Erik Levold

The behaviour of an offshore steel pipeline material subjected to bending and stretching

Guidelines have been worked out on how to design sub-sea pipelines in fishing-rich areas subjected to the possible interference by trawl gear or ship anchors. One topic of special interest for the offshore industry is pipelines first subjected to impact from an anchor before being dragged along the seabed. After removal of the load, the pipe will be straightened due to rebound and present axial forces. The material in the deformed impact zone will experience a complex stress and strain history, which subsequently can cause cracking, leading to leakage or full failure. To study these topics, full-scale testing is not straightforward and thus a simplified approach is chosen as a first step in the present study. Motivated by the observed local curvature in impacted pipelines, three-point bending tests of plate strips cut from a typical offshore pipeline have been carried out and the strips subsequently stretched to a straight position. One objective of these tests was to investigate whether cracking in the plate strip could occur after such a loading sequence. Material tests with specimens taken in different directions and at different locations in the actual pipe were carried out to calibrate an appropriate constitutive relation (taking anisotropy and kinematic hardening into account) and a simplified fracture criterion. Numerical simulations of the complete loading sequence were finally carried out and the predicted response was validated against the experimental data.

Damage and Failure in an X65 Steel Pipeline Caused by Trawl Gear Impact

Offshore pipelines subjected to accidental impact loads from trawl gear or anchors may experience large global deformations and large local strains, creating a complex stress and strain history. In this study experiments and numerical simulations have been carried out to investigate the impact of a pipeline which is subsequently hooked and released. Material and component tests have been performed to investigate the behaviour during impact, and to observe if/when fracture occurs. The pipes were first impacted in a pendulum accelerator at varying velocities before they were pulled straight in a tension machine. Fracture was found in the impacted area of all the pipes during straightening. Material tests were done to determine the characteristics of the X65 grade steel. Numerical simulations showed excellent compliance with the impact phase, while the load level in the stretching phase was a bit overestimated.Copyright

Submarine Pipeline Installation JIP: Strength and Deformation Capacity of Pipes Passing Over the S-Lay Vessel Stinger

The development of deep water gas fields using trunklines to carry the gas to the markets is sometime limited by the feasibility/economics of the construction phase. In particular there is a market for using S-lay vessels in water depth larger than 1000m. The S-lay feasibility depends on the applicable tension at the tensioner which is a function of water depth, stinger length and stinger curvature (for given stinger length by its curvature). This means that, without major vessel up-grading and to avoid too long stingers that are prone to damages caused by environmental loads, the application of larger stinger curvatures than presently allowed by current regulations/state of the art is needed. The work presented in this paper is a result of the project “Development of a Design Guideline for Submarine Pipeline Installation” sponsored by STATOIL and HYDRO. The technical activities are performed in co-operation by DNV, STATOIL and SNAMPROGETTI. The scope of the project is to produce a LRFD (Load Resistant Factor Design) design guideline to be used in the definition and application of design criteria for the laying phase e.g. to S and J-lay methods/equipment. The guideline covers D/t from 15 to 45 and applied strains over the overbend in excess of 0.5%. This paper addresses the failure modes relevant for combined high curvatures/strains, axial, external pressure and local forces due to roller over the stinger of an S-lay vessel and to sea bottom contacts, particularly: • Residual pipe ovality after laying, • Maximum strain and bending moment capacity. Analytical equations are proposed in accordance with DNV OS F101 philosophy and design format.Copyright

Submarine Pipeline Installation Joint Industry Project: Global Response Analysis of Pipelines During S-Laying

The development of deep water gas fields using trunklines to carry the gas to the markets is sometime limited by the feasibility/economics of the construction phase. In particular there is market for using S-lay vessel in water depth larger than 1000m. The S-lay feasibility depends on the applicable tension at the tensioner which is a function of water depth, stinger length and stinger curvature (for given stinger length by its curvature). This means that, without major vessel up-grading and to avoid too long stingers that are prone to damages caused by environmental loads, the application of larger stinger curvatures than allowed by current regulations/state of the art, is needed. The work presented in this paper is a result of the project “Development of a Design Guideline for Submarine Pipeline Installation” sponsored by STATOIL and HYDRO. The technical activities are performed in co-operation by DNV, STATOIL and SNAMPROGETTI. This paper presents the results of the analysed S-lay scenarios in relation to extended laying ability of medium to large diameter pipelines in order to define the statistical distribution of the relevant load effects, i.e. bending moment and longitudinal strain as per static/functional, dynamic/total, and environmental load effects. The results show that load effects (longitudinal applied strain and bending moment) are strongly influenced by the static setting (applied stinger curvature and axial force at the tensioner in combination with local roller reaction over the stinger). The load effect distributions are the basis for the development of design criteria/safety factors which fulfil a predefined target safety level.Copyright

