) modeling approach has been carried out for assessing sensor reliability affected by aging.

Casing collapse capacity was identified by Statoil as a critical operational parameter on one of its fields in production. This facilitated the need to re-evaluate the overall well design, specifically the production casing’s collapse capacity. Studies were performed to analyze and objectively increase the documented casing collapse capacity, while maintaining the safety level. Initially, the casing collapse capacity was evaluated using API TR 5C3 / ISO 10400, with insufficient capacity being documented. In order to investigate further, physical material testing and collapse testing were performed. Detailed finite element analysis was used to evaluate the casing collapse capacity, given well specific input parameters. The four critical parameters of axial load, casing ovality, casing wear, and temperature-dependent material properties were identified and the importance of each parameter was mapped. Using the testing results and the finite element models as a basis, structural reliability analysis (SRA) was applied to calculate the probability of failure for casing collapse of the production casing as a function of the differential pressure. The SRA provided results for the spread of the field and for individual wells given specific input on the key parameters of casing ovality, wear and temperature. At the selected target reliability level, the SRA results showed a higher collapse capacity of the production casing relative to conservative calculations commonly used from API TR 5C3 / ISO 10400 for well design. Applying SRA to well design, specifically collapse evaluations, has proven useful in concluding on the probability of failure. The SRA has transformed improved knowledge from testing and measurements to reduced uncertainty and a corresponding reduction in the failure probability. The potential over-conservatism in the conventional deterministic analysis is then avoided, while maintaining the overall safety level. The SRA results were used to assist in the risk evaluation resulting in an allowance for continued production on existing wells.

Floating wind turbine facilities, which are installed in the deep sea area, plays an essential role to promote the green energy application. One of the problems associated with the commercialization of facilities installed in the deep sea is the reduction of the maintenance cost of mooring chain, because breaking of the mooring chain caused by the wear between links leads to enormous economic losses. Therefore, it is necessary to establish a quantitative wear evaluation method for mooring chains. Experimental facility to reproduce the wear caused by sliding between links in actual scale applied for floating wind turbine, which had been proposed by the authors, was updated and the wear tests was conducted by setting some tensile force conditions between the links. Besides, procedure of the nonlinear finite element analysis was improved to estimate the behaviour of wearing between links. From experiments and numerical analysis, it has been confirmed that the tensile force between links is an important factor of the wear amount between links.

Chain corrosion-fatigue often governs the design of the mooring systems of floating production units, and it is assessed based on a stress-life or S-N approach. For integrity management along the lifetime of the assets, a Fracture Mechanics (FM) approach can be more appropriate, especially if cracks were detected during in-service inspections. An essential parameter of FM is the Stress Intensity Factor (SIF), which becomes fundamental to evaluate the response of a cracked link. The aim of this paper is to present SIF results for chains under different combinations of cracks and loads. Analytic or handbook solutions exist and provide accurate SIFs for simple geometries and test specimens. However, the particular geometry of chain links makes the Finite Element Method (FEM) more appropriate. The authors utilized the contour integral method together with the general purpose nonlinear FEM code Abaqus, to carry out a large amount of analyses to obtain SIFs for different chain sizes, material grades, crack shapes, crack depths and crack locations along the links. In addition to this, SIFs were derived in combination with other degradation phenomena such as large pitting or interlink wear. As an outcome, empirical equations were developed to predict the SIFs of propagating cracks in mooring chains under a large variety of scenarios. This allows engineers to assess the criticality of fatigue cracks using suitable crack growth models, and hence evaluate their fitness-for-service or need to implement remedial actions.

A preliminary investigation of a low-weight carrying marine platform concept was conducted by means of experiments and approximate theories. The platform consists of a circular elastic tube with circular cross-section, covered with an air-supported membrane deck. A vertical skirt is added along the circumference of the model to avoid air-leakage. We refer to this concept as floating solar island. Solar islands have recently gained interest. The models were subjected to waves. Both regular and irregular wave tests were conducted. Tests with the absence of the membrane (floater only) were also conducted. The general behaviour and failure modes were investigated. Failure modes include over-topping with flooding as consequence, as well as out-of-water incidents and membrane wear with air leakage as consequence. A systematic variation of wave conditions revealed for which wave conditions flooding occured. Out-of-water incidents of the skirt were not observed. Vertical accelerations were measured at eight positions along the circumference of the model, and the heave, pitch and first flexible mode motions were re-constructed by modal theory. The modal responses were compared to theory based on linear potential flow assumption, both for the floater model only, and with simplified theory accounting for the air-cushion of the island in heave. The theory was able to predict the global behaviour reasonably well, although important discrepancies were observed. More detailed studies, involving experiments with more instrumentation and development of theory, must be conducted in case deeper understanding of this relatively complex, hydro-elastic concept is needed.