EnginSoft - Conference Abstracts

EnginSoft Users' Meeting 2008
Le Tecnologie CAE nell'Industria

The use of FEM Analyses in predicting the mechanical behaviour of offshore pipelines

Vitali Luigino - Saipem Energy Services
Bruschi Roberto - Saipem Energy Services

Abstract

In the recent years, the offshore pipeline industry has been under pressure to provide solutions for: demanding material and line pipe technology problems, installation technology to safely tackle the ultradeep waters challenge, quantitative prediction of reliable operating lifetime for pipelines under high pressure/high temperature conditions and remedial measures to tackle considerable geo-morphic and human activity related hazards. Today new links, until now unthinkable, and ultra-deep subsea development as well (definitely more than 80% of new projects), are coming up. The most relevant project experiences (engineering, main contracting, construction, operation) have been deeply analysed to establish a guideline for the needed technological development. New R&D efforts are addressed into two major development areas:

• Very long large diameter high pressure trunklines for ultra deep waters crossings, and
• Steel risers, flow and inter-field lines for ultra deep water production systems.

Future pipelines are being planned in very difficult environments i.e. crossing ultra-deep water and difficult geo-seismic-morphic conditions. Project experience has clearly shown that potential cost savings in material, installation and special subsea works are strictly linked to advanced engineering analysis and related design criteria possibly based on limit state principles. Competent engineering is now more than ever addressed to ensure to meet stringent and quantitative safety targets, imposed by the international requirement for sustainable development. As regard design criteria, new rules have been issued for offshore pipelines. These codes provide rational design criteria and limit state equations with associated partial safety factors, see for example DNV Offshore Standard OS-F101 and OS-F201. In most circumstances, minimum wall thickness requirements are related to containment of internal pressure. Actually, for ultra-deep water conditions, failure modes related to the external pressure are more relevant than bursting: design checks against cross section collapse and propagation buckling under external pressure alone, and ovalisation buckling under combined bending, axial and external pressure loads, are the relevant ones. At design stage, FE models, suitably calibrated to experimental tests, can be used as a numerical laboratory for predicting the strength capacity and the pipeline behaviour during the installation and operational phase. Advantages with respect to experimental tests are related to a better control of the relevant parameters affecting the pipe behaviour, a fine monitoring of loads and deformations along both the pipe axis and pipe circumference, and a larger amount of information as regard both stresses and strains on the pipe model at a cheaper cost. Furthermore, this approach allows reducing costs, to design the pipeline safely and to properly assess the technical and economical feasibility of the project. In this paper all the previous issues are discussed in term of FEM calculation approach.