Web viewNumerical modelling of the dynamic interaction between jack-up vessel legs and the ... which temporarily supports the ... where coupled pore fluid pressure

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CONTENTS

Boundary layer profile background and mathematical tools........................................3

Modern jack-ups and their dynamic behaviour investigating the trends and limits of moving into deeper waters...........................................................................................4

Tools for managing VDL limitations in challenging Australian soil and environmental conditions.....................................................................................................................5

Jackup going on location analysis...............................................................................6

On the role of jack-up hull-in-water radiation impedance during spudcan impacts with the seabed...................................................................................................................7

Numerical modelling of the dynamic interaction between jack-up vessel legs and the seabed.........................................................................................................................8

Review of historical jack-up rig spudcan penetration predictions in Malaysian waters 9

Re-visiting Yolla – New insights on spudcan penetration..........................................10

A centrifugal model study for bearing capacity of the spudcan foundation in single clay and sand over clay.............................................................................................11

A FE procedure for calculation of fixity of jack-up foundations with skirts using cyclic strain contour diagrams.............................................................................................12

Non-Linear versus linear foundation modelling in dynamic response of an AJ70 jack-up...............................................................................................................................13

Spudcan penetration analyses for two case histories in sand overlying clay............14

Use of novel spudcan shapes for mitigating punch-through hazards........................15

Development of a force resultant model for spudcans on dense sands based on centrifuge studies.......................................................................................................16

Overcome jack-up rig installation problem in difficult soil condition in east sea – a case history................................................................................................................17

Modelling the effects of pauses during spudcan penetration on the further installation behaviour.................................................................................................18

Case study of spudcan re-penetration analysis using large deformation finite element approach....................................................................................................................19

3D large deformation finite element analysis on jack-up reinstallation next to an existing footprint.........................................................................................................20

The 15th International Conference: The Jack-Up PlatformDesign, Construction and Operation 15th – 16th September 2015City University London, UK 1

Seabed remediation for safe sequential jack-up vessel installations in the harbour areas..........................................................................................................................21

Theoretical study on the effect of a weak layer on the vertical bearing capacity of spudcans in clay........................................................................................................22

Verification of an elasto-plastic vessel impact analysis method against a known incident......................................................................................................................23

Earthquake assessments of jack-ups using ISO 19905-1:2013 screening procedures...................................................................................................................................24

Update on revisions to the MWS Rig Move Document.....Error: Reference source not found

Re-visiting Yolla – Managing storm stability; geotechnical assessment....................25

Design considerations of DOL and VFD jacking system...........................................26


Paper 1Tuesday 15th September 09.15 – 09.40

BOUNDARY LAYER PROFILE BACKGROUND AND MATHEMATICAL TOOLS

J. Andrew Breuer, Joseph H. Rousseau & James N. Brekke

ABS

The Offshore Committee of Society of Naval Architects and Marine Engineers is engaged in a number of initiatives associated with the technology of determining wind forces and moments. This includes a revision to the Guidelines for Wind Tunnel Testing of Mobile Offshore Drilling Units, the use of computational fluid dynamics (CFD), and the development of empirical tools. The purpose of this initiative is to vet these new technologies so they can be incorporated into the regulatory approval procedures by class societies and flag and coastal regulatory agencies

The wind boundary layer profile has been expressed in a number of formulations. Of these, the power law profile formulation is the one that has been used most often in the design of jack-up rigs and other offshore units. This paper presents the history of the boundary layer profile, how it was incorporated into existing standards, and rigorous mathematical tools needed to calculate wind forces and moments with a method acceptable to regulatory agencies.



MODERN JACK-UPS AND THEIR DYNAMIC BEHAVIOUR

INVESTIGATING THE TRENDS AND LIMITS OF MOVING INTO DEEPER WATERS

T. Koole, M. van der Kraan

Shell Global Solutions

This paper describes an analysis of modern jack-up designs. Trends throughout the various designs are identified and justified. The environmental loads and dynamic response of these modern designs are analysed using the methodology as described in current guidelines. Furthermore, the water depth limit of current design philosophy is investigated. This research shows that as water depths increase, jack-ups become more wind-dominated in terms of environmental loading. A more detailed wind load calculation method then the currently used projected-area method is therefore recommended. Furthermore the need for increasing chord spacing is identified in an attempt to retain a stiffness dominated structure. This however increases the risk of problems associated with Rack Phase Difference (RPD) which occur during jacking operations and therefore illustrates a conflict within the design of jack-ups as the industry heads for deeper and deeper waters.



