A series of model tests was carried out on a 1 40 scale model of a gravity base structure (GBS) The model was instrumented to measure global horizontal loads and overturning moments about its base, and local pressures and water elevations at a number of locations on the model structure The model was subjected to a series of regular and irregular waves Groups of high waves were also selected from the irregular spectra and run in isolation The objective was to compare the structure's response to this group to that from the same group found in the full length spectrum This is referred to as the snapshot technique Generation of model waves is complicated by wave reflections from the test facility boundaries The snapshot technique was found to be useful in determining the amount of reflection buildup during an Irregular run In regular waves a technique was developed to minimize these effects Providing quantitative data from such test programs is particularly complicated by the non-linear nature of the wave loading mechanisms such as steep and breaklng waves, slamming and wave impact The paper focuses primarily on techniques of data acquisition and analysis
The gravity base structure (GBS) is a large heavy structure which rests on the ocean floor and remains stable under its own weight with the topsides facility supported some safe distance above the sea level Such structures are generally constructed of concrete and have greater capital costs than other fixed bottom structures such as the jacket type However, the GBS presents an alternative design for more hostile environmental conditions such as Ice infested waters where local oil storage is required but cannot be accommodated by a jacket structure
Mobi1 Oil Canada proposed to develop the Hibernia Field off the coast of Canada using a gravity base structure A preliminary design of the structure proposed by Mobil Canada In 1985, Illustrated In Figure 1, consists of a concrete caisson extending from the ocean floor to a distance of 5 meters above the water surface This large caisson supports four columns which In turn support the topside facilities
The Ocean Engineering Laboratory of the Institute for Marine Dynamics (IMD) carried out a series of tests on this GBS model [I] The primary objective of the test program described In this document was to investigate and evaluate techniques associated with model testing of gravity base structures for purposes of engineering design The test program addressed aspects of GBS model testing such as
the quality of waves being generated in the test facility,
deslgn procedures for the model and instrumentation,
reduction and analysis of the test data
The effects of the test facility's boundaries on the model waves are a concern because of the physical size of the model During a test run waves are diffracted and reflected from the model Although these waves are genuine In the development of the diffraction process, they impinge on the wavemakers and other facility boundaries and are reflected back to the model itself, thus affecting the measured loads
