Global Dynamic Performance of Flexible Risers Employing Coupled Analysis

2015 ◽  
Author(s):  
Sigve Orheim Drabløs ◽  
Katharina Haakenstad ◽  
Nils Sødahl ◽  
Kjell Herfjord ◽  
Oddrun Steinkjer
Keyword(s):  
Material Behavior ◽  
Harsh Environments ◽  
Coupled Analysis ◽  
Flexible Pipes ◽  
Analysis Scheme ◽  
Global Performance ◽  
Flexible Risers ◽  
Current Loading ◽  
Platform Motions ◽  
Slender Structures

Flexible risers are inevitable in the operation of floating production units in intermediate water depths in harsh environments. The operation of flexible pipes under these conditions is challenging, and reliable and accurate analysis of the global performance of risers is vital. The main purpose of this paper is to describe an analysis method for the global performance of flexible risers taking advantages from advanced floater motion modelling and comprehensive modelling of the risers. The analysis scheme has been employed on a semi-submersible platform, operated in harsh environments in the North Sea and all its flexible risers and umbilicals have been analyzed. The platform motions are generated by means of a coupled vessel/slender structure model in which the vessel force model is introduced into a finite element model of the slender structures including the mooring lines and all its flexible risers. In this way all relevant coupling effects from damping, restoring and current loading on the slender structures are consistently included in the platform motion predictions. Subsequently each riser is analyzed in separate models with high element mesh resolution. The platform motions, in terms of low frequency and wave frequency motions obtained from the coupled model, are applied consistently with wave and current loading in the detailed riser model. In the detailed riser models, bending stiffeners are modelled with non-linear material behavior. The diffracted wave field is included in the wave loading of the risers. The stick-slip bending of the flexible pipes have been modelled for a selection of risers and the effect of this on the riser hang off loads and the curvature in the bend stiffener region is discussed. The main responses looked into have been the curvature in the bend stiffener region, the hang off loads and interference between risers and platform pontoons. The riser responses obtained from the coupled analysis scheme performed in the present work is compared to more standard analysis schemes where extreme offsets and transfer functions are used to generate platform motions.

Characterisation of the Wave Environment in the Fatigue Analysis of Flexible Risers

2005 ◽  
Author(s):  
John M. Sheehan ◽  
Frank W. Grealish ◽  
Russell J. Smith ◽  
Annette M. Harte
Keyword(s):  
Cost Effective ◽  
Critical Issue ◽  
Fatigue Analysis ◽  
Harsh Environments ◽  
Flexible Pipes ◽  
Water Developments ◽  
Key Issues ◽  
Flexible Risers ◽  
Offshore Industry ◽  
Unbonded Flexible Pipes

As the offshore industry moves towards deeper water developments and continues to embrace harsh environments, unbonded flexible pipes are increasingly being utilised as a cost effective riser solution. Furthermore, with the advent of issues such as non-pristine annuli environments, the fatigue performance of these flexible risers is becoming a critical issue. This paper presents an overview of the comparisons between deterministic and stochastic global fatigue analysis techniques. Methods used to perform both deterministic and stochastic analyses are outlined, from performing the global analyses to using local models to generate armour wire stresses and subsequent fatigue damage. The paper identifies the key issues in the analysis performed and presents key results and conclusions with regard to the characterisation of the wave environment in the global fatigue analysis of flexible risers.

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.

Characterizing the Wave Environment in the Fatigue Analysis of Flexible Risers

2005 ◽  
Vol 128 (2) ◽  
pp. 108-118 ◽  
Author(s):  
John M. Sheehan ◽  
Frank W. Grealish ◽  
Annette M. Harte ◽  
Russell J. Smith
Keyword(s):  
Cost Effective ◽  
Critical Issue ◽  
Fatigue Analysis ◽  
Flexible Pipes ◽  
Water Developments ◽  
Key Issues ◽  
Flexible Risers ◽  
Offshore Industry ◽  
Unbonded Flexible Pipes

As the offshore industry moves towards deeper water developments and continues to embrace harsh environments, unbonded flexible pipes are increasingly being utilized as a cost effective riser solution. Furthermore, with the advent of issues such as nonpristine annuli environments, the fatigue performance of these flexible risers is becoming a critical issue. This paper presents an overview of the comparisons between deterministic and stochastic global fatigue analysis techniques. Methods used to perform both deterministic and stochastic analyses are outlined, from performing the global analyses to using local models to generate armor wire stresses and subsequent fatigue damage. The paper identifies the key issues in the analysis performed and presents key results and conclusions with regard to the characterization of the wave environment in the global fatigue analysis of flexible risers.

