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Stavanger, Norway

Dybwad J.,Wood Group Kenny Norge | Bryndum M.,Wood Group Kenny Norge | Hollingworth R.,Marathon Oil
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2014

During the periodic inspection of the Alvheim subsea system 2013 a number of cracks were observed at the Mid Water Arch (MWA) tether anchoring arrangement. The MWA and associated anchor block are critical design elements. Detailed investigations were initiated in order to determine future development of the cracks and their severity. The application of advanced non-linear finite element analysis as part of the inspection and maintenance strategy resulted in significant cost savings compared to a solution based on immediate mitigation action. This paper describes the background for occurrence of these cracks and the analyses used to determine their development: • The cracks are located in non-loadbearing locking brackets. The function of the brackets is primarily to secure the pins connecting the top part of the tether hinge to the anchor block. • During construction the locking brackets were welded to the pin and to the tether hinge. This way the non-structural element became part of the load bearing system resulting in very high stresses in the bracket and subsequent crack development. It could not immediately be excluded that the cracks observed could initiate further cracking into main bearing parts of the hinge. • FE modeling using Abaqus [1] was used to analyze the criticality of the situation. Non-linear material properties and removal of elements were applied in order to simulate crack initiation and crack growth. The system was analyzed by modelling the load paths from initial assembly on land, installation loads and finally the loads during operation. Removal of elements was introduced to replicate the crack growth pattern observed on ROV still photos from periodic surveys 2012 and 2013. The analysis documented the principle mechanism behind the crack development and further demonstrated that the risk of failure of any of the load bearing elements was negligible. The results of the analysis provided the necessary documentation for the appropriate precautions and at the same time plan for execution of mitigation measures which would have minimal economic impact. Copyright © 2014 by ASME. Source

Ratnayake R.M.C.,University of Stavanger | Garten T.,Wood Group Kenny Norge | Barre A.,Wood Group Kenny Norge
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2015

Subsea systems' functional failure consequence classification (FFCC) and initial screening has been a stringent requirement for subsea systems operating on the Norwegian Continental Shelf (NCS). Hence, it is of great importance to establish approach(es) for the subsea systems' FFCC. The study performed in this manuscript focuses on adapting existing guidelines, which are available in NORSOK Z-008, to develop an approach for performing the FFCC for a subsea system. A case study has been carried out which is closely aligned with a real-time project to perform subsea manifold related FFCC. The approach has been developed with the help of existing internal documents, data, information, and the experiences of the subsea systems' owner as well as requirements pertaining to regularity authorities' related guidelines, other existing similar work and established standards. The manuscript also illustrates a framework of the work process and illustrative analysis results. © 2015 by ASME. Source

Gil C.,Wood Group Kenny Norge | Tornes K.,Wood Group Kenny Norge | Damsleth P.,Wood Group Kenny Norge
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2014

A study has been performed to better understand ultimate bending moment and strain capacities of pipelines in relation to criteria defined in the design codes. An 18″ HPHT flowline was designed to undergo global buckling on uneven seabed and to resist trawl gear interference. The high temperature (155 degC) and pressure (300 bar) posed considerable design challenges for material selection and design criteria. A CRA-lined X60 CMn pipeline was selected for the project. The pipeline was of seamless manufacture for which the stress/strain characteristics are subject to the effect of Lüders bands. The DNV-OS-F101 code covers a wide range of D/t but does not specifically address Lüder's material behaviour which could significantly reduce the bending moment capacity of pipe. The global buckling and trawl pull-over FE analysis results indicated the pipe was highly utilized, requiring excessive amounts of seabed intervention at great cost to meet the DNV LCC criteria. Detailed FE simulation of limit states for local buckling and strain localization of a 3D solid element pipe model was performed, with both Roundhouse and Lüders material properties, to investigate pipe capacity in relation to that stipulated by the design codes. The pipe moment capacity was established by obtaining the moment curvature relationship by bending the local pipe section subject to internal pressure until the maximum resistance was reached. Imperfections were introduced to initiate local buckling at the desired location. To determine strain concentration factors and strain localization, the effects of thickness changes and weld misalignment were also studied. The DNV OS-F101 LCC moment criterion formulation computes a decreasing moment capacity for increasing internal pressure. It has been suggested in the literature that this is correct for higher D/t but the criterion may be conservative for pipes with lower D/t. The combination of Lüders material with low D/t is not specifically addressed by any design code. Clarification of these aspects will provide a better understanding of the risk of failure for highly utilized seamless pipelines and allow for modified design criteria that will reduce seabed intervention costs. The results of the study showed that a higher bending moment criterion and associated strain criterion could be adopted for the design that allows for the higher initial strain caused by Lüder's plateau. The ultimate bending moment capacity of low D/t pipe with Lüder's material was found to be similar to that of Roundhouse material due to work hardening. In addition, it was demonstrated that the potential strength of the CRA liner could enhance the moment capacity of the seamless pipe. Copyright © 2014 by ASME. Source

Brankovic M.,Wood Group Kenny Norge | Vervik S.,Wood Group Kenny Norge | Loken E.,Wood Group Kenny Norge | Damsleth P.,Wood Group Kenny Norge
Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE | Year: 2015

A pipeline system consists of the flexible pipe and all associated ancillary components up to the connecting components at battery limits. The global system design deals with aspects related to the overall flexible pipe system - not cross-section or component design. Key system design issues to address are specified in ISO 13628-11:2007 (API 17B) and include general system design requirements, flowline design requirements, and interfaces involving different contractors and suppliers. Unlike unbonded flexible risers that operate in tension, flexible pipe laying on the seabed will be subject to compressive forces and global buckling as for rigid pipelines. The global flexible pipeline system design will use relevant design codes such as ISO 13628-11:2007 and API 17B / 17J. Yet, the terminology and what such design encompasses can be interpreted differently amongst the community of flexible designers, installers and operators. Failure modes, for example, are not perceived in the same way. While axial compressive force may not be allowed for a riser design, it is a normal flexible pipe response to expansion when pressurized on the seabed. Pull-in and connection forces and moments may be perceived as maximum loads by installers and subsea equipment suppliers but the pressure-induced hydrotest and operational loads may exceed these and overstress the piping leading to a HISC failure. The system functional requirements for flexible pipelines are similar to rigid lines but when it comes to a flowline laid on the seabed, exposed to pressure and temperature, the methodology for establishing accurate predictable global behaviour is still under development in the industry. To perform the global in-place design of a flexible flowline, a detailed understanding of the effects of pressure and temperature on the unbonded flexible construction with respect to its bending stiffness and expansion and the limiting criteria stipulated by the manufacturer is needed. The flexible's behaviour can be simulated using a 3D FE model capturing the effects of seabed surface variations, interaction with soil, and entire load history from installation, to flooding and hydrotest to filling with product at a range of pressures and temperatures including start-up shut-down cycles. Whereas general requirements are stipulated in API 17, specific guidelines applicable to global buckling design are found in DNV-RPF110 for rigid pipelines. This predictive analysis methodology using risk-based design criteria provides the basis for a robust design that can accommodate large variations in installation tolerances, uneven seabed, varying soils, etc., while providing all concerned with a sound understanding of the flexible behavior and interface loads throughout its design life. © 2015 by ASME. Source

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