Extreme environmental loading of fixed offshore structures

HSE initiated a study in May 2018 to develop guidance on the assessment of structural failure from extreme environmental loading on fixed offshore platforms. Recent advances in both metocean statistics, and the physical understanding of extreme wave events, have demonstrated that commonly applied methods of assessment can be non-conservative and that wave impact on the deck needs to be accounted for.

The key objectives of the study were to:

establish a robust approach to the prediction and management of the structural integrity of fixed offshore installations exposed to the 10,000-year wave event, based on a review of state-of-the-art methods, including procedures for the management of structures which are unable to withstand the 10,000-year event
enable assessment of the risk associated with extreme loads acting on fixed offshore installations, for inclusion in the demonstration of whether the 'as low as reasonably practicable' (ALARP) requirement has been met, and identification of offshore installations with the highest risk of structural failure from extreme environmental events
enable demonstration and assurance that the risks to offshore installations from extreme environmental loading are being controlled and that suitable action is being undertaken to ensure the safety of the offshore workforce

The work was undertaken by the Offshore Consulting Group (OCG) and the scope included participation in the LOADS joint industry project, which is a leading industry effort to develop guidance on the 10,000-year event for incorporation into relevant offshore standards, such as the ISO 19900 series.

The scope of work entailed the following components:

review of methods for the prediction of the extreme environmental loads acting on fixed offshore installations
- limitations of and differences between current code (API, ISO and NORSOK) requirements which do not represent the state-of-the-art knowledge for wave-in-jacket and wave-in-deck loading
- procurement and analysis of metocean data by metocean specialists
- overview of metocean research and current knowledge
- derivation of 10,000-year crest heights, wave particle velocities profile
- application of metocean data: statistical analysis of hindcast metocean data and identification of areas for further development
- requirements for the combination of different extreme loading events to derive the overall target reliability level for use in safety cases
development of recommendations for a risk-based framework for assessing and managing the integrity of fixed offshore installations:
- survey of operators' structural data for all fixed platforms on the UKCS
- development of guidance on the evaluation and management of the risk of structural failure associated with extreme environmental loading, including procedures for the management of structures which are unable to withstand the 10,000-year event

Thirteen reports providing recommendations on the assessment of extreme environmental loading of fixed structures are available. These are:

Summaries of the two components of the study are presented in the reports Extreme Environmental Loading of Fixed Offshore Structures: Summary Report, Component 1 and Extreme Environmental Loading of Fixed Offshore Structures: Summary Report, Component 2. Recommendations on the methodology for the assessment of extreme loading based on the findings of the study are provided in the document Extreme Environmental Loading of Fixed Offshore Structures: Guidance .

The study made the following conclusions:

traditional methods for calculating loading, such as defining regular wave events using Forristall (1978) heights and/or second-order Forristall (2000) crests, are usually non-conservative in deep water but can be conservative in shallow water
the application of the silhouette method for calculating wave-in-deck loads is usually non-conservative
the application of irregular wave models provides a consistent estimate of the loading across the return periods, water depths and platforms that have been considered
the application of a deterministic irregular wave event that is a plunging breaker leads to a consistent and slightly conservative estimate of wave-in-jacket and wave-in-deck loads

The high-level findings from the study are being incorporated into ISO 19901-1 which is currently being revised.

A risk-ranking of all UKCS fixed platforms for the inundation of the topsides and safety critical equipment at a 10,000-year return period, based on a consistent estimate of extreme total water elevation (the sum of crest, surge and tide) and which included effects beyond second-order and wave breaking, indicated the following:

approximately 40% of platforms have some inundation of the topside at a 10,000-year return period based on point statistics
approximately 20% of platforms have more than two metres of inundation of the topside at a 10,000-year return period based on point statistics
approximately 15% of platforms have some inundation of the topside at a 1,000-year return period based on point statistics

The results indicate that there is a need to reassess fixed structures for extreme environmental loading using the procedure defined in the study, but it is also recognised that the application of the advanced techniques involved is not straightforward and requires both a further stage of development, including wider validation and calibration of the procedures, and training on their implementation.

To facilitate this, an offshore industry network on the environmental loading of offshore structures, the Metocean Network, was launched on 30 September 2020 with the support of the Energy Institute and it will convene at regular intervals. The network made a very strong start with a large number of participants from the UK, Europe, North America and Australia. The aims of the Metocean Network are to:

provide a technical forum/network for the subject of the environmental loading of offshore structures for all stakeholders, including regulators, operators and contractors
enable the exchange of industry experience and information
assist the development of industry practice on the environmental loading assessment of offshore structures
disseminate information through the organisation of industry seminars and conferences
publish relevant technical information and guidance for the safe operation of offshore structures

The creation of the Metocean Network will promote better understanding of control of the risk of structural failure from extreme wave loading through reliable structural assessment, and the implementation of suitable mitigation strategies, where appropriate, to ensure the safety of the offshore workforce. It is important to note that the network should enable the development of recommendations on the integrity management of both fixed and mobile offshore installations. The network is open to all stakeholders and it is in the interest of all to participate in it.

Contact

Stakeholders who have not yet joined the Metocean Network are encouraged to contact Dr Cameron Stewart of the Energy Institute to be added to the participants' list: e-mail: [email protected]

For further information on the Extreme Wave Loading Study and the Metocean Network, please contact Dr Alexander Stacey: e-mail: [email protected]