7 tips for a successful offshore operation

This article is the final in a series of four articles about workability and monitoring of offshore operations. It showcases a full monitoring campaign together with live determination of workability.

The series follows the arrangement of Wagner’s ring cycle. The first article sets the stage in a lighthearted manner, whereas the second and third present the two major players. In this fourth article, all will come together in a splendid climax.

The first article provides a somewhat tongue-in-cheek introduction to monitoring campaigns and the things that typically go wrong. The second article introduces gap-bridging monitoring. According to this concept, the purpose of a monitoring campaign must be to bridge the gap between the on-board and the desktop engineering reality. The third articlepresents the workability of offshore operations. Workability expressed in terms of sea state parameters, such as allowable significant wave height, comes with limitations (no pun intended). It was argued that the wave height experienced offshore is very much different from the wave height used in desktop engineering studies.

Tip 01 – it is not the waves that restrict workability, it is the physical limitations to the offshore spread

The elephant in the room of the third article is that wave height or sea state is seldom a direct limitation on the offshore operation. Instead, workability is primarily limited by real, physical restrictions. For instance, the stresses in the steel should not exceed the yield stress. Or, no green water is allowed on deck. Or, vessel accelerations should be limited for crew comfort level. A great many restrictions may apply to an offshore operation.

This article is about these real physical limitations of offshore operations and how they can be directly applied to determine workability objectively.

Unfortunately, for most offshore operations, there is no such thing available as a yield stress forecast, or a green water forecast, or an acceleration forecast. These operations will have to make do with the only forecast available: the weather, current and wave forecast. The wave forecast is typically expressed in terms of sea state parameters, such as wave height and period. Therefore, engineers venture to connect the physical restrictions, such as vessel accelerations, to sea state parameters through simulation models. The drawbacks and limitations of this approach were the topic of the third article.

Flow diagram for offshore decision-making based upon allowable wave parameters
Example of allowable wave height for going-on-location of a jack-up
Tip 02 – Ask for a workability forecast, not a wave forecast

We can actually create forecasts of the restricted items, such as a structural stress forecast or an acceleration forecast. Doing so eliminates the need for expressing workability in terms of sea state parameters. These forecasts can be used to objectively determine workability based upon the real, physical restrictions as opposed to the engineering construct of allowable sea state parameters.

We revisit the case study of the soft-pinned phase of a jack-up going on location. In this condition, the hull is subjected to hydrodynamic actions while the motions are restricted by the legs and seabed. The governing physical criterion for workability in this condition is the stress in the part of the legs that interfaces with the hull.

Simulation models are used in engineering studies to determine workability. The model includes:

  • Hydrodynamic actions (wave force, dependent upon incoming wave energy per direction and period)
  • Hydrodynamic reactions (added mass and radiation damping)
  • Structural response (stiffness)
  • Mass

The simulation model is governed by the equation of motions, which ensures dynamic equilibrium.

Weight, buoyancy and environmental actions are imposed upon the model. The simulation model is used to find the responses of interest. In this case, the responses of interest are the bending moment and axial load in the leg. These, together with the cross-section of the leg, yield the actual stresses in the steel.

Model of a jack-up in soft-pinned condition

Most models used in our industry are either linear or linearizable. This means that a factor, dependent upon wave direction and period, can be determined to relate incoming wave energy to the required output, such as stress or accelerations. This factor is called a transfer function. A transfer function should be seen as a dense form of the model.

The transfer function is dependent upon wave direction and frequency. It has the same dimensions as the 2D wave spectra, which were introduced in depth in the third article. It can thus be readily applied to the 2D wave spectra. This way, a forecast of any physical parameter, such as stress and acceleration, can be performed if forecasted 2D wave spectra are available. But are they?

Tip 03 – Any wave forecast is based on 2D wave spectra. Ask your forecast provider for this additional data for free!

Most of us are (all too) familiar with environmental forecasts used offshore. Long lists of numbers and graphs, providing upcoming wave, current and weather for coming days. These forecasts are based on intricate models of the sea and air. Similar to a FEM model, the sea and air are discretized into small areas. Within each area, conditions are uniformly defined. To keep track of the waves, which move from one area to the next, the wave energy for every direction and period is to be kept. Oceanographers, apart from brilliant scientists, are also great bookkeepers.

