Experimental Investigation of Fatigue Performance of a 300 mm2 Copper Power Conductor

Electrical power cables are used in conjunction with floating units for provision of energy to installations on the sea bed, power from land to the floater, or export of power from a wind turbine to land. Power cables that are linked to a floating unit are subjected to fatigue loading from the waves and due to the movement of the vessel in the waves. Fatigue strength needs to be verified for design. Fatigue performance of a 300 mm2 stranded copper conductor was investigated. The experimental work included fatigue tests of individual wires and full cross section conductors including unlubricated and lubricated conductors. Individual wires from different layers were tested in tension-tension mode with stress ratio R = 0.1. Full cross-section conductors were tested in cyclic reversed bending with constant tension at ends, simulating the loading at the top end of a conductor hanging off a floating structure through a bellmouth. The objective of this paper is experimental assessment of the fatigue strength of a 300 mm2 copper conductor and to investigation of the mechanisms of fatigue crack initiation and growth in individual wires. At the time of submission the test program was still in progress, and conclusions are tentative only. An updated paper with complete results will be published at a later stage.

An Investigation of the Tolerance of Riser to Corrosion Pitting

When risers are designed it is common for corrosion to be accounted for by including a corrosion allowance in the wall thickness [3]. However, when designing risers which are subject to fatigue loading from various sources, simply allowing extra thickness in the wall is inadequate to protect against the accelerated fatigue crack growth driven by corrosion. This paper illustrates a methodology for assessing the fitness for service of a steel catenary riser with various levels of pitting corrosion. The methodology uses FEA tools, as well as classical fracture mechanics, to predict the rates of crack growth and arrive at predictions of life. Once corrosion begins and pits form, the structure may experience an increase in crack growth rate, caused by the influence of the chemistry of the produced fluid on the steel and by the stress effects of the pit geometry. Further complications arise if extreme storms cause riser stresses to exceed yield, which then requires the use of strain based methodology. The results of the illustrative study demonstrate that riser designs should account for the potential of accelerated crack growth where there is a potential for pitting corrosion, and that by only adding a corrosion allowance to account for loss of burst capacity, an inadequate design can easily result.

Qualification of UOE SAWL Linepipes With Enhanced Collapse Resistance for Ultra Deepwater Applications

Large diameter UOE pipes are being increasingly used for the construction of offshore pipelines. Since oil discoveries are moving towards ultra-deepwater areas, such as Pre-Salt in Brazil, collapse resistance is a key factor in the design of the pipelines. It is known that the cold forming, and the final expansion in the UOE linepipe manufacturing process, reduces the elastic limit of the steel in subsequent compression. Due to this, the DNV collapse formula includes a fabrication factor that derates by a 15% the yield strength of UOE Pipes. However, DNV also recognizes the effect of thermal treatments and the code allows for improvement of the fabrication factor when heat treatment or external cold sizing (compression) is applied, if documented. This paper presents the qualification of UOE pipes with enhanced collapse capacity focusing the use of a fabrication factor (αfab) equal to 1. TenarisConfab has performed a technology qualification process according to DNV-RP-A203 standard “Qualification Procedures for New Technology”. The main aspects of the qualification process are presented in this paper which included significant material and full scale testing, including combine load testing, and final analysis. The qualification process achieved successful results and this will allow use of a fabrication factor equal to 1 directly in deepwater and ultra-deepwater offshore pipeline projects with a possible reduction in material and offshore installation costs and also potentially enhancing the feasibility of many challenging offshore projects.

Whistling of Pipes With Narrow Corrugations: Scale Model Tests and Consequences for Carcass Design

Pipes for gas production and transport with a corrugated inner surface, as used in flexible pipes, can be subject to Flow-Induced Pulsations when the flow velocity is larger than a certain velocity. This onset velocity is dependent on the geometry of the corrugations, the operational conditions and the geometry of the topside and subsea piping. In this paper, small-scale tests performed on corrugated tubes are reported. The tested geometries include both “classical” profiles, similar to the inner profile of agraff flexible risers, and profiles with less typical variations, such as narrower and/or deeper cavities, or irregular pitch. These tests were performed in order to evaluate the validity of a prediction model developed earlier for the onset of pulsations, for corrugated pipes with these kinds of atypical variations, which are found on a new type of carcass designs. The mechanism of Flow-Induced Pulsations in corrugated pipes is discussed, as well as the principle of the prediction model. The experimental results show that the validity of the model remains reasonable in most cases, except when the cavities are very narrow. In this case, the model becomes overly conservative. This limitation can be attributed to the fact that, for very narrow cavities, the cavity opening becomes too small compared to the boundary-layer momentum thickness, effectively destroying any instability of the shear layer. Furthermore, the shift towards higher frequencies of the acoustic source term due to narrower cavities, and the possible coupling with higher acoustic modes, is considered. The results of the analysis are used to evaluate the onset velocity and whistling behavior of a newly developed carcass design of flexible risers. A previous analysis has indicated that the particular geometry profile of the new design improves the whistling behavior by pushing the onset velocity outside the typical operational envelope of flexible risers. The analysis confirms that the new design will be less prone to whistling than flexible risers with classical agraff carcasses.

