PSI - Issue 14

Praveen Bhat et al. / Procedia Structural Integrity 14 (2019) 656–663 Praveen Bhat, S. Karunanithi/ Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

ONGC has been operating in the western offshore of India for more than four decades. Many of the oil & gas fields discovered in the western offshore basin in the past have matured over a period of time resulting in decline of production rate of the fossil crude. At the same time, aggressive exploratory campaign has helped in identifying newer offshore finds, many of them being marginal in nature. Considering the factors related to techno-commercial constraints for offshore development of marginal/satellite fields and recent low oil price regime, the need for newer development methodologies is trigged. In context of the same, IEOT-ONGC took up a study for assessing feasibility for re-location of three existing non-producing offshore platform to newer locations having water depths in the range of 22-34m, as presented at Table 1.

Table 1. Feasibility Study for Relocation to new fields.

Water Depth (m) at original location

Platform to be relocated

New Location

Water Depth (m) at new location

A A B B B C C C

25.00 25.00 19.00 19.00 19.00 19.00 19.00 19.00

Y Z X Y Z X Y Z

25.10 33.84 22.65 25.10 33.84 22.65 25.10 33.84

In the context of relocation of existing platform, the plugging & abandonment shall be completed prior to removal of platform. Before re-locating the existing platform to the new location, it is of paramount importance to follow the abandonment and decommissioning procedures meticulously. The platform relocation process will include the abandonment and decommissioning aspects like well plugging and abandonment (P&A), purging of hydrocarbons from pipelines and equipments followed by platform removal which will be performed in stages as reverse order of installation of platform. For assessing the structural adequacy of the three platforms at the relocation sites, integrity check analysis has been carried out as per API RP-2A, (WSD). An in-place static global analysis has been carried out for 100 year extreme storm condition for 8 directions of wave, current and wind with other design loads and marine growth of 100mm from (+) 6.0m to (-) 30m & 50mm from (-) 30m to mud-line ( Ref. 2 ).

Nomenclature A, B & C Name of platforms for which the relocation feasibility is to be ascertained X, Y & Z Name of new locations where Platforms A, B & C are proposed to be relocated

2. Abandonment and decommissioning

2.1 General Decommissioning of offshore structures is being carried out in Gulf of Mexico and North Sea region for quite some time. In Gulf of Mexico alone, operators have responsibly performed their decommissioning obligation, having removed approximately 4000 ( Ref. 3 ) platforms by now. ONGC, in first of its kind project in India, intends to employ the established philosophy for successfully decommissioning some of its platforms and relocating them to suitable locations for continued usage. The main activities involved in decommissioning of offshore platforms and allied facilities are outlined below: • Engineering/surveys