Fatigue Mitigation Design of Deepwater Steel Catenary Risers

2006 ◽  
Author(s):  
Xiongliang Yao ◽  
Liping Sun
Keyword(s):  
Vertical Motion ◽  
Design Criterion ◽  
Steel Catenary Riser ◽  
Fatigue Design ◽  
Induced Motion ◽  
Steel Catenary Risers ◽  
Fatigue Mitigation ◽  
Mitigation Design ◽  
Stress Concentration Factors ◽  
Mitigation Methods

With the development of deepwater offshore industries, Steel Catenary Riser (SCR) concept offers great advantages over others and has been widely deployed worldwide. More than 100 SCRs have been installed in the deepwater fields including West of Africa, Gulf of Mexico, and Offshore Brazil. Fatigue is still the number one governing design criterion of the deepwater SCR regardless the floater types (SPAR, TLP, SEMI, and FPSO). Typical SCR fatigue damages include vortex induced vibration (VIV), host floater motion induced, floater vortex induced motion (VIM) fatigue, floater vertical motion induced VIV, installation, and other kinds of dynamic loading imposed to SCR. The success of the SCR design depends primarily upon the fatigue design and its mitigation methods. This paper summaries the viable fatigue mitigation design for deepwater SCR. Detailed discussion is given to different kind of fatigue damages of SCR. Focus of fatigue design of each type of deepwater floater is pointed out. Detailed discussion is given to the selection of design methods, SN curves, stress concentration factors, influence of sweet and sour service, and post-processing of fatigue sensitive welds. Different potential and practical fatigue design mitigations have been evaluated and discussion. The most viable options have been pointed out. Some examples based on actual projects experience are given to demonstrate the design principles and fatigue mitigation methods.

Fatigue Analysis Technique of Deepwater Steel Catenary Risers

2010 ◽  
Author(s):  
Jing Cao
Keyword(s):  
Vertical Motion ◽  
Fatigue Analysis ◽  
Design Criterion ◽  
Steel Catenary Riser ◽  
Start Up ◽  
Analysis Technique ◽  
Induced Motion ◽  
Steel Catenary Risers ◽  
Stress Concentration Factors ◽  
Sour Service

With the advances in deepwater offshore industries, Steel Catenary Riser (SCR) offers advantages over others and has been widely deployed worldwide including Gulf of Mexico (GoM), West of Africa (WoA), and Offshore Brazil. SCR concept has been applied to all types of deepwater floaters. Among analyses and design criteria, fatigue is still the number one governing design criterion of the deepwater SCR regardless the floater types (SPAR, TLP, SEMI, and FPSO). Typical SCR fatigue analyses include first order and second order floater motion induced fatigue; vortex induced vibration (VIV), deep draft floater vortex induced motion (VIM) fatigue, floater vertical motion induced VIV, slugging fatigue, start-up and shut-down, installation, and other kinds of dynamic loading imposed to the SCR. This paper presents the state-of-art fatigue analysis requirement and technique of different kinds of fatigue analysis for deepwater SCRs. Detailed discussion is given to different kinds of fatigue damage of SCRs. Focus of fatigue analysis of each type of deepwater floater is pointed out. Detailed discussion is also given to the selection of SN curves, stress concentration factors, influence of sweet and sour service, and post-processing of fatigue sensitive welds. Several examples of SCR fatigue analysis are given to demonstrate the analysis techniques and practices.

Dynamic Optimization of Steel Catenary Risers Based on Reliability Using Metamodel

2010 ◽  
Author(s):  
Wenqing Zheng ◽  
Hezhen Yang
Keyword(s):  
Dynamic Optimization ◽  
Computational Cost ◽  
Minimum Cost ◽  
Probabilistic Constraints ◽  
Optimization Approach ◽  
Element Analysis ◽  
Steel Catenary Riser ◽  
Reliability Based Design ◽  
Minimum Cost Design ◽  
Steel Catenary Risers

Reliability based design optimization (RBDO) of a steel catenary riser (SCR) using metamodel is investigated. The purpose of the optimization is to find the minimum-cost design subjecting to probabilistic constraints. To reduce the computational cost of the traditional double-loop RBDO, a single-loop RBDO approach is employed. The performance function is approximated by using metamodel to avoid time consuming finite element analysis during the dynamic optimization. The metamodel is constructed though design of experiments (DOE) sampling. In addition, the reliability assessment is carried out by Monte Carlo simulations. The result shows that the RBDO of SCR is a more rational optimization approach compared with traditional deterministic optimization, and using metamodel technique during the dynamic optimization process can significantly decrease the computational expense without sacrificing accuracy.

