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On-bottom stability design of submarine pipelines
... With the breakthrough of technology and the development of market, the application of bonded flexible pipes is very extensive, which can be used as dynamic riser, submarine manifold system and floating production system connection, oil pipeline and spanning hose, there is a trend to replace the pure steel pipeline. However, the bonded pipeline is light in weight and bears wave-current hydrodynamic load directly on the seabed [3]. Without considering the additional weight, it may lose stability because of excessive lateral displacement, and buckling failure is more likely than steel pipe. ...
... By choosing the counterweight scheme of submarine pipeline and establishing the pipe-soil model with ABAQUS, the in-situ stability of offshore flexible pipeline is analyzed by an example, and compared with DNV code [3], the accuracy of the research method is verified. The factors that have great influence on the in-situ stability of pipeline are analyzed. ...
... Flexible pipes need to be added more weight because of their lighter weight. According to the absolute stability method in DNV RP F109 (2011) [3], the in-situ stability of flexible pipeline is calculated. Finally, the weight of the pipeline under external environmental loads and considering the depth of the pipeline is obtained. ...
As an important part of underwater oil and gas transportation system, the in-situ stability of submarine pipelines is particularly important. Comparing with steel pipe, flexible pipe has the advantages of high strength, corrosion resistance, light weight, coil supply and laying. In this paper, the finite element software ABAQUS is used to calculate the initial in-situ stress balance of soil, pipe weight, internal pressure load analysis and horizontal buckling simulation. The transverse horizontal displacement and stress distribution under high temperature and high pressure in flexible pipeline operation are calculated. The results show that the temperature difference and pressure difference in pipeline operation are the main causes of pipeline failure. The results show that the relevant parameters of seabed soil have different effects on pipeline failure, and the friction coefficient of soil and pipeline is more obvious.
... The soil should provide enough ultimate bearing capacity to avoid an excessive embedment into the seabed, especially in the pipeline laying process [3] . Moreover, during the in-service period, the lateral soil resistance should be large enough to balance the hydrodynamic forces from severe waves and/or currents to avoid the pipeline displacing from its original location [4] . In a more general sense, the pipeline instability should include not only the aforementioned on-bottom stability, but also the tunnelerosion underneath the partially-embedded pipeline, the vortex-induced vibrations (VIVs) of the free spanning, and even the global buckling of a highpressure/ high-temperature (HT/HP) pipeline under deepwater conditions. ...
... In the on-bottom stability design [4] , the bearing capacity of the pipeline foundations has ever been evaluated with conventional bearing capacity theories for the strip footings with flat bottoms [7] . The numerical modeling of the vertical pipe-soil interactions [8] indicated that the failure of the pipeline foundations is often in a general shear failure mode, especially for soft clayey soils or cohesionless sands, i.e., the plastic shear zone underneath the pipe extends gradually to the soil surface with the increase of the downward load. ...
... In the design practice for the on-bottom stability of submarine pipelines by Det Norske Veritas [4] , the fundamental principle is the quasi-static decoupling between the hydrodynamic loads and the corresponding lateral soil resistance. When the drag force becomes larger than the lateral soil resistance, the lateral instability of the pipeline would be triggered. ...
The stability of a submarine pipeline on the seabed concerns the flow-pipe-soil coupling, with influential factors related to the ocean waves and/or currents, the pipeline and the surrounding soils. A flow-pipe-soil coupling system generally has various instability modes, including the vertical and lateral on-bottom instabilities, the tunnel-erosion of the underlying soil and the subsequent vortex-induced vibrations (VIVs) of free-spanning pipelines. This paper reviews the recent advances of the slip-line field solutions to the bearing capacity, the flow-pipe-soil coupling mechanism and the prediction for the lateral instability, the multi-physical coupling analysis of the tunnel-erosion, and the coupling mechanics between the VIVs and the local scour. It is revealed that the mechanism competition always exists among various instability modes, e.g., the competition between the lateral-instability and the tunnel-erosion. Finally, the prospects and scientific challenges for predicting the instability of a long-distance submarine pipeline are discussed in the context of the deep-water oil and gas exploitations.
... Table (3-1) gives formulae valid for all water depths and the shallow and deep water approximations to the general solution. Figures (3)(4) and (3)(4)(5) show the direction of velocity and acceleration in an airy wave, and Figure ...
... The hydrodynamic force coefficients used in the design of submarine pipelines are empirical by nature. One of the authoritative guidelines for pipeline design recommended values is given by DNV 1981 [5] as: ...
... The This method assumes the following [5]: The response on the pipeline in the sea state is principally controlled by the following non-dimensional parameters [5]: ...
... These coefficients depend on the Reynolds number Re= U x D/ν, the Keulegan-Carpenter number KC= U x T/D (with ν and T the kinematic viscosity and the wave period, respectively, and U x is the velocity peak in a wave period), the relative roughness and the relative trench depth [5]. For Re>10 6 , coefficients do not depend on flow regime and for the reference configuration of a smooth pipe laid on the bed (neither trenched nor spanning), the relation given by Det Norske Veritas (DNV) recommendations [6] and valid in the KC range 7-50 is commonly adopted: ...
... These coefficients depend on the Reynolds number Re= U x D/ν, the Keulegan-Carpenter number KC= U x T/D (with ν and T the kinematic viscosity and the wave period, respectively, and U x is the velocity peak in a wave period), the relative roughness and the relative trench depth [5]. For Re>10 6 , coefficients do not depend on flow regime and for the reference configuration of a smooth pipe laid on the bed (neither trenched nor spanning), the relation given by Det Norske Veritas (DNV) recommendations [6] and valid in the KC range 7-50 is commonly adopted: ...
... Forces evaluated from computed pressures were compared with Morison's expressions (Eqs. 1 and 2), using velocity and acceleration values in an undisturbed position. A first estimation for the constant coefficients C D , C M and C L were obtained by applying the least mean square method and, since no suggestions are found in case of protected pipeline, they were compared to the values derived by DNV's recommendations [6] for an unprotected pipe over the seabed. The effect of the mattress installation over a submarine pipe can be expressed in terms of reduction of the drag, added mass and lift coefficients in comparison with unprotected pipes. ...
... In order to minimize the number of parameters used in the empirical model, a self-evolving neural network (SEANN) is used. The performance of the proposed model will be compared with conventional BPN networks as well as the method based on DNV recommended practice (Veritas, 2011). ...
... A different approach is proposed by Brennodden et al. (1989), in which the passive resistance is related to the work done by the pipe as it undergoes lateral movement. A similar approach is adopted in the DNV code (Veritas, 2011). The major drawback of this approach is its reliance on empirical parameters that are difficult to determine in engineering practice. ...
... An attempt to develop a simple model for the lateral stability of un-trenched pipes was made by Gao et al. (2011), based on the DNV code (Veritas, 2011). In the model, a new parameter was proposed, the 'coefficient of ultimate lateral soil resistance ðηÞ', which encapsulates the frictional and passive components of soil resistance. ...
... The submarine cable stability was verified in accordance of DNVGL-ST-F101 [7] and DNVGL-RP-F109 [15], although these standards/recommended practices are not applicable to submarine cables, especially with regards to the safety objectives. ...
... DNVGL-RP-F109 [15] considers excessive displacement due to the action of the hydrodynamic loads to be a serviceability limit state. In the context of a crossing cable, the serviceability limit state is reached in the following cases: ...
