The Torsion Handbook, Understanding and addressing torsion in the handling of cables, umbilicals and flexible pipes

This document was a deliverable in the Torsion JIP (Joint Industry Project) carried out by SINTEF from 2018 to 2022 on behalf of Aker Solutions, Equinor, Hellenic Cables, NKT, Petrobras and Ørsted. It will be freely available for download here, from 18th April 2023. The document gives a comprehensive description of why torsion appears when handling (producing, transferring, loading out, installing, and in some cases operating) power cables, umbilicals and flexible pipes. The document also provides a first generation of approaches to design products and handling operations to prevent torsion related issues

Algorithmic formulation of clay and sand pipe–soil interaction models for on-bottom stability analysis

This paper presents a new algorithmic formulation of the clay and sand pipe–soil interaction models recommended by the DNV-RP-F109 code for dynamic on-bottom stability analysis of submarine pipelines. The pipe–soil force update algorithm is formulated within the framework of computational elasto-plasticity and applies Backward-Euler integration to ensure stability and robustness for large time step sizes. Algorithmic optimization techniques are developed by utilizing a closed-form solution and subincrementation. A numerical verification study covering full cyclic displacement ranges of a 12 inch pipeline is presented. The new formulation is shown to increase the time step size by a factor of up to 50 compared to commercial software tools for on-bottom stability analysis. This achievement will be particularly beneficial for long-duration 3D nonlinear time domain on-bottom stability analysis.publishedVersio

Formulation and Application of Finite Element techniques for Slender Marine Structures Subjected to Contact Interactions

The main purpose of this work is to formulate and apply new computational strategies for two contact-governed problems where existing finite element software suffer from poor efficiency and lack of robustness. The first problem is concerned with trawl board pull-over interaction of subsea pipelines while the second deals with reeling analysis of history-dependent flexible pipes. Previous numerical models for trawl-pipe interaction based on simplified geometry modeling has struggled with contact-related convergence problems. A contact element with a continuous description of the trawl board contact geometry and the pipe geometry was therefore developed. The assumption of a rigid contact geometry for the trawl board and the use of line-line and line-point contact kinematics resulted in good numerical efficiency properties. The ability to predict the pull-over responses depends heavily on the modeling of the trawl board hydrodynamic loads. A fairly advanced six degree of freedom load model with precomputed hydrodynamic coefficients was therefore established. An extensive simulation work was carried out to validate the trawl-pipe computational strategy and to identify sensitive model parameters. Regarding the former, the proposed numerical model was demonstrated to predict pull-over load impulses within a 10% margin of model test measurements and was thus concluded to be capable of describing the relevant effects of the pull-over. The sensitivity study revealed that the interaction behavior was greatly influenced by the board-pipe friction coefficient, the tension level in the wire between board and trawl net, the towing line drag properties and the direction of over-trawling. Due to the sensitivity of the input parameters, it was concluded that a proper validation against experimental tests is necessary for future work of similar kind. Further studies should aim to quantify the degree of non-conservatism present for nonperpendicular crossings and attempt to improve current design load recommendations by including more model parameters. Reeling operations with history-dependent material behavior and extensive contact interactions along the material transport route are often not feasible to simulate with conventional finite element software. This relates to contact-related convergence problems and the need for long meshes with small and equal-sized elements giving poor numerical efficiency. These issues were successfully solved by developing a Lagrangian-Eulerian beam formulation that enabled for a virtually fixed mesh in space. The proposed formulation was subjected to various benchmark tests where it was demonstrated to provide similar accuracy as the conventional Lagrangian method. In recent years, subsea contractors have experienced torsional failures in spoolbasevessel load-out operations of flexible pipes. An idealized finite element model was therefore established to gain insight into such operations and to identify the mechanisms responsible for the generated torque. Torsional failures were identified for three different mechanisms and strategies to avoid them were proposed. A comparison study against a physical load-out operation should be conducted in future work to quantify the ability to predict the torque and to reveal possible model deficiencies

Simulation of Trawl Loads on Subsea Pipelines

The main objectives in this thesis was to investigate the effect of oblique trawl board crossings, increased trawl board added mass due to seabed proximity and the effect of a more rectangular trawl board geometry. In addition a new hydrodynamic load model which handles the seabed proximity and forward speed in a more consistent way was examined. All simulations in this thesis are performed by means of the computer software SIMLA. A brief description of methods applied in SIMLA and nonlinear finite element analysis is therefore included. The thesis contains also a chapter which describes trawling concepts and trawl boards used in Norwegian waters. Design loads from trawl gears on subsea pipelines are nowadays based on recommendations from the DNV-RPF111 code. Simulation models with a 4500 kg polyvalent trawl board were established to verify the DNV recommendations for free spans of height 0 m and 1 m. The simulations demonstrated that increasing trawl board added mass due to seabed proximity did not have any influence on neither pull-over loading nor pipeline response. The effect of a rectangular trawl board geometry was most pronounced for a span height of 0 m because the duration increased by 0.5 s and the horizontal pull-over force was kept constant throughout the pull-over. A slighty larger pull-over loading compared to the polyvalent board was observed for a span height of 1 m. Oblique trawl board crossings were examined for 6 different hit angles. The major finding was that a perpendicular crossing did not predict the largest pull-over load. On a general basis the simulations for a span height of 1 m underpredicted maximum pull-over force, duration and pipeline displacement compared to the DNV-RP-F111 recommendations. The 0 m span height simulations indicate that DNV predicts a different shape of the load time history and is slightly nonconservative in terms of maximum pull-over load. The new hydrodynamic load model which includes the effect of forward speed and seabed proximity was used to simulate a perpendicular trawl board crossing. Here the span height of 0 m indicated that the DNV-RP-F111 code is nonconservative in terms of the pull-over load. The simulation for a span height of 1 m was however in very good agreement with the DNV-RP-F111 code in terms of duration and horizontal pull-over load. Therefore it is recommended that future simulations are based on the new hydrodynamic load model

Real option analysis on offshore day-by-day contracts in the North Sea : is there additional value for the charterer?

