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ELUGA HOCHTIEF Offshore aims to develop and operate offshore wind turbine installation vessels. These


vessels will be able to efficiently load, transport and install, as well as maintain and repair, wind turbines at locations offshore. The first vessel is expected to be operational in 2012. The self-propelled, heavy lift jack-up vessel with Dynamic Po- sitioning (DP) will be equipped with heavy lift crane gears. To provide a stable platform, the vessel is able to jack itself out of the water with four legs, each of which is 90 m in length. Resistance, propulsion, seakeeping and manoeuvring tests were performed to study the characteristics and to advise on possible design improvements. Thruster interaction tests were carried out which is of major importance because it determines station-keeping in wind, waves and current. Thruster efficiency losses, due to the nearby hull or wake of another thruster were in- vestigated, as well as any losses when the thruster wake is aimed towards a jack-up leg. Tests were performed in transit condi- tion and with the legs lowered to certain draughts below the vessels.


Current load tests Current loads on the jack-up legs were shown to be a significant part of the total current load on the vessel. BELUGA HOCHTIEF Offshore chose to use triangular lattice structure legs that have numerous benefits. However, it is difficult to obtain loads on such a complex structure because empirical models cannot be used because of the variety in diameters and angles. In addition, shielding or interaction effects between the leg members are diffi- cult to quantify. Flow around individual members and the interaction between them are dependent on the Reynolds number, which differs for each member diameter. Model testing is also challenging because the scale dependency of the results needs to be understood.


Large-scale model tests During the tests the vessel was equipped with four, detailed lattice structure legs. To determine the possible scale dependency and to gain


2010


MARIN’s news magazine for the maritime industry July 2010 no. 100


Start Renewable Energy team (RENT)


more insight into current loads on the leg itself, one leg was built at larger scale (1:15). The large-scale model is 4 m long and the spudcan has a width of almost 1 m. To investigate loads at different Reynolds numbers a large range of velocities was tested (0.2 m/s up to 6 m/s model scale). To measure the large range in forces a purpose-built measurement frame was constructed. Towing tests were performed at three different headings and at two differ- ent lengths of the leg. With this extensive test program the loads on the complete leg and on parts of the legs could be obtained. This is important to estimate the current forces during DP operations when the lengths of the legs change. Furthermore, scale dependency is determined and results provide reliable insight for the forces on the leg at full-scale. Results are also used for further validation of MARIN’s in-house CFD research program.


Complex CFD calculations CFD calcula- tions have been carried out at the same scale as the model tests. For the lattice structure a computational grid of 21 million cells is constructed to be able to capture the cylin- ders with the different diameters, the rough teeth on the main chords and the complete spudcan.


These large computational grids require large CPU times. This is now possible with MARIN’s High-Performance-Computing cluster consisting of 1,400 processor cores. A first impression of the CFD results illustrates the velocity in the wake of the jack-up leg. Verification, grid convergence studies and validation of the CFD results are in progress. Experience gained during this extensive test and CFD program opens up the possibility of offering CFD calculations in combination with model tests.


Velocity in the water around the lattice leg structure at two different depths 2010 Holtrop-Mennen


Ground-breaking method


Cooperative Research Navies continues code improvement and validation 20 years on Quality checking CFD CFD helps optimise yacht design process


Publication Report no. 100


report 23


VALIDATED MODELS


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