Assessment of Loading on the Carcass and Pressure Armour due to Environment Swelling of Polymer

Flexible pipes consist of multi-layer structures comprising polymer extrusions, tapes and insulation that are trapped between layers of helically-wrapped steel wires. Under certain pressure and temperature service conditions, and when the polymers are in contact with certain chemicals (such as supercritical CO2), these polymer layers may experience significant volumetric changes caused by permeation, solubility and absorption of chemicals into the polymer structure. The swelling of the polymer barrier can result in a significant increase in load on the carcass and pressure armour that may compromise the integrity of the pipe. This paper investigates the effect of the volumetric changes in the barrier and how that can influence the extra loading exerted on carcass and pressure armour under service conditions. A simple analytical model has been proposed to predict the extra loading acting on the carcass and the pressure armour. The proposed procedure has been validated using FE results. Experiments have been performed using a three-layer simulated setup and full-scale pipe to investigate the load acting on the carcass and pressure armour. A brief description of the test program and the results are presented. Tests clearly show that there is only a limited increase in hoop stress in the carcass or the pressure armour in pipes immersed in acetone, although the barrier swelled considerably under unconstrained conditions. An empirical procedure has been proposed to account for this swelling retardation behavior. The effect of the PVDF barrier swelling in pressurized supercritical CO2 environments is discussed.

Large Diameter Pipeline PLEM Design for Remote, Arctic Application

While subsea production template and manifold designs have come to be dominated by standardized solutions tailored for specific hardware, the design of Pipeline End Manifolds (PLEM) remains largely project-specific. Nevertheless, some trends in PLEM design for large-diameter pipelines in moderate water depths have emerged in the past years in the North Sea and elsewhere; namely, large stand-alone structures on suction pile foundations with diverless spoolpiece tie-ins. This arrangement has proven successful on numerous projects; however, the move to remote arctic fields of significant production capacity and long design life introduces new design drivers that warrant a “fresh approach” to PLEM design. The developments currently being considered for the arctic will have to deal with: - Remote location making mobilization of installation assets a significant cost driver such that separate installation of pipeline and PLEM is relatively unattractive - Harsh conditions and short weather windows for installation favoring designs that reduce the number of separate installation steps and vessels - Poorer access for maintenance and repair during the operating life favoring designs that are modular and that allow recovery of critical components using the smallest possible intervention vessels. When combined with envisioned field production lives of 40 to 50 years, this means a very different set of design drivers will apply to the PLEM design. This paper presents an alternative PLEM design developed to overcome these challenges by: - Integrating of the PLEM with the pipeline to work around current industry limitations for large diameter diverless tie-in connector systems and to minimize ROV rotated sealing surfaces subsea in normal operation, - Introducing plug technology to remove the critical dependence on long-term trouble-free performance of large bore valves, - Introducing driven pile foundations to reduce structure size, prevent long-term settlements and eliminate the need for separate pipeline support frames by maintaining the pipe centerline close to the mudline, - Modularizing the system such that key components (all remaining valves) can be retrieved without complete shutdown of flow and such that installation / intervention can be performed using a wide range of vessels, and - Incorporating lessons learned from the successful design of a North Sea vertical diverless pig launcher unit. This paper presents an overview of the alternative PLEM design and discusses the status of the technologies required.

Drilling Riser Design and Analysis for Deepwater Applications

A riser is a fluid conduit from subsea equipment to surface floating production systems such as spars, TLPs, and semi-submersibles. It is a key component in a drilling and producing system. Drilling risers include the applications in marine drilling (low pressure) and tie-back drilling (high pressure). This paper discusses drilling riser design and analysis for a deepwater application. This paper first discusses the configuration of marine drilling and tie-back drilling risers. It then presents the drilling riser design procedure and analysis methodology. The riser design and analysis cover the riser tensioner setting, marine operation window, strength and fatigue, etc. A marine drilling riser example is used in the paper to demonstrate the design and analysis for a deepwater application. This paper shows the dynamic strength analysis results for the riser. It then identifies governing locations for the riser design. A tie-back drilling riser example is also provided to illustrate its global dynamic performance. This paper finally discusses the design and analysis challenges of a drilling riser for a deepwater application.

