TechnipFMC (United Kingdom)

Abstract Flexible pipelines are often subjected to cover requirements on the seabed due to protection or stability issues. These requirements may be satisfied using a range of techniques, e.g. covering the pipe with rocks or soil. Due to pressure and thermal expansion in operation the pipe may be susceptible to lateral or upheaval buckling (UHB) out of the cover which may violate the cover requirements and could ultimately be detrimental to the flexible pipe. The present paper is concerned with the design of reliable and cost-efficient cover solutions for unbonded flexible pipes. A new DNV guideline will introduce “Allowing Irreversible Displacement” design approach which allow for the pipe to move within the cover provided that the end-of-life limit states for pipe and cover are satisfied. The paper includes a description of how a model following that approach may be defined, including considerations on load case matrix and a discussion on relevant sensitivity cases to ensure conservatism in the cover design. A case study is included to show how the principles of the guideline may be applied in UHB design.

The interface shear strength of polypropylene pipeline coatings and marine sandy soils was investigated through direct and surface-over-soil interface shear box testing. Polypropylene specimens were acquired by removal from existing manufactured steel pipes and test soils were fabricated to closely resemble typical compositions and particle size distributions of North Sea marine sediments. The test sands varied according to their coarse particle fractions, with 0, 15 and 35% being retained on a 0·4 mm sieve. Testing was carried out at the very low stresses pertinent to pipeline interfaces, between 2·5 and 37·5 kPa, in both loose and dense states. The experimental results suggest a dependency of the interface shear strength on the stress level and relative density, with the coarse particle fraction playing a modest role. Surface characterisation and lack of volumetric deformation suggests that the shearing kinematic is predominantly grain sliding rather than rolling. Interface efficiency was largely constant despite some scatter due to variability in surface specimens. The distinct seams apparent on some of the polypropylene surfaces as inherent manufacturing artefacts had a negligible influence on interface strength. The relationship between interface strength, normalised roughness and Shore D hardness was assessed, discussed and compared with results from other works.

Abstract Super Duplex Stainless Steel (SDSS) is the preferred material utilized in steel tube umbilicals, due to its excellent corrosion resistance and high mechanical strength. SDSS is susceptible to Hydrogen Induced Stress Cracking (HISC) under the influence of a Cathodic Protection (CP) system, especially the welded zone. Porosity (relevant rounded indication) is one of the most common weld flaws that impacts the SDSS weld reject rate based on historical acceptance criteria. A study has been carried out investigating the influence of weld porosity on 25% Cr (SDSS) butt welded tube, under the effect of CP potential by using a Slow Strain Rate Testing (SSRT) technique. The study focused on the correlation between the level of porosity and the material mechanical performance when exposed to a simulated CP system in seawater at an elevated temperature. The impact of HISC was evaluated through an assessment of various tube welds including welds with different levels of porosity quantified by number, size and location of pores, when exposed directly to natural CP system in seawater for 4 and 8 weeks. Post hydrogen pre-charging, the samples were subjected immediately to a modified Slow Strain Rate Test (SSRT). After hydrogen charging and SSRT the resulting fracture morphology was examined using a scanning electron microscope (SEM), and the failure mode was assessed in relation to porosity locations. Experimental results indicate that all hydrogen pre-charged samples with or without pores decrease in ductility after exposure to the CP system which was further confirmed by the fractography analysis. It was observed that susceptibility of SDSS to HISC is broadly similar for welds with porosity or without porosity and brittle phenomena is more pronounced at the exposed surfaces of the samples. The study has demonstrated that the butt weld of SDSS umbilical tube with a controlled porosity level in line with ASME design codes does not show an increased susceptibility to HISC under CP system when compared to welds with no porosity.

Abstract Rigid pipelay vessels use two types of equipment to hold the pipe under tension while it is being deployed on the seabed: tensioners and hang-off clamps (HOC). Tensioners are used to maintain tension while paying out the pipe behind the vessel, while the HOC holds the pipe statically during welding operations. For both pieces of equipment, the tension is held through frictional pads in contact with the pipe. With the necessity to reach greater water depths leading to an overall increase of pipeline weights and stiffness (and therefore top tension), the installation of rigid pipelines has become more challenging. In other cases, such as pipe-in-pipe (PiP), carrier pipe wall thicknesses are often optimised as much as possible while lower grade materials are selected to reduce costs, making it more difficult to provide pipelay equipment settings that can satisfy both grip and pipe local integrity requirements. Simple hand calculations and basic equipment modelling are not sufficient to provide safe equipment settings and optimised allowable sea states. The new DNVGL-ST-N001 [1] (Marine Operations and Marine Warranty) also increases scrutiny on pipelay equipment and pipe interaction, making it imperative to develop more in-depth methodologies for settings calculation. This paper gives an overview of a robust methodology for pipelay equipment settings and demonstrates through practical examples how it improves safety while optimising operations, therefore enabling more efficient pipelay operations and reducing waiting on weather.

