SCRs have been accompanied with drifting platforms since the mid-1990s and were foremost used as export risers for Auger TLP. They can be suspended in longer lengths, taking the demand for mid-depth arches or buoys. SCR is a freely hanging pipe connected to a floating production vas, bents at a prescribed top angle and swimmingly widening down to the seabed at the Touchdown Point ( TDP ) . At the TDP the SCR buries itself in a trench under the significance of SCR ‘s weight and besides the oscillating gesture of the drifting platform.
ROV studies of installed SCRs have shown deep trenches delve the seabed beyond the TDP. Having established that the TDP is critical to the design of the SCR, it should be noted that the TDP is non a individual point on the SCR. The TDP will travel invariably with clip reflecting motions of the drifting platform and SCR. Hence, the term TDZ may be more applicable ( Thethi, 2001 ) . The mechanisms that create these trenches are thought to be produced by the dynamic oscillations of the SCR combined with the scouring and sediment transit effects of the seabed currents. Storm and current action on a deepwater production vas can draw the riser upwards from its trench, or laterally against the trench wall doing an addition in the local riser emphasiss ( Bridge, 2003 ) . The research background reveal that as the riser moves back toward the dirt, the H2O underneath the riser is pushed downward. The gushing action by the H2O can take to soil-water commixture and trench eroding which can besides cut down the strength and stiffness of the dirt.
The seabed response due to riser burden and the trench formation phenomenon are of great significance for safe and economic riser design. Realistic anticipations of the fatigue life of SCRs require an accurate word picture theoretical account of seabed stiffness every bit good as realistic description of load-deflection curve. The usage of a stiff ocean floor gives higher maximal fatigue harm in the critical TDZ compared with an elastic ocean floor. It is hoped that improved modeling of the seabed interaction will give more accurate anticipations of SCR system behavior, in peculiar for fatigue analysis.
A figure of researches have been directed towards understanding the soil/riser interaction. Recently, several dirt stiffness theoretical accounts have been developing every bit good as a series of theoretical account trials to imitate seabed/riser interaction were performed ( e.g. , Willis and West, 2001 ; Bridge and Willis, 2002 ; Bridge et al. , 2003 ; Bridge et al. , 2004 ; Aubeny et al. , 2006 ; Randolph and Quiggin 2009 ) . Although sidelong gestures of the riser affect riser public presentation ( Hale, 1992 ; Morris, 1988 ) , the proposed seabed/riser interaction theoretical account respects merely the perpendicular riser gestures. Theoretical preparation for sidelong opposition under dynamic environment is non yet known.
Cyclic debasement is by and large thought to impact the capacities of structural systems to defy failure under cyclic loading status. Seabed stiffness debasement due to cyclic burden has a important impact on the public presentation of SCRs in the TDZ, and particularly on the riser ‘s opposition to tire. In order to capture this consequence on the riser ‘s response, dirt debasement must be included in the modeling attempts.
SCR failure consequences in decrease or surcease of gross. It may besides take to spillage or pollution and may jeopardize lives. The TDZ frequently proves to be a topographic point where cyclic bending emphasiss are largest and hence a critical location for weariness. Hence, TDZ design must transport a high grade of dependability. SCRs are considered to be technically executable and commercially efficient solutions, particularly when high temperatures and force per unit areas are involved.
Aims and Summary of Research Questions
Riser/soil interaction surveies have been reported by assorted writers. In all these surveies consequences from experimental programmes have been used to specify soil/riser interaction parametric quantities for usage in regular analyses programmes. Such experimental informations are non available in the public sphere. The aim of the research work is to reproduce the complex soil/riser interaction procedure utilizing expressed finite component method every bit good as acquire better penetration into the trench formation procedure. Furthermore, the sidelong opposition of the SCR in the TDZ needs to be assessed at the design phase.
What is the factors impacting the trench formation procedure?
What is the part of sidelong and perpendicular gestures on the flexing emphasis of the SCR?
To what extent sidelong and perpendicular supplantings of the riser are?
What is the relation between the SCR sidelong motion and the embedment of the riser?
The current hysteretic dirt stuff theoretical account adequately captured the consequence of lading magnitude, but has no agencies of stand foring the effects of multiple burden rhythms. It is hence recommended that clip dependence be included in the theoretical account. This can be achieved by integrating the strength debasement effects based on figure of lading rhythm into the bing hysteretic dirt theoretical account.
What is the impact of the strength debasement on the dirt stiffness in sidelong and perpendicular way?
Perform parametric survey for the trenching formation phenomenon.
What is the consequence of the shear strength on dirt stiffness?
What is the consequence of the shear strength on flexing emphasis?
What is the consequence of the shear strength on riser warp?
What is the consequence of trench deepness on flexing emphasis?
What is the consequence of trench deepness on riser warp?