This thesis investigates the value of optionality in day-by-day spot contracts for PSV and AHTS in the OSV-spot market, where the charterer can replace the initial vessel with an alternative vessel in the spot market or renegotiate the contract, by using the real options embedded in the contracts. Even though there are usually replacement costs for the charterer when replacing the initially hired vessel prior to the contract’s expiry, a significantly large spread between the initial contract rate and the current spot rate may still see the charterer benefit from terminating, or renegotiating, the initial contract. We perform an analysis based on a stochastic process, where parameter estimates are based on historic spot rates for various vessel types from 1996 to 2017. In addition to vessel type, we further distinguish between different exercise frequencies/options, where AHTS contracts allow for exercising the termination option every fifth day, and PSV contracts every day. We find that the optionality may add value for the charterer for both PSV and AHTS, especially in markets with high demand, hence high rates. However, we find that the value of the optionality is significant more attractive in the AHTS, despite it lower assumed exercise frequency. This is mainly due to the higher volatility of AHTS spot rates, making thus AHTS has higher probability of exercise the option. We also perform a sensitivity analysis on the parameters, which suggest that replacement cost is the most sensitive parameter affecting the value of the optionality. Additionally, our thesis discusses how a possible renegotiation process might affect the valuation of the optionality. Lastly, our conclusion provides an overview of the analyses performed, and sums up the valuation of the optionality for various contract specifications.nhhma

Formulation and Application of Finite Element techniques for Slender Marine Structures Subjected to Contact Interactions

The main purpose of this work is to formulate and apply new computational strategies for two contact-governed problems where existing finite element software suffer from poor efficiency and lack of robustness. The first problem is concerned with trawl board pull-over interaction of subsea pipelines while the second deals with reeling analysis of history-dependent flexible pipes. Previous numerical models for trawl-pipe interaction based on simplified geometry modeling has struggled with contact-related convergence problems. A contact element with a continuous description of the trawl board contact geometry and the pipe geometry was therefore developed. The assumption of a rigid contact geometry for the trawl board and the use of line-line and line-point contact kinematics resulted in good numerical efficiency properties. The ability to predict the pull-over responses depends heavily on the modeling of the trawl board hydrodynamic loads. A fairly advanced six degree of freedom load model with precomputed hydrodynamic coefficients was therefore established. An extensive simulation work was carried out to validate the trawl-pipe computational strategy and to identify sensitive model parameters. Regarding the former, the proposed numerical model was demonstrated to predict pull-over load impulses within a 10% margin of model test measurements and was thus concluded to be capable of describing the relevant effects of the pull-over. The sensitivity study revealed that the interaction behavior was greatly influenced by the board-pipe friction coefficient, the tension level in the wire between board and trawl net, the towing line drag properties and the direction of over-trawling. Due to the sensitivity of the input parameters, it was concluded that a proper validation against experimental tests is necessary for future work of similar kind. Further studies should aim to quantify the degree of non-conservatism present for nonperpendicular crossings and attempt to improve current design load recommendations by including more model parameters. Reeling operations with history-dependent material behavior and extensive contact interactions along the material transport route are often not feasible to simulate with conventional finite element software. This relates to contact-related convergence problems and the need for long meshes with small and equal-sized elements giving poor numerical efficiency. These issues were successfully solved by developing a Lagrangian-Eulerian beam formulation that enabled for a virtually fixed mesh in space. The proposed formulation was subjected to various benchmark tests where it was demonstrated to provide similar accuracy as the conventional Lagrangian method. In recent years, subsea contractors have experienced torsional failures in spoolbasevessel load-out operations of flexible pipes. An idealized finite element model was therefore established to gain insight into such operations and to identify the mechanisms responsible for the generated torque. Torsional failures were identified for three different mechanisms and strategies to avoid them were proposed. A comparison study against a physical load-out operation should be conducted in future work to quantify the ability to predict the torque and to reveal possible model deficiencies

Algorithmic formulation of clay and sand pipe–soil interaction models for on-bottom stability analysis

This paper presents a new algorithmic formulation of the clay and sand pipe–soil interaction models recommended by the DNV-RP-F109 code for dynamic on-bottom stability analysis of submarine pipelines. The pipe–soil force update algorithm is formulated within the framework of computational elasto-plasticity and applies Backward-Euler integration to ensure stability and robustness for large time step sizes. Algorithmic optimization techniques are developed by utilizing a closed-form solution and subincrementation. A numerical verification study covering full cyclic displacement ranges of a 12 inch pipeline is presented. The new formulation is shown to increase the time step size by a factor of up to 50 compared to commercial software tools for on-bottom stability analysis. This achievement will be particularly beneficial for long-duration 3D nonlinear time domain on-bottom stability analysis