Practical Airgap Prediction for Offshore Structures

Two new methods are proposed to predict airgap demand. Airgap demand is the maximum expected increase in the water surface elevation caused incident waves interacting with an offshore structure. The first new method enables inclusion of some second-order ef- fects, though it is based on only first-order diffraction results. The method is simple enough to be practical for use as a hand-calculation in the early stages of design. Two existing methods of predicting airgap demand based on first-order diffraction are also briefly presented and results from the three methods are compared with model test results. All three methods yield results superior to those based on conventional post-processing of first-order diffraction results, and comparable to optimal post-processing of second-order diffraction results. A second new method is also presented; it combines extreme value theory with statistical regression to predict extreme airgap events using model test data. Estimates of extreme airgap events based on this method are found to be more reliable than estimates based on extreme observations from a single model test. This second new method is suitable for use in the final stages of design.@DOI: 10.1115/1.1710870# Introduction and Background Present airgap design methodologies for floating structures are not standard and rely heavily on empirical knowledge and model tests. Large production semi-submersibles and novel large-volume offshore structures provide design challenges that are well outside the present design experience base. Unlike drilling semi- submersibles, these vessels are generally required to remain on station throughout the most severe weather. Urgency is added by the fact that airgap design problems ~wave impacts! have been encountered on large North Sea semi-submersibles, including the Veslefrikk B platform in the Norwegian sector of the North Sea, the vessel which is the subject of the analysis and model tests presented in this paper. High volume structures, whether fixed or floating, complicate the airgap calculation by significantly diffracting the incident waves. For these structures, ignoring diffraction effects is non- conservative in that diffraction effects generally worsen the airgap demand. Large-volume floating structures, including semi- submersibles and floating production, storage and offloading ~FP- SO's !, vessels, offer the most significant challenge. Two distinct hydrodynamic effects are observed: ~1! global forces and resulting motions are significantly affected by diffraction; and ~2! the local wave elevation, h(t), is also significantly influenced by diffrac- tion. For semi-submersibles, these wave amplification effects are most extreme at locations above a pontoon and/or near a major column. Here, four methods for prediction of airgap demand without use of second-order diffraction are presented and compared with model test results. First, a theoretical overview and motivation section outlines which physical terms are included in conventional first-and second-order diffraction analysis and the implications of the assumptions implicit to various post-processing methods. Some background is then presented which is relevant to all of the methods presented here. This background includes airgap nota- tion, model testing issues and methodologies and some statistical background. Two existing and one new method of predicting air- gap demand using results from only first-order diffraction are then presented and critically compared with model test data. It is con- cluded that second-order effects should be included in airgap pre- diction throughout the early stages of design, but that these meth- ods are inadequate for final design. Another new methodology is then presented which is useful for final design. In the new meth- odology, extreme value theory and regression techniques are com- bined to predict extremes from model test results. Theoretical Overview and Motivation The disturbed water surface in the presence of the vessel, h(t), is assumed to be a sum of incident and diffracted waves, h i and h d , each of which is a sum of first- and second-order components: