This previous article presented a case study for a gap-bridging monitoring campaign to optimize workability for elevated operations of a jack-up. The concept of gap-bridging monitoring was developed to bridge the gap between the on-board engineering reality and the desktop engineering reality.
This article focuses on the gap between the on-board reality and the desktop reality when it comes to workability and decision making. Workability is typically expressed in terms of allowable sea state parameters, such as restrictions upon allowable wave height for a specific wave period. Wave height, wave period and wave direction are examples of sea state parameters. On-board decision making is typically done by comparing the forecasted sea state parameters to the allowable sea state parameters.
It will be demonstrated that these sea state parameters, when used in the desktop reality, have a very different meaning from how they are used on board. Workability, expressed in allowable sea state parameters, is determined in the desktop engineering reality and is applied in the on-board engineering reality. This article is written to explain this gap, to enable engineers to mind the gap. A follow-up article will present an alternative method of defining workability, to bridge the gap.
The case study from this previous article is elaborated upon. A large four-legged jack-up is performing wind turbine installation operations. This involves two or three rig moves every week.
The rig move is a weather-restricted operation. Going-on-location, where the jack-up goes from free-floating condition (as a vessel) to elevated condition (as a bottom-founded structure), is typically the most onerous stage of the operation and can have a significant impact on the duration of the offshore campaign, especially in more challenging seasons.
For this case study, we focus on the part of the going-on-location operation where the jack-up is in soft-pinned condition. 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 maximum allowable bending moment in the legs. An environment is workable if the expected bending moments are below this limit.
Introducing sea state restrictions
Most offshore operations are weather-restricted, which means that the operation may be performed if the waves and weather are below a certain threshold or restriction. On-board decision making is done by considering environmental parameters from a forecast, such as wind speed and wave height, and comparing these to allowable parameters. These allowable parameters, or weather restrictions, are determined through desktop engineering studies.
The restriction on the sea state is most important. A sea state is the combination of all waves that are present at the location. The restriction is typically expressed as a maximum allowable wave height, which depends on the wave period and, possibly, on the wave direction.
This sea state restriction should be determined specifically for the site/area where the jack-up goes on location, as part of the site-specific assessment for installation (SSA-I). This restriction is determined using a site-specific model of the jack-up, taking into account water depth, the loading condition of the vessel, and the soil type that makes up the seabed. However, this restriction is in most cases copied from the design conditions provided in the operating manual of the jack-up, which are based upon a conservative generalized model.
The simulation model is used, in the desktop engineering reality, to perform a big batch of simulations for a range of sea state parameters. Typically, significant wave height, zero-crossing period and wave direction are variably applied. Using sea state parameters to simulate the offshore situation seems great, because thinking about a sea state in terms of these parameters is done on board as well. The on-board engineering reality seems to be applied in the desktop engineering reality through the use of the very same sea state parameters.
Sea state parameters used on board and at the desktop
The term significant wave height (Hs) has many origins. One of those is in human nature. Ask ten people to estimate the wave height of a wind sea just by watching the water. The average will tend to be close to the engineering definitions of Hs. One engineering definition is the average of the highest one third of the observed waves, which can be measured. The most used engineering definition is a measure of the total amount of energy held in a sea state.
Zero-crossing period (Tz) is the period of the combined waves. Wave direction is the main direction from which the waves come. Both Tz and direction have several engineering definitions which are not elaborated upon here.
These sea state parameters are good coffee-machine / smoking room discussion material – at least, for some of us – both at the office and on board. However, the gap between the on-board reality and the desktop reality on this topic is significant and often overlooked.
When discussing wave parameters on board, these are based upon the sea state as a whole, in all its complexity. Terms like significant wave height are a handle to grasp and think about a more intricate reality. Typically, wave heights are assigned to several components of the sea state, or wave systems, each with a different period and direction. Locally generated wind sea and incoming swell components are distinguished. Used on board, the wave parameters are a boiled-down version of the current or forecasted reality.
Contrarily, in the desktop engineering reality, the wave parameters are a starting point, a premise. One cannot apply a wave parameter to a simulation model. Only a sea state can be applied to a simulation model. Wave parameters therefore have to be converted to a sea state, where wave energy is assigned directionality and period. To do so, major assumptions are made as will be shown in the following paragraph.
