Rigid Riser Research Articles

Experimental investigation of the flow-induced vibration of a hybrid riser conveying severe slugging

This paper reports the experimental results of the vibration response of a hybrid riser conveying severe slugging. The hybrid riser comprises a vertical rigid riser made of a transparent acrylic tube and a flexible jumper made of a transparent silica gel tube with a length to internal diameter ratio of 261. The horizontal span and vertical height of the flexible jumper are 83.5 and 45 cm, respectively. The flow-induced vibration tests were carried out in the liquid superficial velocity range of Vsl = 0.098–0.688 m/s and the gas superficial velocity range of Vsg = 0.177–0.649 m/s using non-intrusive optical measurement with high-speed cameras. Five flow regimes are observed, including the severe slugging I (SSI), severe slugging II (SSII), severe slugging III (SSIII), stable flow (ST), and oscillation flow regimes. The vibration response of the flexible jumper is related to the flow regime. As the flow regime changes from SSI to SSII, the absence of the slug production stage results in an augmented amplitude. In contrast, the response amplitude is significantly reduced when the flow regime shifts to ST. For the SSI regime, the response amplitude varies over the stage, and the occurrence of multiple frequencies is associated with the pressure fluctuation as well as the different duration of stages. The maximum amplitude is observed in the liquid fallback stage in spite of the limited duration, and the vibration frequency is close to the fundamental natural one. The liquid slugs of different lengths passing through the jumper at different velocities contribute to the presence of the traveling wave characteristic.

Parametric analysis for free standing riser by a new calculation procedure

A new calculation procedure for the structural analysis of a free standing riser system is presented. The riser model includes a flexible jumper section and rigid riser section as two single continuous segments, which are coupled at the buoyancy can to form the overall riser static model. The equilibrium equations of the flexible jumper and rigid riser are established based on Minimum Total Potential Energy Principle. At each equilibrium configuration, the elastic potential energy of the system will be always equal to the sum of the potential energy caused by the external forces. The coupled equations are numerically solved by finite difference method, using Newton-Raphson technique in an iterative manner. This calculation procedure is evaluated by convergence analysis and the results validated by comparisons with well established commercial code. Parametric studies consider platform span, flexible jumper length, buoyancy force, buoyancy can depth and ocean current velocity effects to evaluate aspects related to simplicity, flexibility and robustness of the proposed methodology. The solution works as an expeditious alternative to enhance efficiency of the numerical calculations used in deep water riser design applications.

Study on the hydrodynamic characteristics of a marine riser under periodic pulsation disturbance

In this paper, the hydrodynamic forces and vortex distributions of a riser subjected to a non-uniform flow generated by an upstream oscillating wing structure are simulated using high-precision code Incompact3d combined with immersed boundary method. The effects of in-line and transverse positions, oscillating frequency and amplitude on riser performance have also been discussed. It is found that with the increase of in-line spacing or the decrease of oscillation frequency and transverse positions, the time-averaged drag coefficient of riser changes from the negative values to positive values. The time-averaged lift coefficient of riser is generally insensitive to oscillation frequency. However, it shows a large shift when changing the transverse positions or the amplitude of the wing is larger than π/6. Moreover, for various in-line positions, oscillating frequency and amplitude, the vortex shedding of the rigid riser, indicated by the main frequency of lift force, is always locked by the oscillating wing. However, when the transverse position is changing, the ratio of main frequency of lift coefficient to the swing frequency does not still remain 1:1. More concretely, the maximum time-averaged drag coefficient and lift coefficient Root-mean-square value of 0.559 and 1.629 is obtained in the simulation respectively.

