Abstract

Structural safety of drilling rigs is a matter of major concern for operators. In spite of best industry practices a number of mishaps occur, sometimes taking heavy toll on investments and human lives. At times marine environment is also endangered due to accidents on offshore rigs. The paper presents a system design for altogether a new class of online monitoring system based on optical fibre technology for total rig structure providing ample opportunity to take pre-emptive action for averting mishaps arising due to metal fatigue and excessive overloading. The system will be very useful in the inspection and testing of new rigs. The system finds application in periodic preventive and corrective maintenance and would reduce the inspection time and expenditure. Such a system gains more importance on account of emerging trends in "re-qualification" of offshore drilling rigs.

Introduction

Safety of drilling rigs, both on-land and offshore, and its operating manpower is a major concern of the operators. In case of offshore rigs especially in the deep sea drilling, the magnitude of associated risk increases many folds and best efforts are made to achieve safest environment on these mammoth structures - fixed and floating weathering extreme environmental conditions. In spite of best practices mishaps occurs due to human, material and design failure affecting man, machine and environment. Operations in the deep sea drilling stretches these limits further and put more demanding safety measures.

Unfortunately, not even one month was passed since the abstract of this paper was submitted for the proposed conference, world's largest 40-story high offshore oil rig P-36 owned by Brazil's state oil giant Petrobras sank due to a powerful explosion in the Roncador oil field 125 km off the coast of Rio de Janeiro sate in the Campos Basin. Ten people were killed and one was seriously injured along with spillage of 395,000 gallons of crude and diesel into the ocean.

Condition monitoring of heavy equipment such as turbines, etc. is being done online but no system has been put into practice for online monitoring of the whole mammoth structure. Perhaps weight and space constraints don't allow for such an elaborate monitoring system. However, the recent accident of Petrobras's P-36 rig off Brazil coast makes a very strong point that whatever be the constraint, the online monitoring system for the whole structure is imperative for the safety of man, machine and the environment.

Why online monitoring

Drilling rigs especially the offshore ones are continuously subjected to various stress causing forces both external and internal. In addition to these mechanical forces arising out of buoyancy, axial, bending, torsion, internal and external pressure, there are number of other sources such as inflammable gases and huge quantity of diesel that can cause havoc in no time. Mechanical forces are well considered along with adequate safety margins at the designing stage. The design is put to non-linear stochastic dynamic response analysis in addition to traditional deterministic analysis to see the effect in extreme sea conditions. In this analysis the dynamic equation of motion is solved in time domain. Time series of relevant response quantities are calculated. These time series form basis for the estimation of extreme design forces and takes care of nonlinear drag, free surface affects and nonlinear wave kinematics. Deterministic and stochastic fatigue analysis provides safety margins for structural integrity over the fatigue life. In spite of such detailed analysis and simulation mechanical failures do occur because it is extremely difficult to simulate various natural and manmade forces acting on the drilling rig accurately making it difficult to predict the overall effect.

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