Going-on-Location and transit

A Going-on-Location (GOL) analysis – part of a Site-Specific Assessment for Installation (SSA-I) – is performed to determine allowable wave conditions for installation of a jack-up at a specific or generalized site. The GOL report specifies the allowable conditions for installation and is directly usable on-board in combination with the environmental forecast.

A hydrodynamic and structural model of the jack-up is used in combination with a state-of-the-art soil interaction model to simulate the operation. Any type of soil can be modeled, including the most challenging rocky seabed. The simulation framework is presented in the 2023 white paper.

Going-on-location is the transition of a jack-up from being a free-floating vessel to a bottom-founded structure. The impact phase, which lasts up to a few minutes, is typically the most onerous part of the operation. The motions, and the resulting impact loads, are highly dependent upon wave direction and period. For swell coming from beam seas, a small wave height may lead to high observed impacts. For choppy wind sea coming from head seas, a high wave height may be acceptable.

To support decision-making, our reports come with detailed requirements onto the weather forecast as well as a Nereon Compass to apply these into a straightforward decision-making process.

Rack Phase Difference (RPD) is an indication of bending moment in the leg. It is typically observed when jacking the hull out of the water and indicates leg splay or sloped seabed. It may also be observed when jacking the hull down after elevated operations. In this case, a cause for RPD has risen during the elevated operations, e.g. due to sliding or scouring.

We support jack-ups that observe significant RPD on one or more of their legs. This support is typically on-board, but could be performed from the office as well. Firstly, the current situation is assessed. Sometimes, it can be shown that the current situation is within limits and will not aggravate. Typically, a pathway of discrete steps is determined towards a safe situation. Steps may include ballasting, offloading and differential jacking. On-board guidance includes direct support following and calibrating the steps.

Jack-ups, when floating, are sensitive to long swells and beam seas. There is no such thing as ‘THE allowable significant wave height’; the allowable sea state depends upon wave period and direction. This outcome of the analysis is presented as a table and as a graph. In addition, to make this insight accessible for use in the on-board decision-making process, it is also presented onto two compass discs on a Nereon Compass.

The Nereon enables direct application of the wind sea and swell parameters from the weather forecast to visually determine workability.

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A jack-up in transit, e.g. for a field move, is a free-floating vessel. Stability and motions deserve due attention. Intact stability should at the least comply with IMO Intact Stability (2008). This is to be assured taking into account the deck layout and ballast plan.

Jack-ups may have appurtenances or extending deck loads for which wave impact is to be prevented. A motion analysis shows the relative distance between the waves and any appurtenance. An allowable sea state is determined to preclude wave impact on critical components.

Jack-up transit is typically, but not necessarily, performed with legs up. The most critical structural load, for both strength and fatigue, occurs in the interaction between the upper guide and the leg structure. Analysis can be used to determine the impact of any move on fatigue life or to determine the allowable sea state for the move