TOOLS FOR MANAGING VDL LIMITATIONS IN CHALLENGING AUSTRALIAN SOIL AND ENVIRONMENTAL CONDITIONS.

Ian Moyle1, Mark Hayward2, Zahidur Rahman2, Dr Yahui Zhang2

1. Origin Energy 2. Noble Denton Marine Assurance and Advisory, DNV GL

Australian continental shelf drilling faces some of the most challenging soil types combined with severe weather. Soils like Calcareous clays and sands present significant risks to jack-up drilling operations during both installation phases (punch-through risk) and operation phases due to cyclic degradation of the soil strength.

This paper will present learnings and specific tools that were developed for the Yolla Field, offshore Victoria, Australia where the above challenges were compounded by pre-existing spudcan craters and close proximity to a production platform. These tools allowed for effective VDL management planning for drilling operations, and emergency planning for extreme weather events.



JACKUP GOING ON LOCATION ANALYSIS

Paolin Tan1, Xiaohong Chen1, Qing Yu1*, Michael Perry2, Han Mu2, Ted Chang3, Marc Wong3, Donghui Chen4

1. ABS, 2. KEPPEL Offshore & Marine Technology Centre, 3. Bennett Offshore, 4.

Genesis Engineering

Instead of using the traditional wave height limit, often referred to as the 5-ft wave, it is possible to develop a more rational definition of operating limits for jackup units going on location through simulations of jackup leg touchdown on the seabed. This paper presents a recent development of analysis methods applied to jackup units going on location. In addition to the detailed analysis method involving the direct time-domain simulation of global motions and dynamics of the jackup whose legs are approaching and impacting the seabed, two simplified methods based on different simplification schemes are also studied. Verification of these simplified methods is carried out using a generic jackup design and representative soil conditions. Results show that both simplified methods perform reasonably well in predicting maximum vertical impact forces in comparison to the more sophisticated method.



ON THE ROLE OF JACK-UP HULL-IN-WATER RADIATION IMPEDANCE DURING SPUDCAN IMPACTS WITH THE SEABED

Kevin Drake and Yahui Zhang

Noble Denton Marine Assurance and Advisory, DNV GL Noble House, 39 Tabernacle Street, London EC2A 2AA

There is an industrial need to have a better understanding of going-on and going-off location issues for jack-up rigs with the hull in the water. To analyse the risks involved, time domain simulations are required in order to represent the important nonlinearities in the interaction between the leg spudcans and the seabed. However, the hydrodynamic added mass and damping properties of a jack-up hull are dependent upon the direction and frequency content of the hull motion. The main difficulty arises from the fact that the frequency dependence cannot be represented in the time domain with constant hydrodynamic coefficients.

There are two main parts in the hydrodynamic loading on the hull: the first is associated with the diffraction problem, namely the loading on the hull in waves in the absence of motions; the second is associated with the radiation problem, namely the loading on the hull brought about by the motion of the hull in otherwise still water. The two parts are combined using the principle of linear superposition. The latter is valid for the low to moderate sea conditions associated with going-on and going-off location. Time histories of the diffraction loading in irregular seas can be computed in a straightforward manner using standard inverse Fourier transform techniques. The radiation problem is less straightforward since it depends on the unknown motion of the hull during the course of the spudcan impacts with the seabed.

Tackling the radiation problem in the time domain is the principal hydrodynamics challenge to be addressed in order that full length time domain simulations of going-on and going-off location can be produced. In this paper the key focus is upon the role of radiation impedance during short duration isolated episodes of spudcan impacts with the seabed. The purpose is to provide guidance on simplifying approximations that may be applied for the episodic modelling of impacts.



NUMERICAL MODELLING OF THE DYNAMIC INTERACTION BETWEEN JACK-UP VESSEL LEGS AND THE SEABED

Jorrit-Jan Serraris1, Mike Woning2

1. MARIN, 2. Deltares

For offshore wind jack-up vessels are being used for the installation of foundations and wind turbines. These jack-up vessels position themselves on the seabed on a daily basis. The operability of these jack-up vessels is limited by the seastate, their motion response and operational criteria such as allowable impact loads on the legs, allowable motions and allowable accelerations. The motion response of the jack-up vessel during touch down and lift off is influenced by the interaction with the seabed.

