A Fluid-Structure Coupled Computational Framework for Fluid-Induced Failure and Fracture

This work extends and generalizes a recently developed fluid-structure coupled computational framework to model and simulate fluid-induced failure and fracture. In particular, a novel surface representation approach is proposed to represent a fractured fluid-structure interface in the context of embedded boundary method. This approach is generic in the sense that it is applicable to many different computational fracture models and methods, including the element deletion (ED) technique and the extended finite element method (XFEM). Two three-dimensional model problems are presented to demonstrate the salient features of the computational framework, and to compare the performance of ED and XFEM in the context of fluid-induced failure and fracture.

Integrity of Weld Overlay of Flexible Joints and Lined Pipe

This paper describes different alternatives to be adopted to assess the integrity of weld overlays of flexible joints and lined pipes in offshore pipeline and riser projects. Protective layers are adopted as an interesting alternative to full thickness corrosion resistant alloys due to the possibility to adopt carbon steel as base material in order to reduce overall material costs. UNS N06625 (alloy 625) is generally selected for internal layers, such as weld overlay steels, lined pipes or clad pipes because of its sulfide stress cracking (SSC) resistance and outstanding weldability. However, unless the long-term integrity of the cladding or overlay as a protective layer can be demonstrated under the intended service conditions, the base material shall also be resistant against sulfide stress corrosion cracking. Due to low resistance of carbon steel to corrosion fatigue in the presence of contaminants in fluid content, the rupture of thickness of CRA (Corrosion Resistant Alloy) layer becomes a failure mode. An Engineering Critical Assessment (ECA) shall be performed in order to assess if circumferential planar flaws in weld overlay regions will not propagate through the CRA layer, thus exposing the base material, when submitted to critical cyclic loads during the service life. Such analysis would involve fatigue crack growth simulation and surface interaction of full circumferential embedded defects to determine the maximum weld overlay pass height to be limited by machining. This limited height of machined layers should guarantee that a full circumferential flaw will withstand the operational fatigue life. However, this is a very time consuming manufacturing process and would implicate additional concerns for long extensions due to out of straightness and out of roundness. Alternatively, the ECA results may be used to determine the flaw acceptance criteria and required probability of detection of volumetric non-destructive testing. Recent developments in ultrasonic inspection were successfully adopted and represent a better solution for alloy 625 weld overlay in terms of project scheduling and manufacturing costs. Radiographic testing may also be used provided it meets the required sensitivity, in terms of image quality indicators (IQI). Anyway, validation tests shall be performed to demonstrate adequate reliability to detect the minimum required flaw height.

Numerical Simulation of UOE Pipe Process and its Effect on Pipe Mechanical Behavior in Deep-Water Applications

Large-diameter thick-walled steel pipes during their installation in deep-water are subjected to a combination of loading in terms of external pressure, bending and axial tension, which may trigger structural instability due to excessive pipe ovalization with catastrophic effects. In the present study, the UOE pipe manufacturing process, commonly adopted for producing large-diameter pipes of significant thickness, is considered. The study examines the effect of UOE line pipe manufacturing process on the structural response and resistance of offshore pipes during the installation process using nonlinear finite element simulation tools.

Predictive Residual Ovality for Reel-Laid Pipelines in Deepwater

Accurate determination of residual ovality is an important parameter for a successful deployment of single pipeline and pipe-in-pipe in deep waters wherein the integrity of empty pipes during installation depends upon the collapse resistance under external hydrostatic pressure. The reel-lay process of installation during which pipeline undergoes multiple strain cycles due to spooling, reeling and straightening has a significant bearing on pipe ovalisation and hence accurate determination residual ovality at the end of straightening process is one of the key inputs. It is industry practice to use numerical finite element analysis techniques to predict residual ovality of pipelines as full scale testing is expensive and time consuming. In view of the importance of residual ovality on the pipeline integrity particularly for deepwater applications, an integrated approach of testing and finite element simulation have been used to identify the correct numerical model that predicts residual ovality accurately. This paper discusses the full scale tests performed which include material testing and bend tests performed to simulate spooling and straightening process and the pipeline deformations recorded using laser measurements at different cycles of bending process. The paper presents a brief summary of numerical finite element analyses performed to validate the test results and the effect of element types and material models used in the finite element analyses on the predictability of residual ovality. The material evolution models and their effect on the predictability of remaining ovality are discussed in the paper. Comparisons are made on the predictive residual ovality for reel lay process on single pipe and pipe-in-pipe. The effect of residual ovality on the pipeline integrity for the lateral buckling limit state under combined bending and external pressure are discussed in the paper.

