One of the technical issues that must be addressed for compliant towers is the wear that may occur due to the relative motion between the piles and pile guides. This paper presents a method for adapting! accepted analytical procedures for wear of mechanical systems and applying them to compliant towers and other offshore structures. This method is used for the Compliant Concrete Tower in North Sea fatigue conditions to show that wear is expected to occur, but can be limited by proper material selection and design and construction detailing.
Pile supported compliant towers have been proposed and are being considered for use in various deep water fields throughout the world (Ref. 1 to 4). These towers obtain optimum "de-amplified" dynamic response by maintaining a structural period on the order of 2 times the wave period. This high period is achieve through the axial flexibility of the foundation system, which consists of long pile/sleeve units. Each of these units is guided along its entire length between the mudline and the attachment point to the structure.
As the tower sways due to wave loads, axial couples are developed in the pile/sleeve system. Relative motion occurs between the pile/sleeves and guides, giving the potential for wear at those locations. For the Compliant Concrete Tower (Fig. 1), the critical location for wear is at the guide nearest the mudline, where the normal force on the Pile is a maximum (Fig. 2).
Wear is defined as a change in the surface contour, such as a groove formed in a flat surface or a flat spot on a curved surface, that results when two bodies are pressed together and relative sliding occurs. Zero wear is not defined to be the complete lack of wear; rather, it is wear of such a magnitude that the surface finish in the wear track is not significantly different than the finish in the unworn portion. Roughly, this corresponds to a depth of the wear scar on the order of one-half the peak-to-peak value of the surface finish.
Once a detailed design of the pile guides at the base has been completed, the appropriate method for determining wear potential will be through testing. However, at earlier stages of the design, simpler methods are required to ensure that wear concerns will not damage overall feasibility of the compliant tower concept.
Although wear is not a typical problem with offshore designs, it commonly occurs with mechanical systems, and accepted methods of calculating expected wear have been in existence for many years. The preliminary analytical methods presented in this paper are adapted from procedures presented by Bayer, et.al., (Ref. 5) for wear caused by loading of constant magnitude and frequency. This approach has been modified for offshore load conditions by using a discretized wave environment over the life of the platform and determining accumulated wear, as is done in a standard fatigue analysis.