People cannot think in terms of large arrays of data. Therefore, wave data is condensed to sea state parameters, such as significant wave height and period. Typically, multiple wave systems such as local wind sea and swell are identified and assigned individual parameters. Even though only wave parameters are presented in the environmental forecast, that does not mean that the underlying data is not available. Since all forecasted wave parameters are based upon a 2D wave spectrum, the 2D wave spectrum is readily available to anyone who is paying for a forecast.

A polar plot presenting the 2D wave spectrum of a real-world sea state
Tip 04 – Ask your local engineering firm for a workability forecast

With the condensed model and the forecasted 2D wave spectra available, creating a workability forecast is straightforward. All physically limiting items, such as steel stress, accelerations, freeboard, etc. can be forecasted and compared to the allowable values, the restriction. If all requirements are met, the sea state is workable.

The forecast can be performed on-shore, in real-time as the wave forecast comes it. It is presented similarly to the wave forecast and shared through e-mail at the same frequency as the weather forecast.

Calypso offers this service for jack-up operations. They create the required models, establish applicable limitations and perform the forecast.

The on-board system of MO4  takes this to the next level. It enables the user to provide loading conditions and heading. It performs the forecast of restrictions live on board, for which Calypso can supply the required applicable limitations and condensed jack-up model.

Flow diagram for offshore decision-making based upon forecast of applicable restrictions
Tip 05 – Check your forecast – measure that which is easiest to measure

The behavior of an offshore operation can be measured through many means. Measurements from strain gauges and load cells can often be revealing for single events, such as a leg impact of a jack-up going on location. However, these are generally less suitable for analysis of longer operations. Waves can be measured directly or indirectly by radar. The most advanced systems provide a measurement that can be used to verify and calibrate the forecasted 2D wave spectra.

The one measurement that is suitable for most types of operations, is most cost-effective, is easiest to apply and most straightforward to analyze is… the motion response measurement.

A Motion Reference Unit (MRU) can be used for measuring quickly varying motions in six degrees of freedom of the offshore unit and the load. MRUs measure accelerations in translation and velocities in rotation. These can be converted to position by integration over time, minding drift. Since gravity is an acceleration, MRUs know where is up and where is down.

It has been argued through this series of articles that every offshore operation should be equipped with at least one MRU. Dozens of MRUs are available on board, right in the pockets of crew members. Every smartphone has an MRU and to many it is somewhat preposterous that not every object offshore is equipped with one.

Calypso, and many others, provide stand-alone MRUs with storage capacity and an internet connection for communicating measurements. The price for most projects is less than $100,- per day.

Tip 06 – Measure only that which you can also model or simulate

This one should go without saying but unfortunately tends to go wrong often. The purpose of a monitoring campaign is to bridge the gap between the on-board reality, which to some extent can be measured, and the desktop engineering reality. In this fourth article, we will finally have to define this desktop reality. Many people think of coffee machines and pocket protectors when hearing this term. 

What people on board think the desktop engineering reality looks like

However, what is meant here by the desktop engineering reality, is the modeled version of the on-board reality. Desktop reality is defined by the models it uses. It is the digital twin of the on-board reality if you will. To bridge the gap means to improve the models. Therefore, only those things that are an outcome of the model, such as accelerations, should be measured. Such measurements can be used to improve the models.

Conversely – before even thinking of a monitoring campaign, a model of the offshore operation should be available. Trying to increase workability, for instance of the going-on-location of a jack-up, through measurements only, cannot be done. A gap-bridging monitoring campaign starts by questioning the uncertainties in the model, as was detailed in the second article.

What the desktop engineering reality really looks like (barstool model of a jack-up)
Tip 07 – Get buy-in for your workability or monitoring project

It has been established at several places in this series of articles that a gap-bridging monitoring campaign is non-invasive and cost-effective. It is also shown that often there is a significant collateral benefit – a monitoring campaign typically includes a four-times-a-day workability forecast. Yet, sometimes it can be difficult to get buy-in from all stakeholders.

Think of what is most valuable to your company. Do you wish to have an optimal, objectively established workability forecast? Or do you wish to ensure optimal workability while increasing operational safety? Let your engineering service provider know and they will be happy to provide you with a proposal with the right focus.

At Calypso, we are always ready to support you in bridging the gap between on-board and desktop reality. We do so through monitoring campaigns, workability forecasts and, most importantly, by always keeping the end-user on-board in mind when writing our reports, such as site-specific assessments (SSA) and leg penetration analysis (LPA). Reach out to us to elevate your jack-up operations.