Design Of Steel Catenary Risers Research Articles

Investigation of trench effect on fatigue response of steel catenary risers using an effective stress analysis

The design of Steel Catenary Risers (SCR) heavily relies on the interaction between the riser and seabed in the touchdown zone (TDZ). To accurately estimate the SCR-seabed stiffness, the soil behaviour must be assessed in two main phases: (1) during cyclic SCR motions, which generate excess pore pressure, leading to soil softening and remoulding in the TDZ, and (2) during inactivity periods throughout the SCR’s lifespan, causing dissipation of excess pore pressure associated with the consolidation state. The latter was neglected by the existing non-linear hysteretic soil models, which are based on the total stress approach. In the current study, a global riser analysis was conducted and the consolidation effect was incorporated using an effective stress framework. Long-term soil stiffness was determined by capturing both the remoulding and consolidation effects, which are respectively associated with the damage accumulation during the SCR cyclic motions and soil strength recovery during the intervening pause period. The constructed model was then combined with a new methodology named Hybrid Trench Model to investigate how the consolidation effect contributes to the fatigue performance in different trench depths. Stochastic fatigue analysis showed that the consolidation effect might increase the damage by around 10%-40% over the long-term assessment.

Fatigue analysis of a steel catenary riser at touchdown zone with seabed resistance and hydrodynamic forces

Under the actual ocean environment, the complicated hydrodynamic forces and nonlinear seabed resistance are two main sources of the structural fatigue damage accumulation for steel catenary risers (SCRs) at touchdown zone, and the significant coupling effect between them is worthy of enough attention. An alternative numerical model of SCRs for the dynamic response prediction as well as fatigue damage analysis with multi-physics field excitations is established in this paper. For a full-scale SCR, a systematic case matrix including different riser parameters, soil parameters and operation parameters is simulated. The separate fatigue damage of the riser with hydrodynamic forces and seabed resistance is analyzed respectively. Then the coupling effect between these two external excitations on the riser's fatigue characteristics is discussed. Finally, a parameter sensitivity investigation regarding the seabed soil properties is carried out under combined cases. The coupling effect between hydrodynamic forces and seabed resistance makes the riser's fatigue issue more serious, much larger than the simple algebraic superposition of the separate fatigue damage with the single excitation, which needs to be considered in the practical engineering design of SCRs.

Nonlinear riser-seabed interaction response among touchdown zone of a steel catenary riser in consideration of vortex-induced vibration

Steel catenary riser (SCR) is a preferred solution for deepwater oil and gas exploitation under harsh marine environment. Its riser-seabed interaction (RSI) among touchdown zone (TDZ) in consideration of the vortex-induced vibration (VIV) of sag bend is at present a research frontier in ocean engineering. This paper integrates Randolph-Quiggin model into a published global numerical riser model, to investigate the nonlinear RSI response among TDZ of an SCR with VIV. The adopted numerical model is validated against some published experimental measurements for VIV and RSI respectively. Three combined cases with top-end platform heave motion and VIV are simulated, and the trench development as well as the SCR's response characteristics among TDZ is analyzed. VIV makes the trench develop more smoothly, and the trench length as well as depth turns smaller. The VIV effect on RSI is relatively significant with violent heave excitation, under which the P-z curves present clearer interaction mode transitions. VIV enlarges the maximum bending moment visibly, and makes the sag bend of SCR dominated by higher-order modes, which are both detrimental to the structural safety. The coupling effect between RSI and VIV on the riser's dynamic response needs to be considered for the design of full-scale SCRs.

Centrifuge modelling of pipe-soil interaction in clay with crust layer

Seabed in regions, such as the Gulf of Guinea and North West Shelf of Australia, may exhibit a crust layer where the undrained shear strength can be an order of magnitude higher than that of the immediately underlying sediment. This can complicate design of steel catenary risers, where fatigue depends on the cyclic vertical stiffness of the pipe-soil interaction. Potential punch-through of the riser into the underlying soft soil may invalidate design assumptions based on the pipe-soil stiffness within the crust layer. The long-term evolution of pipe-soil stiffness within the crust layer, which exhibits similar properties to an over-consolidated soil, is also poorly understood. This paper describes centrifuge model tests undertaken in a clay sample with a crust layer, simulating the punch-through process of a pipe under load control and investigating the pipe-soil stiffness during long-term cyclic loading tests under displacement control. Results confirm that the potential for punching-through the crust layer depends strongly on the relative ratio of pipe diameter to crust layer thickness. The long-term evolution of pipe-soil stiffness showed a steady increase after an initial remoulding stage in contractile soils (normally consolidated and lightly over-consolidated), but a steady reduction in the heavily over-consolidated, more dilatant, crust. The magnitude of pipe-soil stiffness changes (during both remoulding and reconsolidation) is governed by the over-consolidation ratio of the soil and the amplitude of the cyclic displacements. This study provides insights on the relevant cyclic stiffness to consider when assessing SCR fatigue life in over-consolidated soils and soils exhibiting a superficial crust layer.

