1. Introduction

1.1 Background & objectives Different challenges are encountered during jack-up platform installations on critical soil conditions, consisting of a stronger layer overlying a weaker one. Spudcan punch-through type of failure is associated often with sudden penetration of the legs, difficult to control during installation or preloading phase. Consequences from such failure can be drastic, compromising the safety of the personnel on board, and associated with increased project cost.

This paper elaborates on a case history in the North

Sea, where a jack-up platform was planned to be

installed next to a Gas Platform, on challenging

seabed and soil conditions. Due to limited soil data,

new site survey and soil investigation were

conducted. After overall data interpretation, design

of seabed remediation was carried out. As part of

that, gravel banks (GBs) were constructed and the

jack-up platform safely installed and operated at the

site.

2. Jack-up Platform Location 2.1 Jacking position

Several jack-up positions were proposed and

preliminary studied, considering the seabed features

(sand mobility and possible previous gravel

dumping) interpreted from previous site surveys and

soil investigations. After initial evaluations of two

locations, (chosen according to the CPT

performance) the final jack-up platform position

presented in Figure 1 was investigated in details.

Figure 1: Multi-beam bathymetry chart and CPT locations

SITE INVESTIGATION & SEABED REMEDIATION TO AVOID

SEQUENTIAL JACK-UP PUNCH THROUGH FAILURE

L. Kellezi, L. Xu & C. Molina Geo, Copenhagen, Denmark

Abstract Jack-up platforms are extensively used in offshore operations of oil and gas and wind industry. Depending on

the seabed soil conditions, sudden punch through / rapid penetration during the installation can be of major

risk for the stability of a jack-up structure. Due to limited soil data, previous un-identified jack-up operations

and gravel dumping campaigns at a Gas Platform location in the North Sea, a new site survey and soil

investigation were proposed and carried out, applicable to jack-up vessel geotechnical assessments. Due to

critical seabed features and soil conditions, resulting in predicted sequential risks of punch through, design of

seabed mitigation measures / gravel banks (GBs) was carried out, considering it as a way to flatten the seabed,

avoid non-uniform spudcan penetrations and particularly the risk for sequential leg punch through, ensuring

spudcan penetration to approximately spudcan full base contact. The design of the GBs is based on

conventional & numerical, finite element (FE) analyses. Hence, discussions regarding the theoretical and

numerical approaches are outlined. The jack-up platform was safely installed at the location over the

remediated seabed and operated as planned

In Figure 1, the variation in the water depth indicates

the seabed elevation changes within the area of

interest. Such interpretation was possible by the new

multi-beam survey, conducted in combination with

cone penetration tests (CPTs) investigation (CPT

locations shown also in Figure 1).

2.2 Jack-up platform, spudcan geometry and loads

Figure 2: (a) Jack-up platform; (b) spudcan geometry

The jack-up platform has four legs, each with a

spudcan equipped with a trapezium shaped bottom

plate. The distance between the centres of the legs is

28 m and 37 m in width and length, respectively.

Photo of the jack-up platform at the location is

shown in Figure 2. The spudcan has an equivalent

diameter of 6.2 m, giving a full base contact area of

30 m2. The height from the spudcan base (full

contact) to spudcan tip is about 0.9 m. Based on the

site specific assessment / environmental load

analyses, the required maximum preload to be

applied during spudcan installation is calculated to

about 3056 tons/leg.

3. Previous & New Site Surveys & Investigations 3.1 Prior bathymetry & seabed features Part of the debris clearance survey previously performed at the site, is shown in Figure 3. Within the surveyed area, the water depth varies from (30.4-35.2) m, corrected to the lowest astronomical tide (LAT), with a general seabed gradient of about 1˚. The highest seabed inclination was at the southeast of the Gas Platform. Seabed sediments are interpreted to comprise predominantly sand, with areas of gravel and cobbles. Surrounding the Gas Platform, was observed a large area of gravel and debris, as well as areas of sand accumulation created as a consequence of variable currents.