The paper discusses the development of a seabed reaction force model and the coupling of this model to existing hydrodynamic software. The hydrodynamic software and coupled seabed reaction force model are in time domain and 6 degrees of freedom. The coupled model allows to calculate the non-linear seabed reaction forces on the legs and the resulting motion response of the jack-up vessel.

Simulations are performed for an offshore wind turbine installation vessel and operational criteria are applied to determine the operability. T he results of the simulations with the coupled numerical model are compared with results of model tests on a jack-up wind turbine installation vessel with interaction between the legs and the seabed.



REVIEW OF HISTORICAL JACK-UP RIG SPUDCAN PENETRATION PREDICTIONS IN MALAYSIAN WATERS

M.J. Rohani1, F.A. Sha’ari1, A.B.A. Rahman1 N. Chan2, K.K. Tho2

PETRONAS Carigali Sdn. Bhd. 2. Fugro Singapore Pte Ltd

This paper presents a summary of the performance of jack-up rig spudcan penetration predictions in Malaysian Waters over the past ten years. Measured spudcan penetrations at maximum preload from 100 jack-up rig emplacements are compared against the corresponding spudcan penetration predictions. The locations of these jack-up rig emplacements are evenly distributed between offshore East and West Malaysia. The spudcan penetration predictions, which originated from third parties and are presented anonymously, consist entirely of Class-A predictions made prior to the jack-up emplacement. A typical spudcan penetration prediction consists of penetrations based on high and low estimates of the soil strength. Both upper and lower bounds of the predicted penetrations are considered in the comparison against actual data. Possible causes of discrepancy between predicted and actual spudcan penetration depths in Malaysian Waters are discussed. This work is part of an ongoing initiative to further improve jack-up rig spudcan penetration predictions in Malaysian Waters under the PETRONAS Regional Suitability Mapping project launched in 2013.



RE-VISITING YOLLA – NEW INSIGHTS ON SPUDCAN PENETRATION

Erbrich C1, Amodio A1, Krisdani H1, Lam S. and Xu X1. Tho. KK.2

1. Fugro AG Pty Ltd. 2. Fugro Singapore Pte Ltd.

In 2004 the Ensco 102 was spudded along side the Yolla A platform in the Bass Strait, Australia. The uncemented silty carbonate soils at this site resulted in several surprises and insights which were documented and discussed in Erbrich (2005), including deeper than expected penetration and unexpected punch-through events which were both driven by the very high sensitivity of the soil and the transitional drainage conditions. In early 2015 the Seadrill West Telesto was bought alongside the Yolla A platform with its spudcans re-penetrated through the existing craters. An extensive engineering exercise was commissioned to minimise the risk of any ‘surprises’ this time. This included extensive site investigations of the crater in-fill material, including CPT, ball penetrometer and laboratory testing. Detailed re- analysis of the original penetration and of the planned re-penetration process was also undertaken using new analytical tools (based on the CPT resistance directly) and through large displacement finite element analysis using the CEL feature in ABAQUS. This paper will present these assessments in detail and will compare the predictions made with the final field penetration results obtained.

Erbrich, C.T. (2005). Australian Frontiers - Spudcans on the Edge. Keynote Paper, ‘Frontiers in Offshore Geotechnics (ISFOG 2005)’, Eds S. Gourvenec and M.J. Cassidy, Taylor and Francis.



A CENTRIFUGAL MODEL STUDY FOR BEARING CAPACITY OF THE SPUDCAN FOUNDATION IN SINGLE CLAY AND SAND OVER CLAY

Junhwan Choi & Boem-Seon Jang

Department of Naval Architecture and Ocean Engineering, Seoul National University

