Improvement of onboard wave height measurement

To prepare and conduct shore operations in safe environment, it is important to possess an accurate sea state forecast. In this thesis it is assessed whether it is possible to estimate the real time sea state at a vessel, without using any instruments deployed in the water. As a matter of fact, in-situ measurements are not always possible, for example during pipeline installation, the vessel sails a few kilometres a day, therefore the options are restricted to methods which do not include in-situ instruments. To test and validate the methods developed in this thesis real time measurements were performed on the Audacia, a pipeline installation vessel belonging to Allseas, one of the world leaders in pipeline installation. The Audacia was laying pipe nearby the coast of Australia, close to wave buoy in the Julimar Field. Methods to extract wave field information from conventional nautical radars have been developed. It is possible to estimate a directional wave spectrum by using a temporal sequence of radar images, and following, statistical data representative of the sea state, such as the significant wave height and wave direction. However there is no unique relation between the radar images and the amount of energy in the directional wave spectrum, which result in an inaccurate signififcant wave height. In order to improve the directional wave spectrum estimations given by the X-Band radar used on board of the Audacia (Wavex from Miros), the possibility of using an extra measuring instrument to calibrate the energy is investigated. Two measuring devices are considered. - A down looking radar, recently installed below the helideck of the Audacia, measuring the single point surface elevation in vicinity of the vessel. (Radac WaveGuide) - The MRU sensors, which measure the wave induced vessel motions. (Ixsea Octans) It is investigated whether combining the data measured by one or the other device to the Xband radar data result in an improvement of the radar estimated directional wave spectrum, and which combination gives the best results. Method 1: Use the down looking radar measurements. The one dimensional wave spectrum given by Radac is combined with the directional wave spectrum given by Wavex. Since the Radac measurement are performed close to the vessel, the water surface is disturbed. In order to use the Radac to improve the estimated wave spectrum these disturbance have to be taken into account. It is possible to determine a frequency dependent ratio between the amount of energy in each frequency band of the different instruments. This frequency dependent ratio is used to correct the estimated amount of energy at each frequency of the directional wave spectrum. Method 2: Use the vessel motion measurements. In this method the measured wave induced vessel motions measured by the MRU are combined with the directional wave spectrum given by Wavex. To be able to calibrate the amount of energy in the directional wave spectrum, the radar wave spectrum has to be converted into vessel motions spectra.1 Since the responses are assumed to have a linear relation with the incident wave, the ratio between the measured energy and the expected energy in the responses can be used to correct the energy estimation in the directional wave spectrum in a linear way.After correcting the estimated directional wave spectrum with the two methods described above, the significant wave height was calculated and compared to the significant wave height given by the wave buoy in the Julimar Field.The significant wave height based on method one has a strong correlation to the significant wave height measured by the wave buoy in the Julimar Field. Method two showed that the significant wave height obtained by combining the wave field information from the nautical radar with the measured vessel motions has a less strong correlation to the significant wave height measured by the wave buoy. However, the wave spectrum given by the X-Band radar can be significantly improved in the frequency band in which the vessel responds to incident waves.floating structuresoffshore and dredging engineeringMechanical, Maritime and Materials Engineerin