Dynamic Simulation of Free-Spanning Pipeline Trawl Board Pull-Over

The main objective of this effort was to examine if finite element analyses can be used to predict pull-over loads from a trawl board. Here the trawl board was represented by a 6-DOF hydro-dynamic load model, where the mass and drag coefficients are expressed as functions of seabed gap, seabed inclination angle and heading angle. Both seabed proximity and forward-speed effects of the trawl board are hence included. The applied drag coefficients were established by model testing, while the hydro-dynamic mass was found by potential theory calculations. Compared to previous efforts, the hydrodynamic loading on the trawl board is represented in a far more consistent way in this paper. A simulation model which contained a polyvalent trawl board and a free-spanning pipeline was established. Several simulations were performed with span heights between 0 m and 2 m. In all simulations the pull-over force and pipeline response was sampled. The sampled results were thereafter validated by means of the analysis method and pull-over loading proposed in the DNV-RP-F111 code. Some differences could be observed in the response histories, but in summary the numerical model predicts a realistic pull-over. This indicates that the applied hydrodynamic load model captures the relevant effects during the pullover.Copyright

On the Influence of Mechanical and Geometrical Property Distribution of the Safe Reeling of Rigid Pipelines

The reel-lay method is a fast and cost efficient alternative to the S-lay and J-lay installation methods for steel pipelines up to 20″ in diameter. The quality of the pipeline construction is high due to onshore welding under controlled conditions. However, reeled pipelines are subject to plastic straining (up to approx. 2.3%) during installation. It is therefore common practice to specify a minimum required wall thickness to avoid on-reel buckling. For a given pipe outside diameter and bending radius, formulae developed for pipes under pure bending are generally used. In addition, to ensure the integrity of pipelines during reeling, a minimum spooling-on tension is specified and tolerances on pipe properties, such as wall thickness and yield strength, are constrained. Tolerance limits are specified to reduce the likelihood of spooling two consecutive pipe joints, which have a significant difference in plastic moment capacity (mismatch). It has been shown previously that high levels of mismatch can trigger an on-reel buckle [1]. The reliability of the reeling process is indeed related to the uniformity of pipe properties. It can therefore be supposed that more uniform pipe properties may allow reeling of thinner-walled pipes, while achieving the same level of reliability. This issue has been investigated as part of a wider evaluation of reeling mechanics and the development of procedures for optimized assessment of the process, including such aspects as the effect of the geometry of pipelay equipment [2]. This paper explores methods that can be used to evaluate the reliability of reeling a given pipe onto a given vessel. Particular focus is given on the selection of appropriate material variation parameter for the assessment. The concept of an averaging factor is introduced as a means to relate variations in individual wall thickness and yield strength measurements to the variation in pipeline cross-section, which determines the likelihood of buckling. It is suggested that, in the future, this factor could be used as a method for optimizing design for reeling when using higher quality pipe.Copyright

Efficient Finite Element for Evaluation of Strain Concentrations in Concrete Coated Pipelines

MARINTEK has developed software for detailed analysis of pipelines during installation and operation. As part of the software development a new coating finite element was developed in cooperation with StatoilHydro enabling efficient analysis of field joint strain concentrations of long concrete coated pipeline sections. The element was formulated based on sandwich beam theory and application of the Principle of Potential Energy. Large deformations and non-linear geometry effects were handled by a Co-rotated “ghost” reference description where elimination of rigid body motion was taken care of by referring to relative displacements in the strain energy term. The non-linearity related to shear interaction and concrete material behaviour was handled by applying non-linear springs and a purpose made concrete material model. The paper describes the theoretical formulation and numerical studies carried out to verify the model. The numerical study included comparison between model and full-scale tests as well as between model and other commercial software. At last a 3000 m long pipeline was analysed to demonstrate the strain concentration behaviour of a concrete coated pipeline exposed to high temperature snaking on the seabed.Copyright

