executed model test of a governing load identified by a numerical model can be used to verify the predictions. We do not expect the model tests to give exactly the same results as a numerical test but we do expect the same overall picture. If not, we either have perhaps an error somewhere or we have identified a complex physical behaviour whose importance was not fully appreciated from the numerical time domain simulation model.”

As CFD calculations are applied to more and more hydrodynamic design problems, what role does HESS expect from CFD? CFD is playing an increasingly important role in hydrodynamic design and this has been facilitated by the ever-increasing availability of cost effective computer power, he says. “There are parallels here with structural design where we have first seen the decade of linear FEA and then the decade of non-linear FEA and this is all due to increased computing power coupled with vast improvements in the human interface with computers in terms of data visualisation and handling.”

Equus semi

system in combined environments of wind, waves and currents so there is further insight into the overall system response, including the underlying structure-wind- wave-current interactions.”

Greater confidence HESS also looks at the tests in the Offshore Basin to ensure that it has not missed or underestimated any important nonlinearities in the design models that are influential in the response. “When one does model tests on a bespoke system it is not unusual to make some discoveries not foreseen in the design process beforehand.” Then armed with a set of quality model test results HESS can refine numerical models giving it greater confidence that its design is robust.

Several HESS projects at MARIN have also included time-domain simulations of moored platforms and FPSOs. Joel comments on how he sees the relation between model tests and computer simulations as ‘symbiotic’. Both time domain simulations and model tests are needed in engineering design to evaluate the expected performance of the physical structures that we intend to build, he says. “Neither fully replicate all the complexities of a full-scale structure in the physical world but with both we can be reasonably confident that our design is

sufficiently robust in terms of technical integrity.” Neither is perfect but it is all about managing inherent risks to ensure a safe design in as low as reasonably practicable (ALARP) terms, Joel stresses.

With modern design HESS looks at an ever-increasing set of load cases to ensure it has a full screening to identify the governing load cases. Even with a significant timetable of model testing it is not practical to fully screen for every factor due to the time involved in setting op each load case, he says. With time domain simulations on the other hand, it is possible to do a full screening simply because of the available computer power these days and from this numerical screening HESS can identify the key load cases that it needs to model test. These model tests provide an extremely valuable but often underestimated role in the form of QA/QC of numerical models, Joel adds.

Numerical models These days some of the numerical models use large sets of input data, which are often transferred from one numerical tool to another and coupled with embedded assumptions. It is an increasing challenge to check the models. “We have moved a long way from the handwritten calculation sheets where everything was transparent - albeit simple. A carefully

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CFD “But CFD is still at the next level of complexity compared with the aforemen- tioned structural design tools and I think that it will take a while to ‘mature’. Today CFD tools are most valuable when they are calibrated against model tests and then used to evaluate parametric variations. In this context with the increased availability of CFD I see an increased need for model



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