Spudcan foundations are used for operating smooth penetration at seabed and getting sufficient bearing capacity in Jack-up system just like Jack-up rig and Jack-up type Wind Turbine Installation Vessel (WTIV). Conventionally, bearing capacity of spudcan foundation is calculated by using SNAME guideline and DNV standard which are accepted as an industry standard. In this paper, centrifuge model test for bearing capacity of spudcan foundation in two different soil layer observed in southwest sea of South Korea is operated. Penetration behavior of spudcan foundation is simulated in 100 g-level centrifuge model test at single clay layer and sand over clay which are high risk of deficient bearing capacity and punch-through failure. During experiment, bearing capacity of spudcan foundation and two pressures of soil, total pressure and pore pressure, beneath the spudcan foundation are measured. Qc is measured by the Cone Penetration Test (CPT) in 100 g-level and shear strength at clay is measured by shear strength test instrument using torque after centrifuge test for soil properties. Penetration curve and soil properties with depth measured in experiment are presented in this paper. According as shear strength increases with depth, bearing capacity of spudcan foundation increases faster than that of constant shear strength. 3D Finite Element Method (FEM) of Coupled Eulerian-Lagrangian (CEL) method in ABAQUS/Explicit is carried out to be compared with experiment results. Comparing the result of experiment and that of FEM, bearing capacity of spudcan foundation with penetration depth measured in experiment is similar to that calculated in FEM. In addition, bearing capacity calculation according to SNAME guideline is performed and compared with results of experiment and FEM. In contrast to conservative result of SNAME guideline, sufficient bearing capacity is shown in experiment result and FEM calculation. The centrifuge model test results and consideration about practical implication are discussed in this paper.


Papers 10 & 11Tuesday 15th September 14.15 – 14.50

A FE PROCEDURE FOR CALCULATION OF FIXITY OF JACK-UP FOUNDATIONS WITH SKIRTS USING CYCLIC STRAIN CONTOUR DIAGRAMS

H.P. Jostad1*, Ø. Torgersrud1 and H.K. Engin1, H. Hofstede2

1. Norwegian Geotechnical Institute, 2. GustoMSC

Foundation fixity or global soil spring stiffnesses plays an important role on dynamic behaviour and structural utilization of jack-up platforms, especially at large water depths. In order to establish accurate values for this fixity, one needs to account for the complex non-linear behaviour of the different soil layers involved under combined average and cyclic loading. A procedure for establishing non-linear stress-strain relationships to be used as input to finite element analyses of jack-up footings is presented. The calculated non-linear load-displacement relationships (foundation stiffnesses) of the individual footings are divided into cyclic and total (average plus cyclic) components to be used as input to the dynamic and quasi-static structural analyses of the jack-up. A realistic case, the GustoMSC AJ70 accommodation jack-up with 22m diameter footings equipped with 2m long skirts that penetrates into an overconsolidated clay at a water depth of 135m in the North Sea is analysed. The obtained bearing capacity envelope and rotational stiffnesses are compared with corresponding results based on standard industry practice, ISO 19905-1.


Papers 10 & 11Tuesday 15th September 14.15 – 14.50

NON-LINEAR VERSUS LINEAR FOUNDATION MODELLING IN DYNAMIC RESPONSE OF AN AJ70 JACK-UP

H. Hofstede1, S.M. Hoogeveen2 K. Skau3, Ø. Torgersrud3, H. K. Engin3

1. GUSTOMSC, 2. GUSTOMSC US, 3. NGI

The dynamic response of a harsh-environment deep water accommodation jack-up has been evaluated by means of time domain simulations incorporating both a linear foundation and a non-linear foundation model. With its skirted footings the foundation (fixity) contributes to global stiffness as well as global damping. A non-linear foundation model representing rotational fixity is defined by means of a standard kinematic hardening model. The foundation stiffness curve is derived based on analyses as described in the accompanying paper by [Torgersrud et al. (2015)]. Foundation damping is computed by FEA based on soil damping defined on material level. The result from the time domain simulations suggest a significant effect of the non-linear foundation on the dynamic response. Resulting dynamic amplification factors are compared to those determined based on ISO 19905-1 suggested guidance and a significant disagreement is found.



SPUDCAN PENETRATION ANALYSES FOR TWO CASE HISTORIES IN SAND OVERLYING CLAY

Pan Hu, Mark Cassidy, Dong Wang and Sam Stanier

Centre for Offshore Foundation Systems, The University of Western Australia, Australia

