Steel catenary risers - Results and conclusions from large scale simulations of seabed interactions (original) (raw)
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Effects of Seabed Interaction on Steel Catenary Riser
Steel catenary risers are an enabling technology for deepwater oil and gas production. A steel catenary riser consists of a steel pipeline suspended between the vessel and the seabed forming a catenary shape. Tools to analyse and design steel catenary risers show that the point where the steel catenary riser first touches the seabed, termed the touchdown point, has the highest stress and the greatest fatigue damage. Current understanding of pipe/soil interaction is limited and consequently there is concern within the industry regarding the conservatism of the analysis. In particular, the implications of pipe/soil interaction for maximum stress and fatigue damage at the touchdown point are significant. To address these concerns, research has been conducted into the following areas: • Steel catenary riser trenches – using video survey data from installed steel catenary risers to determine the shape of seabed trenches. A steel catenary riser trench profile has been developed for use in finite element analysis. • Pipe/soil suction force – i.e. the bond that forms between the riser pipe and a clay seabed. Experiments have been conducted and a pipe/soil suction model developed for use in steel catenary riser analysis. • Pipe/soil stiffness – test data from the CARISIMA and STRIDE JIPs has been examined and a series of soil stiffness models for static penetration, small and large displacements, and cyclic loading have been developed for use in finite element analysis programs. • Closed form and finite element models of steel catenary risers were constructed to determine the effect of the soil on stress and fatigue damage at the touchdown point. A finite element model of a representative steel catenary riser has been created and analysed using the seabed interaction models developed. The results show that the seabed trench, pipe/soil suction and soil stiffness have little effect on extreme stress in the steel catenary riser during normal operating conditions. However, pipe/soil suction is shown to have a large effect during slow drift motions where the stress in the riser at the touchdown point could double. The results from a closed form seabed model and finite element analysis show that the fatigue life of a steel catenary riser is sensitive to soil stiffness.If the soil stiffness used to model the seabed is too high the fatigue life may be underestimated; conversely, if the soil stiffness is too low the fatigue life may be over estimated.
Effect of Riser-Seabed Interaction on the Dynamic Behavior of Risers
2017
In recent years, the production of oil and gas has been developed in deep water depths which exceed 500m. Deep water developments are being followed strongly in different parts of the world (Caspian Sea, Gulf of Mexico, etc.). The movement of floater causes severe stress at the touchdown point (TDP) in steel catenary risers (SCR). The main objective of this study was to simulate the exact behavior of the riser in the vicinity of the touchdown zone (TDZ) by implementing linear SCR-seabed interaction model. Hence, present study attempted to investigate the riser-seabed interaction during lateral cyclic pipe movements and also the influence of seabed evolution around the TDZ based on the vertical cyclic movements. Moreover, The significance of the soil types in the response of riser pipeline at TDP was analyzed based on the vertical and lateral interaction. The fully non-linear time domain finite element model was utilized to simulate the riser behavior.
Full-scale Model Tests Of A Steel Catenary Riser
2003
Steel catenary risers (SCRs) are an enabling technology for deepwater oil and gas production. Tools to analyse and design SCRs are available which show that the point where the riser first touches the soil, termed the touchdown point (TDP) is critical. However our understanding of fluid/riser/soil interaction is limited, hence the oil and gas industry has concerns regarding the levels of conservatism in SCR design, and margins of safety. The purpose of this study is to examine the interaction between a pipe (representing a section of the SCR), a clay seabed, and the surrounding seawater. This paper documents some of the results and observations from the full scale harbour test riser experiments which examined the 3D effect of fluid/riser/soil interaction around the TDP. The riser, a 110m (360-ft) long 0.1683m (6-5/8 inch) diameter pipe, was draped from an actuator on the harbour wall to an anchor point on the seabed. The top end of the pipe string was actuated using a programmable l...
Steel Catenary Riser-Seabed Interaction Due to Caspian Sea Environmental Conditions
Journal of Rehabilitation in Civil Engineering, 2017
This paper investigates the integrated riser/vessel system which is subjected to random waves. Riser pipelines are the main components of the oil and gas offshore platforms. Whereas Iran country has been located on the fringes of Caspian Sea deep water, therefore study and research in this area is increasingly essential. The fluctuation of floating production causes the intense response and greatest fatigue damage near the Touchdown Point (TDP) where the Steel Catenary Riser (SCR) first touches the seabed. Therefore, analysis the response of SCRs in the TDP is very important to approximate the behavior of the riser. In this study, initially, the structural parameters (wall thickness and diameter) according to design codes due to the intense climatic conditions are obtained. In the next step, Pipe-soil interaction is modelled using a linear model in the vertical direction and Coulomb friction models in the lateral direction. Also, the significance of SCR-seabed interaction in the glo...