Real-Time Riser Fatigue Monitoring Routine: Architecture, Data and Results

Recently, the Modal Decomposition and Reconstruction (MDR) algorithm was developed to accurately estimate fatigue damage in marine risers based on measured acceleration and angular rates at several locations. The greatest benefit for drilling risers can be derived by incorporating the method in an online, fully automated system. In this way, fatigue damage estimates are available to the crew on the rig in real-time for risk quantification and mitigation. To this end, the MDR routine was implemented for online assessment of fatigue damage along the entire riser from acceleration and angular rate measurements at typically 5–10 elevations. This paper discusses the architecture, highlights some measured data and provides results for modes, stress and fatigue damage rate for the Chikyu drilling vessel during two scientific drilling campaigns. These campaigns occurred at the Shimokita site (1180-meter water depth) and the Nankai trough site (1939-meter water depth). To the authors’ knowledge, real-time fatigue monitoring of the entire riser has not been accomplished previously. Robust incorporation of the MDR algorithm into an online computational environment is detailed, including incorporation of top tension and mud weight data from the rig, detection and removal of data errors, and streamlined flow of the data through the computational modules. Subsequently, it is shown by example how the measured accelerations and angular rates are used to determine excited modes, participating modes, stress distribution and fatigue damage along the entire Chikyu drilling riser in an online setting. The technology highlighted advances riser integrity management two steps forward by first using measured data at 5–10 locations and the MDR algorithm to reconstruct stress and fatigue damage along the entire riser, and secondly integrating this approach into a fully automated, real-time computational environment. As a result, drilling engineers are empowered with a tool that provides real-time data on the integrity of the drilling riser, enabling informed decisions to be made in adverse current or wave conditions. Measured data also serves as a benchmark for analytical model calibration activities, reducing conservatism in stress and fatigue in future deployments. Furthermore, cumulative fatigue damage can be tracked in each riser joint, enabling more effective joint rotation and inspection programs.

Shakedown Study of Pipes With External Corrosion Under Cyclic Bending and Internal Pressure

Pipelines and rigid risers are susceptible to corrosion. This is also a concern for pipes onshore and on process plants of floating platforms. Once detected the corrosion defect on pipes under cyclic loads, the analysis carried out to decide on keeping the pipe in operation or repair/replace should consider that the defect may experience cyclic plasticity caused by stress concentrations and thickness reductions. In this condition, ratcheting can cause rapid failure. This paper presents a study combining experiments and different analysis techniques to evaluate the occurrence of ratcheting in pipes subjected to cyclic bending and internal pressure. Experiments with different defect geometries were carried out. Numerical analysis using incremental plasticity and shakedown procedures are presented and compared with the experiments.

Effect of Reeling Installation on Weld Residual Stress in Pipeline Girth Welds

A characteristic of pipeline installation by the reeling technique is the generation of high plastic strain around the majority of the pipeline’s circumference as it is spooled onto a drum, under displacement controlled conditions. It is well-known that the application of sufficiently high amounts of mechanical or thermal energy will “anneal” (relax) weld residual stresses and, therefore, under the gross plasticity experienced during reeling it should be expected that initial girth weld residual stresses will be entirely relaxed during the first reel cycle. The residual stress state needs to be taken into account in Engineering Critical Assessment (ECA) procedures of girth welds when predicting allowable defect dimensions. ECA codes such as DNV-OS-F101 and BS7910 assume the welding residual stress to be equal to the yield strength of the parent material and relaxation of welding residual stress under overload is allowed. However, the treatment specified in DNV is established from load-controlled scenarios and may result in un-realistic allowable defect dimensions in displacement-controlled situations such as reeling. Welding residual stress in reeling ECA is concerning to the subsea pipeline industry. By performing reeling simulations with 3D finite element analyses (FEA), this paper examines the welding residual stress before and after reeling and assesses the extent of residual stress relaxation. It was found that reeling axial strain causes significant relaxation of the weld residual stress at the pipe intrados and extrados. At the saddle points there is a slight disruption to the residual stress field. The full weld residual stress is relaxed from a value equal to the material yield stress, and is replaced by a plastic deformation induced stress of much lower magnitude, typically in the order of 100 MPa or less. The plastic deformation stress is of equal magnitude whether or not the pipe section contains initial weld residual stress and, therefore, it is concluded that weld residual stress can be ignored following the first reel cycle.