The presence of problematic seabed profiles challenges the stability of the rig during spudcan installation. The typical soil profile consists of a relatively thin layer of sand, which temporarily supports the spudcan, underlain by a weak clay layer. Such soil stratigraphy is commonly found to be the main cause of spudcan punch-through, which in turn may lead to substantial damages to the leg structures. The industry guideline ISO (2012) recommends to use either a load spread method or the punching shear method to calculate the peak resistance for a spudcan on sand overlying clay, for which the failure mechanisms were based on small scale experiments. The formation and subsequent progression of the sand plug trapped below a penetrating spudcan is not considered when calculating the penetration resistance in the underlying clay layer. Therefore, the accuracy of the prediction is limited not only by the reliability of the interpreted soil strength parameters, but also by the validity of the design method currently in use. In this paper, comparisons between the methods recommended in the ISO (2012) industry guideline and the full profile prediction method of Hu et al. were made. By back-analysing two field cases, the Hu et al. method shows a significant improvement in predicting the spudcan bearing capacity-depth profile, which could not be achieved by existing methods based on a wished-in-place shallow foundation assuming a single failure mechanism applied repeatedly at different penetration depths. The Hu et al. method captures the essential characteristics of spudcan penetration behavior in sand overlying clay, which in turn enables the potential punch-through failure and the corresponding punch-through distance to be better assessed.



USE OF NOVEL SPUDCAN SHAPES FOR MITIGATING PUNCH-THROUGH HAZARDS

Joonmo Lee1, Youngho Kim1, Shazzad Hossain1, Mark Cassidy1, Yuxia Hu1 Jonghwa Won2, Jong-Sik Park2, Min Jung Jun2

1. Centre for Offshore Foundation System, The University of Western Australia2. Daewoo Shipbuilding & Marine Engineering, Co. Ltd., Korea

Jack-up geotechnical hazards such as unpredicted leg penetration, rapid leg penetration and punch-through continue to occur despite the jack-up industry efforts to minimise these risks. Improvements have been made on jack-up installation guidelines and site specific assessment as documented in the recently finalised version of ISO guidelines 19905-1 and the completed joint industry project ‘InSafeJIP’. Punch-through incidents occur in stratified soil conditions with a surface or interbedded strong (sand or stiff clay/silt) layer gets pushed into an overlying weak layer. Recently, improved design approaches have been developed for predicting penetration depth of conventional spudcans in stratified soils. Once the risk of punch-through failure is identified at a site, mitigation methods are required to allow a safer installation of the rig. In this project, performances of two novel spudcan shapes are being assessed through 1g and centrifuge model tests and large deformation finite element analyses. The aim is to investigate whether the spudcan shape itself can be used to mitigate potential punch-through when compared to a conventional spudcan. Preliminary results are presented in this paper.



DEVELOPMENT OF A FORCE RESULTANT MODEL FOR SPUDCANS ON DENSE SANDS BASED ON CENTRIFUGE STUDIES

Ning Cheng and Mark J. Cassidy

Centre for Offshore Foundation Systems and ARC CoE for Geotechnical Science and Engineering, The University of Western Australia

Jack-up drilling rigs are usually supported by multiple large inverted cone shape foundations with a sharp protruding spigot, i.e., the so-called spudcan. In the present study, a strain-hardening plasticity based force resultant model was developed to predict the load-displacement behaviour of a spudcan under combined vertical (V ) , horizontal ( H ) and moment (M ) loads. This model was developed from the observation of a series of specifically designed centrifuge experiments. A recently developed VHM apparatus was used in this study with an upgraded mechanical system to incorporate high stress testing conditions. The primary merit of the apparatus is the allowance of independent vertical, horizontal and rotational movements of the footing. As such, this study is considered to be the first comprehensive centrifuge study for investigating spudcan behaviour under all VHM loading directions on dense frictional material. A total of 17 individual tests were performed at an acceleration of 100 times that of the Earth’s gravity. The data indicate that the yield surface previously derived from 1- g laboratory tests on loose sand is conservative for dense sand under properly scaled stress condition. Retrospective simulation of the experimental tests verifies the model’s suitability in the stability assessment of jack-up units subjected to combined loading. The spudcan’s behaviour is also compared to previous studies of flat circular obtained at 1- g with the aim of describing the changes of the behaviours.