Previous jack-ups have been operating at the site,

within the area of possibly previous rock dump and

sand accumulations. However, no feedback from the

previous site activity and installations was available.

Hence, based on the soil conditions, depressions /

footprints, created during the previous installations,

were expected, possibly later filled by the seabed

sand movements or any rock dumping. How the

seabed looked one year prior to the current survey, is

shown in Figure 3. Prior to that, it is unknown.

3.2 Prior geotechnical data The available geotechnical investigation from 1984

at the jacket Gas Platform location (Figure 1 & 3),

consisted of a single borehole (BH) B1/1A with CPT

and laboratory tests. B1/1A showed soil conditions

comprising generally of a seabed sand layer of 0.9 m

thickness, overlying firm clay to about 3.3 m below

seabed (bsb), underlain by sand to 6 m depth,

overlying firm clay to about 11.5 m bsb. Higher

strength clay, sand and clay layers are then found

below this depth, to the end of the BH (80 m, bsb).

Figure 3: (a) Previous bathymetry chart; (b) new site survey

and investigation

3.3 New site survey & CPT investigation

Considering seabed mobility, a multi-beam survey

was proposed and carried out within an area of 250

m x 100 m. Due to limited soil data and previous

unknown jack-up site operations, there was many

uncertainties on the soil condition. Hence, a new

geotechnical investigation was proposed and

conducted, with a heavy seabed operated 20 tons

CPT rig (GeoScope). A total of 18 deep push seabed

CPTs with a target depth of (20-25) m bsb were

performed at two optional jack-up positions. The

locations of the new CPTs together with the new

bathymetry are presented in Figure 1. The new

CPTs, as shown in Figure 4, indicate variation in the

soil layering as compared to BH / CPT B1/1A.

Results from the CPTs show that the subsoil

comprises generally top sand layers interbedded

with firm clay layers within the (3.0-5.5) m bsb,

overlying sand to (5.0-8.0) m depth, underlain by

firm clay (10-14) m bsb. High strength clays and

sands are found below these depths.

Figure 4: Cone resistance for CPTs along two cross sections

(Starboard Legs & Port Legs)

4. Seabed Remediation

4.1 Gravel bank (GB) design

Leg penetration analysis were carried out after

performance of the new CPTs, at the considered

jack-up platform locations. Predictions for as-it-was

seabed indicated sequential risks for punch through /

rapid penetrations. Therefore, seabed remediation

measures (GBs), as per concept shown in Figure 5,

were proposed to prevent such risks. The

construction of GBs has proven to be an effective

way for avoiding jack-up spudcan punch-through

type of failure in several offshore locations [Kellezi

et al. 2007], [Kellezi et al 2009], [Kellezi &

Stadsgaard. 2012].

Figure 5: Gravel Bank (GB) concept design

4.2 Final jack-up platform position

Two main positions were preferred initially by the

client, hence the new CPTs performed as shown in

Figure 1. However, it turned out that the GBs could

not be constructed close to the jacket Gas Platform

due to interaction between the Gas Platform

foundation (monopiles) and the GBs. In addition, the

company in charge of the gravel dumping and GB

construction estimated that it was a difficult and

risky task to dump the gravel so close to the Gas

Platform. Hence, these two main jack-up platform

locations were aborted. Under those circumstances,

another position was proposed, called Option F

(Final), as shown in Figure 1, which is the focus in

the current paper. In choosing this position, the first

priority was to ensure sliding stability of the aft legs

towards the Gas Platform, while attempting to

position the jack-up platform legs / spudcans as

close as possible to the new CPTs.

