Requalification Fatigue Assessment Of Semisubmersibles Being Converted For Floating Production Work
Authors: Potthurst Robert , Bainbridge C.A. 
Published: October 2008

Fatigue life assessment of a semi-submersible drilling unit being requalified for a floating production role in accordance with current requirements and legislation is addressed. The contents reflect Lloyd's Register's role as a Certifying Authority and summarise experience derived from case histories and survey records.

The findings of work carried out on stress concentration factors completed in collaboration with UK Health and Safety Executive are mentioned.

Schemes for upgrading critical structural areas together with inspection and repair requirements to minimise the risk of fatigue cracking during service are also presented.

1. INTRODUCTION

Floating production installations are becoming increasingly popular and can provide a cost effective and flexible solution to companies developing deep water and marginal fields.

Floating production facilities (FPF) bring fields with short life spans within viable economic range due to relatively short set - up times and low de-commissioning costs. An added incentive is the ability to re-use at least the main unit on further developments with minimal adaptation.

FPF configurations vary from CALM or SALM buoy moorings to purpose - built or converted tanker installations and semi-submersible units with positional mooring systems.

One development option available involves the conversion of a semi-submersible unit to a floating production role. Lloyd's Register is the Certifying Authority for a number of such projects in the U.K. sector of the North Sea.

Semi-submersible units will normally require upgrading for their new role. Changes in loading conditions will probably mean that a structural reassessment will be necessary. Unlike a drilling unit an FPF would normally be expected to remain on-station requiring particular attention when considering inspection and repair aspects.

Structural fatigue assessment based on the new operational conditions and site environment will be necessary and will normally result in upgrading of the fatigue sensitive areas of the structure. This can be achieved by improvement of critical welds and detail design, without the need for major modifications. The upgrading should also reflect the difficulties of on-station bracing inspection and the need to avoid costly structural repairs during service.

2. CERTIFICATION REGULATIONS

The impact of the latest regulations on the design needs to be addressed and the Certification Authority must judge that the unit is fit for its intended purpose and that a satisfactory standard of safety has been achieved.

For compliance with the U.K. Health and Safety Executive (HSE) requirements and in accordance with the Statutory Instrument (SI) No. 289 [2] the design and conversion of these semi-submersible units will be subject to appraisal, surveillance and approval by the appointed certifying authority.

The current document defining structural philosophy regarding certification is the Department of Energy publication Offshore Installations: Guidance on design, construction and certification, fourth edition 1990 [1].

Lord Cullen, in his report on the Piper Alpha disaster [3], asserted that many regulations are unduly restrictive in that they impose solutions rather than objectives and are out of date in relation to technological advances and also that the principal regulations should take the form of requiring stated objectives to be met. Guidance notes should give non-mandatory advice.