Whistling of Pipes With Narrow Corrugations: Scale Model Tests and Consequences for Carcass Design

2013 ◽  
Author(s):  
Joachim Golliard ◽  
Stefan Belfroid ◽  
Erik Bendiksen ◽  
Casper Frimodt
Keyword(s):  
Prediction Model ◽  
Gas Production ◽  
Scale Model ◽  
Small Scale ◽  
Operational Conditions ◽  
Flexible Pipes ◽  
Cavity Opening ◽  
Flexible Risers ◽  
Onset Velocity ◽  
Operational Envelope

Pipes for gas production and transport with a corrugated inner surface, as used in flexible pipes, can be subject to Flow-Induced Pulsations when the flow velocity is larger than a certain velocity. This onset velocity is dependent on the geometry of the corrugations, the operational conditions and the geometry of the topside and subsea piping. In this paper, small-scale tests performed on corrugated tubes are reported. The tested geometries include both “classical” profiles, similar to the inner profile of agraff flexible risers, and profiles with less typical variations, such as narrower and/or deeper cavities, or irregular pitch. These tests were performed in order to evaluate the validity of a prediction model developed earlier for the onset of pulsations, for corrugated pipes with these kinds of atypical variations, which are found on a new type of carcass designs. The mechanism of Flow-Induced Pulsations in corrugated pipes is discussed, as well as the principle of the prediction model. The experimental results show that the validity of the model remains reasonable in most cases, except when the cavities are very narrow. In this case, the model becomes overly conservative. This limitation can be attributed to the fact that, for very narrow cavities, the cavity opening becomes too small compared to the boundary-layer momentum thickness, effectively destroying any instability of the shear layer. Furthermore, the shift towards higher frequencies of the acoustic source term due to narrower cavities, and the possible coupling with higher acoustic modes, is considered. The results of the analysis are used to evaluate the onset velocity and whistling behavior of a newly developed carcass design of flexible risers. A previous analysis has indicated that the particular geometry profile of the new design improves the whistling behavior by pushing the onset velocity outside the typical operational envelope of flexible risers. The analysis confirms that the new design will be less prone to whistling than flexible risers with classical agraff carcasses.

Analytical Methodology to Evaluate Flexible Risers Fatigue Lives at the Top Region

2019 ◽  
Author(s):  
Fernando Jorge Mendes de Sousa ◽  
Marcos Queija de Siqueira ◽  
José Renato Mendes de Sousa ◽  
George Carneiro Campello
Keyword(s):  
Fatigue Life ◽  
Structural Complexity ◽  
Analytical Methodology ◽  
Life Estimation ◽  
Flexible Pipes ◽  
Fatigue Life Estimation ◽  
Riser Design ◽  
Flexible Risers ◽  
Critical Regions ◽  
Computer Resources

Abstract Traditionally, fatigue life calculations are very expensive in terms of time and computer resources. They are usually performed during riser design phases, when several lines with similar characteristics need to be analyzed. While operating, when problems are detected, fatigue analyses are also necessary to help to decide if any action is needed. In both situations, end fittings and bendstiffeners are usually the critical regions. Considering the high number of flexible risers installed in Brazil and the structural complexity of this kind of structure, a robust and fast methodology to evaluate the fatigue life of flexible risers becomes attractive. In this way, this paper proposes a analytical/numerical methodology to evaluate the fatigue life at the top region of flexible pipes. Using the top imposed motions and taking into account the properties of all structures in the riser, it is possible to evaluate tension analytically. Combining tension and the rotations imposed at the top of the riser, curvatures are determined, and stresses can be calculated. Finally, SN curves and the Miner’s rule for damage accumulation allow the estimation of fatigue life. The obtained results indicate that the proposed methodology is conservative when compared to traditional ones. Also, it is very fast, allowing the fatigue life estimation in minutes.

Flexible Risers Lifetime Extension: Riser In-Service Monitoring and Advanced Analysis Techniques

2017 ◽  
Author(s):  
Hany Elosta ◽  
Thierry Gavouyere ◽  
Pierrick Garnier
Keyword(s):  
Service Life ◽  
Dynamic Behaviour ◽  
Primary Objective ◽  
Original Design ◽  
Analysis Techniques ◽  
Flexible Pipes ◽  
Integrity Management ◽  
Advanced Analysis ◽  
Flexible Risers ◽  
Lifetime Extension