ECA Methodology for Fatigue Design of Risers and Flowlines

2007 ◽  
Author(s):  
C. H. Luk ◽  
T. J. Wang
Keyword(s):  
Fracture Mechanics ◽  
Design Method ◽  
Steel Catenary Riser ◽  
Fatigue Design ◽  
Wide Range ◽  
Fatigue And Fracture ◽  
Level 3 ◽  
Fatigue Loads ◽  
Vessel Motion ◽  
Level 2

Engineering Criticality Assessment (ECA) is a procedure based on fracture mechanics that may be used to supplement the traditional S-N approach and determine the flaw acceptance and inspection criteria in fatigue and fracture design of risers and flowlines. A number of design codes provide guidance for this procedure, e.g. BS-7910:2005 [1]. However, more investigations and example studies are still needed to address the design implications for riser and flowline applications. This paper provides a review of the existing ECA methodology, presents a fracture mechanics design method for a wide range of riser and flowline fatigue problems, and shows flaw size results from steel catenary riser (SCR) and flowline (FL) examples. The first example is a deepwater SCR subjected to fatigue loads due to vessel motion and riser VIV. The second example is a subsea flowline subjected to thermal fatigue loads. The effects of crack re-characterization and material plasticity on the Level-2 and Level-3 ECA results of the SCR and flowline examples are illustrated.

Effects of Nonlinear Riser-Soil Interaction Model on Fatigue Design of Steel Catenary Riser Under Random Waves

2017 ◽  
Author(s):  
Mehrdad Kimiaei
Keyword(s):  
Oil And Gas ◽  
Structural Response ◽  
Interaction Model ◽  
Nonlinear Behavior ◽  
Random Waves ◽  
Steel Catenary Riser ◽  
Oil And Gas Fields ◽  
Fatigue Design ◽  
Time Histories ◽  
Main Components

Steel Catenary Risers (SCRs) are one of the main components in development of oil and gas fields in deep waters. Fatigue design of SCRs in touch down zone (TDZ) is one of the main engineering challenges in design of riser systems. Nonlinear riser-soil interaction models have recently been introduced and used widely in advanced structural analysis of SCRs. Due to hysteretic nonlinear behavior of the soil, SCR system will show different structural response under different loading time histories. This paper investigates the effects of nonlinear riser-soil interaction in the TDZ on fatigue performance of an example SCR subjected to randomly generated waves. Sensitivity of fatigue life of the system, location of the critical node and the maximum stress range to different wave realizations and different soil types are discussed in detail.

Sensitivity Study of Critical Parameters Influencing the Uncertainty of Fatigue Damage in Steel Catenary Risers

2010 ◽  
Author(s):  
Feng Zi Li ◽  
Ying Min Low
Keyword(s):  
Fatigue Damage ◽  
Cost Effective ◽  
Sensitivity Study ◽  
Structural Safety ◽  
Critical Parameters ◽  
Design Parameters ◽  
Steel Catenary Riser ◽  
Bending Stresses ◽  
Steel Catenary Risers ◽  
Touchdown Point

The most challenging aspect of a deepwater development is the riser system, and a cost-effective choice is the Steel Catenary Riser (SCR). Fatigue is often a governing design consideration, and it is usually most critical at the touchdown point (TDP) where static and dynamic bending stresses are highest. Unfortunately, it is also at this region that uncertainty is the maximum. The increased uncertainty casts doubt on the applicability of generic safety factors recommended by design codes, and the most consistent way of ensuring the structural safety of the SCR is to employ a reliability-based approach, which has so far not received attention in SCR design. As the number of basic random variables affects the complexity of a reliability analysis, these variables should be selected with caution. To this end, the aim of this paper is to draw up a comprehensive list of design parameters that may contribute meaningfully to the uncertainty of the fatigue damage. From this list, several parameters are selected for sensitivity studies using the commercial package Orcaflex. It is found that variations in seabed parameters such as soil stiffness, soil suction and seabed trench can have a pronounced influence on the uncertainty of the fatigue damage at the touchdown point.

scholarly journals A New Semisubmersible Design for Improved Heave Motion, Vortex-Induced Motion and Quayside Stability

2011 ◽  
Author(s):  
Qi Xu
Keyword(s):  
Floating Platform ◽  
Natural Period ◽  
Suppression Effect ◽  
Structural Rigidity ◽  
Hull Design ◽  
Induced Motion ◽  
Steel Catenary Risers ◽  
The Difference ◽  
Touchdown Point ◽  
Heave Motion

Recently the semisubmersible has become a favorable choice as a wet-tree floating platform supporting steel catenary risers (SCRs), mainly due to its capability of quayside topside integration and cost-effectiveness. However, it is still a challenge for a conventional semisubmersible to support SCRs, particularly large ones, in harsh environment and relatively shallow water due to its large heave motion. To answer this challenge, a new semisubmersible design has been developed at Technip as a wet-tree floater which achieves significantly improved heave motion and vortex-induced-motion (VIM) performance through hull form optimization while maintaining the simplicity of a conventional semisubmersible design. The difference between the NexGen semi-submersible design and a conventional semi-submersible design is in the blisters attached to the columns, distribution of pontoon volume, and pontoon cross section shape. In the NexGen semi-submersible design, the pontoon volume is re-distributed to minimize heave loading while maintaining sufficient structural rigidity, a long heave natural period and adequate quayside buoyancy. The blisters attached to the columns effectively break the vortex shedding coherence along the column length and therefore suppresses VIM. The blisters also provide much needed stability at quayside and during the hull deployment process, making the hull design less sensitive to topside weight increase. In the present paper the hydrodynamic aspects of this new design are discussed in detail. A benchmark case is presented in which the new design is compared against a more conventional design with the same principal dimensions. It is shown that the heave response in extreme sea states (100-yr hurricane) at the platform center of gravity is reduced by about 30–40%, and at the SCR hang-off locations by about 25–30%. Due to the reduced heave motion, SCRs experience about one third less stress at the touchdown point. A qualitative VIM analytical model is used to predict the VIM suppression effect of the new design. A highlight of a VIM model test for the proposed design is also presented. The reduced heave and VIM significantly improve the riser stress and fatigue near the touchdown point. This new design makes the semisubmersible a more robust wet-tree floater concept, and even a potentially good candidate as a dry-tree host concept in relatively benign environment.