Paper 15Wednesday 16th September 09.00 – 09.25

OVERCOME JACK-UP RIG INSTALLATION PROBLEM IN DIFFICULT SOIL CONDITION IN EAST SEA – A CASE HISTORY

Li Xiang1, Paul Handidjaja1 Teh T.C. 2 Nhut Dinh Quang3

Braemar Technical Services (Offshore), 2. Seadrill Management Services, 3. PV Drilling

Thin and mobile loose Sand over very dense Sand soil condition on the seabed can cause potential damage to the integrity of the jack up with partial spudcan embedment. Due to its strong current and sandy seabed in Nam Con Son Basin, Offshore Vietnam poses significant scouring risk for jack up installation and operation. A case study of spudcan scouring problem at Dai Hung field is presented. Previously reported scouring incident had happened at the proposed jack up location. To mitigate the potential scouring risk, Mass Flow Excavator was utilised to create three cavities at the seabed to provide full embedment for the spudcan. The operation eventually resulted in a two year scour-free attack to jack up operation. This paper summarises the challenges and experience gained from this site preparation and installation.



MODELLING THE EFFECTS OF PAUSES DURING SPUDCAN PENETRATION ON THE FURTHER INSTALLATION BEHAVIOUR

Raffaele Ragni, Britta Bienen, Dong Wang & Mark J. Cassidy

Centre for Offshore Foundation Systems and ARC CoE for Geotechnical Science and Engineering, University of Western Australia

Installation of jack-up platforms may result in a discontinuous process due to either expected or unexpected delays experienced while penetrating. For this reason it is important not only to correctly estimate the response of the soil during penetration, but also the effect that a potential period of consolidation of variable length can have on further penetration.

Modelling this problem numerically would allow for a wider parametric study to be conducted in an economic fashion. However, it requires a large deformation approach, where coupled pore fluid pressure – effective stress analyses are able to describe the buildup of excess pore pressure during penetration as well as relative dissipation during a stage of consolidation, with relative increase in strength on the soil in the vicinity of the spudcan. For this reasons an advanced hypoplastic numerical model within the framework of the critical state soil mechanics for structured clays was implemented. Also, implementing structure into the model, soil degradation due to shear strains can be described, starting the analysis with an intact shear strength which is gradually degraded as the spudcan advances. The response after consolidation is compared with results from experimental tests in a centrifuge environment at enhanced gravity, where a spudcan model equipped with five pressure transducers was penetrated.

Results show that effects of a period of consolidation must be taken into account, as depending on its length this could lead to different scenarios: from a potential induced punch- through, though having one single layer of uniform soil, to a stronger and possibly reliable soil at shallower depth.



CASE STUDY OF SPUDCAN RE-PENETRATION ANALYSIS USING LARGE DEFORMATION FINITE ELEMENT APPROACH

Kee Kiat Tho1 and Nancy Chan1 Yangrui Zhou2, Jiantao Liu2 and Songwang Zhou2

1. Fugro Singapore Pte Ltd 2. China Oilfield Services Limited

A jack-up re-entry into existing footprints warrants a new set of spudcan penetration curves to be generated. In a uniform clay profile, the spudcan re-penetration analysis will typically be performed by applying a degradation factor to the intact strength profile adopted for the original spudcan penetration analysis. However, in layered soil profiles such as those consisting of sand overlying clay, bearing capacity calculation of spudcan re-entry into an existing footprint is complicated by changes in the soil layering during the first penetration. This paper presents a case study of a spudcan re-penetration analysis in a sand overlying clay soil profile using large deformation finite element approach. Spudcan penetration, extraction and re-penetration activities are sequentially simulated in a single numerical model. The degradation of soil strength in clay is modelled using a strain-softening constitutive model. Changes in soil layering and the development of cavity within the footprint are implicitly accounted for in the numerical simulation. The numerically predicted spudcan penetration depth is found to agree well with the actual spudcan penetration depth. In addition, the numerically generated cavity profile is found to closely resemble that measured on site. This case study demonstrates that large deformation finite element is a viable tool for preforming spudcan re-penetration analysis in layered soil profiles.



3D LARGE DEFORMATION FINITE ELEMENT ANALYSIS ON JACK-UP REINSTALLATION NEXT TO AN EXISTING FOOTPRINT

Wensong Zhang, Mark J. Cassidy & Yinhui Tian

Centre for Offshore Foundation Systems and ARC CoE for Geotechnical Science and Engineering, University of Western Australia

The reinstallation of jack-ups next to existing footprints left on the seabed during previous operations is an emerging concern. The uneven soil surface and strength heterogeneity within the footprint causes large eccentric and inclined loading conditions. These can exceed structural capacities or produce dangerous jack-up movement towards a production platform. Advice on safe installation procedures is extremely limited. Despite the fact that a range of experimental modelling has been carried out to address the problem by considering the effect of the major factors in a qualitative manner, a robust and accurate numerical framework is yet to be developed for better understanding of the Jack-up reinstallation process

