June 03 2019 0Comment

Forecasting jack-up barge motions

The first jack-up platform was built in 1950, and since then these barges have been used extensively in the offshore industry. Most often they were used as a stable drilling platform (MODU). At present jack-up platforms are involved in the construction and maintenance of offshore wind farms as well. The rules that are commonly applied on jacking operations of wind turbine installation jack-up vessels (WTIV) are the same as MODUs. However, the operational profile of a WTIV is different: they jack up and down regularly, whereas a MODU jacks only a few times per year. There is a need for better guidelines for this weather dependent operation.

An important limitation of the jacking operation is the impact of the footing (lower end of the legs) on the soil. The vessel moves in waves, which causes the footings to hit the soil with a higher velocity than in still water. Too large impact loads can lead to damage in the jacking system. As you can imagine, damaging the jacking system with the footings close to the seabed is an unwanted and dangerous situation. Therefore, we have to put a limit on the ship motions to prevent damage.

MO4 has developed a module that forecasts the motions of the jack-up. It is flexible: for every water depth, length of leg lowered and loading condition it can determine how the vessel is going to move in waves. This is quite complex, as the characteristics of the vessel change as the legs are being lowered. The lowering causes the roll and pitch period of the vessel to significantly change. It has a direct impact on the motions in waves. For the hydrodynamicsts among us: we again use a frequency domain model of the vessel. We add the hydrodynamic mass and Froude Krylov force to the model. The entire system is solved iteratively in order to properly take the drag of the legs into account.

Recently we have tested our jacking module in collaboration with Jack-Up Barge on the JB-115 barge. J-UB is continuously working on ways to ensure the integrity of their barges whilst maximizing weather windows for improved planning and workability. They find that innovation is a key aspect to deliver excellent customer service. J-UB’s openness to innovation has led to a partnership with MO4.

The JB115 is now working in the Deutsche Bucht. We were able to forecast and measure the motions during the transit and final move. The supplier of the weather forecasts–we always use 2D wave spectra–was BOC. We have been able to compare the weather forecast to buoy measurements, and the resemblance was very good.

Obtaining relevant information from the measurements is not straightforward. Characteristics change as the JB115 changes heading or lowers its legs. We have chosen to show the 1 minute significant motion and a smoother to find the actual significant motion. A significant motion is a similar metric as the significant wave height (approximately 1/3rd of the peaks are larger). MO4 forecasts the significant motions. The MRU measures accelerations and angular velocities, which we use for the comparison and decision support for jacking operations.

A figure with the comparison is shown below. The upper plot shows the heading of the vessel, the three lower plots respectively show roll and pitch velocities, and heave acceleration. The dots are measurements, and the straight line is MO4’s forecasts.

We can make a few observations:

  • There is a clear change in motions as the heading changes or the legs are lowered
  • The motion forecasts smoothly runs through the measurements (dots) and follows heading changes very well

This means our frequency domain model performs well. MO4 compares the forecasted motions with limits from engineering. This removes the need of a lot of simplifications, most notably on the sea state and loading condition, and is therefore more accurate. As engineering is by and large conservatively done, a higher accuracy leads to more workability.

We are very happy with this validation, of course it’s one of multiple studies. Don’t hesitate to send a message to mark@mo4.online if you’d like to learn more.