ABSTRACT

Subsea Isolation Valve (SSIV) is a key safety device for the riser system. This paper addressed the location analysis of the SSIV set on export gas riser/pipeline system which tied back to the Lingshui17-2 semi-submersible (SEMI). The analysis is based on a quantitative risk analysis of the riser release consequences with two possible locations of SSIV. The results of the study have helped determine the location of the SSIV, and reduced the riser risk of the Lingshui17-2 SEMI.

INTRODUCTION

Lingshui17-2 Gas Field is located in South China Sea, with water depth from 1220 m to 1560 m. The Lingshui17-2 Gas Field Development Project comprises of the following principal facilities:

  • A new-built production and oil storage SEMI;

  • Umbilicals and Steel Catenary Risers (SCR);

  • A Subsea Production System (SPS);

  • A 90 km 18" export gas pipeline from SEMI to the trunkline between YC13-1 and Hong Kong.

The LS17-2 SEMI has four floors, which are helideck, main deck, production deck, and mezzanine deck respectively. The 18" export gas SCR is on the North of SEMI, operating pressure of 135 bar.

SSIV is an emergency shutdown valve located in the subsea pipeline that is normally installed below the splash-zone, often on the seabed. SSIV will be incorporated in the gas export riser/pipeline System, to isolate the topside facility from pipeline/riser inventory in the event of a major accident topsides involving release of hydrocarbon. SSIV has been designed to safeguard the facility against uncontrolled gas release. It has been designed as a hydraulic actuated subsea valve.

SSIV provides the required isolation by the action of an actuator, which is mounted on the SSIV. SSIV is designed to operate on a fail-safe closed basis, the trip logic being integrated into the shutdown logic of the topsides control system.

A SSIV on an export gas pipeline may prevent backflow of gas from the pipeline. Thus, a possible fire will have a short duration. The location of the subsea valve is essential for the safety of the SEMI.

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