High voltage submarine cables are increasingly being installed in existing and new offshore oil and gas fields for power supply and control purposes. These power cables are both large and with a high submerged weight, which poses a challenge when designing a safe, maintenance free (economical), and fit-for-purpose crossing over a pipeline. Damage to subsea pipeline crossings caused by deterioration of a crossing support, field joint materials and cover components is well known in the industry, particularly with old pipelines.
Crossing cables over an existing pipeline should be avoided whenever economical and practical. However, it is inevitable in some situations to use the existing pipeline (unburied) as the crossing support to a new cable/umbilical. In these situations, crossing the cable/umbilical over the existing pipeline may be a cost-effective and worthy consideration. However, there are no explicit guidelines or criteria in the industry concerning the acceptable practice of design and construction of crossings. The only clear recommendation is relating to pipeline separation distances.
This paper documents a recent case study of damage of a field joint coating at a crossing of an existing pipeline by a 132 kV subsea cable of 191 mm outside diameter. Investigation of the damage on site revealed that it was caused by lateral movement of the cable under the influence of hydrodynamic forces.
Further to investigation and assessment of the damage of the case study presented here, the paper proposes some guidelines for the safe design and construction of cable crossing. Another objective of this paper is to invite further evaluation of the proposed guidelines so that appropriate crossing design requirements can be further developed and standardised.
... In offshore pipelines, these loads are hydrodynamic loads induced by waves and currents. This load-resistance relationship has formed the basis for various design codes governing the stability of submarine pipelines such as the API RP 1111 [3], DNV RP E305 [4], DNV RP F109 [5], and DNV OS F101 [6]. ...
... This model adopts a load-resistance relationship that ensures the pipe does not displace horizontally i.e. hydrodynamic loading must not exceed pipe and soil resistance. The DNV-RP-F109 [5] which reflects state-of-the-art industry practice and latest research, also utilizes the Load and Resistance Factors Design Format (LRFD). This method places an upper (allowable) limit on pipe displacement due to hydrodynamic loads, with target safety levels given in the DNV-OS-F101 [6]. ...
Pre-installation Stability analyses of pipelines are required to prevent lateral and upheaval buckling in service. In this study, the hydrodynamic forces associated with an offshore pipeline is analyzed, thereby determining limiting steel wall-thickness and submerged weight necessary to prevent collapse and propagation buckling, contain pressure and ensure on-bottom stability. Relevant design equations, Codes and Procedures were integrated to create a comprehensive platform for analyzing lift, drag and inertia forces acting on submerged pipelines. Hence, a user friendly template with multiple design settings has been developed with MathCAD® for on-bottom stability analyses. The analysis tool is based on the absolute lateral stability method in DNV RP F109. A case study of 762 mm x 34 km pipeline to be installed Offshore Escravos, Gulf of Guinea is simulated and analyzed using the design tool developed. Pipeline behavior under different environmental and pipeline conditions such as water depth, wave height, steel and concrete thickness were investigated. The results showed that concrete and steel wall thicknesses are the most critical parameters in the on-bottom stability of offshore pipelines. With a determined optimal wall thickness of 20.6mm, concrete thicknesses of 78.796 mm, 61.386 mm, 53.043 mm and 42.58 mm corresponding to 5 m, 10 m, 15 m and 20 m water depths, respectively were obtained. Also, the results showed that for pipes OD > 32.5 in (825.5 mm) alternative stability methods may be required as the necessary concrete thickness may exceed allowable limits.
... The commonly used approach in the deterministic analysis to overcome this uncertainty is to apply safety factor(s) (for example , see Refs. [1,2]). However, this has several limitations, since it tends to be overly conservative for many practical situations and it does not provide any quantification of the design reliability. ...
... It is becoming increasingly accepted that in the design of a pipeline some movement of the pipe during large storm events should be allowed, particularly as pipelines are flexible structures [2]. Operators can set a tolerable displacement limit as the criterion to quantitatively evaluate the pipeline stability, providing more flexibility to tune the target reliability limits. ...
Offshore pipelines are increasingly being employed to transport offshore hydrocarbons to onshore processing facilities. Pipelines laid directly on the seabed are subject to a considerable hydrodynamic loading from waves and currents and must be accurately designed for on-bottom stability. Confidence in the stability of pipelines requires appropriate models for their assessment and, in this paper, particular emphasis is placed on achieving an integrated and balanced approach in considering the nonlinearities and uncertainties in the pipe structure, the reaction of the restraining soil, and the hydrodynamic loading applied. A statistical approach is followed by developing a response surface model for the pipeline maximum horizontal displacement within a storm, while including variability in parameters. The Monte Carlo simulation method is used in combination with the developed response surface model to calculate the extreme response statistics. The benefit of this approach is demonstrated and also used to investigate the sensitivity of the on-bottom pipeline simulation for a variety of model input parameters. These results provide guidance to engineers as to what uncertainties are worth reducing, if possible, before a pipe is designed.
... Several models have been proposed specifically for shallowly embedded pipes, considering different soil types such as calcareous sands [8][9][10][11][12] and clays [12][13][14][15]. DNV [16] proposed a small-displacement, three-stage soil resistance model. Bruton et al. [17] conducted lateral large pipeline displacement experiments and analyzed the nonlinear characteristics of soil resistance. ...
This study aims to investigate the interaction between reinforced thermoplastic pipes (RTPs) and sandy soil. The mechanical properties of sandy soil in the South China Sea region were determined through shear tests to obtain fundamental data. Subsequently, a specialized experimental setup was designed and assembled to study the pipe–soil interaction, specifically measuring the lateral soil resistance of flexible pipes at varying burial depths. Data analysis revealed the relationship between soil resistance, lateral displacement, and initial burial depth. To simulate the mechanical behavior of the pipe–soil interaction, the coupled Eulerian–Lagrangian (CEL) method was employed for numerical simulations. The research findings indicate that the lateral soil resistance is influenced by the uplift height and accumulation width of the soil ahead of the pipe. Within a lateral displacement range of 0.5 times the pipe diameter (0.5D), the lateral soil resistance rapidly increases, resulting in a soil uplift along the circumferential direction of the pipe. This process not only enhances the load-bearing capacity of the pipe but also increases the accumulated soil resistance, consequently expanding the soil failure zone. Furthermore, the ultimate soil resistance exhibits an increasing trend with an increasing burial depth. Once the pipe reaches a certain burial depth, the uplift height of the soil reaches a critical state. To address the grid distortion caused by soil deformation, numerical simulations based on the CEL method effectively modeled the pipe–soil interaction forces under significant lateral displacements, exhibiting good agreement with the experimental results. This study provides a solution for investigating soil resistance in submarine pipelines, thereby contributing significantly to the design and performance prediction of underwater pipelines.
... A large seabed roughness up to k s /D = 2.6 was considered. It should be mentioned that according to Cheng et al. [12] and Tang et al. [3], most of the existing on-bottom stability design methods, such as the absolute lateral static stability method suggested by DNV-RP-F109 [13], were developed for oil and gas pipelines with large diameters and cannot be directly applied for small diameter pipelines and power cables. Due to the small diameter of power cables and extreme wave conditions, they are typically under high KC numbers up to O(10 3 ). ...
