Bending Stiffeners for Extreme and Fatigue Loading of Unbonded Flexible Risers

2008 ◽  
Author(s):  
Russell Smith
Keyword(s):  
Corrosion Fatigue ◽  
Fatigue Loading ◽  
Rapid Onset ◽  
Offshore Platform ◽  
Fatigue Performance ◽  
Flexible Riser ◽  
Short Notice ◽  
Bending Radius ◽  
Flexible Risers

Bending stiffeners constrain the dynamic radius of flexible risers at offshore platform and subsea interfaces. The extreme bending and fatigue hotspots of a flexible riser occur near these interfaces. Conventional design of bending stiffeners first account for the extreme metocoean environment to ensure the riser does not bend below the minimum allowable bending radius (MBR). The stiffener design for extreme loading normally proves acceptable for less onerous fatigue loading provided the steel armour in riser annulus stays dry. Flexible riser operations have shown that there are several mechanisms for the annulus to loose its dry-state. A flexible riser that is not fatigue tolerant to a non-dry annulus often needs to be replaced at short notice or shut-down due to the rapid onset of corrosion fatigue of the steel armour wires. This paper demonstrates recent advances in designing bending stiffeners so that a flexible riser can sustain life of field operation with a non-dry annulus. These advances are accomplished by optimising the tapered profile of the bending stiffener and accounting for additional stiffening due to bending hysteresis in a flexible riser. A case study is presented to demonstrate the improvement in fatigue performance of a flexible riser.

Axisymmetric Mechanical Behavior of Flexible Risers

2015 ◽  
Author(s):  
Liu Junpeng ◽  
Murilo A. Vaz ◽  
Menglan Duan ◽  
Xiaotian Li
Keyword(s):  
Correspondence Principle ◽  
Mechanical Analysis ◽  
Axial Stiffness ◽  
Prony Series ◽  
Structural Damping ◽  
Flexible Riser ◽  
Damping Behavior ◽  
Polymer Properties ◽  
Flexible Risers

Polymer properties, i.e. viscoelastic characteristics, may pose a non-negligible impact on the axisymmetric analysis, especially on the axial structural damping behavior. An available mechanical model taking viscoelastic into account is established for capturing such parameters as axial strains and forces on each layer, axial stiffness and so on. In this paper, the flexible riser is divided into helical and cylindrical elements to carry out the mechanical analysis due to the complexity of structure and material property. Prony Series is used to describe the creep and relaxation behavior of material viscoelasticity. A viscoelastic solution is obtained using the correspondence principle. Finally, a case study is performed and some interesting results are presented.

Corrosion Fatigue Testing of Flexible Riser Armour: Procedures for Testing and Assessment of Design Criteria

2003 ◽  
Author(s):  
Stig Berge ◽  
Erik Bendiksen ◽  
Jonas Gudme ◽  
Richard Clements
Keyword(s):  
Corrosion Fatigue ◽  
Fatigue Testing ◽  
Design Criteria ◽  
Flexible Riser ◽  
Test Procedures ◽  
Fatigue Design ◽  
Flexible Risers ◽  
Corrosive Environments

Fatigue design of armour wires in flexible risers is reviewed, with particular emphasis on effects of corrosive environments in pipe annulus. Test procedures for corrosion fatigue testing of armour wire and assessment of fatigue design criteria are described.

Technical Solutions Applied for the Treatment of Damaged Dynamic Risers

2002 ◽  
Author(s):  
Thomas S. Taylor ◽  
Michael V. Joosten ◽  
Frank Smith
Keyword(s):  
Corrosion Fatigue ◽  
Treatment Process ◽  
Fatigue Testing ◽  
Production Systems ◽  
Flexible Riser ◽  
Hydraulic Test ◽  
Test Analysis ◽  
Safety Critical ◽  
Flexible Risers ◽  
Technical Solutions

The objective of this paper is to provide operators of floating production systems that utilise dynamic flexible risers an opportunity to review the technology and procedures adopted by PGS Production. These procedures were utilised in order to verify/re-use the damaged/seawater annulus flooded Banff risers. Most of these risers were also classified as being safety critical. The scope of the work carried out covers the development of a seawater displacement treatment system using an inhibitor fluid. The feasibility and efficacy of the flushing treatment were rigorously developed based on hydraulic test analysis using sections taken from an actual flexible riser. An extensive corrosion fatigue-testing programme using seawater/CO2/inhibitor was carried out to establish optimal safe service lives for the “damaged and treated” case for these risers. This paper presents the successful results from this work covering the development of the remedial treatment process and the resulting corrosion fatigue work that has been conducted.

