There is a scarcity of data relating to the hydrodynamic loading due to sacrificial anodes A method of calculating factors to apply to circular cylinder force coefficients, using the discrete vortex method IS described The results are presented for uni-directional and oscillatory flows Equivalent force coefficients are calculated and used in a Morison type analysis of a simple jacket structure in a current and in waves The overall in-line loading was found to be increased by as much as 9% In modest environmental conditions

INTRODUCTION

Steel offshore jacket structures are generally protected against corrosion by sacrificial anodes, an impressed current system or a hybrid of the two Sacrificial anodes are cast from reactive metals (normally zinc or aluminum alloys) which are more electro-negative than the structures they protect, and so corrode in preferance to the structural steel of the jacket Their size is dictated by the desired current and by manufacturing constraints They are long and slender, of trapezoidal or circular section (perhaps of dimension 2 00 x 0 25 x 0 25m) and weigh in the order of hundreds of kilograms The optimum stand-off distance between the underside of the anode and the structure to which it is attached has been found to be in the region of 0 25 to 0 35m Sacrificial anodes are distributed all over the wetted surface of jackets and can number many hundreds even for structures of modest dimensions It has been estimated that the weight of anodes attached to jackets destined for the North Sea comprises between 4 and 6% of the total weight of the structure when ready for launch(1)

Sacrificial anode systems are usually designed with reference to codes or standards I issued by a certifying authority, a government agency, or the owner or operator of the platform A commonly used code is the DNV Recommended Practice RPB401 ‘Cathodic Protection Design(2)’ Whilst codes and guides give detailed information and procedures concerning protection against corrosion, little is specified concentrating the fluid loading relating to individual anodes or their effect on the fluid loading of the structure as a whole

The in-line force, F, experienced by an element of a jacket structure due to waves is calculated using Morison's equation which, in its simplest form, may be expressed as, Equation F (Available in full paper) where A and V are the projected area and the volume of the element, U and U are the local undisturbed velocity and acceleration of the fluid of density p, and CD/ and CM/ are the drag and inertia coefficients respectively The terms on the right hand side of the equation represent the drag force and the inertia force experienced by the member in the direction of the ambient flow Whilst a great deal of research has been carried out to establish values for the force coefficients (see Sakarpa (3) for example, they continue to be a matter of debate.

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