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SIMFAT: SIMultaneous load processes in FATigue estimates

Joint Industry Project

Consolidating a common design methodology accounting for simultaneous load processes.

Industry need

Structures such as riser systems experience fatigue loading from several processes, acting simultaneously. Wind and waves induce floater motions and direct wave loading, currents may induce vortex-induced vibrations (VIV), and internal flow may induce slugging. Additional effects such as heave-induced VIV and low-frequent vortex-induced vibrations (VIM) may also occur. All these processes impose fatigue stress on the structure. However, addressing them in the same analyses is too complex and time consuming for early-phase design stages, and often the functionality is missing in industry-standard tools. Simplified methods available in design codes are over-conservative, while design practice in the industry is varying and may lead to unsafe designs.

A pilot study has been conducted by DNV in cooperation with Equinor, focusing on deep-water steel-lazy-wave risers. This study assessed the fatigue amplification due to simultaneous fatigue from waves and VIV, and from waves and slugging.

More work has been performed in the pilot study which at present is unpublished. The JIP will build on the experience and tools developed in the pilot study, where the main developments relevant for the JIP include:

  • Studies of methods applying spectral moments for describing stress processes and calculating combined damage.
  • Preliminary assessment of effects due to wide-bandedness, non-Gaussianity, and including three simultaneous processes.
  • Establishing tools for numerical simulations of multiple stress processes with varying characteristics, based on synthetic time series generated from stress spectra.
  • Identifying bias for total fatigue damage from different stress spectra combinations using simulations.

Project goal

The overall project goal is to consolidate a common design methodology accounting for simultaneous load processes. The main goal is to cover 2 simultaneous processes, with ambition to cover 3 or more processes in the final methodology. This includes requirements for doing simultaneous fatigue assessments from a design basis perspective, recommendations for how to do design with a conservative simplified approach, and how to do design using more advanced methods. The exact design format for the simplified approach is not worked out fully, but at present the idea is to use linear sum and introduce a separate bias factor accounting for the stress amplification due to simultaneous loading.

The work will also address the methodology in use today (linear sum, quadratic sum, DNV-RP-C203 method) and see when they are applicable. Methodology developed should be relevant for all structures but will only be tested for cases relevant to risers and cables. The aim is to establish a practical “good enough” approach, enabling separate analysis of different load processes in a standard way throughout the industry. Guidelines for more advanced analysis will also be included.

On a high level, the following work packages are proposed.

  • WP1: Design methodology short-term conditions
  • WP2: Design methodology for long-term environment including requirements for design basis and metocean specification
  • WP3: Case studies
  • WP4: Assessment of load effect analysis methodology
  • WP5: Guideline development

Benefits

By participating in the JIP, members can expect the following benefits:

  1. Clear understanding of governing parameters for simultaneous fatigue from different processes
  2. A framework covering the process from writing design basis and metocean specifications to recommendations for performing analyses considering simultaneous fatigue from different processes
  3. Possibility to influence the recommended design methodology for fatigue design
  4. Early-access to findings from the project.

Schedule

The current scope is estimated to be 24 months, depending on feedback from participants. Kick-off is scheduled for the 3rd quarter of 2024, pending the commitment of critical funding and securing partners.

Deliverables

The main deliverable is a guideline with findings from activities listed above. Technical reports from each activity will also be made. It is intended that a synthesis of the guideline will be made publicly available, whereas some results will only be made available to JIP participants. Learnings from all the studies conducted will be shared with the JIP participants. The results may lead to updates of the DNV design codes.

Participants

We aim for a balanced group of companies within the energy sector. DNV will be responsible for managing the project.

Fee structure

The cost, schedule and more detailed information will follow. The final scope and budget will depend on number of participants and on the outcome of the detailed scoping.

Contact us

Sindre Skrede

Sindre Skrede

Engineer

Pål Are Eikrem

Pål Are Eikrem

Principal Engineer Risers and Cables

Join the JIP!

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