The objective of the STREAM (Steel Riser Enhanced Analytics using Measurements) JIP is to provide a measurement based foundation for SCR and lazy wave riser modelling to ensure that the fatigue response is assessed with adequate but not overly conservative parameters. To achieve this objective, the JIP utilizes field measurements from 4 in-service SCRs and 1 SLWR in water depths from 3,000 ft to 5,000 ft. The field measurements correspond to a range of environments including hurricanes and loop currents, riser functions, sizes, VIV suppression coverages and host vessels. The processing commences with data QA, error assessment and data filtration. Riser response is categorized into wave dominated events, VIV events and others such as MIV events. As-built finite element riser models are developed and simulations are conducted using measured motions. The resulting analytical responses are compared with the measured motion and strain data to determine the level of conservatism or otherwise in typical riser wave fatigue analysis. SHEAR7 models driven by measured current profiles are used to compare predicted VIV response to observed VIV amplitudes and frequencies. Analysis results indicate that industry standard fatigue assessment is indeed conservative. Sensitivities are conducted and presented on key design parameters that are known to be conservatively used in design such as hydrodynamic coefficients and SHEAR7 inputs. A set of parameters is derived that not only reduces fatigue damage bias but also improves the reliability in predictions. Recommendations are made with regards to further refinement of analysis parameters and understanding of atypical riser responses. Measured riser response that does not conform to typical wave and VIV spectra are presented and discussed. The combined assessment of full scale field data from multiple catenary risers is an industry first. The results from this JIP offer valuable insight into riser response characterization with potential applications for SCR/SLWR life extension and more efficient new designs. Note that in this paper, the term SCR is often used a generic term to describe both the regular SCR as well as the lazy wave implementation of it, the SLWR.
Offshore production risers are generally designed considering a large margin of conservatism to account for the uncertainty in the design parameters and to ensure robust design. As the Oil & Gas Industry is advancing toward ultra-deepwater there may be a necessity to adopt new technologies and riser configurations to develop the field. Project teams may elect to quantify the degree of conservatism in the designs as an additional measure to improve confidence in the safe and reliable service over the life of the field. Such validation provides confidence in the design performed based on theoretical concepts and software predictions.This paper presents a novel approach that has been undertaken to validate the design of the Caesar-Tonga Steel Lazy Wave Risers (SLWR) tied back to the Anadarko Petroleum Company (APC) operated Constitution Spar located in the Green Canyon Area of the Gulf of Mexico. These risers represent the first application of the steel lazy wave riser technology in the Gulf of Mexico, and it was the first such application tied back to a spar platform.The procedure is based on the monitoring of motion and strain data measured at critical locations along the risers. The measured data is post-processed to identify significant riser motion events that occurred during the observation period. Met-ocean data during this time period is also evaluated to correlate back to the riser motion events. Vessel motions and current data corresponding to these identified events are used to drive a Finite Element (FE) model of the riser, and the corresponding riser motions and strains are calculated. The calculated motions and strains are compared with measured motions and strains to provide confidence in the design methodologies. The procedure as well as the results of the comparison between predicted performance and measured performance is discussed in detail in the paper. The procedure can be applied for design validation and performance and integrity assessment of newly-built or existing risers.
Successful installation of offshore piles is often challenged by the constraints of cost, technical feasibility, availability of suitable installation vessels and the environment. A practical installation procedure is defined as one that utilizes the capacity of available equipment to operate in a wide allowable environmental window. To find such procedures there is a continuing need for research, use of new technologies, and the adoption of new ideas. This paper presents a feasibility study for side launching piles from a conventional cargo barge. Sophisticated nonlinear time domain dynamic simulations formed the basis of the evaluation. Two noted software packages with nonlinear time domain dynamic finite element analysis capabilities were used to predict the trajectory of the launched pile and the resulting impulsive load on the restraining cable/wire. Analytical results from the software programs were compared to provide a first level of validation. The numerical results were in good agreement. Cable/wire properties (size, length, and material) and tug vessel velocity were varied in an effort to minimize the cable loads during the highly dynamic launch event. The study concluded that side launch was feasible and design recommendations are provided.
Substructure modeling using pipe-in-pipe (PiP) elements in a finite element program allows representation of dynamic interaction between riser components. This modeling technique is especially useful when it comes to the design of a complex riser system in deepwater applications. In this paper, the ABAQUS finite element program was used to illustrate the substructure models and the results for dynamic analysis of a classic Spar top tension riser (TTR) system in the Gulf of Mexico subjected to a given Hurricane Rita sea state. Nonlinear contacts between the buoyancy can and compliant guides are represented by two different substructure models: compliant guide surface model with friction and frictionless compliant guide spring model. The effects of centerwell hydrodynamic forces were considered. ABAQUS dynamic results were compared between the PiP substructure model and a conventional structure model treating the buoyancy can and the riser inside as a composite beam. The PiP guide friction surface model with centerwell hydrodynamics theoretically is the most representative model for riser analysis. However, the PiP guide spring model is more computationally efficient. It generates comparable guide loads but produces lower riser fatigue damage than the PiP guide friction surface model. The composite beam model leads to guide loads comparable to the PiP model, but cannot be used to determine the spacer loads between the buoyancy can and riser. The composite model also could underestimate riser stresses and riser fatigue damage within the buoyancy can region. The riser guide loads and riser damages from the calculation models without centerwell hydrodynamics are generally higher than those by the same calculation models with such consideration. It was concluded the PiP guide spring model can be used for riser design in lieu of the PiP guide surface model. The additional fatigue damage contribution from axial tension variation due to guide surface friction could be accounted for by adding a damage factor to the total fatigue damage along the riser.
Pipe-in-pipe analyses of in-place SCR (Steel Catenary Riser) within pull-tube (PT) or pull-in analyses of SCR through PT are computationally complicated problems in offshore development in the application of SCR hanging off through PT from a platform such as Spar. In the past, those analyses have been done primarily using ABAQUS, an advanced nonlinear FEA commercial tool. A benchmark study of pipe-in-pipe analysis, i.e. SCR through PT, was conducted to compare the performance of ABAQUS and Flexcom, a package popular in the offshore oil and gas industry dedicated to flexible structures like risers or mooring lines. Both static and dynamic loading conditions, as well as the pull-in process were included in the paper. Bending moment, tension, shear force, and von Mises stress of both SCR and PT were compared. The comparison shows that Flexcom pipe-in-pipe analysis of in-place SCR within PT or pull-in analysis of SCR through PT can obtain satisfactory results in bending moment and stress for both static and dynamic simulations with close agreement with ABAQUS. While the friction between SCR and PT is not accounted for in Flexcom, its effect is mainly on tension especially of PT, and insignificant for bending moment and bending stress, which governs the strength and fatigue performance of both PT and SCR at the upper hang-off region. This study indicates that it could be adequate to use Flexcom to conduct pipe-in-pipe analysis with much better efficiency for analysis of in-place SCR within PT or pull-in analysis of SCR through PT.
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