Hydrodynamic forces on small diameter subsea pipelines and cables placed near seabed are important for their on-bottom stability design. In offshore environments, these pipelines are usually subjected to extreme wave conditions. The present study investigates hydrodynamic forces acting on a pipeline near a flat seabed subjected to a wave-induced boundary layer flow. The Keulegan-Carpenter numbers (KC) of the wave-induced boundary layer flow are 20, 140 and 200, defined based on the pipeline diameter (D), the maximum velocity of the undisturbed near-bed orbital velocity (Uw) and the period of the incoming oscillatory flow (Tw). Reynolds number is 1 × 104 based on Um and D. A seabed roughness ratio ks/D (ks is the Nikuradse equivalent sand roughness) of up to 0.1 and different gap ratios of G/D = 0.05∼0.5 between the pipeline and the seabed are considered. Numerical simulations have been carried out based on two-dimensional (2D) Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations combined with the k-ω Shear Stress Transport (SST) turbulence model. A preliminary one-dimensional (1D) simulation is carried out to obtain a fully developed wave-induced boundary layer velocity profile, which is used as inlet flow for the 2D simulations. The numerical model is validated against the experimental data reported by Sumer et al. (1991)[1] at KC = 10. Influences of KC, ks/D and G/D on the hydrodynamic forces and the surrounding flows are discussed in detail.
... where W S is the submerged weight of the pipe, μ is the friction factor of soil, Fc is the vertical contact force between pipe and soil, D is the diameter of the pipe, V S is the significant near-bottom velocity amplitude of wave perpendicular to the pipeline, V C is the current velocity, Θ is the phase angle, A s is the significant wave acceleration, C M and C D are the coefficients of inertia and drag force, respectively. The passive resistance F R on sand can be expressed as in the following equation (Det Norske Veritas, 2007): A. Durap and C.E. Balas Ocean Engineering 261 (2022) 112079 ...
The risk assessment of submarine pipeline is carried out considering vertical and horizontal displacements under hydrodynamic forces. The stability of the pipeline is assessed according to limit-state functions using Monte Carlo (MC) simulation. The effects of subsea topography, hydrodynamic loads of waves and currents, pipe-soil interaction, and characteristics of hazardous materials carried by the pipeline were examined. The failure probability obtained by the MC simulation was defined by the exceedance probability of the displacement limit state. The novel part of this paper is the MC simulation which is coupled with a 3-D hydrodynamic numerical model (Hydrotam-3D) developed by Balas (Balas and Ozhan, 2000). The effects of pipe-soil interactions and environmental parameters such as wave-induced currents can be obtained by this model coupling. The most important parameters that affect the results of the simulations are obtained. Hydrodynamic loads that depend on the combined actions of waves and currents are obtained by the hydrodynamic numerical model (Hydrotam-3D) and they are used in the MC simulation, to increase the reliability of the pipeline design.
... Depending on in-situ conditions and water depth, pipelines on the seabed may be subject to wave and current loading forces. These may destabilise the pipe leading to floatation or lateral movement [62]. Lateral motion is resisted by soil friction force and by the passive soil resistance, which is dependent on the pipe embedment. ...
The European Union has set the ambitious target of becoming climate-neutral by 2050 and reducing greenhouse gas emissions by at least 55% before 2030, compared to 1990. Greater energy generation can be achieved by increased reliance on renewable energy, but the transmission of this energy to match supply with demand is a likely bottleneck in maximising renewable energy use. In this paper, we examine medium-voltage DC superconductors as a potential solution for low-loss, high-power transmission of offshore renewables. We look at what has been achieved to date in onshore superconducting cable deployment and what needs to be done for such superconductors to be deployed subsea, with the goals of exporting electricity from offshore wind farms and acting as grid interconnectors. The offshore oil and gas industry represents state of the art in terms of subsea pipe design. This paper explores how the experience of the offshore oil and gas industry can be applied to subsea superconductor cable design and identifies aspects of superconductor design likely to present a challenge to subsea deployment. The key areas identified as requiring research are the development of flexible pipes suitable for cryogenic usage that can withstand the dynamic loading encountered in the marine environment; robust and low-maintenance insulation systems suitable for subsea deployment; and cooling systems to enable pipelines greater than 100 km in length. Although the primary focus of this research is on superconductor cables, the information is also applicable to other subsea conduits requiring cryogenic cooling such as ‘green’ hydrogen.
... Compressed CO 2 can be injected into porous rock formations below the Earth's surface using many of the same methods already used by the oil and gas industries ( Figure 1). The three main types of geological storage are oil and gas reservoirs, deep saline formations, and un-minable coal beds [24][25][26][27][28] . CO 2 can for instance be physically trapped under a well-sealed rock layer or in the pore spaces within the rock. ...
After the CO2 has been captured at the source of emission,
the CO2 would have to be transported to the storage
site using different technologies. In some countries
(i.e. USA) real possibilities exist so that available and
new oil and water pipe lines could be used for such
operations. In practice it means that transportation
could be carried out with motor carriers, railway and
water carriers. If the present experiences are taken
into account and the real situation checked, such
transportation systems are mainly used in praxis. For
maximum throughput and to facilitate efficient loading
and unloading, the physical condition with respect to
pressure and temperature for the CO2 should be the
liquid or supercritical/dense phases. Temporary storage
of CO2 is of importance for finding a comprehensive
solution for long-term storage under various environmental
circumstances. Underground caverns are one
of the possibilities of temporary storage. Geotechnical
analysis of stress and strain changes that are present in
the rocks around underground caverns filled with CO2
under high pressure provides a realistic assessment of
conditions for temporary storage. This paper presents
the analysis described above, for different parameters
relating to underground storage of CO2.
... Most previous studies focused on the optimization of submarine pipeline routes between two points in 3D terrain. The developments focused on a synthesis stage which contains the geometrical representation of a route; the terms of the objective function (total pipeline length, full-scale tests cost, and fitness); the number, length, and location of free spans; the geographic topographical issues; the route geometry; the geophysical and geotechnical data from bathymetry; the obstacles and regions to be avoided; the structural behavior of the pipe; the hydrostatic and environmental loadings; the implementation of slope stability and on-bottom stability criteria; the treatment of fatigue induced by vortex-induced vibrations on free spans along the candidate routes; and the implementation of the screening criteria (Alekseev et al. 1999;Veritas 2007;Vieira et al. 2010;Baioco et al. 2011;De Lima et al. 2011;Baioco et al. 2012Baioco et al. , 2013Baioco et al. , 2014aDe Lucena et al. 2014;Baioco et al. 2015;Rocha et al. 2015;Devine and Haneberg 2016). In addition, the techniques make it possible to select an optimal pipeline route to carry oil and gas in two-phase flow, which includes a production criterion to reduce the pressure loss (Shamir 1971;Dobersek and Goricanec 2009;Zhang et al. 2009;De Lucena et al. 2014;Üster and Dilaveroğlu 2014;Baioco et al. 2015;Ríos-Mercado and Borraz-Sánchez 2015;Wei et al. 2016;Zhou et al. 2017). ...
The construction and development of a gathering and transmission pipe network in a gas field is a hard and critical task that is one of the main procedures to be performed during the gas field's productive life. This paper reports on two mixed-integer nonlinear programming (MINLP) models including the optimization of the topological structure of the pipe network and the optimization of the pipeline routes in three-dimensional (3D) terrain to minimize the total development costs of surface production pipe networks. The models seek to define the number, location, and capacities of manifolds and central processing facility (CPF); the affiliation of wells; the points where manifolds must be installed; the interconnection between wells or manifolds; the sections or obstacles which should be bypassed; and the optimal routes of the pipelines. A new method combining the Floyd algorithm with weighted grid (FAWG) was developed to optimize the pipeline routes in 3D terrain. An increment and verification (IAV) algorithm based on the exhaustion method was developed to optimize the number and location of manifolds and CPF. Two types of connection structures from wells to manifolds and from manifolds to CPF, radial and dendritic, were applied to design the interconnection pipe structure layout between those points. Four types of pipe layouts and their corresponding costs were obtained and compared. Two different scenarios were tested in which the optimal topological structure, pipeline routes, and related design parameters of the network system in 3D terrain were obtained with minimum investments. The results were computationally feasible and innovative compared with models found in the literature.