Corrosion Fatigue of Steel Armours in Flexible Risers

2002 ◽  
Author(s):  
Tore Roberg Andersen
Keyword(s):  
Carbon Dioxide ◽  
Fatigue Life ◽  
Corrosion Fatigue ◽  
Hydrogen Sulphide ◽  
Fatigue Performance ◽  
Corrosive Environment ◽  
Test Program ◽  
Flexible Risers

Fatigue life calculation of flexible risers is normally based on the assumption that the annulus is dry. Experience has, however, shown that the annulus may become water flooded. Carbon dioxide and hydrogen sulphide permeate from the bore and into the annulus, making the annulus environment corrosive. In such conditions the influence of corrosion fatigue on fatigue life has to be taken into account. A test program was performed to investigate the corrosion fatigue performance of steel armour in water containing carbon dioxide. The results showed that the corrosive environment reduced the fatigue life to about 1/10 to 1/50 of that derived in air. The lifetime was significantly shorter in the environment with 0.1 bar compared to 0.01 bar, indicating that corrosion fatigue becomes more pronounced as the corrosivity increases. The work shown that corrosion fatigue has to be considered for flexible risers with annulus that is or may be water flooded.

Qualified Assessment for Lifetime Extension of Flexible Risers

2015 ◽  
Author(s):  
Bernt J. Leira ◽  
Svein Are Løtveit ◽  
Stig Berge ◽  
Dag Fergestad ◽  
Nina Langhelle
Keyword(s):  
Flexible Riser ◽  
Term Operation ◽  
Integrity Assessment ◽  
Design Life ◽  
Outer Sheath ◽  
Flexible Pipes ◽  
Flexible Risers ◽  
Lifetime Extension

There have been many challenges with respect to long term operation of flexible pipes, in many cases with the need to replace or repair them before the end of the initial design life. The number of identified failure mechanisms and observed failure events is also increasing. In the present paper, some typical issues that arise during the process of integrity assessment and qualification for lifetime extension are outlined. More detailed considerations are illustrated in relation to a particular case study. This involves elaboration of the different steps which are required in order to be able to extend the lifetime of a flexible riser for the case of corrosive annulus environments. The particular case of an outer sheath damage is applied for the purpose of illustration.

Corrosion Fatigue Performance of a Carbon Steel in Brine Containing Air, H2S and CO2

CORROSION ◽  
1966 ◽  
Vol 22 (12) ◽  
pp. 325-335 ◽  
Author(s):  
PARVIS MEHDIZADEH ◽  
R.L. McGLASSON ◽  
J. E.LANDERS
Keyword(s):  
Carbon Steel ◽  
Corrosion Fatigue ◽  
Fatigue Performance

Cathodic Protection Design Consideration for an Offshore Flexible Riser Connected to an Impressed Current System

2021 ◽  
Author(s):  
Thierry Dequin ◽  
Clark Weldon ◽  
Matthew Hense
Keyword(s):  
Cathodic Protection ◽  
Sacrificial Anode ◽  
Flexible Riser ◽  
Flexible Pipe ◽  
Linear Resistance ◽  
Flexible Pipes ◽  
Flexible Risers ◽  
Impressed Current ◽  
Host Structure ◽  
Impressed Current Cathodic Protection