Hybrid Ultradeepwater-Riser Configuration Saves Costs, Pushes Depths

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 29389, “Single Independent Riser: A Cost-Efficient Ultradeepwater Riser,” by François Lirola, Eric Revault, and Jean-François Lunven, Saipem, prepared for the 2019 Offshore Technology Conference, Houston, 6–9 May. The paper has not been peer reviewed. Copyright 2019 Offshore Technology Conference. Reproduced by permission. The single independent riser (SIR) is a hybrid riser configuration optimized for ultradeepwater field development. The SIR is composed of a flexible jumper and a rigid part vertically tensioned by means of distributed or continuous buoyancy. This configuration features improved dynamic behavior under fatigue and extreme environmental conditions for the rigid riser section thanks to its compliant shape. The design is versatile and can be staggered easily to comply with design constraints or congested layouts. Introduction The SIR is adapted to both turret-moored and spread-moored vessels as well as most other types. The configuration is also suitable for a wide spectrum of environments, from the mild conditions of west Africa to those in the Gulf of Mexico or the North Sea. The configuration can be designed for production risers, whether single-pipe or pipe-in-pipe, and injection and export risers. The strength of this configuration lies in its compliant, fatigue-resistant behavior. Its steep-wave configuration facilitates the mitigation of potential interference; the relative position of the sag and hog of each neighboring riser can be adjusted while reducing the load at the hang-off location. Overview The SIR features a steep-wave shape. The upper part is a flexible jumper, and the lower part is a rigid, steel-pipe string maintained in a near-vertical position either by means of continuous buoyancy or distributed buoyancy modules. These methods are used instead of a conventional air can in order to provide the up-thrust necessary to keep the riser vertical. The use of continuous or distributed buoyancy implies that each section compensates for its own weight, meaning that the concept is applicable to any water depth for which buoyancy materials are qualified. The transition between the rigid and the flexible parts is achieved with a sub-sea connector. An overview of the SIR configuration is given in Fig. 1. The SIR features the following technical advantages: The design relies on field-proven technology such as rotolatches to connect the rigid section to its anchor or standard shallow-water flexible-jumper technology to perform the transition between the rigid section and the floater. Though the SIR can be deployed in any water depth, the cost of the flexible jumper limits the suitability of the SIR for water depths below 1000 m, which should be considered the minimum applicable water depth. The SIR is compatible with any installation method. The fabrication process of the SIR can be reversed fully, meaning the system can be retrieved for relocation or maintenance purposes. The SIR features a small dynamic response, thus leading to less-stringent fatigue and welding requirements.

Numerical simulation on the forced oscillation of rigid riser with helical strakes in different section shapes

In order to study the helical strakes' suppression effects on the vibration riser, triangle and circular sections helical strakes are proposed. With the technology of dynamic mesh, forced oscillation in steady flow of the riser with helical strakes in different cross-section shapes at low Reynolds number are numerically simulated. The analysis is focused on the lock-in range, as well as the force status and trailing vortex structure under different values of amplitude (A) and oscillation frequency (fe), and draw comparisons to the bare riser's results. Research shows the lock-in range will increase with the increase of amplitude ratio (A/D), the riser with all three different shaped helical strakes have larger lock-in range than bare riser. Before entering into the lock-in region, riser with different shaped helical strakes all have good effects on suppressing the vibrations of riser. Within and after lock-in region, riser with triangle and circular sections helical strakes have smaller lift force than that with rectangular section, which reduced lift coefficient by 65.83% and 83.53%. The trailing vortex structures of the riser with helical strakes are in strength three-dimensional character. Inside the lock-in region, riser with triangle and circular sections have better suppression effect than that with rectangular section. Outside the lock-in region, riser with triangle section helical strakes has the best vortex suppression effect, of which the influence on the flow field is greatest and the vortex-induced force is least.

Optimization design on the riser system of next generation subsea production system with the assistance of DOE and surrogate model techniques

The Next Generation Subsea Production System (NextGen SPS) is a new concept for petroleum development in ultra-deep water (UDW) areas. It can improve the structural performance of riser as well as provide several operational benefits to subsurface well completion (SWC) equipment. The design of NextGen SPS’s riser system which includes rigid riser and flexible jumper—like the free standing hybrid riser (FSHR), is a very important issue for the definition of NextGen SPS. This paper details an optimization design on the NextGen SPS’s riser system, with the assistance of the design of experiments (DOE) and surrogate model techniques. The optimization model pertaining to riser system is formulated firstly. The DOE is a statistical technique that guides a sensitive study on the behavior of the riser system before the optimization analysis. Structural responses are obtained by the fully coupled methodology. Through such a preliminary study, the effective contribution of each design variable at the riser performance will be known and some general conclusive remarks will be obtained. Based on the DOE results, design variables are screened to improve the efficiency of optimization process. Particle swarm optimization (PSO) method is employed to conduct the optimization analysis. In this analysis, surrogate models, which are developed by back propagation neural network (BPNN), replace the time consuming dynamic analysis to predict structural responses. Latin hypercube sampling (LHS) method is adopted to generate training sample and testing sample for the BPNN. NextGen SPS that operates at a depth of 3000 m is used as the case for this investigation. The efficiency of optimization design is improved by DOE and surrogate techniques, and a reduction of approximately 46% for the riser system cost is achieved. The obtained conclusions have applicability in reference to the engineering design of FSHR and the study procedure will provide reference for study on other new structure concept.