... Faktor percepatan spektra Untuk perpindahan pipa maksimal sejauh 10 kali diameter, berat minimum yang diperlukan dapat dihitung dengan rumus berikut[6]:B. Local BucklingLocal buckling adalah deformasi plastis pada sebagian kecil dari panjang pipa yang menyebabkan perubahan besar pada penampang pipa. Local buckling disebabkan oleh kombinasi tekanan eksternal, tekanan internal, gaya aksial, dan bending moment.Member pipa yang mengalami bending moment, effective axial force dan internal overpressure harus didesain agar memenuhi kriteria berikut ini pada semua bagian[7]: ...
... A cable risk assessment will consider a number of factors, including: seabed type, seabed mobility, fishing risk and shipping anchors. Comprehensive guides to cable risk assessment procedures are available in [27][28][29]. ...
This paper presents a review of the main electrical components that are expected to be present in marine renewable energy arrays. The review is put in context by appraising the current needs of the industry and identifying the key components required in both device and array-scale developments. For each component, electrical, mechanical and cost considerations are discussed; with quantitative data collected during the review made freely available for use by the community via an open access online repository. This data collection updates previous research and addresses gaps specific to emerging offshore technologies, such as marine and floating wind, and provides a comprehensive resource for the techno-economic assessment of offshore energy arrays.
... However relatively little research has been published into the performance and behaviour of pipes on rocky seabeds, particularly in contrast to the extent of research into the behaviour on seabeds comprising sand or clay soils. The prevailing approach to the on-bottom stability design of pipes on rocky seabeds proposes a constant coefficient of lateral friction of 0.6 (Det Norsk Veritas, 2007), irrespective of the diameter of the pipe. This approach appears open to refinement. ...
The behavior of pipelines, cables and umbilicals on rocky seabeds has to date received little research attention. This is despite the marine renewable energy and oil and gas industries relying on these ‘pipes’ to cross a variety of rocky seabed types in the presence of extreme metocean conditions. Present design solutions are challenging and costly, yet there remains a track record of in-service failures.
This paper forms part of a wider research effort being undertaken by the University of Western Australia (UWA) into pipe behavior on rocky seabeds. This work includes the effects intermittent gaps have on hydrodynamic forces, the effect of seabed roughness on enhanced boundary layer thickness and the validity of existing hydrodynamic force models for small diameter cables.
In this paper, the lateral resistance of pipes on rocky seabeds is investigated using both physical and numerical testing of model pipes over artificially-created rocky seabeds. Four model pipes of varying diameter have been displaced laterally over 1 m square model rocky seabeds, with a range of pipe to rock diameters. The lateral resistance of the physical pipe tests were recorded using load cells and a digital data-logger. Analysis of the physical test results has enabled comparison to (and refinement of) numerical models as well as improved understanding of the importance of different parameters. Our results show peak frictions above 6 arising under conditions where interface friction is only about 0.3, which contrasts dramatically with the friction value of 0.6 nominated in F109. This work contributes towards generation of new design methods suitable for application to field conditions.
... But these studies mainly focused on the on-bottom stability design, and the recommended practice in standard file or finite element method are usually employed to analyze the pipeline stability. The authoritative design recommended practice DNV-RP-E305 released by Det Norske Veritas in 1988, and new vision DNV-RP-F109 updated in 2010 recommends three methods for pipeline on-bottom stability design, i.e. dynamic, generalized and absolute static methods, respectively. The above three methods are the basic approaches for the study of pipeline on-bottom stability. ...
... A similar behaviour can be observed also for the 274°N headings, with the difference that the discriminant embedment for which the passage between a self-lowering scenario to a possible occurrence of freespans scenario is moved to / 0.15. Figure 14 shows the decreasing of the maximum overcritical crest length with the increase of the / for the two considered headings. A further analysis is presented in Figure 15, which shows the locations where the is exceeded at any time during the seastate for / 0. 15 ...
1. ABSTRACT For offshore pipelines in shallow waters, designed to meet in place stability and integrity criteria, the initial exposure or shelter to near-bottom hydrodynamics may vary over time, in cyclic or progressive attitude: whether significantly embedded after self-lowering or/and free spanning due to underneath scouring, sometimes mined from a trench after floatation or under the influence of seasonal bed-form activity. Time scale for and entity of variation are strictly linked to the severity of environment, water depth, soil type, pipe diameter and weight. This is outstanding from four decades of regular inspections that pipeline operators regularly perform on both strategic continental links and export pipelines from offshore platforms to shore, which are in service for about 10 5 km * year across the sandy sea-bed contours of the North Sea. For these, design and construction, in service integrity monitoring as well, have been commonly backed by field surveys, data analysis and theoretical modelling, sometimes dedicated experimental programs in hydraulic laboratories. The outcome enabled the engineering tools to provide, at design stage, a consistent basis for assurance of safe and satisfactory performance over the design lifespan. In this paper the outcome from a few years integrity management of the Nord Stream pipelines is described, particularly the development of self-lowering across the German territorial shallow waters. The engineering analyses at design stage included one-point-in-space development of soil-pipe interaction (onset, scouring, self-lowering), then integration along the pipe axis of development patterns in relation to 3D hydrodynamic conditions of the site. Survey and data processing definitely provided basis for calibration/fine tuning of the engineering tools currently used to confirm in place stability and integrity of the Nord Stream pipelines across such active sea beds over lifespan. These further contribute to validate the engineering approach to worldwide pipelines in shallow waters, for which: a) changes of exposure to severe near bottom hydrodynamics may threat in-place stability and integrity over the time, b) mitigation as lowering by post-trenching works or other stabilization measures during operation are expensive.
... Lateral soil resistance is one of the fundamental issues in submarine pipeline on-bottom stability design for hydrodynamic loading conditions in offshore environments (Wagner et al. 1989;Det Norske Veritas 2010). Pipeline on-bottom instability behavior in ocean environments is a complex phenomenon, involving significant flow-soil-structure interaction. ...
As offshore exploitation moves to deeper waters, ocean currents become the prevailing hydrodynamic loads on pipelines, and at the same time a sloping seabed is always encountered. The prediction of lateral soil resistance is vital in evaluating pipeline on-bottom stability. Unlike previous pipe–soil interaction models used mainly for horizontal seabed conditions, a pipe–soil interaction model for current-induced downslope and upslope instabilities is proposed by using the limit equilibrium approach. The Coulomb’s theory of passive earth pressure for the sloping seabed is incorporated in the derivation. The model verification with existing full-scale tests shows good agreement between the experimental results and predicted ones. Parametric study indicates that the effect of slope angle on pipeline lateral soil resistance is significant in the examined range of slope angle from –15° to 15°. The critical pipeline embedment and corresponding passive pressure decrease approximately linearly with increasing slope angle.
... Pasqualino [7] Villarraga [8] Cardoso [9] / H D = 0.037 [11] 13 [5] ...
