Lowering and lifting operations through moonpools: Hydrodynamic investigations

Abstract

On construction vessels subsea modules, diving bells, cables and so on are often lifted through a moonpool. The advantage of using a moonpool is that the surface elevation inside the moonpool is shielded against the outside waves. Further, the moonpool is often located close to the vessels center of rotation, which makes the lifting operation less sensitive to vertical motions due to roll and pitch. For some frequencies the water inside the moonpool may be excited in a resonant condition such as sloshing or so-called piston mode. In these cases the benefit of using a moonpool may be outweighed by the large resonant excitation. This thesis compares the hydrodynamic properties between a regular sized moonpool and a moonpool that is significantly larger. First, the results from a model test campaign for a specific offshore operation have been analyzed. The campaign was performed in irregular waves with a construction vessel moored in a stationary position with modules inside the moonpool. The results are indicating that modules are shielded from the incident waves, especially for the high frequent waves where the response is almost cancelled. There are some indications for a resonant piston mode occurring, but with a very low amplification. The largest amplifications are close to 1. Meaning that the wave height inside the moonpool is similar to the wave height of the incident wave.Finally, the vessel has been modeled and analyzed numerically using the potential theory based panel method. The software WAMIT has been used to calculate responses due to radiation and diffraction. The surface elevation indicates that the piston mode is very large for the standard size moonpool compared to the enlarged moonpool. However, when the moonpool is enlarged, a sloshing mode becomes more dominant. Comparing the experimental and the numerical method has proven to be challenging. The model test was performed for a very complex system with a lot of details that are hard to replicate; both from a modeling point of view and with regards to all the physical effects that should be accounted for.