Session: 02-07-01 Reliability of Marine Structures and Components

Paper Number: 130330

130330 - Reliability of Offshore Structures – Requirements to and Limitations of Calibrated Deterministic Code Design and Assessment Approaches 

With the availability of a new detailed, calibrated, and validated SRA model (presented in an accompanying OMAE 24 paper) an opportunity has emerged to (re-)evaluate a number of the assumptions underlying current design recipes for offshore structures exposed to extreme environmental loading.

The new Monte Carlo based SRA model includes direct time domain simulation of wind and waves. The model can, utilizing a cloud-based set-up, analyze storm loading for long periods of time, typically in the order of a million years to allow convergent results for 10,000-year return period maximum loads. The stochastic semi-empirical sea state model embedded in the SRA is calibrated against a 2-year wave basin test program to ensure accurate predictions of extreme wave type, crest height, transient shape development and kinematics. The model includes spilling and plunging breaking waves. In parallel with the sea state analyses, a time domain wind model capable of predicting the fluctuating partly correlated wind pressure around box-shaped geometries (topside structures) can deliver time domain wind pressures acting on above water structures. The model accounts for interaction between waves and wind, taking account of large waves will locally increase near surface wind speed.

Using a generic analysis set-up, a range of historic notional models and assumptions are evaluated against the new SRA model. The work includes evaluation of:

·         A typical 100-year return period Ultimate Limit State case using regular wave theory and codified RSR values. The performance is compared across a matrix of structures and failure mechanisms (different failure mechanism “location” within the water column)

·         As above but with a typical 10,000-year return period Accidental Limit State and no safety factors (unity)

·         Efthymiou 97 FORM based SRA model (JOMAE 97 – the model is previously referenced by ISO 19902 in the 2007 edition of the code). Evaluation of overall performance across a matrix of structures and failure mechanisms. In addition, comparison of individual parameter choices and assumptions

·         LOADS JIP – SRA using fragility curves. Comparison of overall results across a range (matrix) of structures and failure mechanisms. Furthermore, examination and comparison of individual components (values and assumptions) in the method

The evaluation includes a review and discussion of ISO 2394 basic requirements to SRA. Each of the evaluated approaches will be discussed in the light of ISO 2394 requirements.

Finally, the findings of the review are cross referenced against the Gulf of Mexico operational record of hurricane platform collapse, to evaluate if actual operational data align with the review results.

Presenting Author: Jesper Tychsen TotalEnergies, TEPDK

Presenting Author Biography: Jesper Tychsen is a structural engineer working for TotalEnergies Denmark, a leading energy company that provides low-carbon electricity and natural gas to millions of customers. He has been working with integrity of existing Danish offshore structures and design of new structures since the early 1990s. He has extensive experience in assessing the structural performance and safety of offshore platforms pipelines, and other marine structures under various environmental conditions. In the last decade, he has dedicated his work to evaluate structural integrity considering new knowledge of environmental loads, such as breaking waves, that can affect the reliability and lifespan of offshore structures. He has published papers on this topic, and has contributed to the development of new standards and guidelines for offshore structural design. Jesper Tychsen is a passionate engineer who enjoys solving complex problems and finding improved solutions for the energy sector.

Authors:

Jesper Tychsen TotalEnergies, TEPDK
Allan Zeeberg TotalEnergies, TEPDK
Jørgen S. Nielsen TotalEnergies, TEPDK
Jean-Marc Cholley TotalEnergies, HQ