High temperature and high pressure can lead to a lateral global buckling of unburied or shallowly buried submarine pipelines. Because soil resistance determines the deformation and stress distribution of post-buckling pipelines, it is important to characterize the soil resistance in the pipeline global buckling analysis. A series of model tests based on the sand sampled from Bohai Gulf is conducted. The soil resistance to pipelines with different buried depths are measured. A dynamical soil resistance model varying with the buried depth of pipeline is developed, and the influence of buried depth on peak soil resistance and final soil resistance is analyzed. As the built-in penalty interaction model in ABAQUS software cannot simulate the dynamical friction in pipe-soil interaction, a user-defined subroutine VFRIC is developed to implement the established soil resistance model and used to simulate the variation of soil resistance with pipeline displacement for accurate pipeline global buckling analysis. The study shows that the soil resistance model can significantly affect the results of post-buckling pipeline. Because the dynamical soil resistance model has a peak value and an attenuation process, the critical buckling force of post-buckling pipelines with dynamical soil resistance model is larger than pipeline with constant soil resistance, the deformation of post-buckling pipelines with dynamical soil resistance model is more concentrated, and the maximum bending moment and strain are larger.
... The reliability target level (1%) should be met through the design life. Fig. 17 shows the required submerged weight with respect to different design codes, eg 0.5 OD, 1.0 OD and 10 OD [23]. A submerged weight of 1.72kN/m is acceptable for design criteria of 0.5 OD, the calculated probability is 0.6%. ...
The application of non-metallic light weight pipeline (LWP) in subsea oil/gas transmission system is subject to subsea pipeline on-bottom stability problem because of their light weight. Additional weight required for the stabilization of subsea LWP is a critical item to consider when decreasing the cost of the pipeline system. This paper presents an effective approach to determine the additional weight by utilizing a reliability-based assessment of subsea LWP against on-bottom stability. In the approach, a dynamic non-linear finite element model (FEM), including a model of fluids-pipe-soil interaction for the subsea pipeline, is used to study the pipeline displacement response. In-place analysis of a flexible pipe is presented as an example of the authors' methodology. Results show that displacements are largely affected with and without considering the lift force. Additionally, the uncertainties of all parameters used in the model are considered. With 145 cases of FEM calculations being the samples, a response surface model (RSM) is developed to predict the pipeline lateral displacement using the software Design-Expert. Combing with the RSM equation, the Monte Carlo simulation method is employed to estimate the probability of exceeding pipeline stability. To calculate the reliability of LWP for different submerged weights, the method introduces a calibrated factor into the serviceability limit state (SLS) function. The proposed approach can be used to determine the additional weight required for the on-bottom stability of subsea pipelines while considering the uncertainties of all relevant parameters.
... An efficient evaluation of the ultimate bearing capacity of a cylindrical foundation is crucial for predicting subsea structure-soil interaction behaviors, which may significantly affect the on-bottom stability of a submarine pipeline, the configuration of a steel catenary riser (SCR) at its touchdown zone, or the embedment of a circular mooring line into the seabed, etc. In the offshore engineering practices, the possibility for excessive settlement/sinking or floatation of such a subsea foundation should be checked in the design and maintenance stages (Det Norske, 2010). ...
... In-house developed dynamic finite element program Tian and Cassidy [14][15][16] and Tian et al. [17] developed an integrated fluid-pipe-soil modeling Dynamic Lateral Stability package. Dynamic Lateral Stability analysis is considered to be the most comprehensive method because a complete three-dimensional pipeline simulation can be performed for any given combination of waves and currents in time domain analysis (see DNV [18] for details). This in-house package adopted advanced plasticity pipe-soil force-resultant models [19][20][21][22] and Fourier hydrodynamic load models [23] to evaluate soil resistance and hydrodynamic loading, respectively. ...
Pipelines are the critical link between major offshore oil and gas developments and the mainland. Any inadequate on-bottom stability design could result in disruption and failure, having a devastating impact on the economy and environment. Predicting the stability behavior of offshore pipelines in hurricanes is therefore vital to the assessment of both new design and existing assets. The Gulf of Mexico has a very dense network of pipeline systems constructed on the seabed. During the last two decades, the Gulf of Mexico has experienced a series of strong hurricanes, which have destroyed, disrupted and destabilized many pipelines. This paper first reviews some of these engineering cases. Following that, three case studies are retrospectively simulated using an in-house developed program. The study utilizes the offshore pipeline and hurricane details to conduct a Dynamic Lateral Stability analysis, with the results providing evidence as to the accuracy of the modelling techniques developed.
Hydrodynamic forces on small diameter subsea pipelines and cables placed near seabed are important for their on-bottom stability design. In offshore environments, these pipelines are usually subjected to extreme wave conditions. The present study investigates hydrodynamic forces acting on a pipeline near a flat seabed subjected to a wave-induced boundary layer flow. The Keulegan–Carpenter numbers of the wave-induced boundary layer flow are 20, 140, and 200, defined based on the pipeline diameter (D), the maximum velocity of the undisturbed near-bed orbital velocity (Uw), and the period of the incoming oscillatory flow (Tw). Reynolds number is 1 × 104 based on Uw and D. A seabed roughness ratio ks/D (ks is the Nikuradse equivalent sand roughness) of up to 0.1 and different gap ratios of G/D = 0.05–0.5 between the pipeline and the seabed are considered. Numerical simulations have been carried out based on two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes equations combined with the k–ω shear stress transport turbulence model. A preliminary one-dimensional (1D) simulation is carried out to obtain a fully developed wave-induced boundary layer velocity profile, which is used as inlet flow for the 2D simulations. The numerical model is validated against the experimental data reported by Sumer et al. [1991, “Effect of a Plane Boundary on Oscillatory Flow Around a Circular Cylinder,” J. Fluid Mech., 225, pp. 271–300] at KC = 10. Influences of KC, ks/D, and G/D on the hydrodynamic forces and the surrounding flows are discussed in detail.
The assesment of displacement levels of both the vertical and horizontal displacements of
the submarine pipeline has been done. In this modelling study, the effects of subsea
topography, hydrodynamic loads, pipe-soil interaction, the effects of waves and currents on
the pipeline, and the risk assesment were made on sample applications. A new level of
proposal has been presented for impact risk levels of pipes in accordance with the Det
Norske Veritas (DNV) standards used in practice. The risk assessment of the risk levels is
examined in detail. The aim of this study is to evaluate the risk of displacement of both the
vertical and horizontal displacements of the offshore pipeline, as well as the effects of the
different raw materials carried by the pipeline on the pipeline balance, unlike other studies
in the literature. In addition, in this study, the effect of pipe wall thickness on reliability
was examined by sensitivity analysis. The effect of wave and current is investigated
together while emphasizing the soil and pipe interaction in the above mentioned standards
and applied in this thesis study. The thesis has a specific aspect with respect to study of
risk levels depending on wave parameters and is supported by a field application than
Turkey's annual probability of failure. The likelihood probability of limit state functions
with MCB defines the region below the equilibrium state, thus giving the possibility of
annual failure. The conclusion of this thesis study is that the probability that the MCB
result is considered as the whole of the piping system is defined as the displacement of the
pipe individually, in other words, the probability of exceeding the displacement limit.
Pipeline walking induced by heat-up and cool-down cyclic loadings during shutdown and restart cycles is a challenge in the design of relatively short pipelines exposed on seabeds. A major source of uncertainty in the analysis of pipeline walking is the pipe-soil response, which has a significant influence on pipeline walking. To investigate the pipe-soil response during pipeline walking, a set of large-scale model tests are conducted on Bohai sand, and they involve axially moving pipelines with different speeds and cycles. Based on these test results, bi-linear pipe-soil interaction models with normalization of the pipe weight and diameter are proposed for Bohai Bay sand. Subsequently, a comparison of the model test data to existing pipe-soil interaction models is performed, and the existing pipe-soil interaction models are calibrated with site-specific soil properties of Bohai Bay sand. Finally, the application of the site-specific pipe-soil interaction models for a pipeline-walking analysis of pipeline placed on Bohai Bay sand is performed with the finite element method to study how pipeline walking is influenced by these pipe-soil interaction models. Combined with the influence mechanisms, a design model is proposed that considers the potential risks of pipeline walking in pipeline design to reduce costs without compromising reliability.
