Crapps, Justin M. (ExxonMobil Upstream Research Company) | Yue, Xin (ExxonMobil Upstream Research Company) | Berlin, Ronald A. (ExxonMobil Upstream Research Company) | Suarez, Heider A. (ExxonMobil Production Company) | Pribytkov, Petr A. (Exxon Neftegas Limited) | Vyvial, Brent A. (Stress Engineering Services) | Proegler, Jared S. (CRC Evans Pipeline International)
Production and delivery of hydrocarbons in remote locations of the world may involve traversing regions which could expose pipelines to geohazards. These geohazards may include fault crossings, landslides, liquefaction, ice gouging, and frost heave. ExxonMobil has developed a strain-based design (SBD) pipeline technology to design against these geohazards and to enable safe transportation of hydrocarbons across challenging terrain.
Integrity management of strain-based pipelines includes measures such as corrosion prevention, external damage prevention, ground movement monitoring, and geohazards mitigation. Despite preventive efforts, a pipeline may still become corroded or damaged. The damage may reduce the pipeline's strain capacity and a repair method to restore the pipeline's capacity will be required. This paper presents the qualification of the Type B split sleeve, a sealing repair methodology, for strain-based pipelines. The subjects addressed include selecting the Type B split sleeve as a repair candidate, finite element modeling of the repair, sleeve welding with in-service flow conditions, and full-scale proof testing of three repaired pipes.
Production and delivery of hydrocarbons in remote locations often requires transportation of the hydrocarbons across challenging terrain. This may expose a pipeline to geohazards including faults, landslides, permafrost, earthquakes, and ice gouging. Pipelines are traditionally designed for pressure containment - a circumferential load - whereas most geohazards affect a pipeline by imposing loading in the longitudinal or axial direction. In extreme cases, the longitudinal loading can cause significant degrees of plastic deformation. Traditional pipeline design does not consider extreme longitudinal loading and the design methodology must be modified to ensure the pipe is able to withstand all loading conditions.
ExxonMobil has developed a SBD technology (Panico, et al. 2017; Fairchild et al., 2016, 2014; Tang et al., 2014; Macia et al., 2010) for building pipelines with the ability to sustain loads imposed by ground movement. Fig. 1 illustrates the principal difference between stress and strain-based pipeline design. A stress-based pipeline is designed for pressure containment considering hoop stresses such that at the maximum pressure, the stresses will remain below the specified minimum yield stress (SMYS) by a prescribed amount. The difference between the allowable stress limit and the specified minimum yield strength is the design safety margin. For SBD, the pipeline is designed to contain pressure and sustain loading imposed by potential ground movement. It should be noted that while pressure containment is constant, the loads caused by ground movements are typically upset events and often of long return periods; i.e., they are infrequent. When the ground movement does occur, it is allowed to cause yielding and plastic flow (straining) in the pipe up to the allowable strain limit.
Handa, Tsunehisa (JFE Steel Corporation) | Igi, Satoshi (JFE Steel Corporation) | Tajika, Hisakazu (JFE Steel Corporation) | Tagawa, Tetsuya (JFE Steel Corporation) | Hase, Kazukuni (JFE Steel Corporation) | Fukui, Tsutomu (Nippon Kaiji Kyokai) | Aihara, Shuji (The University of Tokyo)
On the background of increasing demand in ship transportation, capacity of container ship is recently growing to achieve low cost shipping. The use of extremely thick steel plates is recently required around hatch side structure to realize ultra large container ships. Brittle crack arrest design concept in container ships has been already common in the last decade, but the rule revision in brittle crack arrest design doesn't catch up with a demand for thicker plates. Industries of ship building and steel manufacturer in Japan have collaboratively investigated about brittle crack arrest toughness, Kca, and already established Kca test procedure. The recent program of the joint research activity focuses on the required Kca value in brittle crack steel plates over 80mm thickness, which has not been prescribed in any ship classification rules, yet. In this project, the required Kca, with which a running brittle crack can be arrested, is experimentally investigated with steel plates of 100mm thickness. A large scale structural models, which simulate a cruciform joint of hatch side coaming and strength deck, were exposed to brittle crack propagation test. In the present work, a part of the research program was picked up. It was supposed that a brittle crack initiated at butt weld of strength deck propagated into hatch side coaming. Two different scales of the structural model specimens, denoted as large-scale specimen and ultra large-scale specimen, were prepared. A brittle crack runway plate simulated as strength deck was designed over 1000mm in ultra large-scale specimen. A running brittle crack at a specific test temperature was examined to be arrested or not in the test plate subjected to a specific tensile stress. Brittle crack behavior was discussed considering the balance of crack propagation driving force and crack arrest toughness based on fracture mechanics. The required Kca value in steel plates over 80mm thickness for hatch side coaming was also discussed.
