Strengthening of Offshore Steel Components Using High-Strength Grout: Component Testing and Analytical Methods

Abstract

Abstract As part of the strengthening of the four Ekofisk Jackets 2/4A, B, R and P, a total number of 75 under water braces/chords were strengthened by grout filling. A special purpose grout type, developed and produced by Densit AS, Denmark, was used. Extreme high compressive strength and large modulus of elasticity characterize this grout type. For validation of the capacity increase and validation of analytical models, large-scale component tests covering tubular joint connections, axial and bending loaded members as well as friction and bond tests were undertaken. Description of the tests and presentation of the test results are given in this paper. Non-linear finite element analyses of the test specimens are also described, resulting in a calibrated finite element methodology. Finite element analyses were excessively used to document that the grout filled components had adequate capacity. The results from these analyses were then incorporated into nonlinear pushover analyses. For this purpose, an equivalent grouted beam element was developed. Introduction This paper is part 6 of 7 companion papers, which together present a comprehensive assessment of the effect of seabed subsidence on the Ekofisk jackets, see Refs. 1 through 6. Seabed subsidence at the Ekofisk field have raised the need to strengthen many of the jacket structures. Several of the platforms will experience a reduced air gap with respect to the 100 year wave crest. The deck structure will become partially submerged due to the passage of such a wave. On this basis further analyses and remedial work were carried out on several of the platforms. A significant part of the repair and remedial work involved filling several tubular members/joints with high strength grout. In order to determine the improvement in stiffness and strength it was necessary to perform full scale testing of grout filled tubular members and joints. Results from bending, axial and bond tests were used for calculating the improvements in member strength and stiffness. These improvements were then applied to those tubular members and joints that had been over-stressed, primarily due to the revised loading condition caused by wave action on the deck structures. The main steps of the grout filling operation offshore are described in this paper. Comprehensive onshore testing was performed prior to the actual offshore operations. These tests were of significant importance for the success of the offshore operations. Grout specification Densit A/S in Denmark produces grout types, which are characterized by extreme compressive strength. These products have been widely used in other applications such as manufacturing of safes, vaults and burglary resistant panels, hard wearing and chemically resistant industrial floor toppings. Ducorit, which is one of the grout products provided by Densit, was used to reinforce jackets in the Ekofisk field in the summer of 1995 with a great success. The Ducorit products are Portland cement based materials of a composition, which allows for mixing the product at low water dosage. The hardened material is characterized by having high strength and durability.