Deep-sea pipelines exhibit lateral global buckling under high temperature and pressure differences. Numerical simulations are an effective and easy way to obtain the deformation and stress distribution of a post-buckling pipeline with a low cost. A relatively accurate and convenient simulation model using the explicit dynamic method was developed based on ABAQUS software to analyse pipeline lateral global buckling. In pipeline global buckling analysis, the dynamic variation of soil resistance as the pipeline moves laterally, the optimized computational length and the use of smooth initial imperfection profiles influence the accuracy of the simulation results. These three key factors were discussed here, and calculation methods were proposed. A CEL simulation model with improved boundary conditions that reduce the influence of wave reflection with a low calculation time cost was built to calculate the variation dynamics of soil resistance throughout the buckling process. Different computational lengths for pipelines with different parameters were simulated, and a fitting function to obtain an optimized computational length was proposed. A method to build the profiles of a pipeline with smooth imperfections was developed. Based on these analyses of the three impact factors, a relatively accurate and convenient simulation model for pipeline global lateral buckling analysis was established. Finally, an engineering case was evaluated to show the application of the simulation model and to test the reliability of this model.
This paper describes the potential global scientific value of video and other data collected by Remotely Operated Vehicles (ROVs). ROVs are used worldwide, primarily by the offshore oil and gas industry, to monitor the integrity of subsea infrastructure and, in doing so, collect terabytes of video and in situ physical data from inaccessible regions and poorly understood marine environments. The paper begins by describing how recent ROV surveys for projects in Australia have gained a new dimension by involving marine scientists in their interpretation. A previously unrecognised influence of marine life on oil and gas pipelines was uncovered, triggering new collaborations between industry and marine science.
This new collaboration prompted a team of international engineers and marine scientists to gather together with West Australian based members of the oil and gas sector and ROV operators, to examine the global scientific value of ROV-collected data. If made available for research, these data have immense value for science to quantify the marine ecology and assist good stewardship of this environment by industry. It was found that most ROV operations are conducted by industry in a way that fulfils immediate industry requirements but which can confound scientific interpretation of the data. For example, there is variation in video resolution, ROV speed, distance above substrate and time (e.g. both seasonal and time of day), and these variations can limit the quantitative conclusions that can be drawn about marine ecology. We examined potential cost-effective, simple enhancements to standard ROV hardware and operational procedures that will increase the value of future industrial ROV operational data, without disrupting the primary focus of these operations.
The ecological value of existing ROV data represents an immense and under-utilized resource with worldwide coverage. We describe how ROVs can unravel the mysteries of our oceans, yield scientific discoveries, and provide examples of how these data can allow quantification of the ecological value of subsea infrastructure. By using these data, we can greatly improve our knowledge of marine biodiversity on and around offshore infrastructure and their environmental impact on marine ecosystems, both of which are particularly important in the consideration and selection of decommissioning strategies. Predicting the environmental consequences of removing or retaining subsea structures after decommissioning relies on an understanding of the ecological communities that have developed in association with these structures during their operational lives. Making industrial ROV data available for scientific research, and collating it in the future using modified protocols, would provide a very positive contribution to both science and industry, allowing the environmental impacts of subsea infrastructure to be quantified. It will also allow industry to contribute to a broader scientific understanding of our oceans, given the location of ROVs in areas that can rarely be accessed by independent researchers. This would provide novel and valuable information about under-researched and little known regions of the world's oceans.<br/
Subsea pipelines rest on or in the seabed, and therefore their behavior and condition depends on pipe–seabed interactions. Pipelines expand and contract during operation and can also be loaded by hydrodynamic action, turbidity flows, or debris flows. These actions are resisted by seabed reaction forces in the vertical, axial, and lateral directions.
In some situations, the pipeline may be deliberately buried, for example, to allow overtrawling, provide thermal insulation, or prevent expansion‐induced buckling. However, in many cases, the pipeline may be laid directly onto the seabed. It may self‐bury through the action of sediment transport, remain only partially embedded into the soil, or some intermittent combination of these effects along its route.
Assessments of pipe–seabed interaction forces for all the abovementioned burial conditions begin with estimation of the as‐laid (or as‐constructed) embedment. Classical methods such as limit plasticity underpin solutions for embedment and the limiting pipe–soil resistance in each direction. The critical uncertainties are usually the operative soil strength—which is affected by the seabed disturbance during the lay (or backfilling) process and throughout the operating life—and the pipeline embedment (or cover depth). Specific geotechnical testing technologies have evolved to reduce uncertainties in pipe–seabed assessments, including tools for determining the near‐seabed (and disturbed) soil strength and the pipe–soil interface strength at very low stresses. The associated analysis methods also aim to quantify the variability of the seabed reaction forces along the pipeline.
The abovementioned predictions of pipe–seabed interaction behavior are performed to feed into structural analyses of the pipeline, which often use a reliability‐based framework. Pipe–soil interaction assessments require close collaboration between the pipeline and geotechnical experts due to the inherent influence of soil–structure interaction, as well as the aim of tailoring the geotechnical assessment to address the particular design criticalities, which vary depending on the seabed and pipeline conditions.
Subsea pipelines are at risk of suffering damage because of
third-party activities such as anchor drop and anchor drag. In
this study, analysis of damage to subsea pipelines due to
anchor drag was conducted using ABAQUS software. The
pipeline located in Madura Strait was considered for the
study, and hence the analysis was performed using the data of
three types of large ships that pass through the Madura Strait.
Global and local analyses were performed to calculate the
maximum displacement, stresses, and strains that occur in the
pipeline because of anchor drag. First, global analysis was
performed to determine the maximum deflection of the
pipeline using a model with line elements, and local analysis
was then performed to determine the stresses and strains in the
dented pipeline based on the conditions obtained from the
global analysis. The integrity of the dented pipeline was
finally determined based on ASME 31.8 and DNV OS F101
standards.
Keywords: Subsea pipeline, Anchor drag, Strain-based
design.
Deterioration of subsea cable, pipeline and umbilical crossings often occur due to relative movement between the crossing members. Any crossing design should aim to achieve a sound, fit for purpose solution that will be maintenance free over the crossing life. Due to the increased density of subsea fields, crossing instances need to be increasingly accommodated. Current subsea design codes are not explicit in the criteria for subsea crossings, beyond recommending pipeline separation distances.
The work within this paper describes two case studies in the novel use of articulated padding applied to the crossing member, using the crossed pipeline as a support and then using the articulated padding resting on traditional grout-bag supports. The results highlight the ability of the articulated padding to provide the required separation on subsea crossings without the need for extra support design. It is also shown that the articulated padding can be used on grout-bag shoulder supports to allow full subsea crossing separation for crossing lays that will undergo large environmental loading conditions, and hence relative motion. The results presented also provide a basis for the development of future industry standards incorporating articulated padding designs.
