4.1 Introduction

This chapter addresses stability analysis of marine pipelines on the seabed under hydrodynamic loads (wave and current) and provides guidelines for pipeline stabilization using concrete coating. It does not address alternative methods such as pre- or post-trenching techniques, mattress covers, etc. Stability is checked for the installation case with the pipe empty using the 1-year return period condition and for lifetime (pipe with concrete) using the 100-year storm.

4.2 Analysis Procedure

There are several basic approaches to determining the required submerged weight for a marine pipeline. One of them is use of AGA Program LSTAB. It should be used in cases where the pipe is partially embedded or pre-trenched as the lift, drag, and inertia coefficients are adjusted for exposure. Regardless of the computer program selected, hydrodynamic stability analysis involves the following steps:

• Step 1: Collect or define environmental criteria for the 1-year and 100-year conditions, including:

  • Water depth

  • Wave spectrum

  • Current characteristics

  • Soil properties

  • Seabed condition

• Step 2: Determine hydrodynamic coefficients: drag ( C _D), lift ( C _L), and inertia ( C _I). These may be adjusted for Reynolds Number, Keulegan Number, ratio of wave to steady current, and embedment.

• Step 3: Calculate hydrodynamic forces, typically, drag ( F _D), lift ( F _L), and inertia ( F _I).

• Step 4: Perform static force balance at time step increments and assess stability and calculate concrete coating thickness for worst combination of...