The world upside-down: Creating a sea state based upon parameters
In the most simple engineering studies, a regular, long-crested wave is used. This is a simple assumption, but typically a bad representation of reality. There is no variability in crest height and period. In engineering terms, all wave energy is applied at a single period and in a single direction. For this example, a wave height of 2 m is applied at a direction of 275 degrees and a period of 6 s.
This can be visualized in a polar plot. The direction is shown by the azimuth angle and the period is shown by the radius, from 3 to 15 seconds. Both the direction and period are considered from the center looking radially outward. This simple regular wave case is thus represented by a red dot on this otherwise blue polar plot, which is called a 2D wave spectrum.
Every wave, or wave component, has a period and a direction. It can therefore always be shown in a 2D wave spectrum. The color indicates the wave energy density, which is a measure for wave height.
When this sea state is applied to the simulation model of the jack-up in soft-pinned condition, it is found that structural strength is exceeded and therefore this situation is not workable. The simulation model was found to resonate with wave loading from this direction and period.
In somewhat more advanced engineering studies, the wave energy is distributed over a range of periods. A wave energy distribution is called a spectrum. In this case, the JONSWAP spectrum is used. The spectrum has a zero-crossing period of 6 seconds and is applied along a 275 degree direction. This is a somewhat better representation of reality, but a more complex assumption. The wave period and crest height are variable, leading to an irregular, but long crested sea state. This case is represented by a radial line in the 2D spectrum, the width of the line being a reflection of the resolution of the grid, not of directional spreading.
The simulation model resonates to this sea state and, as for the regular wave, is not workable.
At a next level of complexity, the assumed spectrum may have directional spreading, typically applied as a cosine to the power n, where n is an even number. Now the waves are short-crested in addition to being irregular. The line on the polar plot has a wider thickness.
The simulation model indicates that this sea state, with the wave energy now distributed over period and direction, is almost workable. The allowable significant wave height for this direction and period is 1.8 m.
It can be observed from the above that the road to a better representation of a generalized reality is paved with assumptions. It only leads to sea states that look somewhat realistic and prevent typical simulation errors such as exaggerated resonance, but in no way are a proper representation of any actual sea state. Actual sea states, as known from the forecast, are typically multi-modal. This means that there are several components including locally generated wind sea and incoming swell.
A real-world sea state
Let’s now consider an example of a real-world sea state, presented as a 2D wave spectrum. This one is obtained from the actual forecast/hindcast model that is used to generate the daily wave forecast as used on board. Note that the sea state parameters as presented in the daily forecast are based upon detailed forecasted 2D wave spectra – a boiled down version of reality.
The wave direction is 275 deg and zero-crossing period is 6 s, just like for the assumed sea states above. The total wave energy, expressed as significant wave height is much higher than in the assumed sea states, at 2.85 m. Clearly, based upon an engineering study that uses assumed sea states, this real-world sea state would not be workable. The significant wave height is higher than allowable.
It can be seen that part of the real-world sea state is a scattered swell, coming from 345 deg around a period of 11 s. The simulation model does not respond strongly to that. The locally generated wind sea part, around a period of 6 s is distributed over a wide direction ranging from 180 degrees to 275 degrees.
This is very different from the assumed sea states above. The wave energy is not concentrated closely around the peak period and main direction, but much more scattered.
This real-world sea state is applied to the simulation model. It is found that this sea state, with a significant wave height of 2.85 m, is in fact workable.
Applying the engineering study on board
The on-board decision making is done by comparing forecasted wave parameters (a boiled down version of a real sea state) to allowable wave parameters (a premise to an assumed sea state).
It is shown that directly applying a real-world sea state to the simulation model is a more objective method for determining the workability. This approach will be elaborated upon in the following article. Calypso implements this approach in workability studies for jack-up operations. The MO4 system makes a similar approach available for decision making directly on board.
This article argues that workability expressed as allowable sea state parameters is a mere indication, a guideline. Objectivity is lost when a go/no-go decision is made merely by comparing the forecasted significant wave height to the allowable significant wave height, at centimeter accuracy. To say the least, workability expressed as allowable sea state parameters is no more objective or accurate than the subjective judgement of an experienced captain.