Jack-up Rig Research Articles

Measured and calculated spudcan penetration profiles for case histories in sand-over-clay

This paper reports five case histories of jack-up rig installation in layered soil profiles where a dominate feature was a stronger sand layer overlaying a weaker clay layer. In all cases a relatively continuous load-penetration profile was measured during installation of each of the three spudcan foundations. Summary site-investigation data is provided and consisted of mainly torvane, minature vane, unconsolidated undrained triaxial and pocket penetrometer tests for determining undrained shear strength of the clays and blow counts for deriving the relative density of sand. A statistical averaging method recommended in the InSafeJIP guidelines was used to provide the best fit of the undrained shear strength profile in the clay as this then allowed for spudcan load-penetration profiles to be estimated without introduction of user interpretation or bias. Sand properties were taken as provided in the original site-investigation report. Comparisons between load-penetration profiles calculated using the industry-standard ISO guideline, more recently proposed mechanism-based calculation method and three-dimensional large deformation finite element simulations are made with the measured data, leading to valuable insights for practitioners for estimating behaviour of jack-up installations in problematic sand-over-clay soil profiles.

A Framework to Select the Best Strategy for Iran's Entry into the Repair and Maintenance Market of Oil Drilling Rigs

The oil and gas service industry revenue is projected to rise to $350 billion in 2020, and a major part of such services belong to repair and maintenance (R&M) of the drilling rigs. Therefore, with regard to the number of jack-up rigs in Iran and the Persian Gulf, the R&M and improvement of these types of rigs become particularly important, especially due to the high lifespan of these rigs, which increases the time frequency of basic repairs. Nevertheless, Iran is suffering from lack of management and knowledge in the field of project resource management in this industry. Therefore, knowing the strategies and effective criteria for this industry is essential. In this paper, 15 criteria affecting the selection of R&M strategy were determined through interviews with experts, and the effect of each criterion on each alternative (seven strategies) was studied, and the most appropriate R&M strategy was then selected through analytic hierarchy process (AHP). The results show that the best strategy for Iran's entry into the R&M market is to perform all basic repairs of the rig, followed by license and joint venture. Finally, the best strategy was applied at R&M of Sina1 rig, comparing the results with those for Iranian rigs such as Modarres and Rajaei, the repairs of which were carried out in foreign yards, indicates the low cost of domestic repairs and the economic justification.

Causes of post-installation penetration of jack-up spudcan foundations in clays.

This paper examines possible causes of additional spudcan settlement after preloading using both centrifuge model tests and small strain finite element analysis, in which spudcan settlement due to cavity collapse, consolidation settlement and settlement due to cyclic loading are incorporated. Back-analyses of seven jack-up rigs in the Gulf of Mexico show that even complete cavity collapse could only explain part of the measured additional settlements in the majority of the cases. Small strain finite element analyses also show that spudcan consolidation settlement is likely to account for even less of the additional settlement than cavity collapse in the sites considered. On the other hand, centrifuge model tests show that large amplitude cyclic rocking has a very significant effect on spudcan settlement, even if half of the preload has been removed. However, this effect cannot be explained by the exceedance of the yield envelope since the loading combination had not exceeded the yield envelope. One possible explanation is the stiffness and strength degradation of the soil under cyclic loading. In view of this, a conservative approach is recommended in instances where large amplitude cyclic rocking, such as that arising from storm loading, is expected shortly after preloading. The presence of lattice legs is found to reduce the spudcan settlement during large amplitude cyclic rocking.

Numerical Investigation of Novel Spudcan Shapes for Easing Spudcan-Footprint Interactions

This paper reports a measure for easing spudcan-footprint interactions when a jack-up rig needs to be installed near existing jack-up footprints. Two novel spudcan shapes, a skirted spudcan with four rectangular holes and a skirted spudcan with six circular holes and sloped bottom profile, were investigated using 3D large deformation finite-element (LDFE) analyses. The LDFE analyses were performed using the coupled Eulerian-Lagrangian (CEL) approach in the commercial finite-element package ABAQUS. After displaying the validity of the analyses against existing LDFE results and centrifuge test data, the efficiencies of the novel spudcans were studied against a generic spudcan shape, including the effects of spudcan offset distance from the footprint and the footprint depth. Both soft and stiff seabed strength profiles were considered, with the undrained shear strength increasing with depth. The potential of the spudcan sliding toward the footprint center during installation was evaluated based on the resultant maximum horizontal force (Hmax) and moment (Mmax) acting on the different spudcans. It is found that, between the two novel spudcans, the spudcan with six holes and sloped bottom profile is more effective at reducing Hmax and Mmax. Any reduction in the resultant horizontal force on the spudcan can generate a large reduction of the moment at the top of a long jack-up leg. The results from this study indicate that the novel spudcan with circular holes and sloped bottom profile has the potential to ease spudcan-footprint interactions without any additional mechanical operations.

