No abstract

Demand for life extension assessments of floating offshore platforms continues to grow worldwide. Conventional structural analysis methods have limited ability to accurately capture non-linear environmental loading, non-linear loading by the mooring and riser systems, and resulting higher order hull responses. The uncertainties are typically managed by the factors of safety applied in the structural analysis. Time domain structural analyses have long promised to improve analysis accuracy and reduce these uncertainties. This paper describes a comprehensive and practical time domain structural analysis methodology applied to a deep-water semi-submersible-type floating platform including results for structural strength and fatigue. In addition, the time domain structural analysis was extended for use in fracture mechanics and the assessment of notional weld flaws to facilitate specification of impactful non-destructive examination (NDE). Present time domain structural analysis methodology employs a response-based finite element analysis (FEA) conducted in the time domain. All external environmental loads and inertial forces are converted to a response-based stress-time history. Previously, conventional time domain structural analysis involves massive computation resources to resolve solutions at every time interval. Present methodology significantly improves computational efficiency to be practical in real-world problems. The improvement is achieved by decomposing the structural response into a set of multiple load components selected on the bases of function for hull motion response and environmental loadings. Structural response in time domain is directly obtained by synthesizing the load components. An actual time domain structural response is captured effectively and efficiently to simulate the strength and fatigue criterion for the structure with consistent environmental loads and hull responses. Utilizing the level of detail provided by the time domain structural analysis methodology, a fracture mechanics evaluation of notional initial flaws (engineering criticality assessments – ECAs) can be conducted providing meaningful technical basis for in-service NDE and life extension assessments. The procedures for fatigue crack growth and fracture documented in BS 7910 were employed to derive the smallest initial flaws (critical initial flaws) that may result in structural failure during a facility's lifetime. A comparison indicates that conventional structural analysis methods provide conservative results for both structural strength and fatigue damage calculations resulting from the linear assumption of environmental loads and hull responses. Present time domain structural analysis methodology provides an innovative, cutting-edge approach providing accuracy and fewer uncertainties, which can be pragmatically used during a typical project.

This paper presents a time-domain S-N fatigue analysis and an approach to reliable and robust engineering criticality assessments to supplement or provide an alternative to S-N fatigue assessments of offshore platform structures based on time domain structural response analysis. It also provides recommendations for industry standards to improve guidance for structural integrity assessments of offshore platforms using fracture mechanics. Demand continues to grow in the offshore industry to attain value from captured operational data for a number of purposes, including the reduction of uncertainties in structural integrity assessments during design and over the operational lifetime of floating offshore platforms. Recent advances in time domain structural analysis technology demonstrate substantially more accurate assessments of non-linear platform loadings and responses with enhanced computational efficiency. The current S-N approach for fatigue design and integrity assessments calculates a fatigue damage factor that does not address how loading occurs over time (ABS, DNVGL-RP-C203). For the present study, engineering criticality assessments (ECAs) based on fracture mechanics theory (BS 7910) are applied utilizing time-domain loading information theory. The ECA returns the smallest initial flaws that can grow to a critical size during a design lifetime, which can serve as an indicator of acceptability during design, a technical basis for in-service inspection intervals and facilitates asset integrity and life extension assessments. Critical initial flaws are calculated using the Paris Law (BS 7910) and cumulative fatigue crack growth in two ways: with and without an integrated and consistent check for fracture instability. The results are compared with those from S-N fatigue analyses and recommendations are provided.

The spectral fatigue methodology is a widely accepted methodology to compute the fatigue life of an offshore platform. The ever-increasing demand for life extension of the existing floating platforms worldwide continues to grow. ABS Guide for Fatigue Assessment of Offshore Structures and DNVGL-RP-C203 have established guidelines for employing finite element analysis (FEA) to calculate fatigue lives using the spectral fatigue method. For complex structural details, the FE models with 2-D elements may not be able to capture the actual geometric details accurately. Hence, detailed FE models with solid (3-D) elements are utilized to capture geometric SCF’s (stress concentration factors) for these locations. The fatigue lives thus obtained using SCF approach with 2-D elements can be highly conservative or inaccurate. To overcome unreliable fatigue results for such complex locations that need using 3-D elements for a better definition of the local structure, this paper presents an extension to the defined guidelines by employing spectral fatigue methodology to 3-D solid elements. The paper also illustrates the applicability of Engineering Criticality Assessment (ECA) using stress-histogram based Fracture Mechanics Evaluation (FME) approach. A comparative study is performed for a critical weld location on an offshore platform using solid 3-D and shell 2-D FE models. First, FEA is performed for both the models to calculate fatigue lives using the S-N curve-based approach. In addition, FME is also performed for the same critical weld location in order to provide a more accurate and reliable solution that will enable clients to plan their in-service inspections and maintenance programs. Also, presented is a comparison of fatigue lives based on the solid and shell element FME.