Thermal Insulation With Latent Energy Storage for Flow Assurance in Subsea Pipelines

Flow assurance is critical in offshore oil and gas production. Thermal insulation is an effective way to reduce heat loss from subsea pipelines and avoid the formation of hydrates or wax deposits that could block the flowlines. This paper presents heat transfer analysis from a subsea flowline with different insulation materials, particularly with nano-enhanced phase change materials (NPCMs) that allow thermal energy storage in the pipeline system. The phase change materials (PCMs) can effectively regulate fluid temperature during production fluctuations or increase the cool-down time during production shutdown. This paper considers a pipe in pipe configuration with different insulation methods; the cool-down times are calculated and compared. The results show that thermal insulation can greatly delay the fluid cool-down process. A significant improvement of cool-down time can be achieved with PCM energy storage under a good conventional insulation layer. Moreover, with nanoparticles in a PCM, the latent energy storage is enhanced thus it takes even longer time for the internal fluid to reach its hydrate formation temperature.

ECAs, FE and Bi-Axial Loading: A Critique of DNV-OS-F101, Appendix A

Controlled lateral buckling in offshore pipelines typically gives rise to the combination of internal over-pressure and high longitudinal strains (possibly exceeding 0.4 percent). Engineering critical assessments (ECAs) are commonly conducted during design to determine tolerable sizes for girth weld flaws. ECAs are primarily conducted in accordance with BS 7910, often supplemented by guidance given in DNV-OS-F101 and DNV-FP-F108. DNV-OS-F101 requires that finite element (FE) analysis is conducted when, in the presence of internal over-pressure, the nominal longitudinal strain exceeds 0.4 percent. It recommends a crack driving force assessment, rather than one based on the failure assessment diagram. FE analysis is complicated, time consuming and costly. ECAs are, necessarily, conducted towards the end of the design process, at which point the design loads have been defined, the welding procedures qualified and the material properties quantified. In this context, ECAs and FE are not an ideal combination for the pipeline operator, the designer or the installation contractor. A pipeline subject to internal over-pressure is in a state of bi-axial loading. The combination of internal over-pressure and longitudinal strain appears to become more complicated as the longitudinal strain increases, because of the effect of bi-axial loading on the stress-strain response. An analysis of a relatively simple case, a fully-circumferential, external crack in a cylinder subject to internal over-pressure and longitudinal strain, is presented in order to illustrate the issues with the assessment. Finite element analysis, with and without internal over-pressure, are used to determine the plastic limit load, the crack driving force, and the Option 3 failure assessment curve. The results of the assessment are then compared with an assessment using the Option 2 curve. It is shown that an assessment based Option 2, which does not require FE analysis, can potentially give comparable results to the more detailed assessments, when more accurate stress intensity factor and reference stress (plastic limit load) solutions are used. Finally, the results of the illustrative analysis are used to present an outline of suggested revisions to the guidance in DNV-OS-F101, to reduce the need for FE analysis.

Enhanced Collapse Resistance for Different D/t Ratios of UOE Pipes for Ultra Deepwater Application

Energy consumption outlook shows that the demand for Oil and Gas is increasing worldwide and since most of the undemanding reserves are already being explored, new reserves means longer distances from the shore and increasing water depths, of up to 3,000 meters. Collapse resistance has become a key factor in the design of pipelines for ultra-deepwater applications. UOE process is commonly used for manufacturing pipelines of large diameter and the cold work involved in this forming process modifies the mechanical properties of the pipes. This paper presents the effect of thermal treatment on final material properties, proving the validity of enhancing collapse for different D/t, as allowed by DNV-OS-F101 αFab, and extending what has been shown as valid on previous studies. In this work, the inputs for the processing strategies are presented, along with coupon compression testing and full scale testing, in order to qualify the selected route as compliant with producing pipes with αFab equal to 1, for usual D/t combinations. An analysis of the predicted collapse pressure compared to the real collapse pressure of the pipes is also presented. The extension of the qualification process achieved successful results and allows the use of a fabrication factor equal to 1 in ultra-deepwater offshore pipeline projects. This enables the reduction of wall thickness, generating reductions in material and offshore installation costs and also potentially enhancing the feasibility of many challenging offshore projects.