Combined Bearing Capacity of Spudcans on a Double Layer Deposit of Strong-Over-Weak Clays

An extreme sea storm process can lead to a jack-up rig under the combined loading condition of vertical load (V), horizontal load (H), and moment (M) to have stability problems. This paper presents the analysis of combined bearing capacities of a circular spudcan on layered clays with a strong layer overlying a comparatively weaker layer. Numerical models combined with displacement- based load tests, swipe tests, and constant ratio displacement probe tests are adopted to calculate the uniaxial bearing capacities, failure envelopes in combined V-H, V-M planes, and failure envelopes in a combined V-H-M load space, respectively. A parametric study on the effects of vertical load level V, the layer strength ratio su,t/su,b, and the hard layer thickness t1 on the bearing capacities is then performed. Results show that the vertical load level is a key factor that influences the values of H and M and the size of the H-M failure envelope. The existence of the underlying weak clay decreases the bearing capacities in all directions, and the vertical capacity Vult is affected more than the horizontal (Hult) and moment (Mult) capacities based on a single uniform deposit. The influence of the underlying weak clay on H-M failure envelope is mainly shown where H and M are coupled in the same direction. In contrast, little difference is observed when H and M are coupled in opposite directions.

Centrifuge Model Study on the Effect of Lattice Leg and Sleeve on the Postconsolidation Bearing Capacity of Spudcan Foundation

Up to now, the postconsolidation bearing capacity enhancement of jack-up spudcan foundation has been explored using centrifuge model tests and numerical analyses, which however ignored the realistic jack-up lattice leg. This paper investigates both typical lattice leg and sleeve effects on the postconsolidation spudcan bearing capacity using centrifuge model tests, by replicating the entire process of spudcan in normally consolidated clay: “penetration–unloading–consolidation–repenetration.” The experimental results show that the lattice leg and sleeve affect the spudcan bearing capacity in two sides compared with spudcan without leg. First, it increases the transient bearing capacity during initial spudcan penetration; second, less postconsolidation bearing capacity improvement is yielded by the presence of the leg. The former effect is of importance on the prediction of jack-up leg penetration, and the latter effect would suggest a lower risk of spudcan punch-through for realistic offshore jack-up rig during preloading and operation period.

Closed-Loop Drilling Offers Advantages in Process Safety and Cost Savings

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 185283, “Leveraging CLD Potentials: Optimizing Economics Through Dynamic Well Control,” by Robert Ziegler, SPE, Weatherford International, and Evelyn Baldwin, SPE, Maersk Training, prepared for the 2017 SPE/IADC Managed Pressure Drilling/Underbalanced Operations Conference and Exhibition, Rio de Janeiro, 28–29 March. The paper has not been peer reviewed. Maximizing the potential of closed-loop drilling (CLD) to achieve dynamic well control and effectively cut costs is an often overlooked opportunity in times of low oil prices. This paper details the operational effects of CLD to demonstrate its ability to yield operational-process-safety improvements and cost savings over conventional methods. While managed-pressure drilling (MPD) often is the tool that finally enables operators to reach their objectives, major cost savings come when MPD is integrated in the well-planning process from the beginning. MPD for CLD Not too long ago, the topdrive was in a similar position on the technology-adoption curve as MPD is today. The industry was in a downturn at that time, too—oil prices were low, cost pressure was high, and horizontal drilling had just taken off. The topdrive rental business was booming, and, the moment the directional phase of a well was reached, the topdrive was mobilized. Drillers soon started to like the additional abilities the topdrive gave them, such as reducing nonproductive time (NPT) and mitigating stuck-pipe events. Even in the cost-sensitive land market, the topdrive is now considered standard equipment, as it has been offshore for quite a while. For MPD equipment, the situation is reversed. While the land market in the US has adopted the rotating control device as standard equipment, offshore in the Gulf of Mexico, where many of the most challenging wells are being drilled, adoption of the technology has been slow. In other areas of the world, however, MPD adoption is moving quicker. Most notably, in loss-prone carbonates of Southeast Asia; in the subsalt environment of Brazil; and, to a certain extent, in Angola, MPD from floaters has been more widespread. The experience from land wells can be transferred safely to offshore surface-blowout-preventer (BOP) operations (e.g., on jackup rigs, tension-leg platforms, tender-assisted drilling, and other platform rigs). The advantage offshore is that the rig moves are much simpler than for land operations, so complex piping for full MPD operations can remain in place.