Mooring chains are Safety Critical Elements (SCE) of offshore Floating Production Systems (FPS). Out-of-Plane Bending (OPB) fatigue has recently been identified as one of the failure mechanisms of mooring chains (Jean et al, 2005), and has become the bottleneck of the mooring fatigue life in many deepwater projects (Brinkhuis et al, 2015; Lassen et al, 2009). In the past decade, extensive experimental and numerical studies have been carried out in the offshore industry to understand and evaluate chain OPB fatigue. However, the majority of these efforts only focus on the interlink behaviors of two adjacent chain links, whereas the presence of the fairlead structures and their interactions with the chain links pertaining to OPB are largely neglected. In the present paper, a numerical study is carried out leveraging non-linear Finite Element Analysis (FEA) to examine the OPB behaviors of mooring chain links in real fairlead structures, on the example of a 7-pocket fairlead wheel (FW). In this study, a comprehensive FEA procedure is established with four 168mm R4S studless chain links and a 7-pocket FW to examine the chain OPB mechanism on a fairlead structure. The model is shown to be able to capture correctly both the interlink mechanics and the contact/interaction between the chain links and the FW. The interlink stiffness curves and the hotspot stress evolutions on both the first and second OPB link are carefully investigated under the full OPB loading cycle. The results reveal that the presence of the 7-pocket FW deteriorates the OPB fatigue due to the additional contact and friction between the chain link and the fairlead structure. In addition, it is demonstrated that the chain link is subjected to significant OPB stresses/damage even if it is fully supported and rested in the pocket of the FW. The implications of these findings on deepwater mooring system design and the optimization are also discussed.
Dragt, R. C. (Netherlands Institute for Applied Scientific Research (TNO)) | Allaix, D. L. (Netherlands Institute for Applied Scientific Research (TNO), Eindhoven University of Technology) | Maljaars, J. (Keppel Verolme BV) | Tuitman, J. T. (Keppel Verolme BV) | Salman, Y. | Otheguy, M.
Fatigue is one of the main design drivers for offshore wind substructures. Using Fracture Mechanics methods, load sequence effects such as crack growth retardation due to large load peaks can be included in the fatigue damage estimation. Due to the sequence dependency, a method is required that represents the sequences of loads in the design or maintenance procedures.
This paper presents a methodology to deal with this challenge. First, a framework is presented for coupling between the design load cases and the Fracture Mechanics methods, resulting into the requirements for loads and load sequences. Second, a 2-stage Markov Chain Monte Carlo model is presented which is able to create realistic loading sequences based on measurement data. The method is elaborated for fluctuating wind loads.
One of the main design drivers for Offshore Wind Turbine (OWT) substructures is fatigue. Current standards (e.g.; DNVGL, 2016; IEC, 2009) specify a wide range of operational conditions, such as normal operation, parked condition and fault conditions, and the environmental conditions (combinations of wind, wave and current conditions) that are to be included in the fatigue assessment. Every single combination is used as input for a time domain simulation, which results in a stress history at given (hot)spots in the structure. Prescribed Stress Concentration Factors (SCF) are used to account for structural details. Standards prescribe that a 1-hour period is simulated (by either simulating 6 times a 10-minute realization or a 1-hour realization) per load combination. The total analysis requires several thousands of individual 1-hour long time-domain simulations, each typically containing several thousands of stress cycles.
The stress history is used to estimate the fatigue damage in a structural detail during its intended lifetime. Fatigue damage is referred to as the utilization of the total fatigue capacity of a structural detail expressed in terms of life. For every stress history, the number of occurring cycles for each stress range are counted using the Rainflow Counting method. The appropriate SN-curve is selected from the design standard and used to determine the number of cycles to failure for each stress range. The damage contribution per range of stress cycles is defined as the ratio between the occurring number of cycles and the number of cycles per stress range. The damage contributions of all stress cycles are summed in agreement with the damage accumulation rule of Palmgren-Miner in order to arrive at the total fatigue damage. The result is multiplied by a Design Fatigue Factor (DFF) in order to arrive at a certain probability of failure, which amongst others accounts for difficulties encountered during inspection or repair of a specific detail and for the risk of structural failure (DNVGL, 2016). The final result is the estimated design fatigue damage for this particular hotspot, for a particular environmental and operational condition.
