Re-Usable and Lightweight Composite Material Lift Bar for the Installation of Subsea Structures

Abstract Steel lift or spreader bars are used as installation aids for lifting complex structures and spools offshore. Typically, the design of such installation aids needs to be bespoke for the equipment to be installed offshore. For this reason, the steel spreader bar may be scrapped after use. Furthermore, during the installation of a heavy spool, a steel spreader bar can weigh up to 40-60% of the total hook load, thus adding significantly to the total weight of the lift and increasing the required vessel capability to perform the installation task. However, by using a composite material design spreader bar the contribution to the hook load can be reduced significantly. This can increase the flexibility of an offshore campaign, as it allows for the use of smaller vessels to perform the same lifting task. Composite materials have good corrosion resistance and mechanical properties, along with high fatigue capacity and resistance to environmental exposure, e.g. sunlight and seawater. These characteristics enable a composite modular spreader bar to have a longer lifespan than the conventional steel spreader bar. The modular design of the composite spreader bar allows it to be re-used thus making it very cost effective relative to steel lift bars. The modular design of the composite lift bar offers the flexibility to easily transport the system globally for use in numerous projects. The paper details design aspects of the composite lift bar including material selection for marine environments, design safety factors, and the novel fabrication methodology. A prototype composite spreader bar was manufactured including a steel flange-to-composite bond connection. This paper also details the comprehensive testing program which was mainly designed to satisfy service needs including DNV qualification requirements. The design life was validated by performing a full-scale fatigue test under cyclic bending load with regular monitoring for any crack initiation. Resistance to an accidental load case is to be assessed by impact testing of the prototype bar. An ultimate failure load test is to be carried out to demonstrate an adequate safety margin and to satisfy code requirements. A trial offshore lift was also performed to capture the behaviour of the composite lift bar in the splash zone.