A Response-Based Method for Developing Joint Metocean Criteria for On-Bottom Pipeline Stability

Abstract

A response-based method for deriving joint metocean criteria for the on-bottom stability design of submarine pipelines is described. The method consists of applying a response function to convert a long time history of ocean wave and current conditions into a corresponding pipe response time history, from which extreme values and the joint metocean criteria can be obtained. In the case of designing for the 100-year return period conditions, the joint criteria are defined as either the value of the steady current to associate with the wave conditions that have a return period of 100 years or the value of the wave conditions to associate with the value of the steady current that has a return period of 100 years. The method involves first determining the extreme values, of the waves, currents and the responses, such as those with a 100-year return period, and then deriving the pair of wave and current combinations by inverting the response function at the 100-year return period level. Response functions for both the traditional static design approach and the state-of-the-art dynamic simulation design approach are presented. Both response functions are expressed in terms of the pipe weight per unit pipe length required for the pipe to be stable. In the case of the traditional approach, this is the weight required to keep the pipe static on the sea bed; but in the case of the dynamic approach, it is the weight that will allow a maximum lateral pipe displacement of three pipe diameters. The dynamic response function was developed from the AGA Level-3 pipeline stability software, by performing pipeline response simulations for many combinations of different sea state and steady current conditions. Joint wave and current criteria obtained from the response functions, for two test locations, are compared and evaluated against both the independent wave and current criteria and the joint criteria values recommended in design codes. A method is also outlined on how the response functions may be used to make estimates of the pipeline reliability with respect to on-bottom stabilization.