Experimental investigation on the effect of spudcan shape on spudcan-footprint interaction

The interaction between a spudcan and an existing footprint is one of the major concerns during jack-up rig installation. The influence of spudcan-footprint interaction has recently been well addressed by a number of researchers. A lack of investigation exists in mitigating spudcan-footprint interaction issues. In the field, stomping and successive repositioning is conventionally used in installing a rig adjacent to an existing footprint. Water jetting and perforation drilling are also sometimes suggested. This paper reports a measure for easing spudcan-footprint interaction issues, with the efficiency of a spudcan with 4 slots tested through model tests carried out at 1g on the laboratory floor. The soil conditions tested simulate soft to moderate seabed strength profiles close to the mudline, varying the undrained shear strength. The most critical reinstallation locations of 0.5D and 1D (D=spudcan diameter) and existing footprint depths of 0.33D and 0.66D were investigated. By comparing with a conventional spudcan, the spudcan with slots reduced the induced maximum moment, horizontal force, and horizontal sliding distance by up to 80%, 40%, and 98% respectively. Critically, no additional operations, such as stomping/repositioning, perforation drilling, water jetting, are required to be performed offshore.

Effect of water jetting on soil properties and spud-can penetration/extraction in various soil conditions: numerical simulation vs. physical model test

ABSTRACTSpud-cans are used for fixing jack-up rigs on seabed. The spud-can needs to be inserted up to the required depth during the installation process in sand to secure enough soil resistance and prevent overturning accidents. In the clay case, its penetration depth is much larger and it typically takes long time to extract it from the large burial depth. In both cases, water jetting is needed to make it possible or expedite the process. To achieve such goals, spud-can may be equipped with water-jetting system including monitoring and control. In this regard, authors developed a numerical simulation tool by using a commercial FE (finite element) program ANSYS and the extended Drucker–Prager (EDP) equation involving water-jetting effect. Authors also conducted small-scale (1/100) physical model tests to validate the developed analytical/numerical model. By comparing the numerical model against experiment, essential physics are drawn with calibrated soil properties including the effects of water jetting. Then, by using the tuned soil properties, the numerical simulations reproduced the essential characteristics and behaviours of the model tests with sand and K(kaolin)-clay. Finally, for further verification, the cases of sand/K-clay multi-layer with and without water jetting are cross-checked against experimental results to show reasonably good agreement.

Underwater noise from geotechnical drilling and standard penetration testing.

Geotechnical site investigations prior to marine construction typically involve shallow, small-core drilling and standard penetration testing (SPT), during which a small tube is hammered into the ground at the bottom of the borehole. Drilling (120 kW, 83 mm diameter drillbit, 1500 rpm, 16-17 m drill depth in sand and mudstone) and SPT (50 mm diameter test tube, 15 mm wall thickness, 100 kg hammer, 1 m drop height) by a jack-up rig in 7-13 m of water were recorded with a drifting hydrophone at 10-50 m range. Source levels were 142-145 dB re 1 μPa rms @ 1 m (30-2000 Hz) for drilling and 151-160 dB re 1 μPa2s @ 1 m (20-24 000 Hz) for SPT.

Foundation punch-through in clay with sand: analytical modelling

Severe punch-through of jack-up rig foundations can occur due to the presence of a stronger sand layer in a bed of relatively soft clay. Analytical estimation of the bearing capacity and leg load–penetration response on such multi-layer stratigraphies is challenging. Accurate mechanism-based models need to be established in each of the layers involved and the effects of the mechanisms in each of the layers on the response in the other layers must be captured. Based on the recently developed failure stress-dependent punch-through models for sand–clay stratigraphies, an extended model is proposed for clay–sand–clay stratigraphies. Half-spudcan particle image velocimetry centrifuge tests and full-spudcan centrifuge tests are used in developing and validating the extended model. The centrifuge test results were discussed in a companion paper and this paper focuses on the analytical developments and prediction assessment. Both spudcan peak resistance (qpeak) and spudcan punch-through depth (dpunch) can be estimated using the model. The predictions by the extended model and by the current industry guidelines are compared against the centrifuge test data. The extended model proposed in this paper outperforms the approaches suggested in the guidelines. An advantage of the proposed approach is that it can be used for either sand–clay or clay–sand–clay scenarios and exhibits excellent performance compared to the model testing dataset considered in this work for both cases. The resulting penetration resistance model is a useful design tool for routine punch-through risk assessment.

