Subsea pipelines-deemed as the safest and most efficient means for transporting hydrocarbons, their integrity and long term maintenance are among the highest priorities for the oil and gas industry. These assets have a design life and are susceptible to damage from various factors during its operation. Operators should have in place a comprehensive Emergency Pipeline Repair System (EPRS). The EPRS should entail rigorous measures to minimize loss of production, the potential consequences of catastrophes and adverse effects on environment in case of an unforeseen event such as pipeline rupture or spill. The system should ensure swift response to restore the mechanical integrity of a damaged section whilst maintaining high HSE standards and minimizing the production shutdown-time duration. Accidental damages call for remedial response actions on an emergency basis. Considering short lengths of pipeline to be replaced, tow technique can offer an effective alternative to the time consuming and expensive conventional pipe-lay barge operations.

An installation method constituting the tow technique is proposed as a part of EPRS to replace damaged section of an operational pipeline. The paper discusses the applicable scenarios, the installation criteria and operational constraints involved in entire set of activities based on the study undertaken. The scenario analyzed involves launching a pre-fabricated pipe string offshore, towing and lay down at repair location. The pipe string is launched from an onshore facility by means of a lead tug, facilitated by rollers placed at designed intervals. The onerous task of pulling the pipe strings through long stretches of shallow water depths emphasizes the need of neutrally buoyant or slight positively buoyant pipe strings. Placing roller supports for the entire stretch is likely to pose several practical and economic constraints. The pipe string is towed to the intended offshore repair location with the assistance of a trailing tug. The pipe strings are laid down at the intended location in a controlled manner by means of winches and sequential release of buoys.

Numerical models of the system for towing and laydown operations are developed and related investigations are undertaken to evaluate the installation parameters, supplementary buoyancy requirements and to assess the criticality at various stages of installation. Further, the limiting weather criteria to carry out the operation are established based on the wave and current imposed hydrodynamic loadings on the strings and several governing parameters. The paper also briefs merits and limitations of the method.

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