The Strengthening of the Ekofisk Centre Bridges Due to the Environmental Impact Caused by the Installation of the Protective Barrier

Abstract

ABSTRACT The installation of the Protective Barrier around the Ekofisk Tank in the Summer of 1989 significantly altered the wave environment as affecting the three immediately adjacent platforms and the four inter-connecting bridges. The change in environmental conditions provided a locally enhanced wave condition to the extent that the maximum free field wave height (crest to trough) was increased from 23.8 metres to around 37.0 metres at the leading edge of the Protective Barrier. Whilst all the bridges had previously been elevated by 6.0 metres, during the major Ekofisk jacking operation in the Summer of 1987, this revised elevation was inadequate in terms of maintaining an adequate air gap for the four bridges immediately adjacent to the Tank after the Protective Barrier had been installed. The change in environmental conditions after the installation of the Protective Barrier meant that two of the inter-connecting bridges would be submerged close to the mid-span by as much as 9.0 metres during a 100-year storm. This changerepresented a unique design condition for the interconnecting bridges, which in turn support large product pipelines, and in turn form an essential part of the overall Ekofisk Transportation System. This paper describes the changes in environmental loading on one of the bridges and supporting pipelines, together with a description of the necessary strengthening measures carried out. INTRODUCTION Because of an anticipated level of subsidence at Ekofisk of about 6.0 metres, the installations within the Ekofisk complex had to be protected against severe environmental action. Figure 1 shows the field layout of the existing platforms and inter-connecting bridges within the Ekofisk complex. In the Summer of 1987 the steel platform decks, and bridges were all elevated by6.0 metres in order to protect those structures against environmental wave loading, and this project has beenpreviously described as the Ekofisk Jacking Project (1,2,3). In the Summer 1989 the Protective Barrier was installed around the Ekofisk Tank in order to provide protection to the equipment on the Tank decks located at +90 metres and +100 metres above seabed level. Figure 2 shows a plan and elevation of the Protective Barrier around the Tank. An account of the main design and installation aspects of this structure has been given previously (4). The installation of the Protective Barrier significantly altered the wave environment as affecting the platforms and inter-connecting bridges immediately adjacent to the Tank. The effects of this installation on the adjacent2/4R, 2/4P and 2/4G platforms have been described previously (5). Generally, the effect was to increase theenvironmental wave loadings on the platforms by 50%, 20% and 15% for the 2/4R, 2/4P and 2/4G platforms, respectively. The purpose of this paper is to describe the changes in environmental design conditions for the inter-connecting Ekofisk bridges between the Tank and the 2/4R, 2/4P, and 2/4G platforms, and to outline the method of strengthening that was required in order to resist the new environmental loading.