Both physical and mathematical models are used in the design of offshore structures and vessels to obtain data on the motions to be expected in various sea states. It is rare for any detailed measurements to be made of the motions subsequent to the commissioning of the vessel. The adequacy of original design assumption largely unknown and improvements which should come from these operational measurements are not made.
This paper describes the instrumentation of the 'uncle John' semi-submersible support vessel to measure the wave response in operational conditions. Full scale data of the vessel motions and the environment are presented for a variety of vessel and sea conditions. In addition, the paper provides equivalent numerical and model data to provide comparisons.
During the design process for offshore vessels physical and numerical models are used to predict the behaviour of the vessel. The operational behaviour of the vessel may well deviate from the predicted behaviour due to assumptions and simplifications originally made in the analytical processes used in design. For example, various non-linearities will have been ignored in analytical work, scale effects in model testing and shallow water or very steep waves may also have been excluded.
The objective of the work described in this paper was to undertake a series of full scale measurements of the wave response of a semi-submersible operating in the North Sea. These full scale measurements were then compared with those predicted from small scale physical model tests and also those predicted from a numerical model.
The vessel chosen for this study was the Houlder Offshore ?Uncle John? semi-submersible support vessel, see Figure 1. This vessel was ideal as British Maritime Technology possessed a data base of results having been responsible for both the numerical and physical model studies which took place during the vessel design. An earlier study indicated that there was reasonable agreement between the full scale and model test significant responses. The purpose of the present study was to examine more closely the agreement of the spectral and probability characteristics.
It was realised at the start of the project that it would be very difficult and expensive to repair or replace faulty equipment offshore. Hence non-redundant pieces of the measuring system installed on board were duplicated.
Eighteen measurement channel were recorded by the data logging system. These channels comprised:
3 environment channels:- wave height, wind speed and wind direction.
8 vessel motion channels:- sway, surge, heave, roll and pitch displacements, roll, pitch and yaw angular accelerations.
7 vessel dynamic position (dp) computer signals:- surge, sway and yaw position errors, surge and sway dp force, target heading and yaw moment.
A block diagram of the instrumentation system is given in Figure 2.