Root-Cause Analysis of Offshore Pipeline Failures

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 188717, “Offshore-Pipeline-Failure Root-Cause Analysis,” by Emmanuel Iyeh, SPE, Addax Petroleum, and Grant Adams, JEE Subsea, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed. This paper evaluates potential causes of failure for nine pipelines operating in shallow waters (8 to 14 m) in the Gulf of Guinea. The authors develop an analytical method to identify root causes and provide recommendations for pipeline design and placement. Methodology Once the subject pipelines had been identified, field visits and failed-section analyses were conducted. Pipeline properties and conditions are provided in Tables 1 through 5 of the complete paper. Mechanical-failure, hydrodynamic, and third-party-interference assessments were performed. Design-stability assessment was reviewed and gaps identified. A further stability assessment of the pipelines was performed in accordance with Det Norske Veritas- Germanischer Lloyd Recommended Practice (DNVGL-RP) F109. The authors then developed the best remedial option by use of a scoring matrix based on weighting with respect to business drivers. For a new tie-in spool, a subsea flexible spool can be used. This approach has been used safely and successfully to accommodate instability. This approach may be adopted for pipelines, with the following caveats: An unbonded subsea flexible spool verified for use in a multiphase system should be used. A flexible spool should be combined with a rigid riser. The flexible spool does not represent a solution to pipeline instability but rather a method to prevent failure of the riser caused by a mobile pipeline. The pipeline may continue to move over time, and the flexible spool may eventually enter tension and fail. An unstable pipeline is also at significant risk of fatigue if instability continues. A flexible spool could be used with a concrete mattress. This will allow a degree of movement (e.g., if the mattresses take some time to settle into the seabed). The position of the flexible tie-in can also be monitored and, in the event that the pipeline continues to move after mattresses have been installed, will give warning that the pipeline is still unstable. Analysis Results The majority of the studied failures occurred between the months of May and October, the rainy season in the Gulf of Guinea. This season features the region’s most extreme weather, likely contributing to the failures. Most failures involved pipeline movement or riser kinks.

FEM Analysis and Experimental Tests of Rigid Riser Hanging System

Abstract The article presents the analysis, project, and experimental examination of an original rigid riser for Coil Tubing Pipes. The principle of riser operation is based on the use of friction forces. The research included the FEM analysis of the designed riser, calculations of the required bolt tensions, and checking the effect of the clamping force on stress distribution in the pipeline. The results of computer simulation were verified on a specially designed test rig. The described riser design was implemented on the LOTOS Petrobaltic platform, thus eliminating the need for purchase and installation of expensive elastic risers.

Risk Assessment and Reduction for an Innovative Subsurface Well Completion System

In recent years, many oil and gas fields have been discovered in ultra-deep sea (UDS). Some of these fields are evaluated to have no commercial value if existing oil field development approaches are used, especially while the oil prices remain low. A new alternative field development solution, termed as Subsurface Well Completion (SWC) system, is proposed with the aim to produce oil and gas in a cost-effective manner in UDS. This system primarily consists of four parts: a tethered subsurface platform, the rigid riser, SWC equipment and flexible jumper. Obviously, central to the evaluation and application of the new SWC technology is the inherent risk relative to acceptance level. In particular, an uncontrolled release of hydrocarbons to sea, which may lead to catastrophical consequences involving personnel risk, environmental damage and economic losses, is a main contributor to the total risk and of great concern to the offshore petroleum industry. As for the new SWC system, any failure will not be a direct source of risk for the personnel on the surface installation due to its offset feature. In this context, this paper proposes a quantitative risk assessment (QRA) framework to assess such uncontrolled releases to sea with regard to the SWC system for an oil field in the production phase based on the new Subsurface Tension Leg Production (STLP) facility. According to the QRA results presented in this paper, the identified scenarios representing uncontrolled releases to sea are subsea wellhead leaks, rigid riser leaks, subsurface wellhead leaks, releases from X-mas tree and flexible jumper leaks. Among these scenarios, subsea wellhead is found to be the high-risk area. Compared with the established risk acceptance criteria (RAC), the environmental risk levels for the subsea wellhead’s leak lie within the As Low As Reasonably Practicable (ALARP) region while other risks are all below ALARP limits, which means that there is a need for improved consideration of the existing design with regard to the subsea wellhead area, and the corresponding risk reduction measures are proposed. Furthermore, the sources and effects of uncertainties are reviewed and sensitivity studies are carried out to illustrate the effect of some of the important assumptions in the risk model. It can be found that some assumptions made are conservative or optimistic while others are unknown. However, the final QRA results can be regarded as somewhat conservative. This paper concludes that the new SWC technology has a distinct advantage with respect to the leakage duration time in UDS, and thus mitigates the environmental and commercial impacts to a large extent. Besides, relaxed design requirements for the X-mas tree and flexible jumper can be accepted. It is also concluded that there are no serious and major commercial losses for all the identified accidental release scenarios, which is of great importance and attractiveness to oil producers.

