Assessment Of An Analysis Of An Instrumented Drilling Jackup Conductor

Abstract

ABSTRACT This paper presents an assessment of some of the results of an internationally funded joint industry project to measure the dynamic behaviour of a drilling conductor run from a large jack-up platform operating in deep water at the North Sea. Three different elevations spanning the conductor were instrumented using environmentally protected strain gauges while the rig was operating. The objective was to gather data for selected storms for subsequent spectral and statistical analyses and to validate finite element analysis of a drilling conductor system. The response amplitude operators (IWO) from the analysis showed good agreement with the measured RAOS. This agreement increases confidence in the validity of the loading and response calculation methods currently available for the design of drilling conductors. In the frequency analysis, the computed natural periods showed good agreement with those measured under different drilling conditions. Measurements showed that the natural frequency oscillation of the conductor was mainly in the first mode of vibration. In the time domain analysis, the computed bending loads were found in good agreement with measured values particularly at the middle and lower gauged levels. The analytical results showed that for small waves the dynamic amplification was substantial when comparing static and dynamic analytical techniques. However, for large design waves the dynamic amplification was found to be small and the static analytical technique was found to be accurate in predicting environmentally induced loads. INTRODUCTION Designed originally for shallow waters, jack-up platforms are increasingly used in harsher and deeper waters for exploration and exploitation of offshore oil and gas fields. A new generation of jack-up platforms can operate nowadays in water depths up to 360 feet. Conducting drilling activities in harsh environmental conditions at the North Sea pose challenging problems for the design of an adequate conductor system capable of withstanding the structural and extreme environmental loads. The conductor which represents an environmental barrier and structural support for the subsequent casings must withstand the environmentally imposed loads of waves, current and wind combined actions. Current practice of drilling from a jack-up platform uses a set of concentric casings known as the casing programme. The outer casing or conductor commonly of 30" diameter is followed by successively longer and smaller diameter casings until the pay zone of the well is reached. The tubing running inside the last casing and reaching the pay zone is usually referred to as the production tubing. The casings are cemented in the holes up to specified depths. Another feature of jack-up drilling practice is the mudline suspension system usually fitted a few feet below the mudline, where the weight of the casings below the mudline is supported and hence the weight supported at the surface is reduced. The mudline suspension system which consists of a set of casing hangers also provides a disconnect point at the mudline so that the conductor and casings extensions from the seabed to the surface can be removed when the platform moves off location.