In footings of weariness, SCRs are really sensitive to environmental burden. The process for fatigue analysis is basically deterministic. Therefore, this research work concentrates on the dependability index associated with the current design pattern of fatigue analysis of SCRs.
What is the impact of the deterministic and uncertainness modeling of the SCR on the fatigue life?
What the beginnings of uncertainness associated with fatigue life computations?
Develop apprehension of the effects of different environmental and geotechnical variables on the fatigue behavior of the SCRs.
To rede an effectual reliability-based weariness appraisal methodological analysis to help in the design and analysis of SCR in deepwater.
What are the most sensitive factors impacting the public presentation of the SCR?
What are the virtues of utilizing the interfacing between FE package and dependability package?
The undermentioned undertakings will be undertaken as a portion of the proposed research:
Undertaking 1: SCR-soil-fluid interaction job
Using LS-DYNA, an expressed finite component codification in analyzing riser-soil-fluid interaction
Backup program: by experimentation look intoing sidelong dirt opposition.
Undertaking 2: Finite Element theoretical account ( FEA ) of SCR in Orcaflex package.
Construction of the SCR in OrcaFlex package.
SCR/seabed interaction modeling ( additive and nonlinear springs )
Analyze the impact of the dirt strength debasement.
Perform parametric survey:
Consequence of the shear strength on dirt stiffness.
Consequence of the shear strength on flexing emphasis.
Consequence of the shear strength on riser warp.
Consequence of trench deepness on flexing emphasis.
Consequence of trench deepness on riser warp.
Undertaking 3: perform fatigue life anticipation of SCR based on S-N curves, Figure 1. Fatigue life is computed, utilizing the “ Rainflow numeration ” technique. OrcaFlex package is used for the clip sphere analyses. The clip sphere analysis is followed by “ rainflow numeration ” for a node near the touchdown point to specify a weariness lading map.
Undertaking 4: dependability based-fatigue appraisal methodological analysis
Determining the uncertainnesss associated with the fatigue life computations.
Construct the SCR probabilistic theoretical account.
Probability distributions of random variables used.
Interfacing with external deterministic analysis plan.
Perform Monte Carlo Simulation for the sensitiveness analysis.
Probability of fatigue failure of SCR.
Methodology and Approach
In order to look into riser-soil-fluid interaction analytically, a programme of geotechnical FEA affecting riser-soil should be used. The key for this survey on fatigue appraisal is the proper rating of sidelong opposition encountered by the riser as it embeds itself into clay. In soft clay the sidelong opposition of dirt to riser motion when it responds to vessel motion at the top, is straight related to the colony of the riser. This sort of behavior is best investigated through finite component ( FE ) modeling. Theoretical preparation for sidelong opposition under such dynamic environments is non yet known.
The interaction between SCR, dirt and local fluid create an highly complex fluid-structure interaction job, which is yet to be to the full quantified by experiment or by analysis. Such job does impart itself to a continuum mechanics analysis attack where the mechanical, geotechnical and hydrodynamic facets of the job can be explicitly represented. This allows each person phenomenon to be isolated and investigated to find their significance on the incorporate system response. This simulation of this interaction would necessitate multiple functionalities in a individual package tool as stiff organic structure and concentrated dirt continuum stuff features, intermittent contact mechanics that incorporated clash and incompressible fluid response and gesture of a stiff boundary through the fluid medium.
The above functionality can be readily applied in high terminal FE codes that usage an expressed clip integrating strategy. Explicit clip integrating is considered more applicable to the riser-soil interaction job as it entirely dependent on the alteration in burden in a clip measure and the mechanical conditions at the start of the clip increase.
One drawback with utilizing FE analyses is the clip devouring facet of this type of analysis, peculiarly if many different analysis tallies have to be performed. In order to get the better of this job two FE theoretical accounts should be utilized, a harsh mesh theoretical account and a all right mesh theoretical account. The harsh mesh is used for majority of the analysis runs at all the geotechnical subdivisions, as the tally clip with this theoretical account is short. Typical tally times with the harsh theoretical account were about 20 proceedingss compared to more than 8 hours of the finer theoretical account. The chief purpose of these theoretical accounts is finding the sidelong force versus sidelong supplanting response from the coarse and all right theoretical accounts.
Three different attacks can be considered for measuring the sidelong opposition for riser in the TDZ:
A individual “ clash factor ” attack, in which the sidelong opposition is related to the submersed weight of the grapevine, and the dirt type. This first attack is instead simplistic, as it does non take into history pipe embedment, which could be really variable depending on the grapevine belongingss, dirt strength and installing conditions. It is hence non farther considered in this research ;
A two-component theoretical account, dwelling of a skiding opposition constituent, and a sidelong inactive force per unit area constituent.