4.3 Interpreted soil profiles for virgin and disturbed

seabed

The soil conditions at the site are interpreted based

on all the soil data available, old and new. The

undrained shear strength cu of clay layers is derived

from CPT net cone resistance, upon definition of an

appropriate cone factor. Nk = (15-20), (derived based

on the laboratory tests), applied for the upper and

lower bound soil parameters, respectively. This clay

strength would fit with an average equivalent

isotropic shear strength. Regarding the friction angle

ϕ for the sand layers, relative density Dr, is

interpreted first from the CPT data. The peak ϕ is

then based on the Dr. The values of ϕ utilized for the

following analyses are re-assessed by reducing the

peak strengths, or deriving the critical state ϕ as a

difference of the peak and dilatancy values. This is

because ϕ depends on the volumetric changes related

to the stress level. On high stress levels, dense sands

dilate less than on low stress levels. However for

loose sands the dilation is smaller, reducing to zero

for high stress levels. The ϕ values measured in the

laboratory testing are the peak values obtained while

the sand is free to contract / dilate. However, to

account for the fact that in order to mobilize failure

into the clay, large strains within the sand may

result, leading to a drop in ϕ. Therefore, the highest

values that are applied in the current bearing

capacity computations are however the critical state

ϕ, i.e. the angles that correspond to the failure

associated with no volumetric changes, taking also

into account the reduction for the footing size effect

as recommended by [SNAME, 2002]. In general, the lower / upper bound characteristic

soil parameters for sand and clay soils are selected

as a cautious estimate of the value affecting the

occurrence of the relevant limit state [Eurocode 7,

2004]. As the virgin seabed is disturbed by the

activities like sand accumulation, rock dumping and

previous un-identified jack-up installations /

operations, the designed GBs are to be located

partially on the virgin seabed and partially on the

disturbed seabed. Hence, generalized lower bound

soil profiles are interpreted applicable to all four

legs, covering both virgin and disturbed soil

conditions as shown in Table 1 (Note2 and Note3). Table 1: Generalized lower bound soil profile and parameters

interpreted from remediated seabed for all legs (Note: 1

remediated seabed level starts; 2 disturbed seabed level starts; 3 virgin seabed level starts)

Soil

Type

Layer

Depth

Eff. Unit

Weight,

γ’

Friction

Angle,

ϕ

Undrained

Shear Strength,

cu

[m] [kN/m3] [˚] [kN/m2]

Gravel1 7.0-2.0 9.0 40 -

Sand /

Rock2 2.0-0.0 9.0 30 -

Sand3 0.0-0.9 9.0 30 -

Clay 0.9-3.3 9.0 - 55

Sand 3.3-6.0 9.0 35 -

Clay 6.0-11.5 9.0 - 70

Clay 11.5-12.7 9.0 - 225-125

Sand 12.7-20.3 9.0 33 -

4.5 Interpreted soil profile for remediated seabed

Based on the new CPTs, preliminary spudcan

penetration predictions showed sequential risk for

punch through / rapid penetrations at different load

levels. In addition, due to non-uniform seabed and

slopes, caused by sand seabed mobility etc. spudcan

legs were expected to suffer also rack phase

difference (RPD) issues during preloading.

Therefore, various challenges were encountered

when designing the GBs. First, the variation in the

soil conditions across the area, especially the

thickness of the top sand layer. Furthermore, the

design should account for extra uncertainties,

originated by the lack of CPTs at the SF_3 & SF_4

spudcan locations (Figure 1). The final jack-up

location was determined after the termination of the

CPT campaign). As an additional constrain, the

design should be optimized to a maximum gravel

volume of 30000 m3 as requested by the authorities.

The GB design is outlined in section 5, while the

final designed geometry of the GBs at the four legs,

nearest and further from the Gas Platform are

summarized in Table 2. Considering the seabed level

variation, due to previous rock dump and sand

accumulaitons, the GBs are designed referring to the

virgin seabed level of 34 m LAT. The top level of

the GBs is expected to be at the same level,

corresponding to 27 m LAT, for all four legs.

Table 2: Designed geometry of GBs according to virgin seabed

level (Note: 1 interpreted from disturbed seabed due to

previous sand accumulation or rock dumping used as part of

GBs)

Leg

No.

