Pile Driving Dynamic Loads On Offshore Structures

ABSTRACT Reducing conservatism in jacket designs based solely on environmental loads can make other loading mechanisms which were not of concern in the past a dominant design parameter. One of these mechanisms is associated with the dynamic loads induced during pile driving. Acceleration and strain measurements during installation of two jackets in the North Sea illustrate that both tubular joints and sacrificial anode attachments can be subjected to a high level of fatigue damage during pile driving operations. The measured vibration response during pile driving can be simulated by performing time-domain dynamic analysis of the jackets. A load pulse with a duration of 0.2 sec is applied at the first pile guide inside the jacket leg. The U. K. Department of Energy T curve appears to be more appropriate for use to predict fatigue damage during pile driving than the API X curve. INTRODUCTION During the past decade, the offshore industry has made several advances to eliminate uncertainties and reduce conservatism in the design of fixed jackets. Some of these advances include:Reduction of wave forces through the use of better environmental databases which allowed taking directionality and joint probability into account;Reduction of wave shear forces through the optimization of water line geometry by taking advantage of the wave cancellation phenomenon;Use of better analysis and component databases so that joints are designed closer to their expected strengths;Use of a large derrick barge to lift the jacket made some redundant bracing, which were only needed for installation purposes, unnecessary;Reduction of jacket leg wall thickness while increasing the platform lateral stiffness through the use of heavy wall main piles or additional pile inserts; andReduction of jacket dynamics by using more accurate soil models and foundation designs. It is, therefore, not surprising nowadays to see much lighter jackets as compared to equivalent designs of ten years ago. Additional reduction in the weight, particularly for shallow water jackets, is expected through the implementation of the Load Resistance Factored Design (LRFD) procedures, which has been recently developed by API. All of these advances are intended to build lighter jackets while maintaining a high level of reliability under the well-known operating, environmental, and installation loading conditions. Problems can, however, be expected when loads that, in the past, were not major design concerns become significant design parameter. One such loading condition which was not normally considered in the jacket design process and is not explicitly covered by any of the existing codes of practice is dynamic loads during pile driving. Fatigue damage during pile driving has been noted as the cause of cracking of secondary attachments such as hand rails, grout lines, and sacrificial anodes. Vibration monitoring of the Beryl B platform illustrated that vibration induced by pile installation were significant in several areas [1].