Dynamic response of offshore jacket structures under

random loadsAhmed A. Elshafey a, Mahmoud R. Haddara b,*, H. Marzouk c

Faculty of Engineering, Minufiya University, Shebin Elkom, Egypt

b Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X5

c Faculty of Engineering, Architecture and Science, Ryerson University, Toronto, Ontario, Canada

Abstract The dynamic response of a scale model of a jacket offshore structure

is investigated both theoretically and experimentally.

The experiments were conducted both in air and in water.

The model was subjected to random wave loads. Froude’s law of modelling was used to obtain the dimensions of the scale model based on the dimensions of an existing structure

The effects of varying the structure’s weight, and the characteristics of the wave loading were investigated. The structure’s weight was changed by adding weights to the structure’s deck. A finite element model was designed to determine the dynamic response of the model

The reaction force at the foundation was estimated from strain measurements and compared with the finite element calculations. Fair agreement was obtained.

Objective is to develop a method for structural damage detection using the free vibration response of the structure. The free vibration response will be obtained from the stationary random response of the structure using the random decrement method

Continued…….

The present paper reports the results of an experimental program that was carried out at Memorial University of Newfoundland to investigate the dynamic response of a scale model of a fixed offshore structure.

Effects of varying the wave spectrum parameters and the weight of the structure have also been studied. Strain gages were used to estimate the reaction of the foundation. Experimental results were compared with the results obtained from a finite element model

IntroductionOu J, Long X, Li QS, Xiao YQ. Vibration control of steel

jacket offshore structures with damping isolation systems. Engineering Structures 2007;29:1525–38.

Developed a damping isolation system for the control of the vibration of a steel jacket offshore platform structure. A 1/10 model of the structure was fitted with the damping isolation system and tested on a shaking table.

Ice and earthquake loads were simulated. Numerical simulations were conducted and the numerical and experimental results were compared.

Numerical simulations for the undamped and the damped structure were obtained using a single degree and two degrees of freedom system, respectively. Simulations and experimental results were in good agreement.

While the damper design was discussed in detail, few details were given about the model of the jacket structure.

The study, reported above, is one of a few papers in the literature that report experimental investigations of the dynamic response of scale models of fixed offshore structures

Experimental setup

The prototype structure is a four-legged jacket with a combination of diagonal and k-bracings.

The model was scaled using dimensional analysis according to Froude’s law. The model was fabricated from Acrylonitrile Butadiene Styrene (ABS) pipes and its deck is made of Acrylic sheets.

The main legs, the bracings, and the deck are constructed using pipes with external diameter of 1in, 7/16in, 1/2in , and 5/8in , respectively. All pipes have the same thickness of 1/16in . The deck was made of 1/2in Acrylic sheets.

The total height of the model including the deck is 2.663 m. The four legs at the bottom form a rectangle, 1.000 m by 0.636 m. The structure consisted of five levels separated by horizontal bracings.

The main legs were slanted from the base until a height of 1.953 m to form a rectangle with dimensions of 0.667 m by 0.300 m. The main legs were kept vertical from a height of 1.953 m to the top of the deck.

The deck has a rectangular shape with a length of 0.966 m and a width of 0.600 m. Weights were placed on deck to change the mass of the model.

Continued…..

Four accelerometers and eight strain gages were used to measure the motion and the strains of various parts of the structure.

Two accelerometers were mounted on deck and the other two were mounted on one of the main legs. The strain gages were mounted on the main legs just above the points of support.