Towards the Understanding of the Steady Tilt Phenomenon in Semi-Submersibles

Atlar, Mehmet (1986) Towards the Understanding of the Steady Tilt Phenomenon in Semi-Submersibles. PhD thesis, University of Glasgow.

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Dissatisfaction with the existing rules governing the intact stability of semi-submersibles has created one of the major research areas in recent years (post 1970). At that time several stability tests on models had shown that capsizing of a semi-submersible with minimum stability index in maximum environmental conditions had a very low probability due to its inherently good motion characteristics. This finding encouraged operators and designers to put pressure on the regulatory authorities and classification societies to relax the design rules by reducing the metacentric height (GM). This would provide more deck load and possibly improved motion characteristics. However, during several of these stability tests it was noted that, especially with low values of GM, the models developed a "steady tilt" in regular waves which could be as high as 10 - 15 and that it then rolled about this tilt angle. This tilt was worst in short and steep regular waves but could be observed in a confused seaway, although it was then periodic in that it occurred most commonly at certain wave frequencies in the spectrum. This phenomena was called "slowly-varying tilt". This behaviour was potentially dangerous since it could affect the motions non-linearly leading to large angles of inclinations and the deck edge becoming immersed; two conditions which could lead to dangerous stability problems, increased mooring tensions, structural damage due to slamming and operational difficulties with risers, helicopters, etc. The majority of the research studies were originally designed to explore various aspects of the dynamic behaviour of semi-submers-ibles and the tilt effect was merely observed incidental to these tests. Thus the data recorded were of limited scope and in some cases of a conflicting nature. No documental cases of tilt on semi-submersibles in service had been recorded. As a result the various theoretical approaches to the problem lacked good experimental verification and no clear guidance regarding the reasons for tilt had been developed. This thesis attempts to extend both the experimental and theoretical knowledge of this poorly understood and potentially dangerous phenomenon. The first chapter of the thesis is of an introductory nature where the existing rules which govern the intact stability of semi-submersibles are reviewed and attention is drawn to the need to explore some dynamic aspects of the stability of the semi-submersibles with emphasis on the tilt behaviour. The second chapter presents an historical review of past developments in the study of tilt behaviour. The results obtained from each tilt study are discussed with reference to the theory and experimental details which are provided in the appendix of this chapter. The conclusions drawn from this chapter determine that the primary requirement for the understanding of the phenomenon is some accurate experimental work devoted entirely to the tilt problem so that some of the conflicting reports in the early studies could be clarified and form a basis for a sound theoretical approach. The third chapter of the thesis presents the experimental work carried out with a twin-circular 4-columns per hull semi-submers-ible model. In order to provide a reliable database for the present and future studies systematic tilt measurements were obtained over a wide range of regular beam seas and varying GMs. Systematic force tests on the model hulls at various hull spacings and a number of other exploratory tests were carried out in order to clear up several hydrodynamic aspects some of which have been reported in previous studies. In the fourth chapter, a theoretical analysis is presented with reference to the previous theoretical approaches and the test results carried out in this thesis. It is concentrated on the wave-induced loads in terms of the oscillatory (first-order) and the steady (second-order) components which are believed to be mainly responsible for the tilt behaviour. The main emphasis is placed on a proper determination of the wave-induced tilting mechanism which causes the steady tilt response and the determination of a minimum GM needed to limit tilt to some specified angle. The theoretical methods for the prediction of oscillatory forces and resulting motions and the main components of the steady tilting moments are presented. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Naval engineering
Date of Award: 1986
Depositing User: Enlighten Team
Unique ID: glathesis:1986-77349
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 09:11
Last Modified: 14 Jan 2020 09:11

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