Height,

H

Top /

Bottom

Level

GB Top

Diameter

GB

Slope

Aft GB

NW

Slope

[m] [m

LAT] [m] [-]

SF_1

5.0+2.01 27 / 34 20 1:2.5 1:1.6 SF_2

SF_3

SF_4

The generalized soil profile, with the lower bound soil parameters applicable to the design, interpreted for the remediated seabed at the four legs, is shown in Table 1 (Note1). For the gravel material ϕ = 40˚ was chosen based on interpretation of the data for the stress dependency of ϕ for such material. The actual gravel material was expected to be loosely compacted, but relatively uniformly graded material. However, it consists of high strength particles meaning that the particle interlocking is not easy destroyed by crushing. Hence, the chosen strength was considered realistic. The deformation modulus for the gravel was interpreted to E=35000 kPa.

5. Spudcan Penetration Analyses

5.1 Conventional analyses Conventional bearing capacity analyses are often based on limit equilibrium methods, which are a state of equilibrium corresponding to a failure criterion that defines the strength of the soil. Failure is usually interpreted the state when the strength is fully mobilized along the entire failure surface. The conventional penetration analyses carried out follow the guidelines given in [EN ISO 2016] and [SNAME

2002], together with author’s experience developed by hundreds of predicted jack-up sites and feedback data. The derived lower bound generalized soil profile given in Table 1 have been used in the analyses. In the assessments, soil hardening / softening as a result of spudcan load and penetration has been disregarded and the preload is considered as static load. To conventionally define footing penetration depth versus load, the calculation of static bearing capacity of the spudcan at various depths is carried out based on [Hansen 20002] as given in equation 1. Q/A=0.5γBNγsγdγiγbγgγ+qNqsqdqiqbqgq+cNcscdcicbcgc (1)

Where: Q = bearing capacity of foundation base; A = foundation area, B = foundation width; γ = unit weight; q = vertical overburden; c = cohesion; sγ, sq, sc = shape factors; Nγ, Nq, Nc = bearing capacity factor; dγ, dq, dc = depth factors; iγ, iq, ic = load inclination factors, bγ, bq, bc = base inclination factors; gγ, gq, gc = ground inclination factors. The spudcan is assumed to be a circular footing with a flat bottom. The effect of the actual spudcan shape is taken into account. Squeezing of clay layer underlying the seabed sand and the GBs during footing penetration is also considered and implemented. The punch through mechanism was analyzed based on the load spread factor (SF) & punching mechanism (SNAME 2002), and author’s experiences. During spudcan penetration through the sand layers a sand plug is assumed to develop and lead the spudcan penetration through clay. This is confirmed by the FE modellings and centrifuge testing (at different universities). The conventional analyses utilize the thickness of the soil plug estimated by the FE results. Punching failure is investigated by varying SF, which depend on the ratio between the spudcan diameter and the sand thickness, the average strength of the sand and clay layers etc. SF, finally adopted for the conventional analyses, has been determined from the vertical peak bearing capacities derived from subsequent FE small strain analyses. 5.2 Finite element (FE) analyses FE modelling of the spudcan penetration is carried out with Plaxis 2D 2016, as an alternative to conventional analysis. Based on the interpreted lower bound soil profiles, FE analyses are conducted for all four legs designing heights / shapes of the GBs. Plaxis Plastic (small strain) analyses are performed in order to determine the lower bound peak bearing capacity to punch through / rapid penetrations. In addition, updated mesh (UM), large deformation analyses, are carried out, to confirm the conventional risk for sequential punch through, and adjust the conventional solution for multilayered soil conditions, while correcting for soil backflow. Due

to multi-layered soil profiles and considering the program limitations, some assumptions are made in building the FE models in Plaxis. 2D axisymmetric modelling of the spudcan-soil interaction is applied. In such modelling the GBs are simplified to truncated cones. Sand is modelled as Mohr-Coulomb (MC) drained material and clay as MC undrained (Tresca) material, utilizing effective unit weights for the soil layers. Mesh has been generated using 15-noded triangular FE and mesh sensitivity analyses conducted. The spudcan is modelled as a weightless elastic body. Soil backflow, which cannot accurately be modelled in Plaxis, where important, is interpreted and implemented conventionally. The FE calculations for virgin / disturbed or remediated seabed are carried out in the following phases: initial stress condition; placing spudcan with full-base-contact with the disturbed seabed or top of GB; applying vertical prescribed displacement at the spudcan full-base; calculate the reaction force. 5.3 Conventional and FE results for virgin / disturbed and remediated seabed