Modified multiple generalized regression neural network models using fuzzy C-means with principal component analysis for noise prediction of offshore platform

A modified multiple generalized regression neural network (GRNN) is proposed to predict the noise level of various compartments onboard of the offshore platform. With limited samples available during the initial design stage, GRNN can cause errors when it maps the available inputs to sound pressure level for the entire offshore platform. To obtain more relevant group for GRNNs training, fuzzy C-mean (FCM) is used. However, outliers in some group may interfere the prediction accuracy. The problem of selecting suitable inputs parameters (in each cluster) is often impeded by lack of accurate information. Principal component analysis (PCA) is used to ensure high relevance input variables in each cluster. By fusing multiple GRNNs by an optimal spread parameter, the proposed modeling scheme becomes quite effective for modeling multiple frequency-dependent data set (ranging from 125 to 8000 Hz) with different input parameters. The performance of FCM-PCA-GRNNs has improved significantly as the results show a 25% improvement on the spatial sound pressure level (SPL) and 85% improvement on the spatial average SPL than just GRNNs alone. By comparing with data obtained from real engine room on a jack-up rig, the FCM-PCA-GRNNs noise model performs better with around 16% less error than the empirical-based acoustic models. Additionally, the results show comparable performance to statistical energy analysis that requires more time and resources to solve during the early stage of the offshore platform design.

Spud-can penetration depending on soil properties: Comparison between numerical simulation and physical model test

Spud-can is used for fixing jack-up rig on seabed. It needs to be inserted up to the required depth during the installation process to secure enough soil reaction and prevent overturning accidents. On the other hand, it should be extracted from seabed soils as fast as possible during the extraction process to minimize the corresponding operational cost. To achieve such goals, spud-can may be equipped with water-jetting system including monitoring and control. To develop such a smart spud-can, a reliable numerical simulation tool is essential and it has also to be validated against physical model tests. In this regard, authors developed a numerical simulation tool by using a commercial program ANSYS with extended Drucker-Prager (EDP) formula. Authors also conducted small-scale (1/100) physical model tests for verification and calibration purpose. By using the numerical model, a systematic parametric study is conducted both for sand and K(kaolin)-clay with varying important soil parameters and the best estimated soil properties of the physical test are deduced. Then, by using the selected soil properties, the numerical and experimental results for a sand/K-clay multi-layer case are cross-checked to show reasonably good agreement. The validated numerical model will be useful in the next-stage study which includes controllable water-jetting.

Radiated noise levels from marine geotechnical drilling and standard penetration testing

Geotechnical drilling is a common part of site investigations prior to marine construction. A small, solid core is extracted from shallow depth for examination at the surface. During standard penetration testing (SPT), a sample tube is hammered into the ground at the bottom of the borehole. The number of blows needed for the tube to penetrate a fixed depth relates to the hardness of the ground and is termed the standard penetration resistance. Recordings of the drilling and SPT operations of a jack-up rig situated in 12 m of water were obtained with a mobile recording system at 10-50 m range and 10 m depth below the sea surface. Geotechnical drilling (120 kW, 83 mm diameter drill bit, 1500 rpm, 16-17 m drill depth below the seafloor consisting of sand then mudstone) had a radiated noise level (using geometric spreading) of 142-145 dB re 1 μPa rms @ 1 m (30-10,000 Hz). SPT (50 mm outer diameter of the test tube, 15 mm wall thickness, 100 kg hammer, and 1 m drop height) exhibited a radiated noise level of 152-160 dB re 1 μPa2s @ 1 m.

A study on the fatigue analysis for a vertical caisson on FPSO subjected to the nonlinear wave loading

A cylindrical type hull appurtenance attached to the side hull like a seawater caisson or a riser tube is subject to the drag force and inertia force defined by the Morison equation, as well as global hull girder loads. When performing fatigue analysis for a structure subject to Morison load, the nonlinearity of drag term in the Morison equation should be taken into account for an exact structural safety assessment in an appropriate way. This research proposes a time domain approach (Level II method) in order to take into account the nonlinearity, as well as a proper combination of local hotspot stress induced by the Morison load and global hull girder stress. It is based on a representation of irregular wave as a combination of a large number of regular waves. It enables for reflecting phase angle differences of particle velocities along the vertical tube. The phase difference with global stress is also taken into account. The method also includes the vertical motion of the caisson and the wave elevation above the mean water line. The nonlinearity of drag term can be treated by linearizing the drag term in the frequency domain analysis. For the frequency domain approach, Level I method using a linearization coefficient is proposed to yield a fatigue damage equivalent to the time domain analysis. It incorporates the different contributions of horizontal velocities along the vertical tube on the hotspot stress at a connecting bracket on the side hull. The proposed methods are verified through a caisson in FPSO example and various comparative studies. Various comparative studies are performed for a verification of spectral code, an evaluation of rain flow counting effect and the combination of motion and stretching effect, a verification of rain flow counting(RFC) code and linearization coefficient. Through these case studies, it is found that the proposed spectral fatigue analysis with linearized Morison’s force results in a good agreement with time domain analysis with a drastic reduction of computational time. In this paper, a FPSO model with a seawater caisson is used as a verification example. The proposed methods are expected to be applied to many similar tubular structures on the side of floating offshore structure, such as, riser guide tube, protection frame for riser guide tube, collision protector and so on. In addition, jacket or jack-up rig consisting of tubular members could be good applications.