An effective stress analysis for predicting the evolution of SCR–seabed stiffness accounting for consolidation

Steel catenary risers (SCRs) are an efficient solution to transfer hydrocarbons from deep-water seabeds to floating facilities. SCR design requires an assessment of the fatigue life in the touchdown zone, where the riser interacts with the seabed, which relies on reliable estimates of the SCR–seabed stiffness over the design life. Current models for SCR–seabed stiffness consider only undrained conditions, neglecting the development and dissipation of excess pore pressures that occur over the life of the SCR. This consolidation process alters the seabed strength and consequently the SCR–seabed stiffness. This paper summarises experimental data that show that long-term cyclic vertical motion of an SCR at the touchdown zone leads to a reduction in seabed strength due to remoulding and water entrainment, but that this degradation is eclipsed by the regain in soil strength during consolidation. The main focus of this paper is on prediction of the temporal changes in seabed strength and stiffness due to long-term cyclic shearing and consolidation, to support calculations of SCR–seabed interaction. The predictions are obtained using a framework that considers the change in effective stress and hence soil strength using critical state concepts, and that considers the soil domain as a one-dimensional column of elements. The merit of the model is assessed by way of simulations of SCR centrifuge model tests with over 3000 cycles of repeated undrained vertical cycles in normally consolidated kaolin clay. Comparisons of the simulated and measured profiles of SCR penetration resistance reveal that the model can capture accurately the observed changes in SCR–seabed stiffness. Example simulations show the merit of the model as a tool to assess the timescale in field conditions over which this order of magnitude change in seabed stiffness occurs. It is concluded that current design practice may underestimate the seabed stiffness significantly, but the new approach allows rapid checking of this for particular combinations of SCR and soil conditions.

Response-based environmental conditions for the design of steel catenary risers

This work presents a semi-analytical methodology to select design environmental conditions based on long-term cross-section utilization ratios at the TDZ (Touchdown Zone) for steel catenary risers. This approach uses simplified analytical models to calculate time series of short-term utilization ratios, defined according to the DnV-OS-F201 (2010) standard. After processing these time series, long-term utilization ratios can be determined with relatively low computational cost. By evaluating long-term utilization ratios, it is possible to define short-term design environmental conditions, defined as short-term conditions for which the extreme riser responses are equal to the long-term ones. This kind of methodology may represent a substantial change to the traditional focus given to riser design, which is based on responses obtained from extreme environmental conditions, instead of on the extreme responses.

Fatigue performance of deepwater steel catenary riser considering nonlinear soil

The touch down zone (TDZ) and top connection point of the vessel are most critical part of fatigue damage in the steel catenary riser (SCR). In general, the linear soil model has been used to evaluate fatigue performance of SCRs because it gives conservative results in the TDZ. However, the conservative linear soil model shows the limitation to accommodate real behavior in the TDZ as water depth is increased. Therefore, the riser behavior on soft clay seabed is investigated using a nonlinear soil model through time domain approach in this study. The numerical analysis considering various important parameters of the nonlinear soil model such as shear strength at mudline, shear strength gradient and suction resistance force is conducted to check the adoptability and applicability of nonlinear soil model for SCR design.

Dynamic characteristics of pipe-soil interaction for steel catenary riser in touchdown zone

Steel catenary riser (SCR) is one of the most popular and economic risers in the development of deep-water oil and gas field. As a difficulty in the SCR design, the pipe-soil interaction at the touchdown zone is affected by the soil strength, riser diameter, upper floating body, and other factors. Conventionally, the pipe-soil interaction was mainly studied based on the static response of the riser under a linear seabed which was inconsistent with the actual situation. Therefore, a dynamic characteristic model was built in the paper for more realistic simulation of the pipe-soil interaction. And experiments were conducted to verify the reliability of the numerical model. Based on the verified model, the dynamic process of the nonlinear time-domain was analyzed. Different parameters, such as the soil strength and dynamic loads, were changed to analyze their effects on the pipe-soil interaction in the whole process which provided a certain foundation for the SCR design.