Improving time-domain prediction of vortex-induced vibration of marine risers

The prediction of the vortex-induced vibration (VIV) of marine risers becomes a critical issue as the offshore exploration and production moving into deepwater and ultra-deepwater regions. In this paper, a time-domain model, based on a forcing algorithm and on high Reynolds number experimental data, was further developed to predict the VIV of rigid and flexible risers. The forcing algorithm was integrated into a global-coordinate-based finite-element program. At each time step, the hydrodynamic forces on a riser, including added mass, lift and drag forces, were calculated for each element based on two non-dimensional state variables—the amplitude ratio and the reduced velocity. The state variables were determined from a zero up-crossing analysis of the time history of the cross-flow displacement. Validation studies were carried out for a full-scale rigid riser segment in a uniform flow and a flexible riser in a stepped current. The predicted motions of the risers were compared with experimental data and the motions predicted by other numerical models.

Prototype Reynolds Number Vortex-Induced Vibration Tests on a Full-Scale Rigid Riser

Slender offshore structures in deep water subjected to currents may experience vortex-induced vibrations (VIV), which can cause significant fatigue damage. Extensive experimental researches have been conducted to study the VIV in the past several decades. However, most of the experimental works have small-scale models and relatively low Reynolds number (Re)—“subcritical” or even lower Reynolds number regime. There is a lack of full understanding of the VIV in prototype Re flow regime. Applying the results with low Re to a full-scale riser with prototype Re might have uncertainties due to the scaling effects. In addition, the surface roughness of the riser is also an important parameter, especially in critical Re regime, which is the case for prototype risers. In the present study, two full-scale rigid riser models with different surface roughness ratios were tested in the towing tank of MARINTEK in 2014. Stationary tests, pure crossflow (CF) free oscillation tests, and forced/controlled motion tests were carried out. Several conclusions could be made: The drag coefficient is dependent on the Re number and surface roughness ratio. At critical and supercritical flow regimes, the displacement amplitude ratio is less sensitive to Re than that at lower Re. The displacement amplitude ratio in subcritical flow regime is significantly larger than that in critical and supercritical flow regimes. Two excitation regions for the ‘smooth riser’ and one excitation region for the “rough riser” are identified.

Development of nanocomposite for rigid riser application: Diallyl bisphenol A modified Bismaleimide / epoxy interpenetrating network and its nanocomposite (NH2-MWCNT)

High performance composites have drawn substantial attention from the offshore industry for the development of rigid riser. This paper focuses on the development of a cost effective nanocomposite matrix system with a Tg of about 200 °C for potential use as a matrix in composite risers. An interpenetrating network (IPN) was developed by introducing Diallyl bisphenol A (DBA) modified Bismaleimide (BMI) into the Diglycidylether of Bisphenol A (DGEBA) epoxy matrix and it's nanocomposites were developed by introducing MWCNT (such as unfunctionalized and NH2-functionalized) into the BMI/DBA-Epoxy IPN. The effect of NH2-MWCNTs on the thermal and mechanical properties of NH2-MWCNTs/BMI/DBA/epoxy nanocomposite IPN were investigated and compared to un-functionalized MWCNTs/BMI/DBA/epoxy and BMI/DBA/epoxy IPN. Dynamic mechanical analysis shows significant increase in storage modulus as well as Tg (increased by 50 °C) with the addition of NH2-MWCNT in comparison to BMI/DBA/epoxy IPN. Mechanical studies indicated that the incorporation of NH2-MWCNT into BMI/DBA/epoxy resin not only improved the tensile (more than 2.5 times) and flexural strength (more than 2 times) but also significantly improved the impact strength of the composite (more than 3 times) in comparison to pure BMI/DBA/epoxy resin. It is noteworthy that the NH2-MWCNT/BMI/DBA/epoxy nanocomposite IPN resin exhibit improved thermal and mechanical properties, indicating that this novel toughened IPN resin system have great potential to be used as a matrix for advanced functional composites for rigid riser applications in offshore applications.