Floating Drilling, Production, Storage and Offloading (FDPSO) is subject to combined loads including wind, wave and current loads, which affect the fatigue damage of FDPSO mooring system respectively. In this paper, the effects of wind and current loads to the fatigue damage of FDPSO mooring system are presented by counting damages under wave-wind, wave-current and wave-wind-current cases. Firstly, according to the time domain coupled analysis method, the numerical reconstruction and extrapolation are carried out by using the model test results and ANSYS AQWA software, and tension time- histories of mooring lines under the three cases are obtained. Secondly, the fatigue load spectra under short-term sea states are counted out by Rain-flow counting method. Then the fatigue damage of mooring line is calculated according to T-N curves and Miner linear cumulative damage theory.
FDPSO is placed in oilfields for a long time and several mooring lines are employed to provide the principal resistance to displacements induced by the environmental loading. Providing the FDPSO mooring systems are subjected to alternating loads at sea, its fatigue strength must be considered in the design and production process. Several studies have been conducted on the fatigue strength of mooring lines by different methods. In time domain, Luo (1990) studied the non-Gaussian characteristic of fatigue load spectrum. Lassen (1996) used Gaussian distribution model to fit the relationship between fatigue damage and service time of steel chain. Vazquez-Hernandez (1998) predicted the fatigue life of a floating structure mooring line by different statistical methods, Dirlik, Rain-flow, Narrow-band and Wideband included. Mathisen (1999) pointed out that the wave-frequency load causes more fatigue damage than the low-frequency part. Gao (2006) investigated the catenary mooring fatigue damage of a semisubmersible under the combined wave-frequency load and low- frequency load. Taking the correction into consideration, Qiao (2014) compared the fatigue life of two types of catenary. This paper points out the effects of wind and current loads to the fatigue damage of FDPSO mooring line which is multi-section catenary.
Internal solitary waves usually transform into an internal wave train when propagating through the continental shelf/slope. However, such phenomenon is rarely taken into account in previous design of marine structures. This paper aims to construct an evolution model to describe the evolution of internal wave trains. To be specific, in consideration of various influential factors, such as benthic topography, friction, dissipation, et al., a mathematical model is built to describe the evolution of wave trains and numerical experiments are carried out to validate the method by analyzing the leading internal wave and the induced velocity fields. It is shown that the numerical results of the wave train displacement and the velocity fields are in good agreements with the observational data in South China Sea. In the meantime, the sensitivity of dissipation and friction to the model is discussed.
Internal solitary waves propagating over underwater topography, such as a sill or continental shelf/slope can produce internal wave trains. Specific to South China Sea (SCS), these wave trains propagate westward across the Luzon Basin to South China Sea shelf break, and the corresponding surface expression has been detected with satellite synthetic aperture radar (SAR)(Liu et al., 1998). Many observations showed that internal wave trains occur frequently and exist widely, which has resulted in obvious impacts on the operation of offshore structures. For instance, the investigation of (Xu et al., 2013) has shown the impact of internal solitons on offloading system, and operations need to be assessed during the design of FLNG in soliton active area.
As the foundation for assessing the hydrodynamic action between internal wave trains and floating structures, the characteristics of internal soliton trains have been studied preliminarily by far (Liu et al., 1998). However, many issues are not yet clear. First, the factors such as dissipation, shoaling, and friction play a role during internal soliton evolution. But previous scholars have different opinions about which factor should be added into the Korteweg-de Vries (KdV) equation, which is one of the most popular equations to analytically describe internal solitary waves. Besides, the comparisons are very few between numerical results (especially induced velocity fields) and the observational data from actual sea areas. Finally, selecting the coefficients of dissipation and friction are often subjective due to the lack of clear criterions. Thus, it is necessary to discuss the sensitivity of numerical model to the two coefficients.
Lee, Sung-Je (Daewoo Shipbuilding & Marine Engineering (DSME)) | Lee, Jeong-Dae (Daewoo Shipbuilding & Marine Engineering (DSME)) | Jun, Seock-Hee (Daewoo Shipbuilding & Marine Engineering (DSME)) | Yoo, Kwang-Kyu (Daewoo Shipbuilding & Marine Engineering (DSME)) | Joo, Young-Seok (Daewoo Shipbuilding & Marine Engineering (DSME)) | Han, Sung-Kon (Daewoo Shipbuilding & Marine Engineering (DSME)) | Park, Sung-Gun (Daewoo Shipbuilding & Marine Engineering (DSME))
The Steel Catenary Riser (SCR) concept has many advantages compared with other riser concepts and has been widely used for decades in deep water oil and gas production. Once the SCR has been installed in deep water field, the replacement of SCR is almost impossible so SCR should have enough fatigue strength. Therefore, the prediction of fatigue life is very important for developing SCR. The Vortex Induced Vibration (VIV) is an important source of fatigue damage for SCR. For riser system, the fatigue damage due to VIV can be calculated with the use of S/W SHEAR 7. This program was developed for predicting the VIV response based on various theoretical models and experiments. Parameters for VIV response calculation in this program were also determined through various experiments and experiences. For better understanding of VIV response, it is necessary to investigate the effect of parameters which affects the analysis result. This paper summarizes the results of parametric study performed to enhance the understanding of relationship between each parameter and fatigue analysis result.