Equivalent linear soil stiffness in fatigue design of steel catenary risers

For the development of the oil and gas fields in deep water, steel catenary risers (SCRs) are considered a versatile riser system option. However, fatigue design of SCRs in the touch down zone is an engineering challenge considering the complexity of the seabed–riser interactions. Advanced nonlinear interaction models have recently been developed for fatigue design of SCRs. However, the use of these nonlinear models tremendously increases the computational costs. Traditional fatigue design procedures for SCRs are based on linear soil models for the seabed and though these are simple, they usually lead to conservative results. The main aim of this study is to investigate the concept of using “equivalent linear soil stiffness” instead of the complex nonlinear seabed–riser interaction model, for the early stages of fatigue design of SCRs. The procedure is examined using two example SCRs in different water depths. The structural responses of the systems with nonlinear and equivalent seabed models due to regular vessel motions are compared first and then the contributions of some key input parameters for evaluation of the equivalent linear stiffness are investigated. In addition, an attempt is also made to propose a simplified formula for the equivalent linear soil stiffness for the SCR systems.

Experimental and numerical investigation of vertical pipe–soil interaction considering pipe velocity

The repeated motion of steel catenary risers (SCR) at the touchdown zone is one of the key factors considered in SCR design. Soil behaviours under different pipe velocities further complicate the accurate assessment of pipe–soil interactions. In the present research, a vertical pipe–soil interaction model test system was developed in this research study to investigate the effect of pipe velocity. The experimental results under soft clay condition indicated a close relationship between soil behaviour and pipe velocity, and the increase of cyclic number was also attributed to the progressive softening and trenching effect of soil until relative stabilisation. A new backbone curve model considering pipe velocity was obtained based on the validated coupled Eulerian–Lagrangianmethod, and the accuracy of the model was verified by the experiment results.

The coupling VIV analysis of SCRs with rigid swing

With the development of deepwater oil and gas exploration, Steel Catenary Risers (SCRs) become preferred risers for resource production, import and export. Vortex induced vibration (VIV) is the key problem encountered in the design of SCRs. In this study, a new model, the rigid swing model, is proposed based on the consideration of large curvature of SCRs. The sag bend of SCRs is assumed as a rigid swing system around the axis from the hanging point to the touch down point (TDP) in the model. The torque, produced by the lift force and the swing vector, provides the driving torque for the swing system, and the weight of SCRs provides the restoring torque. The simulated response of rigid swing is coupled with bending vibration, and then the coupling VIV model of SCRs is studied in consideration of bending vibration and rigid motion. The calculated results indicate that the rigid swing has a magnitude equal to that of bending vibration, and the rigid motion affects the dynamic response of SCRs and can not be neglected in the VIV analysis.

OrcaFlex를 이용한 심해 SCR 구조 해석

The design challenges when attempting to obtain sufficient strength for a deepwater steel catenary riser (SCR) include high stress near the hang-off location, an elevated beam-column buckling load due to the effective compression in the touchdown zone (TDZ), and increased stress and low-cycle fatigue damage in the TDZ. Therefore, a systematic strength analysis is required for the proper design of an SCR. However, deepwater SCR analysis is a new research area. Thus, the objective of this study was to develop an overall analysis procedure for a deepwater SCR. The structural behavior of a deepwater SCR under various enviro nmental loading conditions was investigated, and a sensitivity analysis was conducted with respect to various parameters such as the SCR weight, weight of the internal contents, hang-off angle (HOA), and vertical soil stiffness. Based on a deepwater SCR design example, it was foun d that the maximum stress of an SCR occurred at a hang-off location under parallel loading direction with respect to the riser plane, except for a wave dominant dynamic survival loading condition. Furthermore, the tensile stress governed the total stress of the SCRs, whereas the bending stress governed the total stress at the TDZ. The weight of the SCR and internal contents affected the maximum stress of the SCR more than the H OA and vertical soil stiffness, because the weight of the SCR, including the internal contents, was directly related to its tensile stress.

Fatigue analysis of steel catenary risers based on a plasticity model

The most critical issue in the steel catenary riser design is to evaluate the fatigue damage in the touchdown zone accurately. Appropriate modeling of the riser-soil resistance in the touchdown zone can lead to significant cost reduction by optimizing design. This paper presents a plasticity model that can be applied to numerically simulate riser-soil interaction and evaluate dynamic responses and the fatigue damage of a steel catenary riser in the touchdown zone. Utilizing the model, numerous riser-soil elements are attached to the steel catenary riser finite elements, in which each simulates local foundation restraint along the riser touchdown zone. The riser-soil interaction plasticity model accounts for the behavior within an allowable combined loading surface. The model will be represented in this paper, allowing simple numerical implementation. More importantly, it can be incorporated within the structural analysis of a steel catenary riser with the finite element method. The applicability of the model is interpreted theoretically and the results are shown through application to an offshore 8.625″ steel catenary riser example. The fatigue analysis results of the liner elastic riser-soil model are also shown. According to the comparison results of the two models, the fatigue life analysis results of the plasticity framework are reasonable and the horizontal effects of the riser-soil interaction can be included.