Analysis of mooring and steel catenary risers system in ultra deepwater
2012
With the gradual depletion of oil and gas resources onshore as well as shallow offshore waters, oil exploration is gradually moving deeper into the seas. One of the major means of oil exploration at such locations is by way of Floating Production Storage and offloading (FPSO) system. Because of the ever increasing depths of exploration and the prevailing harsh environmental conditions, there is a need to constantly re-evaluate or develop new methods for mooring system and riser analyses. There are several methods available which are well tested for the analysis of systems operating in shallow to deepwater using catenary or finite element approach in both frequency and time domain. These have been reviewed and the method considered to be most relevant for the purpose of this research has been identified for further development. Based on this a methodology a quasi-static and dynamic analyses of single and multicomponent mooring and steel catenary risers system in ultra deepwater has been developed. The dynamic equations of motion were formulated based on the modified Lagrange's equation and solved using the fourth order Runge-Kutta method. Because of the dearth of experimental data at such water depth, the developed methodology for line dynamics has been validated using relevant published data for finite water depth. These techniques are then applied to the analysis of a mooring and steel catenary risers system of an FPSO unit in 2500m of water offshore Nigeria and also the Gulf of Mexico both in the frequency and time domain. The results were found to be practical and compare reasonably very well between the two approaches.
Iranian Journal of Oil and Gas Science and Technology, 2017
A steel catenary riser (SCR) attached to a floating platform at its upper end encounters fluctuations in and near its touchdown zone (TDZ), which causes the interaction with the seabed. Subsea surveys and the analysis of SCR’s indicated that the greatest stress and highest damage occurred near the touchdown point (TDP), where the SCR first touches the seabed. Nowadays, the linear seabed spring is carried out, and it is assumed as a flat seabed. Improved nonlinear hysteretic seabed models have recently been proposed, which simulate the different stiffness in the seabed response in the TDZ. In this study, an advanced hysteretic nonlinear SCR-seabed soil interaction model has been implemented to simulate the exact behavior of the riser in the vicinity of the touchdown zone. This paper focusses on the seabed trench, which develops progressively under the SCR due to repeated contact. Also, different important parameters such as water depth and material of riser have been investigated bas...
2015
Steel catenary risers (SCR) are widely used in deepwater oil and gas production. Due to environmental loading the riser may be subject to six degrees of motion; however, in the touchdown zone (TDZ), the vertical penetration into the seabed and uplift are two of the main components. The riser-seabed-water interaction near the touchdown zone is one of the main concerns in fatigue life design of SCR. During upward displacement, suction develops under the riser and a trench might be formed when it separates from the seabed near the touchdown point (TDP). In subsequent downward movement, the riser penetrates through this trench to the seabed. Therefore, modeling of suction and trench formation is very important. In most of the existing models these factors are incorporated using empirical relationships. It is also recognized that the available finite element (FE) modeling techniques for this large deformation problem are computationally very expensive, although the penetration resistance...
Ships and Offshore Structures, 2016
A coupled numerical analysis has been carried out for the structural responses and motion behaviour of a classical spar structure subjected to irregular waves represented by JONSWAP (Joint North Sea Wave Project) spectrum. The motion of the floating body is restrained by the four catenary mooring lines with the generation of tension due to change in their nonlinear profile. The surface of the spar is represented by hydrodynamic pressure panels while cables are discretised using a series of Morison elements. A comprehensive sensitivity analysis under two sea depths is carried out by changing (1) the length of mooring lines, (2) the vertical position of fairlead point, (3) incident angle of long crested unidirectional single spectrum, and (4) the number of short crested sub segmented spreading spectra. The effect of second-order hydrodynamic loading on the structure is also taken into account in the analysis procedure by using quadratic transfer function.
Response of nonlinear offshore spar platform under wave and current
Ocean Engineering, 2017
Several advanced floating structures have been proposed and developed with varying cost effectiveness and productiveness in deep water exploration. Among them, Spar platforms have been accepted as an efficient platform for the exploration. Many research works have been conducted on floating structure but a few on Spar platform. Nonlinear dynamic analysis of a 3D model of floating Spar platform structure is a resourceful tool to predict the responses, where the main body of the Spar hull and mooring lines are considered as an integrated coupled system. To define accurately the interaction between the Spar and mooring lines, coupled dynamic analysis was found to be appropriate for studying responses in the deep sea. Numerical simulation and motion analyses were carried out with the ABAQUS/AQUA. The responses of Spar platform were extracted and evaluated in time histories along with Response Amplitude Operator (RAO). The behaviours of coupled Spar platform have been investigated under real sea environments for increasing water depth to ultra-deep together with the load variability employing sea current for surge, heave, pitch and mooring tension responses. Motions show the consistency in the behaviour of Spar platform responses. Surge response indicates the static offset of the platform due to the static current force under wave plus current. The current force compresses oscillations and reduce heave and pitch magnitude. For larger water depth the platform responses reduce significantly due to the increased damping of mooring line.