Reliability Analysis and Design for Pipe-in-Pipe Pipelines With Centralizers

In this paper, a simplified reliability model is developed to identify how the pipe-in-pipe component uncertainties (manufacturing tolerances of centralizer thickness) influence the fatigue life of the system. The focus is on the reliability analysis with respect to the centralizer thickness. In order to reduce the complexity of the problem, only the centralizer thickness is considered to be a random variable. A limit function is formulated based on the three dimension (3D) finite element analysis. With the help of the probabilistic method, the correlation between the centralizer thickness and the failure probability is investigated. Two examples on pipe-in-pipe pipeline system are analyzed. The first one presents the relationship between centralizer thickness and failure probability for inner and outer pipes. The second one is an application of six mile pipe-in-pipe pipeline system. The failure probability of the fatigue is estimated. The influence of the centralizer thickness decreasing with time due to the abrasion, creep wear and elastic deformation is also considered when computing fatigue life and failure probability. The maximum fatigue damage ratio is calculated based on all trial samples generated considering manufacturing tolerances. If the maximum fatigue damage ratio is less than or equal to the allowable fatigue damage ratio, the failure probabilities with respect to the given centralizer thickness is negligible and the design is acceptable if only considering the influence of the given centralizer thickness. In addition, numerical results show that the maximum fatigue damage ratio possibly exceeds the allowable fatigue damage ratio considering manufacturing tolerances although the deterministic fatigue damage ratio is less than the allowable fatigue damage ratio.

Fatigue Life Assessment of Single Layer 60512 PVDF Barrier in Unbonded Flexible Risers

The polymeric barrier is one of the key components in a flexible pipe, the sound function of which is essential for the containment of the transported medium, ensuring no leakage to the environment which could result in undesired consequences. According to API 17J the barrier design must be able to sustain certain static and dynamic strain conditions however; the actual design or the fatigue assessment of the barrier is not covered within the standard. Since the barrier is subjected to the same dynamic loading as the pipe the durability and integrity of the barrier is a key issue that needs to be addressed during barrier design for dynamic risers. This paper discusses a fatigue life assessment procedure for a barrier made of copolymer Grade 60512 PVDF. A flexible pipe barrier is manufactured by continuous extrusion of polymer onto a metallic carcass. The carcass has a spiral structure with an irregular outer profile. As such, the extruded polymer on this irregular surface inevitably gives non-uniform thickness and geometric anomalies where the polymer has flowed into gaps in the carcass. During pipe loading such anomalies act as stress concentrations and become critical locations for fatigue crack formation. The evaluation of the effect of the barrier profile shape on the fatigue durability of the barrier is therefore an essential requirement, in particular, in cases where the barrier consists of a single extruded polymer layer. Within the procedure outlined in this paper, the fatigue assessment of the barrier is made using the local plastic strain behaviour. The maximum stress concentration factor and the acceptable profile for the extruded barrier are selected to provide adequate safety margins for the project specific loading conditions. A procedure has been proposed to predict the local plastic strain of the barrier using global service loading data.

Steel Steep Wave Riser as an Alternative Configuration for FPSO’s Compliant Risers

The work was focused in the chase for alternative configurations that could resist to the high FPSO motions in the Brasil’s Pre-Salt harsh wave environment, and that could also be less compliant laterally when compared to the SLWR solution. A case study was taken where an infield 8 inch SLWR configuration has been taken for comparisons. After adjusting the SSWR (Steel Steep Wave Riser) main characteristics such as top angle, buoyant section length, buoyancy modules geometry and spacing, feasible configurations have been obtained. For a configuration to be considered as feasible, a set of verifications have been carried out including extreme events, wave fatigue, vortex induced vibration and installation. The verification was performed considering several riser top connection positions and azimuths along the FPSO riser support balcony. The interference with neighboring risers has been also taken as an important issue, but was taken solely for comparison with the SLWR configurations. The installation phase has been focused including the stages of bottom connection, normal pipe lay and the connection at the FPSO. The main problems associated to the installation phase of the steep wave configuration were identified and addressed in the discussion presented. As the SSWR configuration has a fixed point at the sea bottom, two different solutions for this connection have been studied, and the final choice is described. The main differences between SSWRs and SLWRs, and the possible advantages of the SSWR configuration are discussed and a direct comparison is presented.