As subsea pipeline developments continue to move into deep and ultra-deep water locations, there is an increasing need for the accurate prediction of expected pipeline fatigue life. A significant factor that must be considered as part of this process is the accumulated fatigue damage experienced by a pipeline weld during installation, i.e. before commencement of operations.
Previously, the installation-related damage for a pipeline could only be estimated onshore using appropriate finite element models and assumed environmental loadings. The drawback of this traditional approach is that it often results in an over-conservative value for the predicted installation fatigue damage sustained by the pipeline, and so leads to a reduction in the estimated fatigue life for the pipeline. The primary source of the over-conservatism is the fact that limiting sea-states must be applied as part of the fatigue analysis. Such requirements are often overly stringent given that there is only a small probability of normal lay operations being carried out offshore during such severe sea-states.
Advanced subsea engineering company, MCS, now offers a quantum leap forward with the development of a pipeline installation and fatigue monitoring system for use on-board pipe laying vessels.
This software tool can be used to estimate the fatigue damage suffered by the pipeline during the installation phase based on certain measured inputs (such as vessel position, tension, current profile, sea-state etc.). The primary output from the system is the fraction of the pipeline fatigue life that has been used-up during the installation process. Further secondary outputs from the system include typical installation parameters such as pipe tension, stresses, strains, etc. These outputs are provided in pseudo-real-time based on the latest measured inputs. The system can also provide vessel positioning advice in order to keep the pipeline on-route.
The on-board tool itself is mostly based on the capabilities of the MCS pipeline installation analysis software PipeLay. PipeLay is an integrated engineering tool designed specifically for the analysis of a range of pipeline installation scenarios. The software combines an intuitive Windows™-based Graphical User Interface (GUI) allowing rapid data specification, with powerful analytical capabilities. PipeLay has been developed with significant input from many of the biggest names in the pipeline installation world, including Allseas, Heerema, Saipem, Technip and Petrobras.
From the viewpoint of the user, PipeLay is designed as an engineering tool rather than a finite-element package. Inputs are all in engineering terms and the software automates many of the tasks associated with building complex finite element models, running and post-processing multiple load cases and presenting results in a concise report-ready format. These features contribute to the significant labor-saving potential of the software.
In terms of analytical capabilities PipeLay has a number of advanced features which allow for more detailed modeling as well as for a reduction in conservatism. For example, the program offers a number of different options for modeling the contact between barge/stinger supports and the pipeline. These options range from the simplest model where only a single finite-element node is restrained by an individual support to the most complex one where a number of different nodes are restrained by a support and so the support reaction is distributed over a length of pipeline rather than being just applied to a single point. The program can also model both fixed stingers where the motions of the stinger are completely defined by the motions of the vessel and articulated stingers where the stinger itself becomes included in the finite-element model and so can move independently of the vessel under its own weight, etc. There is a sophisticated pipe-in-pipe and pipe-on-pipe capability in PipeLay that can account for the presence of bulkheads and centralizers as well as being able to model pipe wall interaction and axial motion between the relevant pipes. PipeLay’s seabed model allows for flat, sloping and arbitrary seabed profiles. Sea beds in the program can be considered to be either elastic or rigid and there are also a number of options available for modeling seabed friction. A useful feature of the program’s analysis engine is the automatic iteration on model configuration to satisfy some user defined criteria. This feature involves the analysis engine changing a particular aspect of the model in a way that a user specified parameter such as top tension or bending stress falls within a certain range. The fatigue analysis capability is another key feature and it can be used to determine the damage sustained by critical joint welds during the journey from the firing line to the seafloor. Both normal lay operations and so called staged operations such as free start-up can be considered for fatigue analysis.
The on-board pipeline installation and fatigue monitoring system is also based on components of the MCS software Optima, which is an on-board riser operations management and planning tool that has been supplied for use on-board a range of drilling vessels and production facilities. Optima and PipeLay are both based on the finite element (FE) analysis engine of the MCS software Flexcom which has been widely used and accepted throughout the offshore oil and gas industry for more than 20 years.
A key underlying principle of this advanced on-board package is that it has been designed to operate offshore with a minimal level of operator intervention. This high level of automation is reflected in the fact that the system operates on its own in terms of acquiring the necessary data, performing the various simulations and storing the subsequent results.
The simulations themselves are based on finite element models of the various installation scenarios which are created on-shore in the office using PipeLay. During the simulations the models are subjected to a set of measured conditions which are acquired from a variety of vessel systems such as the tensioner system. Some of these simulations are dedicated to tracking the fatigue damage at each weld during installation while others provide pseudo real time results/vessel positioning advice. All results/output are displayed on a dedicated on-board operator’s interface and also are stored in suitable databases for future review and analysis.
The single greatest benefit of this on-board monitoring system is that the predicted installation fatigue damage sustained by the pipeline is computed offshore during the actual installation project and so is based on accurate measurements rather than conservative assumptions. This removal of conservatism can ultimately result in an extended estimation of the fatigue life for the pipeline.
MCS expects that the use of such an on-board system will become a more integral part of future pipeline developments as both operators and installation contractors strive to evaluate/demonstrate the pipeline fatigue damage associated with installation.
Robert O’Grady joined the MCS Software Division in 2005 after graduating from the National University of Ireland, Galway, with a degree in civil engineering. He is the Product Manager for the MCS pipeline installation analysis software PipeLay. In this role he is responsible for all aspects of the product development, including client liaison, identification of customer requirements, leading technical developments, customer training and strategic planning. In 2007, he won the award for “Young IT Practitioner of the Year” at the Information Technology Association Galway (ITAG) Industry Awards.