Laminated Rubber Properties For Structural Offshore Applications

ABSTRACT An extensive research program of environmental and mechanical fatigue tests has been performed specifically to investigate the performance of laminated rubber structural bearings for offshore applications. The tests were designed to simulate the service requirements of a laminated rubber ball joint which has been designed as a key structural component of the Deep Water Gravity Tower, a production platform designed for water depths of 1000 – 3000 ft. The durability of laminated rubber/metal units in sea water is studied with and without strong cathodic protection. Potentially adverse effects of cathodic protection on exposed metal/rubber edges are discussed. Mechanical fatigue tests in sea water have shown the behavior of the material to be stable and reasonably predictable under compressive dynamic stress up to 55 MPa with simultaneous dynamic shear strain of up to 200 %. The key performance requirements of high compressive stiffness combined with low shear stiffness were found to be maintained, even in the presence of deep cracks. The program of work reported here extends the technology of laminated rubber structural bearings to provide confidence for the use of this material is even more critical designs. UNITS AND NOTATIONS S.I. units are used throughout this paper. It is reminded that:1 MN = 100 metric tonnes = 225 kips1 MPa = 10 bars = 145 psi Crack growth rate is measured in nm/c = 10−9 meter per cycle. INTRODUCTION Laminated rubber is a composite material made of alternating layers of rubber and metal. It is able to support large compressive forces, perpendicular to the surface of lamination, with small deflections. In the other direction, it remains very flexible and offers little resistance to imposed tangential displacement. Structural laminated rubber bearings have successfully been used over the last 30 years for many applications in civil, mechanical and marine engineering. For instance, they are widely used as bridge supports, allowing the deck to accommodate displacements due to imposed loads or deformations, without significant restraint from bridge piers. Another typical civil engineering application is their use for isolation of equipment or buildings from external vibration of mechanical or seismic origin. More recently, this composite material has been developed to an even higher standard of quality and precision for the aerospace industry where they are used for rocket jet deflectors and for helicopter rotor bearings. This latter application is a spectacular example of the reliability which can be obtained, even for laminated bearings subjected to extremely severe cyclic loadings. In the offshore industry, the use of rubber has grown rapidly during the last ten years and it is now used for many applications, such as : supports for topside modules or complete deck superstructures; fendering and mooring system; energy absorption device for installation by lifting or barge transfer of modules or complete deck; protective anti-corrosion sheathing of steel pipes; drilling riser joints and other downhole applications. In above examples, rubber is used either for its resistance to corrosion or for the possibility to design parts supporting large deformations (Ref. l).