Model test for lateral soil resistance of partially embedded subsea pipelines on sand during large-amplitude lateral movement. The lateral soil resistance exerted on partially embedded pipes during large-amplitude lateral movement is the key parameter for the analysis of controlled lateral buckling attributable to axial compressive thermal stress. A series of large-scale model tests were performed to investigate the responses of pipe segment with different initial pipe embedment and different pipe weight during large-amplitude lateral movement. The lateral and vertical displacement and lateral soil resistance were measured during the test. The test results showed that breakout resistance and the trajectories of model pipe depend on initial pipe embedment and pipe weight; however, residual resistance is influenced only by pipe weight. In addition, the lateral soil resistance is idealized to two types of the lateral soil-resistance models on the basis of different initial embedment depth, namely the trilinear and quadlinear soil-resistance models, for deeper and shallower initial pipe embedment, respectively. A modified formula is presented to calculate breakout resistance. Finally, according to the failure mechanism of pipe soil interaction during lateral movement, a new formula for predicting residual resistance has been developed. The predicted results are well fitted with test results.
An approach for the screening of subsea pipelines against on bottom lateral instability and free spanning is presented in this paper. The approach is based on the use of a nonlinear finite element model. Combined stresses/lateral displacement acting on offshore pipelines due to combined hydrodynamic loads including wave/current effects are computed using the finite element model for both on bottom stability and free spans. Results are compared with those obtained from pipeline design codes. A case study is presented for an actual pipeline off the shore of Saudi Arabia. Results show that computed stresses/lateral displacements are within the specified code values. The proposed approach can be a valuable tool for the pipeline designer/operator for assessment of pipeline stability and free spans.. Keywords: subsea pipelines, on bottom stability, free span, finite element method.
It is increasingly recognized that the state of the seabed surrounding an on-bottom pipeline may change during the operating life of the pipeline. For seabed sediments that are soft and fine-grained, the strength may vary through episodes of pipeline movement due to consolidation effects. For seabed sediments that are mobile due to waves and currents, the burial state and the adjacent seabed topography may vary due to sediment transport and scour.
These changes in the strength and topography of the surrounding seabed alter the exposure of the pipeline to hydrodynamic loads and ambient cooling, as well as the level of geotechnical support and insulation provided by the seabed.
The design relevance of these changes in seabed condition is amplified by modern design approaches in which the pipeline itself can be tolerably mobile — for example in a dynamic onbottom stability approach or through engineered schemes of global buckling and axial walking.
This paper illustrates the interactions between the geotechnical and sediment transport processes and the resulting global pipeline behaviour. Two interactions are considered: the long-term axial walking behaviour on soft soil, and the long-term insulation and temperature profile on a mobile seabed.
The examples highlight the potential for over or underestimation of various inputs to a pipeline design when these temporal changes in pipe-seabed condition are overlooked. Emerging analysis methods for pipeline-seabed interaction that incorporate these temporal effects can lead to more reliable and cost-effective design.
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On-bottom stability is one of the most critical elements in the design of submarine pipelines. Empirical research into vertical and horizontal stability during installation and operation of both buried and unburied structures began with ancient harbor structures. This entered into a new phases with the mid-19th Century transatlantic cable crossings and ocean outfalls of the 1890s. While sanitary and environmental engineers have looked at the behavior of effluents in the sea, the increasing development of offshore petroleum production since about 1950 has sparked most of the theoretical and empirical studies of on-bottom stability that are identified in this chapter and that constitute the operational extensions to Terzaghi’s classic work (24).
THE ON-BOTTOM STABILITY and long-term integrity of offshore pipelines resting on the seabed with an almost-flat or an uneven submarine topography, due to permanent or active bed-forms, is a topical issue within the engineering design flow of many projects. The complexity of geo-morpho-seismic environments commonly leads to a strict interplay between the scientific fields of study including marine soil mechanics, seabed morphodynamics, ocean engineering, and mechanical design of the pipeline.
There is drift toward moving offshore structures operating sites to deep water that brings subsea systems and types of apparatus to meet more severe environment than onshore. At this moment, climatic condition and seabed state affect trenching efficiency so trenching process is need to make steady progress in a short time. This paper is research on estimation about construction performance of waterjet trenching machine mounted on ROV trencher. Optimal number of nozzles that can maximize trenching efficiency is selected by considering clearance and angle of nozzles through CFD. Then verified effectiveness of waterjet apparatus on the result of trenching depth and velocity by model test analogized performance for construction work of waterjet trenching machine.
The subsea pipeline industry has recognised for over two decades that sediment transport, scour and liquefaction mechanisms have potentially significant impact on the validity and accuracy of existing pipe-soil interaction models under extreme metocean conditions. This is highlighted in the recently-published DNV-RP-F109 code in Sections 4.4-4.5. Appendix B of the code also discusses the need for specific models to be used in calcareous soils. The STABLEpipe JIP is presently in Phase 2, with the objectives of undertaking research activities which could achieve a step change in the science and engineering of subsea pipeline stability design. A summary of the proposed work and present status of the JIP will be presented. Copyright © 2010 by The International Society of Offshore and Polar Engineers (ISOPE).
For over a decade it has been recognised that our existing models and tools for subsea pipeline stability design fail to account for the fact that non-cohesive seabed sediments tend to become mobile well before the onset of pipeline instability. Despite ample evidence from both laboratory and field observations that scour and erosion have an important role to play in pipeline/soil interaction, very few models have been presented which account for the tripartite interaction between the fluid and the pipe, the fluid and the soil, and the pipe and the soil. This paper presents details of a novel 2D pipe-soil-fluid (PSF) interaction algorithm, which has been developed to offer a more accurate model of the evolution of soil profiles around the pipeline compared to existing hysteresis friction spring approaches (such as the Verley model) which ignore sediment transport and scour. The PSF model has been designed to minimise computational cost compared to continuum soil FEA approaches, but still enable the profile of the soil around the pipe to be tracked. A large number of parametric 2D CFD models have been run to generate seabed shear stress profiles as a function of seabed and pipe geometry under different wave and current flow conditions. By developing algorithmic approaches based on Shield's criteria to replicate the results of these CFD analysis results, the PSF model also incorporates sediment suspension and transportation into a pipe-soil response model, without requiring the concurrent solution of the Navier Stokes equations in a CFD model. The model has significant potential to demonstrate to operators that conventional stabilisation methodologies and techniques are too conservative for pipelines on non-cohesive sediments, such as those which cross Australia's North West Shelf, where extreme metocean conditions driven by frequent tropical cyclones result in the requirement for extensive primary stabilisation measures and additionally expensive secondary stabilisation measures. Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE).
The large-amplitude lateral soil resistance between an on-bottom pipeline and the seabed is an important design parameter in assessing pipeline behaviour during lateral thermal buckling or under the impact of a submarine slide. This paper describes a series of centrifuge model tests that shed light on the underlying behaviour during large-amplitude lateral pipe movement. It is shown that at large displacements the lateral response is governed predominantly by the passive resistance of the growing berm of soil ahead of the pipe. Using a new analysis of this growing soil berm, based on conservation of volume, the 'local' embedment of the pipe relative to the top of the idealised soil berm is defined. In this way, the normalised lateral pipe-soil resistance, H/suD, from tests encompassing a range of pipe weights and initial embedments follows a single trend line. This idealisation of the response is more consistent than the usual terminology of a pipe-soil friction factor.
