Over the years we have developed an excellent working relationship with a European Construction and Engineering company that specializes in major construction projects worldwide. In addition to General Commercial projects they construct Transportation Infrastructure including High Speed Rail Bridges and Tunnels, Port Construction as well as construct and maintain Off Shore Wind Electrical Generation installations.
A common method for installing Off Shore Wind Generators starts with driving a 4 M (13 Ft.) diameter foundation pile with a 2″ (50 mm) wall thickness into the sea floor. This can never be done so the stanchion is perfectly plumb, so a tapered transition piece is installed over the pile, made plumb, and high strength grout is used to fill the gap and bond the two pieces together. The mast with the generating equipment is then rigged in place and attached to the transition piece:
Although the area of grout surface contact is huge, over time the grout tends to break down from the dynamic forces generated by the wind and waves. When the transition piece slides down over the pile, and steel-to-steel contact is made between the two pieces, there is the worry that the very high stresses from the dynamic forces could result in structural failure.
In order to support the transition piece, mast and wind generator weighing approximately 3,200 kN (720,000 Lb), as well as to allow stress relief from the dynamic effects of wave and wind, our resilient LDS, Low Dynamic Stiffness Rubber Bearings were designed to be installed between brackets fixed to the upper and lower sections:
A very complex stress analysis of the connection between the upper and lower sections was performed before guaranteeing a fatigue life of 20 years. It turned out that the spring rate (or cushioning effect) of the LDS Rubber Bearings became the most critical element in the support system.
After a year and a half of manufacturing and testing prototype bearings, the final design was determined to be 200 mm x 200 mm x 160 mm tall (8″ x 8″ x 6 1/2″) LDS Rubber Bearings with (8) internal bonded steel plates. The LDS Rubber used is the same formulation that we use to manufacture our standard product line for HVAC mechanical equipment and floating floor/wall/ceiling architectural isolation systems as well as complete building isolation bearings. Considering the harsh environment of this installation, to re-confirm the longevity of the material, we re-ran all physical properties tests for tensile strength, elongation at break, accelerated aging and compression set. In addition, we ran a series of long term relaxation tests on actual bearings to verify the minimum 20 year performance required.
In order to guarantee the performance of the LDS Rubber Bearings, we were required to provide a series of load/deflection tests on every bearing manufactured:
Each bearing was loaded to 150% of design load (9) times: Three pre-conditioning cycles, followed by three cycles at 0° and three cycles with the bearing at a 1° incline. The last 0° and 1° test runs were photographed and the results plotted and submitted for approval:
The acceptance criteria for the bearings was very stringent. The tested spring rate needed to be within +/- 10% of the design spring rate, and considering that industry standards allow a difference of +/- 5 durometer in molded rubber parts results in a difference in spring rate of approximately +/- 15%, careful control of durometer became essential.
In order to achieve this level of accuracy and repeatability, every step of the manufacturing process from mixing the compound, preparing the steel plates for bonding, loading the molds and vulcanization needed to be carefully controlled. We followed our standard Quality Control procedures used for the manufacture of structural building isolation bearings as shown in our AB-104-2 Bulletin and we realized a very low rejection rate.
We initially manufactured and tested (30) bearings for a trial installation on one wind generator. After operating for approximately 12 months in the North Atlantic Sea, data collected from strain gages and displacement monitors installed along with the bearings verified that the system was performing as designed. As a result, we received an order to manufacture and test (1362) bearings for installation on (50) wind generators.
The weather is so severe in the North Atlantic between October and March that it makes installation of the bearings impossible. In order to have the bearings delivered before the weather deteriorated, we were asked to deliver the bearings only 16 weeks after order placement… not a lot of time to process 16,800 Kg (37,800 Lb) of rubber and steel. We manufactured several molds and worked 3 shifts, 24 hours per pay, 7 days per week to meet the delivery requirement.
Projects like this do not come along every day, but we handle them no differently than projects that do: Each and every application receives careful design, manufacturing and testing…
We look forward to working on your next application.
Director International Sales