A novel high performance bismaleimide/diallyl bisphenol A (BMI/DBA)–epoxy interpenetrating network resin for rigid riser application

A new class of interpenetrating network (IPN) resin system was developed by mixing tetrafunctional epoxy resin (TGDDM) with diallyl bisphenol A (DBA) modified bismaleimide (BMI) for rigid riser applications at high temperatures of more than 280 °C.

Fractographic Analysis of Weld Metal and HAZ Regions of API X-80 Steel Subjected to Simulation of the Reel-Lay Method

The reel-lay method is a process commonly used for rigid riser installation. During this process, riser materials are subjected to high strain levels and associated plastic damage which can affect their structural integrity. The aim of this work was to evaluate the fracture surfaces at weld metal and heat-affected zone regions of API X-80 steel three-point bend specimens, SE(B), subjected to reel-lay method simulations. The pre-existence of circumferential planar defects at 12 o’clock position was considered and the J-integral was used to simulate the reel-lay condition at the defects. Different strain levels considering conditions less and more severe than the real one were also studied. The results showed a great influence of J magnitude on fracture surface morphology. The increase of J magnitude led to a greater local surface damage on the specimens and stable crack growth during reel-lay simulation. High loading conditions should be avoided in the operation in order to prevent further structural damage of the materials.

Analysis of Vortex-Induced Vibration of Riser using Spalart-Almaras Model

Vortex-induced vibration is natural phenomena where an object is exposed to moving fluid caused vibration of the object. Vortex-induced vibration occurred due to vortex shedding behind the object. One of the offshore structures that experience this vortex-induced vibration is riser. The riser experience vortex-induced vibration due to vortex shedding caused by external load which is sea current. The effect of this vortex shedding to the riser is fatigue damage. Vortex-induced vibration of riser becomes the main concern in oil and gas industry since there will be a lots of money to be invested for the installation and maintenance of the riser. The previous studies of this vortex-induced vibration have been conducted by experimental method and Computational Fluid Dynamics (CFD) method in order to predict the vortex shedding behaviour behind the riser body for the determination of way to improve the riser design. This thesis represented the analysis of vortex induced vibration of rigid riser in two-dimensional. The analysis is conducted using Computational Fluid Dynamic (CFD) simulations at Reynolds number at 40, 200, 1000, and 1500. The simulations were performed using Spalart-Allmaras turbulent model to solve the transport equation of turbulent viscosity. The simulations results at Reynolds number 40 and 200 is compared with the other studies for the validation of the simulation, then further simulations were conducted at Reynolds number of 1000 and 1500. The coefficient of lift and drag were obtained from the simulations. The comparison of lift and drag coefficient between the simulation results in this study and experiment results from the other studies showed good agreement. Besides that, the in-line vibration and cross-flow vibration at different Reynolds number were also investigated. The drag coefficient obtained from the simulation results remain unchanged as the Reynolds number increased from 200 to 1500. The lift coefficient obtained from the simulations increased as the Reynolds number increased from 40 to 1500.

Numerical Vortex-Induced Vibration Prediction of Marine Risers in Time-Domain Based on a Forcing Algorithm

Vortex-induced vibration (VIV) of marine risers poses a significant challenge as the offshore oil and gas industry moves into deep water. A time-domain analysis tool has been developed to predict the VIV of marine risers based on a forcing algorithm and by making full use of the available high Reynolds number experimental data. In the formulation, the hydrodynamic damping is not treated as a special case but simply an extension of the experimentally derived lift curves. The forcing algorithm was integrated into a mooring analysis program based on the global coordinate-based finite element method. At each time step, the added mass, lifting force, and drag force coefficients and their corresponding loads are computed for each element. Validation studies have been carried out for a full-scale rigid riser segment and a model-scale flexible riser. The numerical results were compared with experimental data and solutions by other programs.

