Fatigue Life Enhancement of Semisubmersible Rigs

ABSTRACT Details of engineering techniques of cost-effective, safe and reliable means of improving structural fatigue lives of spaceframe semisubmersible drilling rigs have been presented. Furtner advancement to fatigue life calculations has been validated and proposed. An integrated procedure for rig upgrading has been outlined. Different ways of achieving extended fatigue life and the attending cost impact has been examined. INTRODUCTION Market rates of semisubmersible drilling rigs and the cost of building new rigs do not complement each other. This present situation is expected to continue for significant time. Cost estimates have shown that current day rates can justify the investments required to upgrade older rigs to present day standards, both in terms of operational and regulatory requirements. Consequently, rig owners are seriously thinking about the engineering feasibility of such upgrading and if possibly what would be the engineering involvment, costs, safety, operational reliability and structural integrity. An in-depth study/investigation was undertaken which has carefully considered the followings points:Past fatigue performances of all key joints.Regulations today and future trends.Recent research into fatigue strength improvements.Advances into fatigue calculation methods. The results were integrated into a systematic engineering solution procedure which yields an inexpensive, safe and reliable way of extending and enhancing the fatigue lives of the older rigs. ECONOMIC CONSIDERATION The demand for semisubmersible drilling rigs has grown steadily and cautiously from its lowest level of June 1987. Associated day rates have moved up proportionately but still lagging behind as the curve for supply still exceeds tile market demand for these rigs (Fig 1). Nevertheless, the supply of rigs is reducing gradually due to planned retirement of older rigs and a virtual stop in the new buildings. Assuming that demand and supply curves continue in the same way as have been so far these curves will not converge till significantly in the future. This will not warrant additional supply of right for several years. However, supply may need to be stabilized before and it is prudent to be prepared in advance. The day rate trend does not indicate any sharpe rise in the short term and consequently anticipated day rates in 2 to 3 years time will only be a fraction of what is required for new building of rigs to be economically viable. Under the circumstances, therefore, supply of rigs will not be increased. Supply can be stabilized by revitalizing the older rigs ready to retire. This means engineering solutions must be found to be able to upgrade the older rigs up to today's regulatory standards and customer's specifications at a cost that can be supported by present day rates. UPGRADING REQUIREMENT First we need to establish what kind of upgrading/enhancement is required to these older rigs which will satisfy present day demands for efficient, safe and reliable drilling operations. This approach can be subdivided into two categories.