Abstract

Subsea system consists of multiple inter-connected subsea structures. Hence, the foundation deployed for supporting subsea structures is designed with stringent requirement on settlement performance. This paper presents a case study of adopting probability approach to shed light on the impact of manifold suction pile sinking due to pile top seal failure. The impact assessment identified the tie-in subsea structures which would be subject to high risk of damage based on different pile sinking scenario, allowing mitigation plan to be reasonably devised during project execution phase. Recommendations are included for practical reference.

1. INTRODUCTION

A subsea system consists of multiple inter-connected subsea structures. Hence, the foundation deployed for supporting the subsea structure is designed with stringent requirement on settlement performance. The impact of foundation settlement shall be evaluated at local as well as system levels.

In this paper, a case study, deriving from a deepwater project i.e. Liuhua 29-1 (LH29-1) located offshore South China Sea, is presented. The LH29-1 subsea manifold is an eight-slot single-header subsea manifold connected to seven wells via a network of subsea production and control systems comprising of the followings:

• Production system: Christmas tree (XT), production/flexible flowline, pipeline end terminal (PLET), spool/jumper etc.

• Control system: umbilicals, hydraulic flying lead (HFL), MEG flying lead (MFL), Electrical/Optical flying lead (EFL, OFL), umbilical termination head (UTH), umbilical termination assembly (UTA) etc.

Figure 1 shows the subsea production system of the LH29-1 deepwater project. The LH 29-1 production flowline is tied in to the existing Liuhua 34-2 subsea system and then Liwan 3-1 subsea system. The LH29-1 control system is tied in directly to the Liwan 3-1 control system.

The subsea manifold has a submerged weight of approximately 162 MT and is supported by an 11-m long suction pile with an outer diameter of 7.6 m and wall thickness of 32 mm. Figure 2 shows the interface arrangement of the subsea manifold and the suction pile.

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