Lateral buckling design for flowlines in deep and ultra-deep water often relies on the use of sleepers or distributed buoyancy modules as buckle initiators. However, these methods can be associated with complex engineering, manufacturing or installation and may not be cost-effective, especially when several buckle triggers are required.
The Residual Curvature Method (RCM) has often been used for shallow water projects but has rarely been proposed for deep and ultra-deep water due to the challenges in terms of installation, landing orientation on very soft soils associated with deep water, and the resulting in-place initiation reliability.
This paper presents how the Residual Curvature Method can be designed for deep and ultra-deep water flowline installation to ensure a controlled as-landed residual curvature (RC) configuration on the seabed, resulting in a reliable lateral buckling initiation for flowlines operating under high internal pressure and temperature (HPHT) conditions. In particular, the mitigations proposed to address the RC over-rotation challenges in the sagbend during installation will be presented. The RC feature integrity assessment considering high top tension combined with dynamic sea state is also covered, along with the creation of the proposed RC design with a reel-lay vessel. Other installation considerations specific to deep and ultra-deep water are discussed. The case study is the Mero 2 project offshore Brazil.
Subsea rigid pipelines subjected to high pressure and high temperature (HPHT) during operation will try to expand axially. Where the pipeline is exposed on the seabed, it can buckle to release this expansion. The challenge for the design engineer is to control the locations and sizes of each buckle via buckle initiation devices at specific locations along the pipeline route. Several buckle initiation methods are available to help mitigate pipeline in-service expansion.
The Residual Curvature Method (RCM) is a technique of installing pipelines with an imposed residual curvature (RC) to initiate a controlled lateral buckle during operation. Residual curvature sections are traditionally achieved with reel-lay vessels by not fully straightening the pipeline over short sections.
The entire RCM engineering process is split into three assessment phases, as per Fig. 1.
