This paper presents experimental techniques and resultant data from an ongoing programmed of large scale laboratory tests on marine risers in waves. The experiments are being carried out in the No. 3 tank of the National Maritime Institute in England which has a water depth of 25 ft (7.6 m). The range of models used represent full size risers of between 500-2000 ft (150-600 m) supported by semi submersible or tension leg platforms. The measurements made include the wave elevation, surface platform motions, riser top and bottom end tensions and a large number of displacements and bending stresses along the riser length in long crested regular and irregular waves. Both in line (to the waves) and transverse riser displacements and bending stresses are presented. Measured data for a scaled riser from (500 ft) 152 m water depth are presented and compared with two dimensional riser dynamics computations based on finite element techniques and on modal analysis. The measured data reveal a significant level of transverse riser dynamic displacements and bending stresses. These are shown to be consistent with the effects that would be produced by fluid loading due to vortex shedding along the riser length.
The almost total absence of reliable prototype or experimental data on riser motions and stresses in waves is generally acknowledged to be a serious drawback for the credibility of the large number of marine riser computation methods that are currently available. There is a large body of small scale test data for the fluid loading on riser cross-sections (circular or otherwise) in planar oscillatory flow and small wave tanks. (References 1, 2, 3 for example). Such data can be criticized from the point of view of not simulating the full scale Reynolds number correctly or that the experiment does not adequately model the riser section in motion within fluid which is itself in motion due to wave action. In general, small scale tests make it difficult to simulate a long riser adequately with the correct "boundary conditions" at the surface vessel, sea bed and along the riser length. At the other extreme, there are some current proposals to acquire full scale data (4, 5) on risers operating in the ocean environment. Although, the acquisition of full scale data has the potential to provide the ultimate validation tool for riser analysis methods, there are a number of uncertainties regarding the manner in which the full scale raw data can be converted into a form suitable for comparison with riser analysis. The uncertainties are associated with the difficulties of acquiring sufficient current and wave data in a multi-directional sea to adequately describe the incident wave process. Even if sufficient data have been obtained for this to be done, the lack of reliable mathematical models to describe the consequent fluid loading on the riser and to interpret its motions makes it difficult to use the data in verifying or improving the techniques of marine riser analysis. England (6) gives some indication of these difficulties for full scale data.