Due to economic efficiencies in laying, untrenched pipelines are finding increased popularity in the transportation of offshore oil and gas, though their design for on-bottom stability remains critical. During a storm complex fluid-structure-soil behaviour is exhibited with an integrated assessment of the pipeline load and displacement required. Traditionally, hydrodynamic fluid loading and structural analysis has been conducted independently of the geotechnical assessment. Invariant hydrodynamic loading calculated prior to the analysis for the initial pipe position does not account for any movement calculated. This paper introduces a "balanced" modelling technique to demonstrate and quantify the advantage of a coupled assessment. Soil-pipe interaction and fluid-pipe interaction are modelled with integrated numerical modelling approaches developed for on-bottom pipeline. Both models are implemented into commercial finite element package ABAQUS as user subroutines. ABAQUS structural elements model the pipe itself. This balanced modelling method is quantified for an example 1.0 m diameter pipeline, simulated over a length of 1250 m. Conditions simulated are typical of those found offshore Australia. Comparisons between analysis methods are presented for a range of water depths. Copyright © 2010 by The International Society of Offshore and Polar Engineers (ISOPE).
The stability assessment of the 40 in. North Rankin A trunkline, operated by Woodside Energy Ltd., has provided better insight into fluid-soil-pipe interactions during extreme storm events. The resulting conclusion of the work is that the trunkline, a major subsea natural gas artery in Australia's Northwest Shelf since its installation in 1982, can continue to be operated safely for the next 30 years from a hydrodynamic stability point of view. This conclusion was reached after substantial study and physical model testing was performed considering the tripartite interaction between fluid, seabed, and pipeline. To provide vital information feeding into the stability analysis, a physical model testing programme was developed, and a new world-class hydrodynamic testing facility designed, constructed, and commissioned at the University of Western Australia. This facility allows the replication of near-seabed conditions during tropical cyclones in controlled laboratory conditions, and observation of the interaction between ocean, seabed, and pipeline. Tests were performed using a range of pipeline embedment profiles, storm realizations, and pipe fixity conditions simultaneously to model hydrodynamic loading onto the pipeline and seabed scour. These data were then used in the three-dimensional numerical modelling of pipeline response using finite-element analyses, which included the effects of seabed instability.
This paper discusses the on-bottom stability of subsea lightweight pipeline (LWP) on sand soil seabed subjected to a combined load of wave and current. Hydrodynamic loads were applied on an LWP to test the on-bottom stability of the pipe. The tests of LWP were carried out with different pipe outer diameter (OD) and different pipe submerged weight. The lateral movement of pipe experiences three characteristic periods based on the lateral displacement of pipe: (1) slightly shaking with sand scour; (2) crossover sand breakouts; (3) laterally moving within a displacement of 2.0 × OD or the pipe will continue to lateral movement after the lateral displacement of 2.0 × OD. A lateral displacement of 2.0 × OD is selected as the displacement criteria for the on-bottom stability of LWP because the pipe will be continuously laterally moving in case the lateral displacement of pipe is larger than 2.0 × OD for LWP, which is different from the on-bottom stability of a rigid steel pipe. Based on the test results, an empirical correlation was established to describe pipeline's lateral on-bottom stability. Three-dimensional ABAQUS dynamic finite element simulations of an LWP on flat seabed are carried out. In accordance with the experimental observations, a modified correlation suggested in DNV-RP-F109 code is used for simulating the lateral resistance of LWP on sand soil surface. The effects of lift force on the pipe when the pipe contacts the seabed were considered. A MathCAD worksheet based on the generalised lateral stability method in DNV-RP-F109 was used to assess the pipeline on-bottom stability. The comparison results of MathCAD and finite element analysis (FEA) showed that the FEA simulation gives a good agreement with test results when the lateral pipe displacement is less than 2.0 × OD. The modified FEA model can be used for the on-bottom stability analysis of LWP to reduce conservatism.
The ultimate anti-slide capacity for a submarine pipeline on the sloping seabed is one of the main concerns in the global buckling design. A mechanical-actuator facility has been designed and constructed for physical modeling of the axial pipe-soil interaction. The phenomenon of "pipe-trembling" was observed in the process of the pipe's axial movement, which is more prone to occur for the smoother pipes. Based on dimensional analyses, a dimensionless factor is proposed to describe the anti-slide capacity of the pipeline on a sloping sandy seabed. Experimental results indicate that both the slope angle and the pipe roughness have much effect on the axial pipe-soil interaction behavior and eventually on the anti-slide capacity. Unlike the lateral pipe-soil interaction, the slope angle has slight effect on the axial soil resistance to the partially-embedded pipeline in the examined range of slope angle (-9° - +9°). Copyright © 2015 by the International Society of Offshore and Polar Engineers (ISOPE).
While the subject of scour, erosion and sedimentation around subsea pipelines has been studied for many decades, a reviewof the literature shows a number of areaswhere relatively little existing publishedwork has been presented, including the variation in seabed shear stresses around the shoulders of subsea pipeline spans. This paper presents the results of 3D CFD modelling using ANSYS FLUENT, which has been undertaken for a parametric range of 24 cases, including 6 different pipe/span geometries, as well as a range of steady current and wave cases at a range of incident angles. The effect of span presence is described using the previously published concept of a seabed shear stress amplification factor α at the span shoulder which extends a distance of βD around each shoulder, together with a related amplification factor γ for the mid-span region. Analysis of the results focusses on evaluation of seabed shear stress amplification factors compared to far-field ambient values. The results have relevance to the prediction of span growth rates and complement previously published experimental work under live-bed conditions. The present results have the potential to provide insights into span growth under clear-water conditions which have significant potential to result in seabed profile changes due to slow-processes induced by persistent ambient metocean conditions.
In some areas of the world, subsea pipeline hydrodynamic stability is one of the most fundamental aspects of pipeline design. Several design approaches can be used to analyse pipeline stability, ranging from simple force-balance calculations to complex dynamic finite element analysis. Effective and accurate pipeline stability analysis is essential to avoid the potentially high costs associated with complex stabilisation solutions.
As it stands the current engineering practices using absolute stability or generalized stability do not explicitly account for pipeline stiffness, hence do not distinguish any difference in response between flexible and rigid pipelines of similar SG and diameter under a given metocean condition.
Observations of recent WGK dynamic stability analyses show that pipelines that have identical properties but very different stiffness will exhibit significantly different lateral response under the same metocean conditions.
Studies investigating sensitivity of predicted lateral response versus the number of storm random seed realisations have also been observed to show little or no correlation between the peak wave velocity and peak pipeline response.
Recognising that in directionally spread wave conditions, peak wave events do not tend to be associated with the widest wave crests in a seastate, this paper will investigate the nature of wave velocity and crest width characteristics leading to pipeline lateral displacement events (width and net movement) within a storm, and how this varies with pipeline stiffness.
Further understanding of the mechanisms driving pipeline response are therefore expected, based on the hypothesis that both pipeline stiffness and the crest width of waves capable of causing pipeline displacement are important parameters. It is further anticipated that this will help refine the existing design approaches for flexible pipelines which may not be adequately addressed using existing industry design methods, as well as providing recommendations for the minimum length of 3D dynamic stability models required to be considered in order to accurately capture the longitudinal length scales of importance.
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This paper describes a numerical study investigating the effect of sediment transport and associated changes in the local seabed profile on the drained breakout resistance of subsea pipelines. Limit analyses were conducted assessing the breakout response of a pipeline placed on a cohesionless Mohr-Coulomb material considering different seabed profiles around the pipeline. These profiles were determined from surveys of a pipeline on an erodible seabed. The parametric study shows the relative importance of various parameters describing the seabed profile geometry, including the local pipe embedment and the adjacent slope of the seabed. Significant changes in drained resistance occur due to changes in local pipeline embedment resulting from scour induced pipeline lowering and/or sedimentation. The seabed slope local to the pipeline also has a strong impact. The assumption of a flat seabed can lead to predicted seabed resistance that differs significantly from the actual value, accounting for a more natural seabed profile.
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