Wave-Impact Loads on Large-Diameter Horizontal Cylinder Placed Above Still Water Level
Authors: Davies R.L. , Martin J.G. 
Published: October 2008

ABSTRACT
This paper gives information on results of studies performed in relation to the installation of the Protective Barrier around the Ekofisk Tank, in particular the wave loads and wave impact forces on large diameter cylinders. The studies have been conducted on 10.15m in diameter, 40m long cylinders (prototype scale), placed above still water level. in such a way that they could be partially submerged by storm waves. Both vertical and horizontal impact forces have beenanalysed. The paper presents the model tests which have been undertaken as well as typical results. The paper also presents the theoretical model which has been set up and results from the corresponding mathematical simulation.

INTRODUCTION
The Ekofisk 2/4 Tank is 1inked to adjacent platforms by four bridges (See Fig.l). The combined effect of the subsidence at theEkofisk field and of the wave enhancement created by the Protective Barrier results in the bridges being partially submerged by the storm waves (See Fig.2). Studies were undertaken in order to evaluate the consequences of partial Submergence of the bridges; two concepts were envisaged : the shroud concept where the bridges would be enclosed in a large protective cylinder. or the bare bridge concept where the bridges will be made as transparent as possible to the waves by stripping away all cladd1ng and other unnecessary obstructions. The bare bridgesolution is the one which has actually been implemented at Ekofisk; however, studies concerning the shroud concept were carried up to a basic design stage.
In the shroud concept. the bridges were totally enclosed in a protective cylinder (See Fig.3). resting on the Protective Barrier at one end and at the adjacent jacket at the other end. Depending on the size of the bridge to be protected, the necessary shroud diameters were ranging from 8 to 12 meters. the maximum Submergence (referring to the bottom chord of the shroud) being in the order of 14 meters. The designer was then faced with the problem of the determination of the wave forces on large diameter cylinder submitted to impact loads. A bibliographic review showed that there had been many publications dealing with the problem of wave impact loads. that the theory was really well established, and that model tests had been conducted. However, the studies and model tests mainly dealt with the problem of vertical slamming forces on horizontal jacket members situated in the splash zone, the aim being to determine slamming coefficient for the design of those members. In our case we had to dea1 not only with the impact force, but with thewhole time history of the wave force over awave cycle. with the reaction at both ends of the shroud, with oblique impacts and also with the horizontal component of the force. Based on existing publications a theoretical model was set up. At the same time model tests were undertaken. as they were deemed necessary to calibrate any the oretica1 mode1. The mode1 tests. the theoretical model and thecorresponding results are presented hereafter.