An Innovative "No Hot Work" Approach to Hull Repair on In-Service FPSOs Using Sandwich Plate System Overlay
Authors: Kennedy Stephen J. , Bond James , Braun Dale , Noble Peter G. , Forsyth J. David 
Published: October 2008

The Sandwich Plate System (SPS), comprising two (steel) plates bonded to a continuous elastomer core, forms a much stiffer and stronger system than a single steel plate. The SPS doesn't need closely spaced stiffeners and, as compared to the single steel plate, loses strength less rapidly per unit thickness of corrosion loss of a steel plate. An SPS Overlay bonds a new plate and elastomer core to an existing, suitably prepared plate to give substantially increased strength and extended service life. An SPS Overlay is proposed to rehabilitate areas of the bottom of the ConocoPhillips FPSO INDEPENDENCE. Rather than the usual welds, a structural adhesive is used to bond the steel components together until the elastomer core sets and cures. This eliminates hot work. Placed between existing stiffeners, the SPS Overlay reduces the stresses due to hydrostatic pressure and the new inner plate restores or extends the corrosion life.

Introduction

Floating Production Storage and Offloading units (FPSOs) and Floating Storage and Offloading units (FSOs) have become common tools for offshore oil production in areas worldwide over the past 10 to 20 years. Today's total fleet consists of approximately 150 units, of which 80 are FPSOs and 70 are FSOs. The use of these units as a cost effective means of developing offshore fields continues to accelerate.

The majority of F(P)SO field development projects during the 1980's and early 1990's used surplus tanker tonnage, which had often already spent more than 20 years in transportation service. As F(P)SOs, many of these units are planned to be in continuous service without drydocking for a further 10 to 20 years.

An increasing number of these projects use new construction, rather than conversion from the diminishing supply of existing tanker tonnage of suitable size, configuration and quality, for long-term development of major fields. As of the fall of 2002, 23 FPSOs (11 newbuild and 12 conversions) and 5 FSOs (3 newbuild and 2 conversions) were under construction. Notwithstanding the new construction, the ability to maintain these assets on site and in full production over the long term is very important. Where a typical shuttle tanker may have a day rate of $20,000 to $25,000/day and a deep water Mobile Offshore Drilling Unit (MODU) may earn $150,000 to $200,000/day, one day of lost production on an FPSO can easily be valued at $2,000,000.

While the appropriate engineering analysis, structural renewal and corrosion protection work was completed prior to entering service, structural deterioration has occurred at an unexpected rate in a number of units, requiring remedial action. Industry understanding of fatigue and corrosion of F(P)SOs continues to expand and the lessons are being incorporated into new designs as they are produced. Nevertheless, it is important to recognize that structural repairs on site are likely to be required in even the best-designed and maintained units. A further complication of F(P)SO operations, as compared to trading tanker operations, is that hydrocarbons are present in the cargo tanks as long as production is underway.