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

Paper Number: 130327

130327 - Development of Calibrated Deterministic Design Approach for Design and Assessment of Offshore Structures Exposed to Environmental Loading 

Historically, design formats for offshore structures are relying on notional safety, often experience-based in nature and influenced by heritage design methods, safety factors, wave load recipes, etc.

Today, none of the commonly used structural design codes is built around an official, available to the user, calibrated safety format. One argument for the situation being acceptable is that an approximate (notional) safety level is present, as e.g. a 10,000-year return period “accidental” regular wave event with unity safety factors will ensure safety levels at approx. 10^-4 pr. annum.

Research in recent years has proved that extreme waves, even in deep water, in many regions of the world may potentially be breaking waves. Breaking waves are transient loading events with high kinematics above mean sea level, higher than regular wave theory can predict. A design format anchored in use of regular wave theory cannot represent the dynamic and high velocity nature in a breaking wave event. The existence of breaking extreme waves calls for an updated design philosophy, as load statistics based on regular waves, normalized mudline statistics (“global” loads), etc. will not be representative for failure mechanisms which are dominated by response driven by extreme breaking wave events.

A novel Monte Carlo based SRA model including transient breaking waves is used to develop and calibrate a new deterministic approach with safety factors linked to target failure probabilities.

Across a range of generic structures (space frame jackets, tripods/mono-towers), each with a range of failure mechanisms over the height of the structure, a methodology delivering a controlled and uniform failure probability has been developed. The method applies to both jacket structures and local elements (appurtenances) exposed to environmental loading anywhere in the water column.

The challenge developing a design format accounting for loading from irregular waves, is that different wave types may drive design loads for different structures/failure mechanisms. A concept based on a single wave type will be conservative for some failure mechanisms/structures, and accurate for others. An accurate approach shall for each failure mechanism/structure being designed/assessed identify the design loads across all relevant wave types. The site relevant wave types are represented by focused non-linear design waves. Waves are generated feeding linear “Newwave” to a non-linear solver. For each directional sector, a series of equally probable site-specific Newwave’s with variable wave period and wave height is developed. The non-linear solver will turn these linear events into a range of non-linear events. Typically the shallow (long period) Newwave’s will deliver non-breaking focused wave events. Increasing the Newwave steepness, the non-linear events will transit into spilling breakers and ultimate plunging breakers. Importantly, the design wave types are anchored in the site-specific long-term statistics and water depth, etc.

The key design action is to expose each potential failure mechanism to the range of developed design events. With design events being 3D focused waves, the wave position giving highest response is to be identified. Having completed the design process, each failure mechanism must be designed to resist the load effect generated by the most critical wave event. Some failure mechanisms will be designed by one wave type/position and other failure mechanisms by others. This approach has turned out very robust across structures and failure mechanisms. The approach has been validated successfully even to deliver design loads for local elements at high elevations (e.g. riser spools etc.), which are notoriously known to be problematic to control.

The design format has been calibrated across a range of target annual failure probabilities, allowing the designer to look-up safety factors dependent upon the acceptable annual failure probability. The methodology further includes a simple correction for actual system effects, i.e. properly account for the statistical effect of potentially several competing failure mechanisms.

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