The Steel Catenary Riser (SCR) is simple and cost effective system among the riser systems. For many decades, SCR has been adopted for several offshore projects. Most rules and regulations for riser design, however, only specify the functional requirements and basic analysis method or initial scantlings. They do not suggest detailed analysis methods for riser fatigue analysis. Therefore, it is imperative that engineers select appropriate analysis procedure to get valid results. In general, for riser, the fatigue damage due to environments can be divided into two damages. They are from the wave action and vortex phenomenon.
In case of vortex phenomenon, there are two terms. They are Vortex Induced Vibration (VIV) acting on riser and the other one is Vortex Induced Motion (VIM) acting on floater. This paper focuses on VIV fatigue analysis.
Zhu, Ling (Wuhan University of Technology, Collaborative Innovation Centre for Advanced Ship and Deep-sea Exploration) | Pan, Man (Wuhan University of Technology) | Zhou, Han-wei (Wuhan University of Technology) | Das, Purnendu K. (Wuhan University of Technology, ASRANet Ltd)
This paper concentrates on the transverse strength of the cross-deck structures of an ore carrier in which buckling damages were found on the cross-deck strip plating and stiffeners. Analytical approaches are utilized to obtain the ultimate strength of the structures at five collapse modes. With the initial imperfections and load distribution considered, non-linear finite element method (FEM) is used to investigate the transverse ultimate strength of the whole cross-deck structures. FEM results are consistent with those by the simplified first-principle-based method. Besides, advanced first-order second-moment method is adopted to perform the structural reliability analyses from which the outcomes cover the sensitivity factors, safety indices and probabilities of failure. It is found that the buckling initiation is identified at hatch end coaming and upper deck, according with the phenomena on the damaged ore carrier. The purpose of this paper is to develop a structural reliability-based procedure for the preliminary ultimate strength analysis and to make predictions for the potential damages on the crossdeck structures.
With the increasing demands and economic benefits of ores, ore carriers have gradually become the main means of transporting ores across oceans and thus need to be specially designed. Since the ore carrier has the trend of being ultra large scale, its global hull strength cannot be accurately assessed by the normal methods like beam theory and part-cabin Finite Element Analysis (FEA). Similar to the real load conditions, the results from the FEA of whole ships are more convinced than those from the FEA of local structures. However, the FEA of whole ships is not advisable for the local strength assessment especially at the preliminary design stage, due to its long modeling cycle and low efficiency. As one of the most important transverse members of ore carriers, the cross-deck structures are required to have adequate local strength to resist tremendous transverse forces imposed on the main deck. The permissible stress approach was adopted in the traditional assessment of the local structural strength (Hughes and Paik, 2010). However the ultimate strength cannot be predicted and verified by this method. For the local strength assessments of ore carriers, Do et al. (2013) studied the ultimate strength of the outer bottom of a Very Large Ore Carrier (VLOC) designed by the Common Structural Rules (CSR) and pre-CSR methods, to investigate the difference between different standards. With the aid of FEA and comparisons with the existing experimental results, comprehensive studies have been carried out and a semi-analytical method has been proposed to evaluate the ultimate strength of deck and bottom structures for 46 oil tankers and bulk carriers by Zhang and Khan (2009) and Zhang (2016).
This paper proposes using modified pipe-in-pipe (PIP) system to mitigate vortex induced vibration (VIV). Numerical simulations are carried out to examine the effectiveness of the proposed method. Firstly, a semi-empirical oscillator model is developed and validated by an experimental test of a single pipe. The validated model is then extended to the modified PIP system, which is simplified as a structure- tuned mass damper (TMD) system in present study. The governing equation of vibration is solved in the time domain using MATLAB/Simulink programming. The results demonstrate that the carefully designed PIP system can significantly suppress VIV of offshore cylindrical structures up to 84%.