The demand for the lifetime extension of flexible pipes is increasing due to the need to extend the lifetime of the existing production fields. There have been many challenges with the lifetime extension of flexible pipes after the end of the initial design service life due to the inherent conservatism with the common analysis approach, safety factors and operation beyond the design limits. A lifetime assessment should be performed on flexible risers for re-qualification during the original design life if the design envelope is exceeded or there is a need for lifetime extension. Hence, a systematic approach for lifetime assessment execution is established to determine the integrity level of the flexible risers and define the recommended actions required, such as mitigations, repairs or monitoring to maintain an acceptable risk for the required extended service life based on consistent methodology. The primary objective of this paper is to present a riser integrity management field-proven technology to monitor the riser’s behaviour in-service in addition to the advanced analyses guidelines to form a basis for the lifetime extension of flexible risers. The primary objective for the integrity management is to manage and control the risk of failure by detecting failure at an earlier stage when preventive action can be taken to avoid failure propagation. In addition, it is demonstrated that the primary hot-spots for the dynamic behaviour and fatigue life assessments of the flexible risers are primarily in bend stiffener regions and the touchdown zone (TDZ) due to large tension fluctuations caused by vessel motions and cyclic movement in the TDZ. Therefore, analysis techniques have been developed in two primary areas: advanced bend stiffener modelling using pipe-in-pipe (PIP) to model the sliding friction and the bend stiffener/flexible pipe’s annular space and flexible pipe–seabed interaction modelling using a non-linear seabed model. Therefore, the flexible riser’s lifetime extension assessment will be based on more reliable models that reflect the realistic and dynamic behaviour of the flexible risers. Consequently, these advanced analysis techniques can be used for new designs or lifetime extension of flexible pipes.

Numerical investigation of friction joint between Basalt Fiber Reinforced Composites and aluminum

2016 ◽  
Vol 231 (5) ◽  
pp. 543-551
Author(s):  
Andrei Costache ◽  
Christian Berggreen ◽  
Ion Marius Sivebæk ◽  
Kristian Glejbøl
Keyword(s):  
Finite Element ◽  
Basalt Fiber ◽  
Element Model ◽  
Fiber Reinforced ◽  
Pullout Force ◽  
Flexible Pipes ◽  
The Coefficient Of Friction ◽  
Flexible Risers ◽  
Offshore Oil Industry ◽  
Unbonded Flexible Pipes

Flexible risers are used in the offshore oil industry for exporting hydrocarbons from subsea equipment to floating production and storage vessels. The latest research in unbonded flexible pipes aims to reduce weight by replacing metal components with composite materials. This would result in lighter and stiffer flexible risers, which would be well suited for ultra deep water applications. This paper develops a new finite element model used for evaluating the efficiency of anchoring flat unidirectional fiber reinforced tendons in a mechanical grip. It consists two flat grips with the fiber reinforced tendon in between. The grips are pressed against the composite and the pullout force is ensured through friction. The novelty of the paper is represented by the detailed investigation of the influence between the coefficient of friction and the pullout force. By comparing numerical and experimentally obtained results, it is possible to show the importance of friction decay in the grip. Improper contact between the grips and composite is also taken into account and leads to good agreement between numerical and experimental results. This study shows how to avoid over-estimating the efficiency of such grip by using dry friction in finite element models.

Consistent Free Span VIV Fatigue Analysis of Flexibles

2017 ◽  
Author(s):  
Mário Caruso ◽  
Xu Han ◽  
Nils Sødahl
Keyword(s):  
Fatigue Life ◽  
Cross Section ◽  
Fatigue Analysis ◽  
Life Assessment ◽  
Response Analysis ◽  
Stick Slip ◽  
Analysis Scheme ◽  
Industry Practice ◽  
Current Loading ◽  
Calculation Procedures

The fatigue life assessment of free spanning flexible products, such as subsea cables and umbilicals, due VIV requires special attention to the structural properties due to the stick/slip behaviour of helix elements in bending. Essential parameters for assessment of stick/slip effects in free span VIV response are the structural damping in the stick regime (i.e. umbilical behaves as a solid cross section due to friction between the helix elements) as well as the additional damping introduced by the hysteretic damping due to the stick/slip behaviour in bending. Furthermore, consistent fatigue stress recovery considering the stick/slip behaviour in bending is essential for fatigue life predictions. The consistent evaluation of stick/slip behaviour requires more sophisticated calculation procedures due to the non-linearity it introduces. Hence, industry practice has been to consider simplified, linear calculation procedures. However, future flexibles utilization may be much benefitted by a consistent stick/slip treatment in free spanning VIV fatigue assessments, as it may allow for longer allowable free span lengths or longer fatigue life. The overall objective of the paper is to establish a consistent free span VIV analysis methodology for flexibles in compliance with requirements given in ISO 13628-5 ‘Subsea Umbilicals’ and the overall philosophy of DNV-RP-F105 ‘Free Spanning Pipelines’. A consistent fatigue analysis scheme for VIV in free spans is outlined using commercially available state-of-the-art computer programs for free span VIV response analysis (FatFree) and cross section stress analysis (Helica). The performance of the calculation scheme is demonstrated by case studies in a complex long-term current loading environment. It is shown that consistent treatment of the mechanical properties of flexibles is essential for VIV fatigue life assessments of free spans.

