Large Diameter Deepwater Gas Pipelines Subjected to Global Buckling Behavior

In general, gas export pipeline designs have low restrictions concerning the flow assurance requirements, i. e., hydrate formation is not a great concern once processes in production platform facilities can significantly decrease the water content in the gas to be exported. Thus, these pipelines have only a small thickness of a single or multilayer anticorrosive coating and export gas at low temperatures. However, high pressures are required in order to overcome long distances and to increase the production flow rates. Large diameter gas pipelines submitted to high pressures even with low associated temperatures can be susceptible to global buckling, mainly if the pipelines are simply rested on a seabed of low resistance. This scenario characterizes strictly the gas pipelines installed in Brazilian Pre-Salt fields, where currently a relevant amount of export lines is operating in these conditions. Post-installation and operating pipeline surveys have identified marks on seabed confirming the buckle formation in some gas pipelines. In addition, axial movements of end equipment (PLETs) have been also observed. These issues require at least a verification and confirmation of the assumptions and predictions made in detailed design phase. This paper aims to present evaluations of the global buckling behavior of large diameter deepwater gas pipelines. Lateral buckles on very soft clayey seabed and displacements in ends and crossing locations are addressed in this work. Finally, numerical analyses confirm that gas pipelines structural integrity has not been jeopardized.

Impact of Soil Modeling on Fatigue Design of Lazy Wave Riser Systems

The Liza risers comprise production risers, water injection risers and gas injection risers, and a lazy wave configuration is selected considering FPSO motion, reservoir fluid and overall project execution requirements. During operation, the risers are expected to move cyclically with small vertical displacement amplitudes (e.g. 0.1% to 1% of the riser diameter), and a key design issue is the fatigue life of these risers at critical locations including the touch-down zone which will be governed by the seabed stiffness. The role of soil response on fatigue life of riser with buoyancy has been investigated through nonlinear finite element and comprehensive lab and field testing program. Published methodologies for determining seabed stiffness values for risers concentrate more on larger amplitude motions based on the design requirements of steel catenary risers. The paper presents the sensitivity of the fatigue life at TDP to various soil model and provides insight in the results. Also included is the importance of site specific soil investigation in the context of design of riser.

Challenges and Lessons Learnt From the Design, Fabrication and Installation of Pipe Walking Mitigations

This paper is based on the experience made during the execution of a project recently completed in West Africa, where a number of production and injection lines (10” and 6”, in 650m water depth) were found susceptible to pipe walking. To stop pipe walking, measures have been taken, consisting in connection points installed in line during the laying of the pipelines (J-laying) and then anchoring structures, post-installed. The Pipe Walking Mitigation Structures (PWMS) are skirted structures, sized to provide anchoring forces up to 95mT, with a max 32m × 17m footprint, and a dry weight up to 150mT. This paper presents the engineering, fabrication and installation of these PWMSs (8off). The engineering was driven by layout and schedule constraints. The layout, that at the start of the detailed engineering was almost frozen and was already quite congested, did not allow the use of solutions based on anchoring the pipelines at their ends. Therefore, the chosen solution was the placement of anchors along the pipeline. In not more than 6 months, the detailed engineering had to go through the screening between alternative concepts, the validation of the chosen solution and the engineering for the procurement, fabrication and installation activities. The tight schedule required to maximize the post-installation of the structures. Only the connection points were laid at the same time of the installation of the pipeline, and were installed 4 months ahead of the installation of the anchoring structures. The schedule has benefited of a standardization of the structures that made the fabrication and the installation engineering easier. To give additional flexibility to the schedule, the latest time by when, during the operation, a PWMS was to be installed, was defined, considering the expected anticipated number of shutdown/restart cycles and the maximum displacements that could be accommodated by the tie-in structures. Only 2 out of 8 PWMS were required to be installed before the start-up, whilst the installation of the others could be postponed, to a maximum of 4 years from the start of the operations. Furthermore, on a number of locations, only the connection points were installed, where the anchoring structures can be retro fitted, should they be required based on monitoring of the pipeline behavior during operation. Pipe walking is a well understood phenomenon — However, there are uncertainties in the key parameters impacting the results. This paper discusses the main contributing factors, and how the uncertainties have been tackled. A monitoring plan has also been laid down — The purpose is to verify the design of the mitigations and also to gather in service information that may allow to defer further the installation of the PWMS or even to avoid their installation. The post installed anchoring structures are quite large and heavy structures. A close interfacing between design engineering and installation engineering — since the early inception phase — allowed to achieve a safe installation and an accurate positioning of the structures, with the tight installation tolerances that were a key for the PWMS to provide the intended function. The paper presents the lessons learnt, from the engineering, the fabrication and the installation of the PWMS. The paper provides also some recommendations for further optimizations of the proposed solution, which will allow savings in costs and schedule in future projects. Pipe walking is a ratcheting phenomenon — As such, the foreseen time by when the accumulation of the pipe walking could become excessive can be assessed, and this time can be used to calibrate the response models and gather more reliable data about the operation of the system. The solution proposed allows to minimize the initial investments as only the connection points are to be installed together with the pipelines, while it maximizes the postinstallation of the mitigations, so not impacting the schedule for pipeline installation. A modular design of the structures would allow the use of lighter support vessels/construction vessels which will give more flexibility for a deferred installation of the mitigation structures, as retrofitting.