Abstract Flexible risers are regularly used to produce oil and gas in subsea production systems and by nature interconnect the subsea production system to the floating or fixed host facilities. Unbonded flexible pipes are made of a combination of metallic and non-metallic layers, each layer being individually terminated at each extremity by complex end fittings. Mostly submerged in seawater, the metallic parts require careful material selection and cathodic protection (CP) to survive the expected service life. Design engineers must determine whether the flexible pipe risers should be electrically connected to the host in order to receive cathodic protection current or be electrically isolated. If the host structure is equipped with a sacrificial anode system, then electrical continuity between the riser and the host structure is generally preferred. The exception is often when the riser and host structure are operated by separate organizations, in which case electrical isolation may be preferred simply to provide delineation of ownership between the two CP systems. The paper discusses these interface issues between hull and subsea where the hull is equipped with an impressed current cathodic protection (ICCP) system, and provides guidance for addressing them during flexible pipe CP design, operation, and monitoring. Specifically, CP design philosophies for flexible risers will be addressed with respect to manufacturing, installation and interface with the host structure’s Impressed Current Cathodic Protection (ICCP) system. The discussion will emphasize the importance of early coordination between the host structure ICCP system designers and the subsea SACP system designers, and will include recommendations for CP system computer modeling, CP system design operation and CP system monitoring. One of the challenges is to understand what to consider for the exposed surfaces in the flexible pipes and its multiple layers, and also the evaluation of the linear resistance of each riser segment. The linear resistance of the riser is a major determinant with respect to potential attenuation, which in turn largely determines the extent of current drain between the subsea sacrificial anode system and the hull ICCP system. To model the flexible riser CP system behavior for self-protection, linear resistance may be maximized, however the use of a realistic linear resistance is recommended for evaluation of the interaction between the host structure and subsea system. Realistic flexible linear resistance would also reduce conservatism in the CP design, potentially save time during the offshore campaign by reducing anode quantities, and also providing correct evaluation of drain current and stray currents.

Local Buckling in end Expansion of Subsea Pipelines

2014 ◽  
Vol 69 (7) ◽  
Author(s):  
Jaswar Koto ◽  
Abd. Khair Junaidi ◽  
M. H. Hashim
Keyword(s):  
Crude Oil ◽  
Axial Force ◽  
Oil And Gas ◽  
Local Buckling ◽  
Offshore Platform ◽  
Offshore Pipeline ◽  
Specific Rule ◽  
Subsea Pipelines ◽  
Study Development

Offshore pipeline is mainly to transport crude oil and gas from offshore to onshore. It is also used to transport crude oil and gas from well to offshore platform and from platform to another platform. The crude oil and gas horizontally flows on the seabed, and then vertically flows inside the riser to the offshore platform. One of current issues of the oil and gas transportation system is an end expansion caused by the axial force. If the end expansion occurs over it limit can cause overstress to riser. This paper explores the effect of axial force toward local buckling in end expansion. In the study, development of programming in visual basic 2010 firstly was constructed using empirical equation. The programming code, then, was validated by comparing simulation result with actual data from company. As case study, the end expansion for various thicknesses of pipes was simulated. In this programming, DNV regulation is included for checking either design complied or not with regulation. However, DNV regulation doesn’t have specific rule regarding the end expansion but it is evaluated under load displacement control under strain condition.

Riser Concept Selection for FPSO in Deepwater Norwegian Sea: A Case Study

2018 ◽  
Author(s):  
Arvind Keprate ◽  
R. M. Chandima Ratnayake
Keyword(s):  
Literature Review ◽  
Geographical Location ◽  
Weather Conditions ◽  
Large Vessel ◽  
Concept Selection ◽  
Flexible Riser ◽  
Norwegian Sea ◽  
Daunting Task ◽  
Selection For

Selecting a riser concept for FPSOs stationed in deep water has posed challenges, due to the high hydrostatic pressure and large vessel payload. One of the major factors governing the riser concept selection for deepwater FPSOs is the geographical location and weather conditions prevalent in the region. For example, the free hanging flexible riser has been mostly used in the moderate environments of offshore Brazil, while concepts like the SCR and Hybrid Riser Tower (HRT) are dominant in the calm weather conditions of the West of Africa (WoA). Selecting a riser concept for an FPSO stationed in harsh weather conditions like those of the Northern Norwegian Sea is a daunting task. This is due to the large vessel offsets and dynamics, which are directly transferred along the riser’s length to its base, thereby causing considerable fatigue damage to the riser. The main aim of this paper is to recommend a suitable riser concept, which may be hooked to an internal turret moored FPSO stationed in water of 1500m depth and in the harsh environmental conditions of the Northern Norwegian Sea. The recommendations are based on the literature review and the case study performed in the manuscript. On the basis of the literature review, a lazy wave configuration of flexible riser and Steel Lazy Wave Riser (SLWR) has been considered as a viable riser concept. Thereafter, a case study is performed to compare the two riser concepts, on the basis of vessel payload, fabrication cost and installation cost.

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