Radial Instability of Flexible Pipes With Defects in the High Resistance Bandage and External Sheath

2017 ◽  
Author(s):  
Marcelo Favaro Borges ◽  
Otaviano Luis Talgatti ◽  
Amauri Mosquen
Keyword(s):  
Deep Water ◽  
High Resistance ◽  
Radial Strain ◽  
Mechanical Tests ◽  
Field Application ◽  
Local Analysis ◽  
Axial Stiffness ◽  
Flexible Pipes ◽  
Compressive Loads ◽  
Flexible Risers

Exploration and production of oil and gas in ultra-deep waters will face several technical challenges. One major concern of the static failure of flexible pipes is the occurrence of damage on external sheath and high resistance bandages that in some cases can generate radial instability of those structures. The new ultra-deep water fields will require a better understanding of those failure mechanisms and relationship between compressive loads and defect sizes to guarantee a safe operation, otherwise the expected service lifetime will not be achieved. Besides that, historical data of in-service failures of this type of equipment shows that several flexible pipes have to be early replaced due to missing of proper data to evaluate damaged structures. Therefore, even worse results are expected on ultra-deep water field application. Flexible risers comprise multiple structural layers, which combine leads to characteristics of resistance, tightness and desired flexibility, both for its installation and operation. Regarding the mechanical strength, flexible riser structure must withstand several load modes acting together or isolated. Within this context, axial compression acting individually or combined is the responsible for radial instability of flexible pipes. Radial instability occurs mainly when the flexible pipe suffers damage on the outer layers, which are responsible for radial strain resistance. This damage on the external sheath and bandages occurs due to the launching procedure, project or material failures, wearing, excessive loading, abandonment procedures or possible falling. Damages on the external bandage layer, together with axial compressive load may lead to catastrophic failures due to radial strains, as well known as birdcaging or lateral bucking, thereby leading to a complex local analysis in the search for solutions capable to predict riser behavior. Therefore, this study intends to build a relationship between the size of the defects and compressive loads for flexible risers that leads to birdcage formation, which consequently reduces the pipe expected life. The measurements were performed in full scale mechanical tests of two sizes of flexible risers. After that, finite element method models calibrated and validated with mechanical tests data, were used to extrapolate the results for other possible defect scenarios. The case studies for an analysis of the relationship between compressive loads and sizes of defects which lead to radial instability and consequently to pipe stiffness decreasing, were two flexible pipes of different sizes, widely used on offshore applications, with produced defects. Besides that, thirty-two conditions were analyzed through the model developed with variations in the size of defects, according to riser geometric limits in length and width. The results indicated that the radial instability in flexible pipes with defect on high resistance bandages does not reach the failure criterion for axial stiffness if compressive loads are limited to a threshold. Also, the defect size on bandage of flexible pipes subjected to compressive loads influences the radial instability, reducing the stiffness up to five times according to obtained results, especially depending on its length and without significant dependence of the width. One of the simulated conditions presented a change on the deformation distribution located near the manufactured defect, indicating another type of instability known as lateral buckling.

Annulus Testing for Condition Assessment and Monitoring of Flexible Pipes

2004 ◽  
Author(s):  
Jon Olav Bondevik ◽  
Sigmund Lunde ◽  
Rune Haakonsen
Keyword(s):  
Barrier Layer ◽  
Nitrogen Pressure ◽  
Condition Assessment ◽  
Reliable Operation ◽  
Flexible Pipe ◽  
Flexible Pipes ◽  
Assessment And Monitoring ◽  
Flexible Risers ◽  
Paper Address ◽  
Vacuum Testing

The Norwegian operator Norsk Hydro has more than 80 flexible dynamic risers and service lines in operation at different platforms. Riser integrity monitoring programs have been established for the flexible risers in order to ensure safe and reliable operation. SeaFlex has performed annulus testing on a large number of these risers as a part of the programs. The free annulus volume of a flexible pipe is defined as the volume between the extruded internal pressure barrier layer and the extruded external sheath subtracted the volume occupied by pressure- and tension armor, tape and eventual other layers. Two methods are presently used by the industry for annulus free volume testing of flexible pipes, namely nitrogen pressure testing and vacuum testing. Both methods identify trends of volume reduction with time and to detect annulus flooding. Annulus testing has proven to be an efficient and reliable tool for detecting annulus flooding, blocked vent ports and outer sheet damages. This paper address the challenges related to annulus testing of flexible pipes, advantages, experiences and how such tests and the results are used for condition assessment and monitoring of the risers.

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