Sensitivity studies of SCR fatigue damage in the touchdown zone using an efficient simplified framework for stress range evaluation

Steel catenary risers (SCRs) are widely used in deep water. Several sources of nonlinearities make SCR fatigue design challenging. Limited understanding of the influence of the various input parameters on the structural response of SCRs leads to unnecessarily high conservatism in design. Also, time consuming numerical simulations are usually performed to assess SCR fatigue damage which is inefficient, especially for early design stages.A simplified framework for fatigue analysis of SCRs in the touchdown zone (TDZ) has been developed previously, using artificial neural networks. The approach may be used to efficiently estimate maximum static and dynamic stress ranges in the TDZ, from which the fatigue damage can be deduced. Comparison of the maximum static and dynamic stress changes for a given input motion allows quantification of the dynamic amplification factor (DAF). This paper explores the sensitivity of the maximum dynamic stress ranges and DAF to the key dimensionless groups of input parameters and also certain individual input parameters. The study illustrates the usefulness of the proposed framework in understanding SCR behaviour in the TDZ, providing guidance on optimisation of SCR design from a fatigue perspective. The paper also reflects on the potential benefits of using DAFs for SCR fatigue design.

A new method for strake configuration design of Steel Catenary Risers

A number of studies of vortex-induced vibration (VIV) having important effect on the fatigue performance of riser have performed analytically as well as experimentally in the last decade. However, there are few research results on the configuration design of VIV suppression devices such as fairings, strakes, and etc. Therefore, a systematic design procedure of strake configuration (i.e., location and length) using the modal approach for steel catenary riser (SCR) is proposed in this study. The proposed method considers the dominant mode for VIV suppression through modal analysis of bare riser to find appropriate location and various design parameters are introduced to calculate appropriate length of VIV suppression devices. From the numerical example of SCR, it is shown that the required strake length from the optimal location for VIV suppression of SCR is much shorter than that of the conventional method. This is achieved by staggered strake installation at the effective location. Furthermore, no additional calculations or engineering cost are required for the proposed method, because the proposed method follows exact same analysis procedure of the conventional one except using the modal information of bare SCR obtained from initial analysis of bare SCR.

Three Semisubmersible Floating Production System Options With Steel Catenary Risers

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 25265, ’Field Development Using Semisubmersible Floating Production System With Steel Catenary Risers in Western Australia Harsh Environment,’ by Alaa M. Mansour, Shankar Bhat, Dharma Pasala, and Dhiraj Kumar, Intecsea, prepared for the 2014 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed. Semisubmersible floating production systems (SFPSs) with steel catenary risers (SCRs) are attractive field-development solutions, especially in deeper water of Western Australia. However, SCR design is challenging in the offshore Western Australia environment because of severe cyclonic environments and persistent swells. This paper presents a comparison of three different field-development alternatives that use SFPSs with SCRs in deepwater Western Australia. These alternative solutions are a conventional semisubmersible hull design with steel lazy wave riser (SLWR), a deep-draft semisubmersible hull design with conventional SCRs, and a novel semisubmersible hull design with conventional SCRs. Introduction Building robustness in an SFPS, especially when personnel stay onboard during cyclonic events, requires the system to be designed to survive events with return periods larger than the typical 1,000 years adopted for survival events in the Gulf of Mexico. Events with 10,000-year return periods or greater need be adopted for Western Australia. Events with 10,000- year return periods have higher significant wave height and longer peak periods, both of which increase the SFPS motion response, especially in the heave direction. This, coupled with a design location in moderately deep water (<5,000 ft), makes SCR strength design very challenging. In addition, the persistent swell in the Western Australia area makes SCR fatigue design more challenging. There are potentially three alternatives to overcome the SCR strength and fatigue challenges. One option is to improve the SFPS motion by using a very deep- draft semisubmersible (VDDS) while maintaining a conventional SCR design. Another option is to use conventional semisubmersible design and adopt an SLWR. The third alternative is to use conventional SCRs with a novel semisubmersible hull design that does not require a deep draft but offers benign motion response, especially in the heave direction. In this paper, the three different hull/riser combinations are investigated and compared for a presumed wet-tree gas field with a host facility in 4,000 ft of water. The free-hanging-solid-ballast (FHSB) semisubmersible design fundamentally consists of a conventional semisubmersible hull with four columns connected by a ring pontoon at the keel level. The semisubmersible’s motions and overall performance are enhanced by use of a robust, low-tech feature: namely the FHSB tanks (Fig. 1). The FHSB tank is attached to the hull below keel level by means of chains, which pass through guide fairleads at the keel level and connect to deployment winches at the top of the columns. The addition of an FHSB tank to the semisubmersible results in a significant increase in the semisubmersible heave natural period, mainly through its own weight and added mass from its large surface area, while controlling the heave response in the wave-frequency range below 16 seconds.