A range of sands were tested in direct and interface shear at very low stresses to determine the interface shear strength of polypropylene pipeline coating counterfaces and to evaluate interface efficiency. Polypropylene has a wide range of applications as a coating material in the offshore environment, so quantification of interfacial strength is an important component for geotechnical design. Direct shear tests show classic peak-postpeak stress-displacement and stress-dilatancy behaviour whereas interface tests show an elastic, perfectly plastic type behaviour for both loose and dense samples with no appreciable volumetric response. Interface efficiencies generally range between 0.3 and 0.7 dependent on both grain size and stress level. Normalised roughness is used to relate the surface roughness to the grain size and shows that the greater interface strength with smaller grained sands can be explained by their greater effective roughness. The relationship between stress ratio and normalised roughness for sand-polypropylene resembles established relationships for sand-steel interfaces.

Many natural and man-made slopes are planted with vegetation, and it is known that this can increase the stability of slopes under static conditions. There is anecdotal evidence that vegetated slopes also perform better than fallow slopes during earthquakes. However, the study of the dynamic behaviour of slopes planted with species having dichotomous (‘woody’) roots is relatively rare owing to the extreme expense and difficulty involved in conducting full-scale dynamic testing on shrubs and trees. In this paper, dynamic centrifuge testing and supporting numerical modelling have been conducted to study this problem. In the centrifuge modelling, ABS plastic rods are used to simulate repeatably the mechanical properties of real roots. The numerical modelling work consisted of two parts. First, a computationally-efficient beam-on-non-linear-Winkler-foundation (BNWF) model using existing p–y formulations from piling engineering was employed to produce a macro-element describing the individual root and soil interaction both pre- and post-failure. By adding contributions from the different root analogues of different diameters, smeared continuum properties were derived that could be included in a fully dynamic, plane-strain continuum, finite-element model in a straightforward way. The BNWF approach was validated against large direct shear tests having stress conditions simulating those in the centrifuge at different potential slip plane depths. The conversion to smeared properties for global time-history analysis of the slope was validated by comparing the continuum finite-element results with the centrifuge test data in terms of both the dynamic response and permanent deformations at the crest, and these demonstrated good agreement. Owing to the simplicity of the BNWF approach and its ability to consider variable root geometries and properties, along with variation of soil properties with depth, it is suggested that the validated approach described will be useful in linking individual root–soil interaction characteristics (root strength and stiffness, diameter variation, root spacing and so on) to global slope behaviour.

Abstract The present work was carried out with the objectives of supplying to CP designers data collected in situ, in the actual environment in which the protected structure is deployed. Sensors arrays have been deployed at six different sites from 35 m to 900 m depth. The measurements are based on the deployment of a CPC sensor which records the coupling current between a cathode and an anode, and the cathode potential. The effect of temperature and pressure on calcareous deposit properties was put into evidence and was useful to explain the different pseudo polarization curves. For carbon steel: The results are broadly quite in a good agreement with standard and recomendation1,2. For stainless steel: The initial current was independent of the location; the steady state current demand at -600 mV/Ag.AgCl depends strongly on the environment ; the current demands tent to decrease with time in tropical and arctic zones, but increased in sub-tropical area and the current demand at -950 mV/Ag.AgCl does not depend on the location.

Pipelines installed by the reel lay method are plastically deformed during installation. The nominal level of plastic deformation is determined by the vessel equipment geometry and pipe dimensions. The natural variation of wall thickness and yield strength determines the potential differences in bending stiffness (also called mismatch) that can occur between adjacent pipe joints. These mismatches cause a localized peak in strain and can drive gross deformation of the pipe, which may result in a buckle if not addressed at the engineering stage. The slenderness of a pipe and the strain hardening capacity determines the capacity of a pipe to handle the effects of mismatches during reeling A minimum wall thickness for reeling design equation has been defined for seamless pipe and has a proven track record and demonstrable reliability. There is a recent increase in the level of interest in cold worked pipe such as HFI/HFW, which appears to be an attractive cost effective alternative to seamless pipes. HFI/HFW potentially has inferior strain hardening properties due to cold forming, but have superior tolerance control of yield strength and wall thickness. This paper presents the results of a reliability based study, demonstrating the applicability of existing minimum wall thickness for reeling criteria, when applied to HFI/HFW linepipe.