Fatigue Assessment of SLWR Riser in Brazilian Pre-Salt: The Impact of Slope Changing Point in SN Curve

One of the main challenges in rigid riser design for Brazilian Pre-salt is the fatigue limit state. At this new production frontier, some key points are imposed as a challenge for riser designers, mainly due to the high level of motions imposed by the FPSO at the riser top in a coupled system with water depth around 2200 meters, and thicker riser’s thermal insulation demanded for flow assurance (which worsens the dynamic response of production risers). Additionally, high contaminant levels in the fluid (CO2 & H2S) demands CRA materials. Within this context, Petrobras has been considering Steel Lazy Wave Riser (SLWR) configuration as a base case scenario for rigid riser projects, since this configuration is able to absorb part of the FPSO motions that would reach the touch down zone (TDZ) and, consequently, making this region much less demanded when compared against Steel Catenary Risers (SCR). In its pioneer deepwater SLWR [1], Petrobras adopted a conservative approach for fatigue assessment that involved degenerated SN curves from DNV-RP-C203, i.e. D curve in cathodic protection with the slope changing point (SCP) shifted to 5 × 106 for external wall and F1 curve in air with SCP at 5 × 107 for internal wall. More recently, both DNVGL and BSI have reviewed their fatigue assessment codes and no longer holds parity between SN curves. BS-7608 Ed. 2014 introduced different SCPs in order to account for a possible non-conservativeness in the assessment of low stresses under variable amplitude in the loading spectra. DNVGL-RP-C203 Ed. 2016 now presents three different bilinear SN curves for the internal wall of pipelines and risers that depends on weld misalignment, while it keeps SCP unchanged. This paper presents a recent case study for a typical SLWR configuration in pre-salt, in order to evaluate the impact of the changes proposed by the new versions of these design codes in the fatigue life of riser girth welds. Results of this work showed that the impact of different positioning of slope changing points in SN curves can have a great importance for riser design, since typical load spectrum lies around this region. Fatigue life could be increased up to twice or three times if one of these codes are adopted instead of the Shifted SN curves. However, the effect of low stresses under variable amplitude loading spectra is still a concern and it should be further investigated.

Walking Anchors: When to Fix One or Both Ways?

Anchors to mitigate pipeline walking often involve very large piles, requiring large crane installation vessels, which bring cost, schedule and HSE risk implications. Optimizing such piles traditionally require substantial FEA effort, and frequently, project decisions are made, and purchase orders are placed based on conservative assumptions. The choice of anchor-to-pipeline connection can significantly influence the anchor sizing. In some cases, allowing some free slide-back displacement when the anchor is being unloaded can substantially reduce the maximum load levels. This paper presents simple analytical calculations that can be used to determine whether this artifice is effective or not for any given condition; how much load reduction can be achieved; and how much slide-back length is required for that. Results for typical, hypothetical pipeline properties are presented. These show that while some cases will see no benefit, in others cases the load can be reduced by over 1 MN by allowing no more than a few centimeters slide-back tolerance. Having the ability to assess multiple options with minimum effort, then use FEA for detailed confirmation (rather than using expensive FEA on a trial and error basis) will allow certain projects to realize significant savings.

Analytical Study for Lateral Buckling of Imperfect Pipelines With Distributed Buoyancy Section

Distributed buoyancy method is one of the buckle initiation techniques used to trigger controlled lateral buckling at planned locations for subsea pipelines operating under high temperature and high pressure (HT/HP) conditions. Deviations from a straight profile for pipelines may be introduced by the pipe-laying vessel’s sway motion during the installation process. In this study, analytical solutions of lateral buckling are deduced for imperfect unburied subsea pipelines with a distributed buoyancy section. The effect of initial imperfections on buckled configurations and typical post-buckling behaviours is illustrated and analysed. The results show that, compared to the case without initial imperfection, lateral displacement amplitude becomes larger when initial imperfection exists. Maximum compressive stress increases when wavelength of initial imperfection is smaller than buckled length of pipeline. However, maximum compressive stress decreases when wavelength of initial imperfection is larger than buckled length of pipeline. So it’s better to introduce longer wavelength of initial imperfection.

