Strain Criteria for Deepwater Pipe Laying Operations
Authors: Perinet Dominique , Frazer Ian 
Published: October 2008

The pipelines are major components within an offshore oil and gas production facility. With the onset of deep water development combined with the full usage of gas, long gas pipelines will be built to transport the gas from deep water offshore fields to onshore liquefaction plants. The S-lay method is a very efficient pipelay technique, particularly for long pipelines, however the criteria used to control the loads in the pipe need to be re-evaluated in order to accommodate the deep water conditions. It is necessary to define a suitably high level for allowable strain, to both reduce the required tension and the size of the stinger in order to reduce the associated risks involved in the laying operation and maintain the overall safety levels. When applying the standard allowable strain level for S-Lay of 0.2%, steel pipelines will remain in the elastic regime, on the other hand strains of up to 2 % are applied to pipes during the reeling process. Consequently from a pure pipe integrity point of view it is possible to apply strain criteria very much higher than the conventional level. This paper will discuss the strain criteria to be applied for deep water S-Lay operations and explore the potential to significantly increase the allowable strain to levels approaching 0.5%. The impact of high strain levels on the overall integrity of the pipeline will be addressed and the influence on such areas as residual curvature, ovality, and fatigue performance will be highlighted. In addition the loads on the pipe in the overbend section, particularly the dynamic loads will be revisited along with the influence of buckle arrestors. Finally conclusions will be drawn and suitable strain levels for deep water S-Lay operations proposed.

Introduction

Due to the relatively remote location many deep water fields have been developed without a means of exporting associated gas, the gas being reinjected into a suitable reservoir for later recovery. To day as deep water fields become more common, and with the non flaring and the gas valorisation policies, which are fully in line with the optimization of the world energy resources, and reduction in CO2 production, the transportation of the gas associated with the deep water oil fields development, is a new challenge which must be addressed. The majority of the deep water fields developed to date, particularly in West Africa, have been oil fields with associated gas, where the valorisation of the gas has been a secondary issue. This situation is now changing as gas valorisation is a major concern for countries with deep water oil and gas reserves. As a result the demand for gas gathering and gas export pipeline systems linking deep water fields to onshore terminals and liquefaction plants will increase, making the pipeline installation a critical factor in this process. The experience of the industry with installation of deep water pipelines has increased significantly over the last 10 years, with a number of projects around the world such as Mardi Gras /1/, Blue Stream /2/, and Independence Trail /3/, as well as a many deep water field development projects in West Africa and Brazil.

Typically a pipeline transporting gas from a deep water field to an onshore liquefaction terminal will be of the order of 100 - 150 km long, and the typical diameter based upon annual throughput :24" for 12 billons standard cubic meter per year20" for 5 billons standard cubic meter per year16" for 2 billons standard cubic meter per year

The required wall thickness for deep water pipelines can be dependant by the water depth when the line is air filled, Table 1 gives some typical values of wall thickness for a 15?? and 20?? pipe as well as the submerged weight both air filled and water filled for a range of water depths.