Assessment of the Theoretical Net Relief Drilling Rate for Conductor Pipes

The following paper deals with the prediction of the theoretical net drilling time for a conductor pipe (CP) of offshore platforms when it plugs. The CP is driven to prevent the looser surface layer from caving in and obstructing the wellbore, because the shallow section of most wells on- and offshore is drilled in unconsolidated sediment. Due to the small CP diameter, the CP plugs very often. Normally the material is drilled out by using jetting, normal drill rigs and sometimes jack-up rigs. As offshore operations time is very important, this paper tries to assess the theoretical net time needed to excavate within the CP as a relief drilling operation, by means of Nishimatsu and Evans cutting theories. The results are compared with publicly available TBM excavation rates and then theoretical drilling rates for different unconfined compressive strength values for CP diameters of 700, 900 and 1000 mm are suggested. The results are considered for drag bits, however, they are only theoretical assumptions and cannot be applied for commercial projects with the same geotechnical properties.

Centrifuge modelling of spudcan–pile interaction in soft clay overlying sand

When a jack-up rig is installed in close proximity to an existing piled jacket platform, the massive soil movements induced by spudcan penetration may damage the adjacent piles. In 2012 the authors reported the findings of an extensive centrifuge model study on spudcan–pile interaction in soft clay. As a seabed profile may consist of soft clay over stiff soils and the spudcan–pile interaction phenomenon in such case is expected to be significantly different from that in soft clay, the earlier study is extended to investigate the interaction of a spudcan with adjacent piles in a soft clay overlying sand profile. Besides centrifuge tests on model spudcans, tests on half-spudcans placed behind the transparent window of the model container have been conducted to evaluate the soil movements and failure mechanism for various model configurations and soil conditions. The experimental results and findings are presented in this paper.

Structural safety of oil jack-up rigs in operation

Structural safety of oil jack-up rigs in operation

Cantilever Force Distribution on Jack-Up Rig

Cantilever is the most important structure of the jack-up rig. Jack-up usually has three working conditions: drilling condition, storm condition, preloading condition. The fulcrums of the cantilever in the drilling condition are less than the other two. So the distribution of the cantilever force in the storm and preloading conditions becomes extremely complicated with the increase of fulcrums. In order to facilitate the evaluation of jack-up rig strength, a simplified analytical method and a finite element method (FEM) are presented to distribute the cantilever force on a jack-up rig during the storm and preloading conditions. For the simplified analytical method, the fulcrum force is obtained from the distance between the fulcrum and the position of the cantilever resultant force. For the FEM, the cantilever is assumed to be perfect rigid body without any strain. The fulcrum force is calculated according to the balance of the fulcrum forces. Both results of fulcrum force are analyzed and compared. It is shown that they are in good agreement. Thus both methods can be applied in distributing the cantilever force to evaluate the strength of jack-up rig.

Plug excavation from the conductor pipes by assessing the specific energy

Conductor pipes are the first part of an oil well to provide the initial structural foundation and they are normally installed into the ground by using impact hammers both onshore and offshore before any drilling operation. However, it is possible that conductor pipes do not reach the design depth and therefore relief drilling is needed. Nowadays offshore relief drilling is performed by using drill strings with drill bit of jack-up rigs. However, this operation is both time-consuming and expensive. This paper analyzes the use of an alternative solution, developed for supporting the installation of conductor pipes. Two mathematical cutting models were used to theoretically assess the net relief drilling time based on two geotechnical parameters, i.e. cohesion and friction angle. By comparing the theoretical results with three drill logs of an offshore project performed in the UK with another drill rig with similar geometries, it is shown that the preliminary models seem to match the net drilling time registered on the field. The results shown in this paper have to be carefully interpreted and cannot be applied to future projects as the net drilling time values have to be considered as theoretical.