Figure 6: Disturbed seabed – Plastic analysis – 0.9 m spudcan

tip penetration: (a) FE model geometry & mesh; (b) total

deviatoric strain

Figure 7: Disturbed seabed – UM analysis – 3.9 m spudcan tip

penetration: (a) Deformed mesh (b) Deformed model geometry

(c) total deviatoric strains

For virgin / disturbed seabed, the conventional penetration analysis shows spudcan tip penetration of approximately 3 m for rather low load level and 11 m for the maximum preload of 3056 tons/leg. The conventional penetration curve, (adjusted with respect to FE small strain capacities) together with the FE results, are shown in Figure 8. From the shape of the curves, two sequential punch-throughs are predicted: at preload levels 1150 tons/leg & 2200 tons/leg, respectively. The sequential punch-through is confirmed by Plaxis UM analyses, (even with the limitation on backflow modelling), able to simulate the soil volume activated during the spudcan penetration. The output FE results are shown in Figures 6 for shallow spudcan penetration, Plastic (small strain) analyses & in Figure 7 for larger penetrations and UM analysis. Based on those analyses, and to ensure no risks for punch-through during installation, GBs are designed

for all four legs. As shown in Figure 5, the concept of GB is related to artificial local increase of the seabed sand layer thickness, increasing this way the fictive spudcan area at the sand clay interface. The size of the GB (thickness and top / bottom radius) depends on the spudcan vertical bearing capacity target, in order to reduce / limit the spudcan penetrations, or avoid them completely, allowing only shallow penetrations, corresponding to spudcan full base contact with the top of the GB. In order to achieve only shallow spudcan penetrations (not more than full base contact) GBs with height of maximum 5 m, (over the disturbed seabed sand layer of about 2.0 m in thickness) seemed to be required for lower bound soil parameters. In this case, for GB top radius of 10 m and slope 1:2.5, the GB lower bound peak bearing capacity to punch through is derived to 3500 tons/leg. This capacity is somewhat larger than the expected maximum preload for the jack-up platform, allowing for any soil variation within the spudcan areas and vicinities.

Figure 8: Conventional & FE penetration curves for disturbed

& remediated seabed for all legs

The output results for the remediated seabed from FE Plastic analyses, are given in Figure 9. The design of the GBs is based on small strain analyses

Soil Plug Sand

Sand

Sand

Clay

Clay

Sand

Clay

Soil Plug

Sand

Sand

Clay

in Plaxis 2D, being on the safe side. Similar failure figures as given in Figure 6 for disturbed seabed.

Figure 9: (a) FE model geometry & mesh; (b) total deviatoric

strain for remediated seabed with Plastic analyses

GBs with smaller heights could have been designed at the location, if some spudcan penetrations up to 3.5 m were tolerated during the installation. This would correspond to the conventional spudcan penetration curve started at 30.5 m LAT in Figure 8. In this case GB heights of minimum 1.5 m could have been possibly sufficient for increasing the vertical spudcan bearing capacity at the second punch through level (about 35 m LAT), to the maximum preload capacity. However, in order to avoid any risks during the installation, client’s preference was to aim for shallow penetration, equal to full base contact to all 4 spudcan legs. Regarding small strain and large deformation (UM) analyses in Plaxis 2D, in small strain, generally the influence of the geometry change on the FE equilibrium conditions is neglected [Plaxis 2016]. This is usually a good approximation when the