Control of marine riser end angles by position mooring

For safe drilling and work-over operations on floating platform, one objective is to minimize the rigid riser angles at the well-head and at the top joint. One way to achieve this is to control the vessel’s position. For shallow water, drilling operations are normally carried out using a moored vessel. In this paper, a system consisting of a vessel, mooring lines and a drilling riser was mathematically modelled. The strategy was to control the vessel’s position by changing the tensions in the mooring lines to ensure that the riser end angles are within safe limits. Numerical simulations and experiments of a moored vessel were performed to illustrate the feasibility of the proposed control strategy.

Parametric study on the axial vibrations of riser suspended and moored by chains (RSAA) configurations

Recently, in order to minimize the influence of the vertical motions in the risers and consequently allow the utilization of FPSOs in deep waters, a new riser configuration called RSAA (riser suspended and moored by chains - in Portuguese), composed of a rigid vertical riser, flexible structures and mooring line segments (top and bottom) was proposed. This configuration presents solutions to the most critical points in a riser design: the top tensions are dissociated from the bending moments at the top region, and the curvatures at the TDP are reduced by utilization of floaters. Feasibility analyses have shown that the vertical riser is the most critical part of the proposed system due to the FPSO high level of vertical motions. These motions are transmitted by the top chains, leading to high levels of axial stress variation due to dynamic tension. Faced with this, a parametric study is vital in order to understand the system’s behavior as well as to establish the main parameters which influence its structural behavior. Analytical methods may require some slight simplifications of the problem to be applicable, but they generally lead to very compact formulae that do explain which parameters influence the results and why and how it does so. Considering some simplifying hypotheses, this work proposes an analytical model to determine axial stress and tension variations at the top of the vertical riser. Neglecting some non-linearities but considering the coupling between axial and transversal vibrations, a random dynamic analysis in the frequency domain can be performed to evaluate the maximum stresses and tensions levels with considerably lower computational costs.

Fatigue reliability analysis of deep water rigid marine risers associated with Morison-type wave loading

This work presents a simple and efficient framework for the fatigue reliability assessment of a vertical top-tensioned rigid riser. The fatigue damage response is considered as a narrow-band Gaussian stationary random process with a zero mean for the short-term behavior of a riser. Non-linearity in a response associated with Morison-type wave loading is accounted for by using a factor, which is the ratio of expected damage according to a non-linear probability distribution to the expected damage according to a linear method of analysis. Long-term non-stationary response is obtained by summing up a large number of short-term stationary responses. Uncertainties associated with both the strength and stress parts of the limit state function are quantified by a lognormal distribution. A closed form reliability analysis is carried out, which is based on the limit state function formulated in terms of Miner’s cumulative damage rule. The results thus obtained are compared with the well-documented lognormal format of reliability analysis based on time to fatigue failure. The validity of using the lognormal hazard rate function in predicting the fatigue life is discussed. A Monte Carlo simulation technique is also used as a reliability assessment method. A simple algorithm is used to reduce the uncertainty associated with direct sampling at small probability of failure values and a small number of simulations. Simulation results are compared with closed form solutions. A worked example is included to show the practical riser design problem based on reliability analysis.

Frontal advance of turf-banked solifluction lobes, Kluane Range, Yukon Territory, Canada

Humus horizons associated with thirteen solifluction lobes on a single slope were radiocarbon dated at fixed distances behind the risers, as well as ahead of the lobes. The dated horizons, together with stratigraphic observations of planar discontinuities inside the lobes and shallow burial of vegetation ahead of the lobes, all suggest rapid advance of lobe fronts. We infer that lobe advance at this site results from slow accumulation of soliflucted material behind a relatively rigid riser, progressive steepening of the riser and build-up of stress, and finally rupture of the front and its collapse onto the slope. Rebuilding of the lobe usually then takes place. The time for a full cycle of development is estimated to be in the order of a few hundred years. Long-term frontal movement appears inversely related to lobe size, and is mainly controlled by soil moisture and sediment characteristics. These findings call into question the usefulness of solifluction lobe advance rates for paleoclimate reconstructions, at least at this site.