A Safe and Cost Effective Diverless Method for Hull Wetted Surface Side Shell Repairs Executed On FPSO Fluminense

The Bijupira and Salema oil fields in the Campos Basin, Brazil, are tied-back to the FPSO Fluminense. The design life of FPSO Fluminense was 15 years, first oil was in 2003. In 2017, Shell as operator and partner Petrobras decided to execute a 5 year life extension project to extend production until 2023. Although the life extension project encompassed many vessel, moorings, and production facility renovation projects, this paper will focus on the repairs of cracks in the wetted surfaces of the hull. Crack repairs were executed during production operations using a novel methodology designed to eliminate the risks of diving operations, while significantly reducing the costs of the offshore hull repairs. Earlier attempts using traditional methods with divers to deploy cofferdams were unsuccessful due to the high safety risk of diving in the persistent 2-plus knot Brazil current. In early 2019, the operator and a team of engineering consultants concluded that a novel repair approach that eliminates diving operations was feasible. The concept methodology was developed, one element of which included positioning and securing a cofferdam over a cracked area of the FPSO side shell. The cofferdam was lowered from the FPSO deck while being guided into position and secured in place by guide wires passed through holes in the side shell. Water ingress into the void spaces was eliminated via a hot tap and winching system that penetrates the side shell from inside the tank. Following the concept development, a step-by-step procedure was written, vetted with the classification society and subsequently used in the tendering process to select an execution contractor. The selected execution contractor constructed a test tank that replicated the actual hull structure. The prototype was used to test the method at full scale, including procedures, communication protocols, and equipment. The learnings were incorporated into both the procedures and equipment. This process was repeated as a training exercise for the offshore personnel and HSSE leaders prior to offshore execution. The execution contractor was mobilized offshore in the first quarter of 2020 and successfully performed the hull repairs. The job was completed as planned, including removal and replacement of several independent sections of stiffened hull plating, which included both flat and curved surfaces. Based upon successful execution, this method has been proven to be a technology that can be used to eliminate the risks of diving and to significantly reduce the costs of future offshore hull repairs. This paper will address the methodical approach taken to develop and integrate the various elements of this novel technology developed jointly by the operator and consultants, as well as the phases testing, pre-mobilization surveys, project execution, and other activities that drive success.

Oil Tanker Simplified Fatigue Assessment with Inspection and Repair Approach and Parameters

The occurrence of cracks in the hull structure of oil tankers is an important concern for the maritime industry because crack propagation will reduce collapse strength of deck-stiffened panels and, consequently, decrease the ultimate hull girder capacity of ship’s structures. Fatigue is an important design criteria for ships to ensure a sufficiently high safety level. Fatigue life predictions of ship’s structural details have traditionally been carried out using S-N approach and the Palmgren-Miner’s rule. The principal objective of such approach is to estimate the time to failure in order to ensure a satisfactory design lifetime of ship’s structural components. Potential cracks are considered to occur in the side shell, in the connections between longitudinal stiffeners and transverse web frame. The main objectives of the present study are to evaluate the fatigue life of vessel’s amidships using the simplified fatigue method, which is based on DNVGL-CG-0129 “Fatigue Assessment of Ship Structures” in order to determine the main cause of the observed cracks on the single skin oil tanker. Fatigue assessment was based on worldwide trade. Longitudinal stiffeners at transverse frames amidships are considered. The results show that fatigue life is generally above 20 years; however, analysis has revealed that the fatigue life of typical stiffener transitions in the side shell is below 20 years. The fatigue lives of side shell longitudinals are regarded as normal for ships built in the period between 1980 and 1990 with extensive use of high tensile steel in the side shell. Inspection and repair proposals of details with fatigue lives below 20 years are advised accordingly. Findings of fatigue analyses provide remaining life assessment, inspection plan definition, determination of repair and modification solutions, and avoiding integrity issues resulting in production downtime and hot work or dry dock.

Analytical Method for Evaluating the Impact Response of Stiffeners in a Ship Side Shell Subjected to Bulbous Bow Collision

This paper addresses a simplified analytical method for evaluating the impact responses of the stiffeners in a ship side shell subjected to head-on collision by a bulbous bow. The stiffeners are classified as the “central stiffener” and the “lateral stiffener” according to their relative position to the bulbous bow. In analytical predictions, it is assumed that the flexural bending of the central stiffener and plate occurs simultaneously. However, the deformation mode of the central stiffener outside the indenter contact region is simplified as linear to derive its deformation resistance. The curved deformation mode of the lateral stiffener is proposed to calculate the deformation resistance and to consider the interaction effect with the plate, which can cause the plate to fracture earlier. Model tests with three specimens (one unstiffened plate for reference and two stiffened plates) quasistatically punched by a conical indenter are performed to validate the proposed analytical method. Resistance-penetration curves and damage shapes for the three specimens are obtained. The experimental results illustrate the effects of the stiffeners on the deformation resistance and fracture initiation of the stiffened plate and the influence of stiffener tripping on the lateral resistance. Moreover, the experimental and analytical predicted results correspond well, suggesting that the proposed analytical method can accurately predict the crashworthiness of a ship side shell subjected to bulbous bow collision.