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16 Jun 2023

Risk in extreme storms: destaffing to protect people and the environment during abnormal environmental events

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Understanding the loading applied to offshore jackets from extreme storms can be challenging, requiring specialist knowledge and expertise to deliver the work scope competently.

In this article, Bryan Horton, our expert in Offshore Structures, deep-dives into the importance of Severe Weather Action Plans (SWAP) to protect people and the environment.

The risk of structural collapse of offshore jacket structures due to severe weather events is a critical and often costly issue. Understanding the Life Safety, Environmental, and Business Risks that it implies is imperative.

The publication of the research results from the LOADS Joint Industry Project (LOADS JIP) in 2022 has provided a better understanding of the loading on the structure caused by abnormal environmental events and the understanding continues to evolve still. The probability of collapse of the offshore fixed structure, below a pre-defined threshold, as a result of those loads being applied has prompted many operators to adopt Severe Weather Action Plans (SWAP). The success of these action plans relies heavily on accurately establishing de-staffing thresholds based on forecast wave heights (Hs) and taking into account the uncertainties associated with different forecast horizons. This involves accurately predicting wave, wind, and current loads affecting the jacket and the wave and wind loads affecting the deck. Interpreting the results of analyses conducted using the LOADS methodology is a specialised engineering task, given that the largest waves do not necessarily give rise to the largest loads.

Why Severe Weather Action Plans are Important

At the heart of most severe weather action planning is the need to evacuate the offshore structure in the event of a predicted extreme weather event that brings unacceptable risk to Life Safety.

This is a no-brainer. Protecting people and the environment is paramount. However, de-staffing for ‘false alarms’ has a significant impact on production and profitability.

Ultimately there is a human responsible for making the decision on whether to de-staff or not based on predictions of the severity of an event that hasn’t happened yet. That’s why it's of utmost importance that these decision-makers are armed with the most accurate data possible to be able to make the right decisions at the right time.

The challenges

Defining a Severe Weather Action Plan is complex. Not only are the results of the contributing analyses difficult to interpret for those not directly involved in it, then translating the results into an actionable SWAP procedure with accurately defined de-staffing limits can be equally difficult.

To avoid confusion and the potential for misinterpretation, Kent has been assisting decision-makers by preparing SWAP summaries.

Not only do the specialist teams review the various analysis results contributing to the SWAP procedure, but they also undertake a critical review of the methods involved in delivering the work.

As an example of some of the findings made in a recent SWAP summary, Kent’s Structural and Technical Safety teams worked together to identify specific misinterpretations of the data that would have led to increased and unnecessary de-staffing levels than the analysis suggested were required.

When dealing with such a critical decision, how do you translate the risk threshold into Hs limits for different forecast horizons? (Accounting for forecast uncertainties)

Decision-makers have two options. Either define the probability of collapse as an annualised probability (Pa)or as a probability of collapse in a forecast storm with a significant wave height (Ps). The standard emerging within the industry is to assign a target Ps in the range 10-3 to 10-4 i.e. a target between one in one thousand and one in ten thousand chance of jacket collapse in a forecast storm with a given significant wave height.

Norsok N006 provides guidance on accounting for forecast uncertainty in setting de-staffing criteria, but only with forecast horizons of 1 – 3 days. So how do decision-makers define de-staffing limits beyond 3 days? With no definitive guidance provided in the present codes and standards, it's essential that clients turn to reliable partners who are able to elucidate these particular criteria. Kent has developed a recipe for forecasting beyond 3 days and we have successfully applied this to inform clients’ SWAPs. Forecast de-staffing limits are also often defined in discrete day increments (e.g. 0, 1, 3, 5, 7, 10 days etc.). Taken literally, this will cause the plot of de-staffing limits (significant wave height versus forecast horizon) to be stepped. Instead Kent recommends to our clients to interpolate the forecast de-staffing limits will smooth out the steps.

Although weather forecasting continues to improve, many of the standards used in the industry are based on older deterministic forecasting. Kent’s portal, AQIMA RISQ, can provide a more advanced ensemble forecast of up to 14 days and significantly reduce the occurrence of de-staffing and manage risk more effectively.

Although SWAP planning is imperative for an intact structure, it is also important to understand how inspections and maintenance programme performance impacts the SWAP. Establishing a link between inspections and maintenance plans and SWAP procedures ensures appropriate decisions can be made on the deferral or advancement of works.

Getting the best from the automation software

To complete the structural reliability analysis many pushover analysis cases need to be run. Using more simplified models to expedite these many runs instead of performing them using industry-standard software is tempting but can bring its own challenges. The response of dynamically sensitive structures can be challenging to model accurately, particularly when using simplified models. If the simplified model is not appropriately calibrated to a more detailed model, an overestimation of the dynamics may lead to an overestimation of the probability of platform collapse. In turn, this can significantly impact how onerous the SWAP destaffing limits are and has associated consequences on destaffing frequency, risk and cost.. Therefore, it is important to ensure that the simplified model is calibrated to a more detailed model to capture the dynamics of the structure accurately. At Kent, we use a hybrid approach where our proprietary structural analysis scripting tools are employed to batch-run pushover analyses en-masse in standard industry software. This is then supplemented by analysis in suitably calibrated simplified models.

Understanding the loading applied to offshore jackets from extreme storms can be challenging, requiring specialist knowledge and expertise to deliver the work scope competently. At Kent, we have a proven track record of delivering these specialist services for our clients, leveraging automation to reduce computational and labour expenses often associated with the work involved. We also help our clients identify previous misinterpretations of data and utilise SWAP summaries to help client decision-makers be most informed on the decisions they are making.

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