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Publisher Offshore Technology Conference LanguageEnglish
Document ID 5705-MSDOI  More information10.4043/5705-MS
Content TypeConference Paper
TitleAn Investigation Into The Dynamics Of The Esv Iolair Wet-Diving Bell During Launch And Recovery
AuthorsD. Vassalos, U. of Strathclyde; D. Dutta, American Bureau of Shipping; and P. Macgregor, U. of Strathclyde
Source

Offshore Technology Conference, 2-5 May 1988, Houston, Texas
ISBN978-1-61399-081-0
Copyright1988. Offshore Technology Conference
PreviewABSTRACT

This paper outlines an investigation into the dynamics of a 3.0 tonnes wet-diving bell during deployment from ESV IOLAIR aimed at improving the weather limits of the operation. The operational weather limit is dictated by the occurrence of cable "snatching" and stands between 1.0m and 1.5m significant wave height. The investigation comprises theoretical analysis, model experiments and full-scale measurements, achieving good correlation between the results of all three approaches. The research findings indicate that the most important factor affecting the existing problem is the submerged weight of the bell. An increase of 1.0 tonne mass, for example, is expected to extend operability to some 3.0m significant wave height. Further improvement can be achieved by reducing the heave motion at the boom tip.

INTRODUCTION

As the development of offshore fields moves into deeper and harsher waters and the technology of subsea production systems improves, more and more hardware will be deployed beneath the water surface. Safe and cost effective deployment of subsea units from surface vessels means overcoming the problems of the passage through the air/sea interface. This passage presents the most critical phase of the launch/recovery operation and has been identified as the largest single source of equipment damage. During this phase, the subsea unit is subjected to impact loads (slamming) and large motions as well as large dynamic forces (snap loads) in the hoisting wire both of which can severely limit operational capabilities. Deployment of subsea units from surface vessels is at present dependent to a large extent on the skill and experience of the operator. Theoretical understanding of the physical phenomena involved is far from satisfactory and measured data is sparse. Attempting to fill this gap, a research team at the University of Strathclyde has been conducting research over the past five years and considerable advances have been achived. At the centre of these is a suite of computer programs capable of simulating the launch/recovery operation from surface vessels and of being used on a formal quantitative design basis as well as for evaluating and appraising existing operational procedures.

It has to be emphasised however, that the deployment of arbitrarily shaped objects from floating vessels in hazardous seaways is a problem of such complexity that the art of engineering must be practised by combining experience, good judgement, analytical flair and ingenuity if useful results are to be obtained, correctly interpreted and the full-scale performance predicted therefrom. The work presented in this paper was tackled with this in mind. It involves theoretical, experimental and full-scale investigations of the launch/recovery operation of an unusually shaped manned wet-diving bell from the ESV IOLAIR with the view to identifying and verifying means for improving the operational capability of the existing system.

The IOLAIR is the emergency support vessel (ESV) for the BP Forties field. It is a self-propelled, dynamically positioned semisubmersible capable of sypporting both air and saturation diving operations. The method of deploying air divers involves the use of a wet-diving bell which is an open sided, cylindrical, enclosed upper section diving stage 2.0m high and 1.5m in diameter, capable of supporting up to three divers to a depth of 50 m (Figure la). Life support and auxiliary services are supplied to the bell via a dedicated air diving umbilical.
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