HotPipe JI Project: Experimental Test and FE Analyses

In the last twenty years, experimental tests and FEM-based theoretical studies have been carried out to investigate the buckling mechanisms of thin-walled pipes subject to internal pressure, axial force and bending moment. Unfortunately, these studies do not completely cover the scope relevant for offshore pipelines i.e. outer diameter to thickness ratio lower than 50. In the HotPipe Phase 2 JI Project, full-scale bending tests were performed on pressurized pipes to verify the Finite Element Model predictions from HotPipe Phase 1 of the beneficial effect of internal pressure on the capacity of pipes to undergo large plastic bending deformations without developing local buckling. A total of 4 pipes were tested, the key test parameters being the outer-diameter-to-wall-thickness ratio (seameless pipes with D/t = 25.6, and welded UOE pipes with D/t = 34.2), and the presence of a girth weld in the test section. For comparison a Finite Element Model was developed with shell elements in ABAQUS. The test conditions were matched as closely as possible: this includes the test configuration, the stress-strain curves (i.e. using measured curves as input), and the loading history. The FE results very realistically reproduce the observed failure mechanisms by formation and localization of wrinkles on the compression side of the pipe. Good agreement is also achieved in the moment capacities (with predictions only 2.5 to 8% above measured values), but larger differences arose for the deformation capacity, suggesting that the DNV OS-F101 formulation for the characteristic bending strain (which is based on FE predictions from HotPipe Phase I) may be non-conservative in certain cases.Copyright

Transverse Deformation of Pressurised Pipes With Different Axial Loads

Pipelines residing on the seabed are exposed to various hazards, one of them being denting, hooking and release of the pipeline by e.g. anchors or trawl gear. As a pipeline is displaced transversely in a hooking event, an axial tensile load resisting the displacement builds up in the pipeline. This study examines the effect of applying three different axial loads (zero, constant, and linearly increasing) to a pipe while simultaneously deforming it transversely. A fairly sharp indenter conforming to the prevailing design codes was used to deform the pipes. These three tests were repeated with an internal pressure of about 100 bar for comparison. Adding an axial load appeared to increase the pipe’s stiffness in terms of the force-displacement curve arising from deforming the pipe transversely. The internal pressure also increased the stiffness, and produced a more local dent in the pipe compared with the unpressurised pipes. All tests were recreated numerically in finite element simulations. Generally, the results of the simulations were in good agreement with the experiments. INTRODUCTION Pipelines are an integral part of the offshore industry and will continue to be so for the foreseeable future. Multiple hazards are present in the waters [1], and close to the coast pipelines ∗Corresponding author may suffer impact and hooking by e.g. anchors or trawl gear [2]. An initial impact typically causes a dent in the pipe, and if the impacting object hooks the pipeline it may displace it significantly, during which membrane forces arise in the pipeline. When the pipeline is released, it recoils back towards its initial position, thereby creating a complex load history. The open literature provides studies on impact against tubular structures of various character, ranging from rectangular cross-sections [3] to the more complicated T-joints [4]. Circular cross-sections are the most common, and are studied experimentally [5], theoretically [6] and numerically [7]. Inclusion of pressure in pipes during impact has also been investigated [8, 9]. Manes et al. [10] attempted to recreate the loading sequence of impact, hooking and subsequent release of an X65 pipline by subjecting strips taken from an actual offshore pipeline to quasistatic three point bending tests. The strips were then pulled straight and checked for fracture, which was present only as minor surface cracks without exerting any influence on the forcedisplacement curves. Later, simply supported X65 steel pipes were subjected to a dynamic impact before being pulled straight in quasi-static tension to emulate the release after hooking [11]. Here, fracture was a dominant part of the problem. When investigating fracture, dynamic effects from springback can be important [8, 12]. During an impact and hooking event the pipeline will deform locally and a dent will form under the impactor, and large Proceedings of the ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering OMAE2017 June 25-30, 2017, Trondheim, Norway

Strength and Deformation Capacity of Corroded Pipes: The Joint Industry Project

Considering the future development for offshore pipelines, moving towards difficult operating condition and deep/ultra-deep water applications, there is the need to understand the failure mechanisms and better quantify the strength and deformation capacity of corroded pipelines considering the relevant failure modes (collapse, local buckling under internal and external pressure, fracture / plastic collapse etc.).A Joint Industry Project sponsored by ENI EP• The full-scale laboratory tests on corroded pipes under bending moment dominated load conditions, performed at C-FER facilities, are shown together with the calibrated ABAQUS FE Model;• The results of the ABAQUS FEM parametric study are presented.© 2013 ASME