A Cold Repair Method for FPSOs
Authors: Weitzenboeck Jan Ruediger , Mcgeorge Dag 
Published: April 2012

The paper reports on a series of Joint Industry Projects and field trials todevelop a bonded patch repair method for metallic load bearing structures onFPSOs. This cold repair method replaces hot-work, thus avoiding anyinterruptions of production. Examples of successful use of bonded patch repairare shown. This experience and the JIP project results have been summarised ina DNV Recommended Practice (RP) on bonded patch repair. The RP providesguidance for cold repair of non-critical defects and installation of secondarystructures. It covers criticality assessment of defect, design of patch, qualification of materials and patch, fabrication on location and inspection. Furthermore, a method has been developed to predict the capacity of bondedpatches. The test results agree well with the numerical predictions.

Introduction

FPSOs are among the most complex marine structures. These vessels arepermanently stationed offshore and are exposed to severe weather. Many FPSOsexperience corrosion and fatigue crack damage. Welded repairs can be quitecostly as hot-work may cause production interruption with considerable loss ofrevenue. Hence there has been an increasing interest in using cold repairmethods. A JIP was run for a number of years to develop guidelines for bondedpatch repair of FPSO structures as reported before by Meniconi et al. (2010)and McGeorge et al. (2009). The project documentation has now been furtherdeveloped into a Recommended Practice by DNV (DNV RP C301, see also (Det NorskeVeritas, 2012)). The new RP provides a framework that allows the application ofbonded patches or bonded repairs on FPSOs and other offshore structures. Thisapproach makes it feasible to postpone (emergency) repairs until the nextplanned maintenance thus making considerable cost savings and minimisinginterruption of daily operations.

The principle of a bonded repair is shown in Figure 1. The original structurewas designed for certain loads. A fatigue or corrosion damage reduces itscapability and the structure needs repair. Instead of welding, a stiff andstrong patch will be bonded onto the structure to restore integrity. The mainapplication of this RP is the repair of offshore steel structures. The RPprovides guidance for cold repair of non-critical defects and installation ofsecondary structures. In the long run it will be possible to expand theapplication to critical defects once more in-service experience has beengained.

An important topic is the lifetime of the repair. Today, no establishedaccelerated test method exists that can reliably predict the lifetime of abonded repair, see also Weitzenböck and McGeorge (2004) and Weitzenböck andMcGeorge (2005). Hence this RP focuses on repairs of non-critical elements andnon-critical damage of critical elements where premature failure of the bondedrepair will not have catastrophic consequences. The paper by Grabovac andWhittaker (2009) sums up 15 years of experience with bonded repairs applied tonaval ships. The authors report excellent long-term performance and believethat (carbon fibre) patch repair should become a standard process for temporarystructural repairs.

This paper presents first some typical repairs done using bonded patches. Thisis followed by an outline of the new DNV RP on bonded repair. Next a decisionmaking process on whether to repair using bonded patches is presented. Andfinally a new method for predicting failure of bonded patches is outlined.