In this study, entire Jack-up footing reinstallation processes were successfully simulated for a first time using the lately developed 3D Large Deformation Finite Element(LDFE) RITSS method at COFS, UWA. During the simulation, large variations in footing load condition were observed together with significant transitions in soil deforming mechanism. More importantly, the quantitative predictions on vertical (V), horizontal (H) and moment (M) loads exerted on the footing matched closely with previous experimental data. This recent development in numerical simulation enables the possibility of accurate sensitivity studies on major influential factors such as footprint geometry, soil heterogeneity and Jack-up structural rigidity. A more comprehensive understanding of Jack-up reinstallation processes is therefore within reach.



SEABED REMEDIATION FOR SAFE SEQUENTIAL JACK-UP VESSEL INSTALLATIONS IN THE HARBOUR AREAS

L. Kellezi1, L. Xu1 & C. Molina1 T.C. Millward2 & M.S. Pedersen2

1. GEO, Copenhagen, Denmark. 2. A2SEA A/S, Fredericia, Denmark

Geotechnical engineering assessment applicable to sequential installation and operation of a four-leg jack-up vessel, on critical soil conditions in a harbour area, offshore Denmark, is the focus of this paper.

When critical soil conditions reveal, design of seabed remediation measures has proven to be a way to avoid jack-up punch through foundation failure. The jack-up vessel is planned located in the harbor for the purpose of being loaded with wind turbine generators (WTGs) and transporting them to the wind farm location. The soil conditions at the harbor site consist of sand overlying silt and clay, with a high level of variability. In addition, the planned vessel location consisted of disturbed seabed from previous jack-up vessels installations.

Some previous gravel / stone beds (SBs) and depressions are found at the site. Due to multi-layered soil conditions, FE analyses are performed, applicable to the design of the remediation measures, (which consist of a combination of excavation and new SBs), parallel to the conventional approach. The fact that sequential installations will occur, is taken into account in the measures design, assuming loading and unloading (pull out) of the spudcans. As the vessel is positioned next to the quay, spudcan – quay wall / sheet pile interaction is also discussed and considered in the design of the measures closest to the quay.

Details on the analyses are outlined and discussed. The remediation measures are constructed and the vessel was sequentially safely installed and loaded as planned, with recorded spudcan penetrations similar to predictions.



THEORETICAL STUDY ON THE EFFECT OF A WEAK LAYER ON THE VERTICAL BEARING CAPACITY OF SPUDCANS IN CLAY

Vaughan Meyer*, Youhu Zhang & Arjen Kort

Norwegian Geotechnical Institute

Soil encountered offshore is usually stratified, often with alternating strong and weak layers. In contrast to a uniform soil profile, the presence of a weaker layer introduces challenges to correctly predict the spudcan load-penetration profile due to alteration of soil failure mechanism. It is therefore important to understand the influence of a weak layer on the vertical load-penetration response and to reflect this influence correctly in design methodologies. This paper presents a theoretical study on the effect of the weak layer in both a two layer (weak-strong) and a three layer (strong-weak-strong) soil profile in clay. Normalised parameters, such as relative layer thicknesses and soil shear strength ratios, are investigated. The paper will discuss the results of the study in the context of the current industry guidelines and reflect on how the results could be applied in engineering practice.



VERIFICATION OF AN ELASTO-PLASTIC VESSEL IMPACT ANALYSIS METHOD AGAINST A KNOWN INCIDENT

Ankor Raithatha & Richard Stonor

Noble Denton Marine Assurance and Advisory, DNV GL Noble House, 39 Tabernacle Street, EC2A 4AA, London

In December 1996, an impact from a pipe-lay barge caused severe damage to the starboard leg of the jack-up unit “Harvey H. Ward” while working in the Tapti Field offshore Mumbai, India. A 1997 Jack-Up Platform conference paper describing the post-incident recovery operations explains that initial attempts at elasto-plastic collapse modelling made at the time of the incident to predict the observed deformation yielded inconclusive results.