Artificial neural network development for stress analysis of steel catenary risers: Sensitivity study and approximation of static stress range

Fatigue design of steel catenary risers (SCRs) is an important challenge especially in the touchdown zone (TDZ). Numerous parameters affect the fatigue damage in the TDZ, including those pertaining to riser motions, riser characteristics and soil properties. So far, only a few sensitivity studies have been published with limited applications, considering small ranges, investigating only a selection of input parameters or failing to examine the interactions between input parameters.This paper aims to test the robustness of previous research and extend the ranges of the input parameters for SCR systems under static loading, by means of numerical simulations. An approximation of the critical stress range in the TDZ defined by the authors previously was refined to assist the sensitivity studies. A large database was created using an automation subroutine coded in Python programming language that links the marine analysis software OrcaFlex and the optimisation software modeFRONTIER. Design of experiment techniques were used for post-processing and quantify the relative effects of the various dimensionless groups and their interactions. An approximation using a series of artificial neural networks is presented; it successfully approximates over 99% of the cases of the database with an accuracy of ±5%.

Fatigue Testing of Full Scale Girth Welded Pipes Under Variable Amplitude Loading

In the fatigue design of steel catenary risers, there are concerns regarding the fatigue damage to girth welds from low stresses, below the constant amplitude fatigue limit, in the loading spectrum and the validity of Miner's cumulative damage rule. These fundamental issues were addressed in a recent joint-industrial project (JIP). A key feature was development of the resonance fatigue testing rigs to enable them to test full-scale pipes under variable amplitude loading. Such tests were performed under a loading spectrum representative of that experienced by some risers, with many tests lasting over 100 million cycles to investigate the fatigue damage due to small stresses as well as the validity of Miner's rule. However, the resonance rigs are only capable of producing spectrum loading by gradually increasing or decreasing the applied load whereas more “spiky” random load sequences may be relevant in practice. Therefore, the program also included fatigue tests in conventional testing machines on strip specimens cut from pipes to compare the two types of loading sequence. This paper presents the results of these tests, conclusions drawn, and recommendations for changes to current fatigue design guidance for girth welded pipes regarding the definition of the fatigue limit, allowance for the damaging effect of low stresses, and the validity of Miner's rule.

Response of steel catenary risers on hysteretic non-linear seabed

The methodologies recommended by existing codes and standards for design of steel catenary risers are considering linear springs in the seabed while it is publicly accepted that the non-linear riser-seabed interaction can have vital influence on stress variation in touchdown area and consequently fatigue performance during the cyclic motions of the riser as the latter is excited by vessel motions under environmental loads. In this study an advanced hysteretic non-linear riser-seabed interaction model has been implemented into the seabed enabling the automatic simulation of different stiffness in the seabed response through the touchdown zone and gradual embedment of riser. Then the impacts of this model on fatigue calculation procedure and fatigue performance in touchdown area has been studied.

Dimensionless groups governing response of steel catenary risers

Steel catenary risers (SCRs) are one of the most cost effective types of risers in deep water, but their design, particularly in the touchdown zone (TDZ), is challenging. Design uncertainties arise due to limited understanding of the influence of various parameters on the SCR response, pertaining to the SCR geometry and structural properties, environmental loading and the seabed characteristics. An improved framework that provides quantitative guidance on how each parameter affects the overall structural response, and in particular fatigue damage in the touchdown zone, would facilitate design and increase confidence in the use of SCRs. This paper details the application of dimensional analysis to SCR behaviour. Suitable dimensionless groups are proposed and validated through a series of numerical tests, comparing the response of similar SCR systems defined by appropriate scaling of parameters. Due to the fundamental effect of the seabed response on fatigue damage, two types of soil models are considered in the study: a linear and a nonlinear model. The study provides a framework for design of SCRs, facilitating sensitivity analyses, enabling better control on parameters and reducing the number of simulations required.