Experimental and Analytical Studies of Wave Impact Forces on Ekofisk Platform Structures
Authors: Murray J.J. , Kaplan P. , Yu W.C. 
Published: October 2008

Experimental studies in a wave basin are described with emphasis on wave impact on jacket platform decks and related structures in the Ekofisk field off Norway. The paper identifies a number of considerations related to the design of scaled physical models to be used in wave tests measuring impact loads. Special procedures for measurement of wave forces as well as techniques used for data analysis, specifically to remove extraneous effects introduced by the model's response to impact loads, are presented. Influence of the large protective barrier on wave crest characteristics around the structure are also provided in the paper. Results and discussion provide information on impact phenomena on various types of offshore structures, apart from the particular one used here. In view of the limited extent of information presently available, present results provide a useful addition to the engineering knowledge of such wave forces, their measurement and analysis.

INTRODUCTION

The Ekofisk complex, operated by Phillips Petroleum Company Norway, is located approximately 200 kilometers off the Norwegian coast near the UK border. The installation is characterized by a large circular shaped barrier housing a storage and processing facility, Within the near vicinity of this protective barrier are several jacket structures supporting production and accommodation facilities. A number of these jacket References and figures at end of paper. structures have access to the facility behind the protective barrier by means of connecting bridges.

The primary concern with the Ekofisk field is subsidence. Since the mid-80's when this problem was discovered, the operators have taken various steps to ensure the continued safe operation of the complex, including raising a number of decks on jackets and installing the protective barrier around the storage and processing facility as it presently exists. However, subsidence is continuing and is consequently increasinghe effective water depth within the field beyond the design water depth, This raises concerns about waves vertopping the protective barrier wall, causing damage to the facility inside and impacting on the decks of the nearby structures. Therefore a requirement to investigate the probability of such an occurrence, as well as to quantify the resulting impact loads on various components of the installation, has been presented.

A model study was commissioned at the Institute forMarine Dynamics in St. John's, Newfoundland by Phillips Petroleum Company Norway to measure and analyze wave impact loads resulting from predicted extreme wave conditions on various components of the Ekofisk facility. The model study involved design, construction and testing of 1:36 scale models of the protective barrier, 2/4 R jacket, connecting bridges and the 2/4 B jacket. Measurements included global and impact loads, water run up on the barrier wall and crest elevations in the surrounding field. The study generated a considerable amount of data, most of which is unique to the Ekofisk condition and is impractical to report here. However, the experimental techniques used to measure and interpret wave impact loads on the deck structures can be of widespread interest. These measured impact loads included resonant effects from the models.