Improved Pipelay Equipment Settings Methodology for Rigid Pipes

Rigid pipelay vessels use two types of equipment to hold the pipe under tension while it is being deployed on the seabed: tensioners and hang-off clamps (HOC). Tensioners are used to maintain tension while paying out the pipe behind the vessel, while the HOC holds the pipe statically during welding operations. For both pieces of equipment, the tension is held through frictional pads in contact with the pipe. With the necessity to reach greater water depths leading to an overall increase of pipeline weights and stiffness (and therefore top tension), the installation of rigid pipelines has become more challenging. In other cases, such as pipe-in-pipe (PiP), carrier pipe wall thicknesses are often optimised as much as possible while lower grade materials are selected to reduce costs, making it more difficult to provide pipelay equipment settings that can satisfy both grip and pipe local integrity requirements. Simple hand calculations and basic equipment modelling are not sufficient to provide safe equipment settings and optimised allowable sea states. The new DNVGL-ST-N001 [1] (Marine Operations and Marine Warranty) also increases scrutiny on pipelay equipment and pipe interaction, making it imperative to develop more in-depth methodologies for settings calculation. This paper gives an overview of a robust methodology for pipelay equipment settings and demonstrates through practical examples how it improves safety while optimising operations, therefore enabling more efficient pipelay operations and reducing waiting on weather.

Integrity Assessment of Subsea Pipeline Dent / Buckle Using ILI Data

Mechanical damage in subsea pipelines in the form of local dents / buckles due to excessive bending deformation may severely threaten their structural integrity. A dent / buckle has two significant effects on the pipeline integrity. Notably, residual stresses are set up as result of the plastic deformation and stress concentrations are created due to change in pipe geometry caused by the denting / buckling process. To assess the criticality of a dent / buckle, which often can be associated with strain induced flaws in the highly deformed metal, integrity assessment is required. The objective of this paper is to evaluate the severity of dent / buckle in a 48” subsea pipeline and to make the rerate, repair or replacement decision. This paper presents a Level 3 integrity assessment of a subsea pipeline dent / buckle with metal loss, reported in in-line inspection (ILI), in accordance with Fitness-For-Service Standard API 579-1/ASME FFS-1. In this paper, the deformation process that caused the damage (i.e. dent / buckle) with metal loss is numerically simulated using ILI data in order to determine the magnitude of permanent plastic strain developed and to evaluate the protection against potential failure modes. For numerical simulation, elastic-plastic finite element analyses (FEA) are performed considering the material as well as geometric non-linearity using general purpose finite element software ABAQUS/CAE 2017. Based on the numerical simulation results, the integrity assessment of dented / buckled subsea pipeline segment with metal loss has been performed to assess the fitness-for-service at the operating loads.

Effect of Flexible Joint Modelling Method on Deep Water Catenary Riser Hang-Off Fatigue Response

The Flexible joint is one of the most widely used hang-off systems for deep water catenary riser for its large rotation and load bearing capacity. The fatigue performance of riser hang-off region and fatigue load on the flexible joint highly depend on the rotational stiffness of the flexible joint. Thus, modelling the flexible joint stiffness to accurately simulate the behavior under cyclic bending cycles is critical in global riser fatigue analysis. The load-displacement relationship of a flexible joint typically follows a nonlinear curve, and it shows hysteresis behavior when subject to cyclic bending cycles. However, in current industry practice, the flexible joint stiffness is modelled either as a nonlinear curve or simplified as a fixed value. These simplified methods sometimes can lead to unconservative or over conservative results in riser design. Modelling the flexible joint stiffness in an accurate approach becomes more important especially when the riser fatigue is critical at the hang-off region. In addition, the design of flexible joint will also be impacted by the fatigue load extracted from global fatigue analysis, which is also largely affected by the flexible joint stiffness modelling method. Thus, modelling a flexible joint by accounting for the nonlinear hysteretic stiffness is recommended. This paper compares the different modelling methodologies of the flexible joint for catenary riser hang-off and presents the impact on fatigue performance considering hysteretic behavior. This study considers the effects of wave amplitude and hosting vessel offset. A case study is also presented on the application of all the modelling methods on fatigue performance of an SCR in the Gulf of Mexico. The fatigue behavior is compared for the different modelling methods considering long term wave motion and platform offsets. The impact on the results from different types of hosting platform is also discussed.

A Design Practice for Subsea Pipeline Subjected to UXO Hazards

This paper presents a design practice for the oil export pipeline (OEP) of Johan Sverdrup Oil Field subjected to unexploded ordnance (UXO) hazards during the pipeline installation period. The UXO (unexploded ordnance) is a potential risk to the oil export pipeline due to its significant impulsive pressure load in a short time. Present paper discusses an unfavorable scenario in which the UXOs are identified during the pre-lay survey stage. It may (and it does) happen due to the survey methods chosen between the initial preliminary and the pre-lay survey. Consequently the original design pipeline routing has to be updated in order to minimize the UXOs’ potential damage to the pipeline. A safety distance between pipeline and UXOs shall be established and maintained. To achieve this, advanced numerical simulation was used for assessing the damage of pipeline under UXO explosion loads. The damage is sensitive to the charge weight and the distance between charge and pipeline. The pipeline route was updated accordingly based on the safety distance and actual locations of UXOs. The new route shall also fulfill all design checks. With the updated pipeline routing, the installation could continue without interruption of the project schedule. The identified UXOs will be subject to later removal before startup of production to further ensure the safety of installed pipeline. The overall design process is presented. Some simulation results from Abaqus Explicit solver are shown in the paper. Conclusions and discussions are included, which may be useful for similar projects in the future.