Lateral stability of a pipeline is achieved by ensuring a balance between external lateral forces and seabed friction. Stability assessment of submerged pipelines has been commonly performed in accordance with DnV RP E305 1988 requirements. Based on the sequence of installation process, which is applied to the pipeline, an increase of embedment depth will potentially occur since the pipe will be flooded with water. This process causes the pipeline’s submerged weight to increase significantly. Based on OTC 5851 paper “Forces on Sheltered Pipelines” the embedment leads to an increase of soil resistance and reductions of hydrodynamic forces experienced by the pipe due to the less exposed area. The reductions in hydrodynamic forces are accounted through the modification of hydrodynamic coefficients. To have this embedment taken into stability assessment, DnV RP E305 has stated several requirements to be fulfilled.

Oleh : M. Munari, R. Gantina, H. Ibrahim, K. Idris, T. Fahrozi

Abstract.

Lateral stability of a pipeline is achieved by ensuring a balance between external lateral forces and seabed friction. Stability assessment of submerged pipelines has been commonly performed in accordance with DnV RP E305 1988 requirements. Based on the sequence of installation process, which is applied to the pipeline, an increase of embedment depth will potentially occur since the pipe will be flooded with water. This process causes the pipeline’s submerged weight to increase significantly. Based on OTC 5851 paper “Forces on Sheltered Pipelines” the embedment leads to an increase of soil resistance and reductions of hydrodynamic forces experienced by the pipe due to the less exposed area. The reductions in hydrodynamic forces are accounted through the modification of hydrodynamic coefficients. To have this embedment taken into stability assessment, DnV RP E305 has stated several requirements to be fulfilled. Another aspect that would contribute to pipeline stability is the nature of environment condition of pipeline surroundings. This has been done by assessing the visual data of the pipeline. Thus, the most suitable conditions of the pipeline have been considered. This paper exemplifies the consideration of those conditions within the assessment of submerged pipeline stability.

Keywords: pipeline lateral stability, submerged pipeline, seabed friction, sheltered pipeline, hydrodynamic coefficients, on-bottom stability, DnV RP E305 1988, OTC 5851.

1. Introduction

An independent on-bottom stability analysis on PT Perusahaan Gas Negara (PGN) offshore pipeline system at East Lampung shore has been carried out based on the measurement of existing environmental condition. The analysis is also based on current data collected by the main contractor during pipeline system installation.

The purpose of this paper is to exhibit the various analyses performed within the pipeline zone. The stability analysis was carried out in accordance to Ref [1] requirements, PGN specifications, and existing environmental parameters. The pipeline had been installed for about four month after its initial installation and has been scheduled to be flooded by water for hydro-testing purpose. Thus, the analysis will consider both the existing (hereby called ‘installation’) and operating conditions, which include:

1. Reviewing the visual documentation of ROV Survey;

2. Soil bearing capacity analysis; and

3. On-bottom stability analysis for various water depths within the zone of concern.

2. Input Data and Criteria

The met ocean data used in the analysis were collected from both PGN supplied data and installation contractor. Environmental data and design criteria used in the analysis are stated in the following sub-section.

2.1 Water Depth

Based on Ref [2], the water depth within the zone of concern is varying from approximately 13 m to 16 m.

2.2 Geotechnical Data

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