Reeled pipelines are submitted to cyclic deformation during reeling and unreeling operations. To assess the risk of ovalisation of the pipe after reeling and unreeling, OCAS investigated the behaviour of the plastic deformed pipe.
Offshore pipe reeling is a more recent technique to install pipes in deeper waters. Steel pipes are welded together onshore, rolled onto a large diameter reel and unreeled into the sea at the desired offshore location. Therefore, a long string of pipes, welded together and spooled from the reel, undergoes a reversible bending loading. When the pipe is plastically deformed, its ovality increases permanently. This ovalisation is a deformation in the cross section of the pipe, while the bending is in the axial direction.
After an analysis of the process, a testing programme to evaluate the material properties under reversible loading and to evaluate the shape of the yield locus in function of the deformation was established. The testing matrix was designed to calibrate a model taking into account the evolution of the yield locus.
The obtained results are implemented in a model for further simulation of the reeling installation process. The material model with the calibrated coefficients allowed to significantly improve the prediction of the ovality.
The scientific paper “Reeling pipeline material characterization – testing, material modeling and challenging strain limitation”, was presented by Ph. Thibaux during the ASME 2014 33rd International conference on Ocean, Offhsore and Arctic Engineering, June 8-13, 2014, San Francisco, USA (OMAE2014-23442). This work was done by OCAS in close collaboration with Heerema Marine Contractors and DNV Norway.
Heerema Marine Contractors (HMC) is entering a new era of laying pipelines using the reel-lay method in order to be more cost effective for deep and shallow water pipeline installation projects. The new Deep water Construction Vessel (DCV) Aegir is designed to be able to reel/J-lay pipelines for a range of pipe dimension and water depth combinations.
It is well known that when a pipeline is installed by the reeling method, it undergoes cyclic plastic straining and the pipe plastically deforms. Due to the applied plastic bending moment, the residual deformation in term of residual pipe ovality after reeling is difficult to predict by the FE analyses without a throughout understanding of the material characterization and changes under cyclic plastic straining. Another challenge of reeling pipeline material is the single-event/cycle strain limitation. The presence of a variation of pipe property and geometry (stiffness mismatch) of the welded line pipes can initiate a high local strain in the base material far above the level of the expected nominal strain.
The paper describes how the material behavior of seamless pipe under reeling cycles plastic strain has been characterized by a comprehensive material testing program including bauschinger tests and perpendicular pre-straining tests. It turns out that for seamless pipe, by looking to the yield stress locus of the material after plastic straining, the reeled pipe material which is initially shown an isotropic behavior in un-strained condition will change and evolve to show an anisotropic behavior. The material in the hoop direction of the pipe will become more hardened than the material in the longitudinal direction of the pipe. The characteristic of cyclic plastic strained material has been modeled and implemented in an user subroutine of a FE analyses software. Discussion and comparison is given between the ovality prediction using the developed material model and the ovality measurement from the full scale bend tests.
The second part of the paper discusses a series of high plastic strain tests (up to 7% strain), both in tension or compression mode, with seamless pipe material. The purpose of the tests is to investigate a possible degradation of material after reeling and challenge the present allowable limits according to the DNV standard code. Main challenge is the setup of the high compression strain cyclic strain tests. Discussion is given to compare and justify the test results against the DNV-OS-F101 limitation of line pipe supplementary plastic deformation with respect to the acceptance of the tensile (elongation break), charpy and hardness test results.
“By using an innovative testing approach, we managed to obtain excellent material data under reeled loading conditions. Once fed into the numerical model, they have led to a very good fit between the model behaviour and the properties observed in the full-scale reeled pipe.”