Vortex Induced Motion of TLP With Consideration of Appurtenances

2014 ◽  
Author(s):  
Jaime Hui Choo Tan ◽  
Yih Jeng Teng ◽  
Allan Magee ◽  
Benedict Toong Heng Ly ◽  
Shankar Bhat Aramanadka
Keyword(s):  
Cfd Simulation ◽  
Scale Effects ◽  
Floating Platform ◽  
Steel Catenary Riser ◽  
Aspect Ratios ◽  
Significant Difference ◽  
Helical Strakes ◽  
Induced Motion ◽  
Set Up ◽  
Comparison Of The Results

Offshore floating platform configurations often consist of geometrically simple and symmetrical shapes which are made complicated by the presence of appurtenances such as helical strakes, tendon porches, steel catenary riser (SCR) porches, pipes, chains, fairleads and anodes on the surface of the hull. Previous studies mainly on spars show that these hull external features affect the Vortex Induced Motion (VIM) performance of the platform significantly. This is to be expected since VIM is controlled by the flow separation on the hull surface and the resulting vortex shedding patterns. Scale effects may also play a role in model tests for bare cylinders or hulls with bare cylindrical columns, whereas previous studies have shown less Reynolds dependence when appurtenances are modelled. This study investigates the effect of hull appurtenances on VIM of a multi-column floating platform, i.e. a Tension Leg Platform (TLP) designed for Southeast Asian environment. Significant difference in VIM behaviors is expected between spars and TLPs since the column aspect ratios are very different and TLPs do not have helical strakes that are commonly fitted on spars. Model testing and Computational Fluid Dynamics (CFD) simulation are used in this VIM study, with the former being the emphasis of this paper. Descriptions of the respective experimental and numerical methodologies are presented and the comparison of the results is made. Further work required to improve the model test set-up and the CFD simulation are suggested. From this study, it is shown that the effect of appurtenances on TLP VIM simulation is important and must be taken into account to obtain realistic results.

A Parametric Study for the Design of Steel Catenary Riser System in Deepwater Harsh Environments

2008 ◽  
Author(s):  
Jie Xia ◽  
Purnendu K. Das ◽  
Daniel Karunakaran
Keyword(s):  
Parametric Study ◽  
Computer Time ◽  
Harsh Environments ◽  
Analysis Tool ◽  
Primary Analysis ◽  
Steel Catenary Riser ◽  
Analysis Process ◽  
Steel Catenary Risers ◽  
Northern North Sea

In recent years, offshore reservoirs have been developed in deeper and deeper water environments, where floating production, storage and offloading (FPSO), semi-submersibles, spars and TLPs are considered to be the most economically viable platforms. Steel catenary risers (SCRs) are being considered for these production units in deepwater development such as Northern North Sea. A variety of uncertainties are associated with material behaviour, environmental loading, hydromechanics modelling, structural modelling, and fatigue / corrosion / wear characteristics, especially around hang-off and touch down areas. SCRs used in conjunction with a semi-submersible or a FPSO in deepwater harsh environments present significant design challenges. The large vertical motions at the FPSO or semi induce severe riser response, which results in difficulty meeting strength and fatigue criteria at the hang-off and touch down point locations. To improve the understanding of SCR behaviour and increase the confidence in the design of such systems in deepwater harsh environments, a parametric study was carried out in this paper to deal with the factors that mainly influence the loading condition and fatigue life of the riser. Two cases, one steel catenary riser connected to a semi-submersible and one steel catenary riser connected to a FPSO, were studied and compared. And weight-optimized configurations were applied for both risers. Riflex combined with DeepC was the primary analysis tool used for the long-term response of the nonlinear structure SCR’s simulations, which is high computer time consuming. Hence, the parameters affecting the efficiency and accuracy of the simulations have also been studied during the analysis process.

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

2019 ◽  
Author(s):  
Rupak Ghosh ◽  
Haydar Arslan
Keyword(s):  
Fatigue Life ◽  
Water Injection ◽  
Field Testing ◽  
Vertical Displacement ◽  
Soil Response ◽  
Fatigue Design ◽  
Steel Catenary Risers ◽  
Design Requirements ◽  
Seabed Stiffness

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

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