A malleability theoretical account attack. This malleability model provides a much more cardinal manner of understanding the mechanisms involved. It allows the analysis of the riser-soil behavior under combined perpendicular and horizontal burden, and that of the way of motion during output. However, the theoretical account for clays is based chiefly on the behavior of shallow level termss, or spud-cans in which a big sidelong motion does non happen. Furthermore, the brickle sidelong opposition observed during the theoretical account trial appears to be partially due to loss of suction on the rear of the pipes, which is non modelled by the malleability theoretical account. Therefore, merely the practical two-component theoretical accounts for the appraisal of the extremum opposition will be used in this research.
In footings of weariness, SCRs are really sensitive to environmental burden. Fatigue life anticipation in SCRs is a complicated procedure affecting many factors. The research process for fatigue analysis is basically deterministic. The right foretelling fatigue life is complicated since it involves many beginnings of uncertainness. There are two distinguishable attacks for handling uncertainnesss in weariness job which are randomised parametric attack and stochastic attack.
The ability to quantify the uncertainness of complex technology constructions subject to built-in entropy in burden, stuff belongingss, and geometric parametric quantities, is going progressively of import in the design and analysis of constructions. Probabilistic finite component analysis provides a agency to quantify the dependability of complex systems. However, for broad credence, probabilistic methods must be interfaced with widely used commercial finite component convergent thinkers, such as ABAQUS, ANSYS, and NASTRAN ( Riha et. al. , 2000 ) . A figure of issues originate when executing probabilistic analysis with external deterministic analysis plans such as commercial FE analysis bundles. In add-on, the finite component consequences are by and large post-processed to measure a step of utile life with tools that can calculate measures such as Fatigue Limit State ( FLS ) .
The stairss required to carry on component dependability rating are:
place the basic opposition and lading variables specifying the job,
find the distribution parametric quantities, i.e. chance map, standard divergence, location parametric quantity, scale parametric quantity, form parametric quantity, etc. , as appropriate, for each basic variable ( if old experience indicates the variable can be treated as deterministic, this is to be preferred ) ,
choice opposition theoretical accounts ( R ) for constituent strength,
choose the most accurate moving ridge force and air current force theoretical accounts ( S ) available for each structural type. Analyze the constructions under the applied burden, for the preferable selected return period, utilizing a sufficient figure of analyses to bring forth an appropriate n-dimensional response surface where N is the figure of adoptive random parametric quantities,
set up the truth of the lading preparation in probabilistic footings,
organize the failure equation ( z-function ) . In simple footings this will be:
omega = XR R -Xs S
where Ten denotes patterning truth and omega & lt ; 0.0 denotes failure,
utilizing a FORM process, measure the chance that omega & lt ; 0.0, i.e. the chance of failure Pf, and the corresponding dependability index I? given by
I? = – I¦-1 ( Pf )
where I¦ ( ) is the standardised normal distribution map, calculate sensitiveness steps, that is, importance factors ( Offshore Technology Report, 1999 ) .
Facilities to be used
OrcaFlex package will be used for the clip sphere and fatigue analysis.
LS-DYNA should be incorporated in ABAQUS and ANSYS ( need license look into ) .
NESSUS package for probabilistic finite component and dependability analysis but there is no licence in our section.
Roll uping full informations of SCR, attached vas and seabed dirt features.
Time consumed in larning LS-DYNA.
In instance of transporting out experiment analyzing the sidelong dirt opposition, the handiness of the topographic point and clay with such shear strength is a challenge.
Lateral dirt opposition and weariness analysis need more reappraisal.
The seabed/riser interaction patterning allows the consequence of physical phenomena such as sidelong opposition, dirt suction forces and perpendicular ocean floor stiffness on the SCR public presentation to be identified and quantified.
Extension of cognition of SCR-seabed interaction to more realistic conditions to better assurance in anticipation of public presentation.
Non-linear seabed/riser theoretical account interaction will find the influence of the seabed profile on SCR weariness.
A little alteration in seabed stiffness can ensue in a little alteration in flexing emphasis, but this causes a important alteration in fatigue life. So, the demand for seabed/riser interaction modeling to be every bit realistic as possible is apparent.
The many advantages and benefits of utilizing reliability-based design methods include the followers:
They provide the agencies for the direction of uncertainness in lading, strength, and debasement mechanisms.
They provide consistence in dependability.
They result in efficient and perchance economical usage of stuffs.
They allow for future alterations as a consequence of gained information in anticipation theoretical accounts, and stuff and burden word picture.
They provide directional cosines and sensitiveness factors that can be used for specifying future research and development demands.
They allow for executing time-dependent dependability analysis that can organize the footing for life anticipation appraisal, life extension, and development of review and care schemes.
They are consistent with other industries, AISC, AASHTO, ACI, API, ASME, aˆ¦ , etc.
Reduced likeliness of failure of SCR-seabed interaction thereby minimising the hazard of loss the containment with the associated environmental impact.
Determination of the factors which have important consequence on the chance of failure ( Pf ) under cyclic burden in the TDZ.