Deepwater Pipelines and Flowlines
Authors: Igland Ragnar T. , Sævik Svein , Bai Yong , Berge Stig , Collberg Leif , Gotoh Koji , Mainçon Philippe , Thaulow Christian 
Published: October 2008

DEEP WATER PIPELINE DESIGN requires cost effective design, installation and operation. This implies challenges with regard to acceptance criteria for materials, welded joints, design requirements for installation and service, testing and verification procedures and safety evaluations.

SINTEF, DnV, and JP Kenny have carried out a three year joint industry project Deep Water Pipeline Design Criteria (DEEPIPE). The project was supported by the Norwegian Research Council, UK Health and Safety Executive, Statoil, Conoco, Phillips, Allseas, ETPM and Mannesmann Rör.

Introduction

The overall objective of the DEEPIPE project was to provide more cost effective design, installation and operation of deep water pipelines and flowlines. The DEEPIPE Project included a pilot study and a main program. The pilot study comprised three subtasks; General assessment - State of the art technology screening; Material technology; Design and construction of deep water pipelines.

The main program included development of a guideline stating design requirements related to installation and operation of pipelines in deep water. The guideline will be developed as recommended practices under DNV OS-F1011 including acceptance criteria for materials, welded joints, design requirements for installation and service, testing and verification procedures and safety evaluations.

Three main issues from this project will be presented in this paper; Fracture capacity of girth welds; Cyclic fatigue during laying and operation; In Service load effects.

Pipe laying - strain controlled loading

The most cost-efficient laying method for offshore pipelines is S-lay, Figure 1. S-lay has been applied in laying of a large majority of pipelines (diameter larger than 16 inches) in the North Sea. With the S-lay method, pipe sections are joined at welding stations on the deck of the vessel. The pipe is deployed over a stinger structure with rollers, to provide a smooth and controlled curvature of the pipe. The pipe is fed over the stinger into the sea by moving the vessel forwards.

Figure 1 S-lay configuration (Available in full paper)

The departure angle ? is essentially a function of the submerged weight of the pipe and coating, and the available force capacity of the anchor system. The stinger is vulnerable to wave loading, and for most of the larger laying vessels the stinger length cannot be extended further. An increase in departure angle must therefore be accommodated by an increased curvature of the stinger. Hence, the curvature and the applied strain to a pipe during laying are essentially determined by the departure angle.

For current pipelines at water depth of 300 – 500 m the nominal strain in the pipe wall during laying is limited to essentially elastic conditions. For deep water pipelines (1500 – 2000 m water depth) using the current generation of laying vessels, the departure angle may be approaching 90° due to limited force capacity of the anchor system. In this situation, considerable plastic strains may develop during pipe laying. It should be recognised that the pipe is loaded in a displacement or strain controlled situation. A more detailed discussion on limiting factors for deep water pipe laying was presented by Damsleth et al. (1999)2.