Abstract

In recent years, many oil and gas fields have been discovered in ultra-deep sea (UDS). Some of these fields are evaluated to have no commercial value if existing oil field development approaches are used, especially while the oil prices remain low. A new alternative field development solution, termed as Subsurface Well Completion (SWC) system, is proposed with the aim to produce oil and gas in a cost-effective manner in UDS. This system primarily consists of four parts: a tethered subsurface platform, the rigid riser, SWC equipment and flexible jumper. Obviously, central to the evaluation and application of the new SWC technology is the inherent risk relative to acceptance level. In particular, an uncontrolled release of hydrocarbons to sea, which may lead to catastrophical consequences involving personnel risk, environmental damage and economic losses, is a main contributor to the total risk and of great concern to the offshore petroleum industry. As for the new SWC system, any failure will not be a direct source of risk for the personnel on the surface installation due to its offset feature. In this context, this paper proposes a quantitative risk assessment (QRA) framework to assess such uncontrolled releases to sea with regard to the SWC system for an oil field in the production phase based on the new Subsurface Tension Leg Production (STLP) facility. According to the QRA results presented in this paper, the identified scenarios representing uncontrolled releases to sea are subsea wellhead leaks, rigid riser leaks, subsurface wellhead leaks, releases from X-mas tree and flexible jumper leaks. Among these scenarios, subsea wellhead is found to be the high-risk area. Compared with the established risk acceptance criteria (RAC), the environmental risk levels for the subsea wellhead’s leak lie within the As Low As Reasonably Practicable (ALARP) region while other risks are all below ALARP limits, which means that there is a need for improved consideration of the existing design with regard to the subsea wellhead area, and the corresponding risk reduction measures are proposed. Furthermore, the sources and effects of uncertainties are reviewed and sensitivity studies are carried out to illustrate the effect of some of the important assumptions in the risk model. It can be found that some assumptions made are conservative or optimistic while others are unknown. However, the final QRA results can be regarded as somewhat conservative. This paper concludes that the new SWC technology has a distinct advantage with respect to the leakage duration time in UDS, and thus mitigates the environmental and commercial impacts to a large extent. Besides, relaxed design requirements for the X-mas tree and flexible jumper can be accepted. It is also concluded that there are no serious and major commercial losses for all the identified accidental release scenarios, which is of great importance and attractiveness to oil producers.

Highlights

  • Introductionultra-deep sea (UDS) oil and gasusing fieldsexisting have been discovered.oilSome these fields fieldsInare evaluated have no of commercial value conventional field of development are evaluated to have no commercial value using existing conventional oil field development approaches [1] (dry tree units or subsea developments, or a combination of both [2]), especially while approaches [1]

  • The main objective of this paper is to propose and apply a quantitative risk assessment (QRA) framework [11] for such uncontrolled releases to sea in the production phase with regard to the Subsurface Well Completion (SWC) system of the Subsurface Tension Leg Production (STLP) facility

  • commercial risk acceptance criteria (CRAC) are established, as shown in Tables 2 and 3. Both environmental risk acceptance criteria (ERAC) and CRAC values are focused on the maximum acceptable annual probability and corresponding release quantity of oil to sea

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Summary

Introduction

UDS oil and gasusing fieldsexisting have been discovered.oilSome these fields fieldsInare evaluated have no of commercial value conventional field of development are evaluated to have no commercial value using existing conventional oil field development approaches [1] (dry tree units or subsea developments, or a combination of both [2]), especially while approaches [1] 1, is proposed and canThe be regarded as context, a subsurface development, which[3,4], is a totally new oilinfield development approach. Regarded as aaims subsurface development, which is a totally new oil field development approach. STLP facility at producing oil and gas in a cost-effective manner in UDS and primarily consistsThe of facility aims at producing oil and gas in a cost-effective manner in UDS and primarily four parts: a tethered subsurface Sea-star platform (SSP), rigid risers, SWC equipment and consists flexible of four parts: tethered. UDS oil and gasusing fieldsexisting have been discovered.oilSome these fields fieldsInare evaluated have no of commercial value conventional field of development are evaluated to have no commercial value using existing conventional oil field development approaches [1] (dry tree units or subsea developments, or a combination of both [2]), especially while approaches [1] (dry oil prices remain low. tree units or subsea developments, or a combination of both [2]), especially whileInoil prices remain this context, the low.

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