The ongoing energy industry is heavily involved in using offshore floating structures which conventionally comprise of many vertical and horizontal cylindrical components such as risers, conductors and pipelines. Many structural failures in these structures are associated with the fatigue damages caused by Vortex Induced Vibration (VIV) (Williamson and Govardhan, 2008; Kamble and Chen, 2016). Various vibration control methods have been then suggested to supress such destructive vibrations. These methods can generally be divided into two broad categories: Passive (require no external power to operate) and Active (additional power input/sensors is required for operation) (Gad- el-Hak, 2000). The former approach is more applicable, much less expensive and easier to install particularly in the field of offshore and marine structures. The literature on the passive vibration control of marine cylindrical structures is quite rich (Gabbai and Benaroya, 2005; Liming, Xingfu and Yingxiang, 2012). All proposed approaches can mitigate VIV to a certain extent. However, they have some limitations which can considerably affect their performances particularly in deep and ultra-deep waters. Table 1 summaries the advantages/disadvantages of the most well-known current VIV suppression devices (Owen, Bearman and Szewczyk, 2001; Kumar, Sohn and Gowda, 2008; Assi, Bearman and Kitney, 2009; Kiu, Stappenbelt and Thiagarajan, 2011; Zhou, Razali, Hao and Cheng, 2011; Azmi, Zhou, Cheng, Wang and Chua, 2012; Sudhakar and Vengadesan, 2012; Bernitsas and Raghavan, 2014; Quadrante and Nishi, 2014; Zeinoddini, Farhangmehr, Seif and Zandi, 2015). It can be seen that many methods are still controversial from both structural and economical points of view. It is important to develop more efficient, cost effective and practical passive control devices.
This work considers results from l:50-scale model tests of the DeepCwind semi-submersible floating wind turbine. Global motion and load trends associated with wind turbine controls are replicated with a numerical model of the system using the National Renewable Energy Laboratory's FAST software. Once the correlation study is complete, the numerical model's aerodynamic properties are adjusted to represent the full-scale NREL 5MW wind turbine. Numerical simulations in the presence of full-field turbulent winds are investigated in an effort to identify differences in controller-induced motion and load trends between model and full-scale systems. Differences are identified and highlight opportunities to improve wind/wave basin model test procedures.
The next frontier in wind energy is deepwater offshore as it provides an energy resource substantially stronger than its land-based and nearshore counterparts in addition to mitigating competing use and visual impact issues (Musial et al., 2006). However, the technical challenges of harnessing deepwater offshore wind are significant with most technologies under consideration employing novel floating foundations. The rigid-body compliance of a floating foundation is far greater than that of a land-based or fixed-bottom offshore foundation giving rise to new engineering obstacles. A challenge of particular interest in this work is that of active blade pitch and generator controls, as the use of standard land-based or fixed-bottom wind turbine control schemes can give rise to platform motion instabilities (Jonkman, 2008). Wind turbine controllers that have been adjusted for use in floating wind turbines can mitigate these issues, but may do so at the expense of altering other dynamic characteristics of the floating wind turbine system (e.g. see Goupee et al., 2017).
A possible means of improving and de-risking wind turbine controllers for floating applications is through wind/wave basin model testing (Nielsen et al., 2006; Roddier et al., 2010; Chujo et al., 2013; Huijs et al., 2014; Muller et al., 2014). The primary difficulty with performing model tests of floating wind turbines is that employing the typical Froude-scaling approach common to floating body experiments yields very low Reynolds numbers for the wind turbine aerodynamics. As such, a test strictly adhering to Froude-scaled techniques will result in diminished turbine performance as compared to the full-scale target (Martin et al., 2014). Certain methods have been devised to replicate the steady-state performance of the full-scale target wind turbine in a Froude-scale environment (Martin et al, 2014; de Ridder et al., 2014). These methods use low-Reynolds number-specific airfoils with slightly larger chord lengths which are tuned to recreate the steady-state thrust vs. rotor speed response. These so-called performance-matched wind turbines do not preserve other turbine performance metrics, such as power or turbine aerodynamic sensitivities to changes in blade pitch setting. This creates difficulties for properly testing full-scale turbine control schemes in a Froude-scale model test of a floating wind turbine. Some have overcome this by using a hybrid approach where the turbine is simulated through servo-mechanical (or similar) means commanded by real-time simulations of the full-scale turbine (Azcona et al, 2014; Hall, 2016; Sauder et al., 2016; Bachynski et al, 2016). This method is not trivial to implement, however, and does assume that the coupled turbine aero-servo-elastic effects are captured properly in the software side of the experiment. For those who wish to test a complete scale model in Froude-scale environments, advances in testing techniques are required in order to properly emulate the full-scale turbine aerodynamics, control schemes and wind environments.