In recent years, NDCL have adopted such elasto-plastic collapse analysis methods as their standard analytical tool when conducting vessel impact assessments for jack-up structures, typically for clients preparing safety cases for the UK HSE and other safety authorities. Whilst NDCL have utilised the commercial non-linear finite element analysis software, Abaqus, and have based their approach on the latest guidance literature for structural analysis using non-linear finite element analysis methods, their analysis has not been tested against any real incidents, until now. This paper describes an analysis of the Harvey H. Ward incident undertaken by NDCL using their current assessment method.

The FEA model was efficiently developed using NDCL’s in-house JUSTAS-to-Abaqus software converter tool and ran on a standard modern desktop computer. The analysis results compare very well against the actual damage profile observed during the 1996 incident, providing both confidence in the analysis approach and an opportunity to reflect upon the progress in technology and understanding of analysis that now permits such work to be conducted in a relatively straightforward manner.



EARTHQUAKE ASSESSMENTS OF JACK-UPS USING ISO 19905-1:2013 SCREENING PROCEDURES

Douglas Stock1, John Stiff2 and Jim Brekke2

1. Digital Structures, Inc 2. ABS

Earthquake analyses are performed for a jack-up in different site conditions and seismic intensities. The analyses are conducted with equivalent jack-up models according to the ISO 19905-1:2013 screening procedures. The footing reactions are evaluated with soil capacities per the ISO standard recipe. Other component force results are compared with reasonable storm load results to assess general performance demands. Sensitivity cases are included to better identify the source of earthquake loads. Recommendations are included to assist in the further development of the ISO standard. A snapshot of jack-up rig site locations is used to identify the earthquake exposure of the worldwide jack-up fleet.



RE-VISITING YOLLA – MANAGING STORM STABILITY; GEOTECHNICAL ASSESSMENT

Amodio A1, Erbrich C.T1, Murgavel V. 1 Moyle I. 2

Fugro AG Pty Ltd. 2. Origin Energy Pty Ltd.

In 2004 the Ensco 102 was spudded along side the Yolla A platform in the Bass Strait, Australia. The uncemented silty carbonate soils at this site required a modified approach to documenting stability during storm events as discussed in Erbrich (2005). In early 2015 the Seadrill West Telesto was bought alongside the Yolla A platform with its spudcans re-penetrated deeply through the remoulded infill material within the existing craters. An extensive engineering exercise was commissioned to assess the storm stability for this new rig which involved a complex interaction between soil consolidation times and strength degradation under cyclic loads, coupled with uncertainty as to exactly what depth each of the spudcans would end up and hence what soil type they would rest on. A variety of anticipated scenarios were assessed, which included the effect of varying the installation preload, varying the return period of the storm and varying the VDL as a function of time. This paper will present the geotechnical work undertaken to unravel this complex puzzle, which led to a detailed installation flowchart that provided guidance on the process required to meet the acceptability criteria for each leg.

Erbrich, C.T. (2005). Australian Frontiers - Spudcans on the Edge. Keynote Paper, ‘Frontiers in Offshore Geotechnics (ISFOG 2005)’, Eds S. Gourvenec and M.J. Cassidy, Taylor and Francis.



DESIGN CONSIDERATIONS OF DOL AND VFD JACKING SYSTEM

Bernard Voon Ee HOW1, Hai Dong XU1, Satish MENON2 and Kok Seng FOO1,2

1. Keppel Offshore & Marine Technology Centre, 2 Offshore Technology Development

The jacking system is a key mission critical and safety critical system on a Jack Up platform (JUP). Due to its massive hull, a typical jacking system uses thirty six, fifty four or seventy two motors working in combination to share the load in elevating or lowering the hull against gravity. Distinct characteristics for jacking system loads include high inertia, large variable loads due to different loading conditions of the rig, guide and pinion friction, cyclic loads due to pinion action, and that the system can experience instantaneous load change due to the environment.

The purpose of this paper is to collect and elaborate on some considerations that jacking system designers may encounter during design of a jacking system powered with Direct-On-Line (DOL) and variable frequency drives (VFD). The design considerations discussed include interaction of the DOL or VFD drive with the braking system, start and stop sequences, load sharing, system reliability, short circuit current ratings and protection, encoder feedback, shaft voltages, bearing currents, vibrations, cable length and reflected wave phenomena, thermal insulation and inverter duty motors. This collection of considerations can serve as a basis for discussion to further enhance jacking system robustness and safety.

Documents

Web viewNumerical modelling of the dynamic interaction between jack-up vessel legs and the ... which temporarily supports the ... where coupled pore fluid pressure