Concrete Coated Submarine Pipelines: Further Advances In Strain Concentration At Field Joints And Relevant Implications On Strain Based Design
Authors: Bruschi R. , Malacari L. Ercoli , Torselletti E. , Vitali L. 
Published: October 2008

ABSTRACT
Recent projects in the North Sea stress the importance of the rationalization of &sign criteria for oil and gas submarine pipelines. From the revien of relevant literature it appears that a key issue in this process is the introduction of strain based criteria, which have sometimes been advocated in the engineering practice. In particular, this is relevant to pipelay conditions which are a topical issue of the coming project scenarios. In fact, in some development projects the allowance for higher strains on the overbend or high usage factors on the sagbend of an "S" lay span, is in discussion these days. In these circumstances, the effect of the concrete coating, in terms of additional bendingsttfjkess and strain concentration at jield joints, may become very important.
The scope of this paper is to investigate the flexural behaviour of concrete coated pipes outside the linear elastic range. New advances in modelling the mechanical behavior of concrete coated joints are discussed. They account for the effect of the non linear stress-strain relationship of the steel at large usage factors curvatures on the strain concentration at the field joint, An approach for the simulation of pipe string during laying and, hence, for the assessment of peak strains at field joint, is finally proposed.

INTRODUCTION
In the early 90's submarine pipeline technology has focused on the foundation of new criteria for pipe installation, Bruschi et al. (1994). 'Tlis interest was triggered by new projects of large diameter submarine pipelines which are envisaged as being ready for operation in the second half of this decade, Ansebni and Bruschi (1993). The installation requires the employment of the most advanced laying equipment, sometimes even beyond the claimed or experienced target capacity.
During the late 80's laying operator were interested in extending the operability of their laybarges with minor upgrading works, while pipeline operators were at the beginning concerned about the pipeline integrity. Then the need to rationalize lay criteria became a must for both parties. In fact, in some circumstances lay criteria were relaxed on the basis of mere engineering judgment and project experience. These objectives find scope in Joint Industry Research projects performed during these recent years. These projects aim at reviewing the design criteria established in the late seventies by introducing limit state methods and reliability analyses, Sotberg and Bnrschi (1992).
Further from study activities which were backing the discussions about the most appropriate criteria applicable to specific project, it came out that upgrading of the laybarges was however necessary. A relevant example is the extension of the ramp and the increase of the pulfing capacity of the tensioner system of the C!ASTORO SEI laybarge in order to face projects like the crossing of the Strait of Gibraltar, Baker (1995), and other projects irr the North Sea, Breivik et al. (1995) and Hauskcn (1995).
Irr the meantime new laybarges and lay equipment have been proposed in the offshore market, often claimed by lay Contractors as being capable to lay pipes safely even in ultra deep waters.