deformations are relatively small as in the case of spudcans installed on top of GBs (Figure 6 & 9) & [Kellezi & Stadsgaard 2012]. However, for large spudcan penetrations (Figure 7) this influence should be taken into account. When large deformation theory is included in the FE program, some special features are considered as explained in [Plaxis 2016] mentioning among others that the FE mesh is updated as the calculation proceed, done automatically in Plaxis when the UM option is selected. These calculation procedures are based on an approach known as an Updated Lagrangian formulation. 6. Review of As-built Gravel Banks (GBs) Based on the current design, the GBs were built as

per Figure 10, where the bathymetry from post-

survey is also shown. The constructed GBs, more or

less approached the design.

Figure 10: (a) Bathymetry for disturbed seabed and the

designed GBs (b) Bathymetry after construction of GBs /

remediated seabed

However, they had some variation in the elevation of

the top surface. In addition, the aft legs GB slopes,

in the northwest direction (towards the Gas

Platform), as noted also in Table 2, were constructed

steeper (slope 1:1.6) than the rest (designed slope

1:2.5), which reduced somewhat the GBs bearing

capacity in that direction. The reason for steeper

slopes towards the Gas Platform was to minimize

the amount of gravel dumped next to or over the

jacket piles. The northwest direction of the GBs was

however, further analysed conventionally &

numerically in 2D & 3D at the most critical section,

checking the aft legs sliding capacities towards the

Gas Platform. As only a sector of the GBs had a

slope 1:1.6, due to 3D effect, and the fact that the

design was based on lower bound soil strength, the

safety to sliding was considered sufficient, while it

was recommended close monitoring of the aft legs

during preloading. 7. Recorded leg penetrations / feedback After the construction of the GBs, the jack-up platform was successfully installed and the measured penetration data for each leg were received. The spudcans were preloaded to the maximum preload. According to the recorded load-penetration data, full-base-contact was almost reached at all spudcan legs. 8. Conclusions and Recommendations A case history on jack-up platform geotechnical assessments and installation in the North Sea is presented. The seabed and soil conditions at the site were critical, consisting of non-uniform seabed due to sand mobility and previous gravel dumping and two sequential sand over firm clay scenarios indicating for risks for sequential punch through / rapid penetrations. Therefore, to identify the seabed features and soil conditions, the existing data were supplemented with new seabed surveys and CPT investigations. Based on the interpreted soil data, GBs were designed avoiding non-uniform spudcan penetrations / RPD issues & sequential spudcan punch through type of failure. There is only one way to avoid the sequential punch through - by designing GBs which can increase the vertical capacity to above maximum preload of the first sand over clay scenario, increasing at the same time also the capacity of the second sand over clay scenario (through the soil plug lateral resistance and overlying pressure as shown in Figure 7), until both sequential peak punch through capacities approach each other as shown in Figure 8 (red curve - final remediated seabed consisting of GBs with 5 m height and 20 m top diameter and slope 1:2.5).

The design of the remediated seabed is carried out based on conventional and FE Plaxis 2D axisymmetric analyses, both, small strain (Plastic) and large deformations (UM). The GBs were constructed as designed, and the jack-up platform safely installed, measuring shallow spudcan penetrations as predicted. This case history shows how overall seabed and soil data can be incorporated deriving a safe solution for an otherwise critical site. 9. Acknowledgement The authors thank Geo for supporting this work and acknowledge Gulf Marine Services and ConocoPhillips UK Limited, for their cooperation. 10. References Eurocode 7, Geotechnical Design – Part 1: General

Rules. EN 1997-1, November 2004. EN ISO 19905-1 Petroleum and Natural Gas

Industries – Site Specific Assessment of Mobile Off shore Units – Part 1 Jack-ups (ISO 19905-1:2016).

Hansen, J. B., A Revised and Extended Formula for Bearing Capacity. Bulletin No. 28. The Danish Geotechnical Institute, 1970.

Kellezi L. & Stromann H. (2003), ‘FEM Analysis of Jack-up Spudcan Penetration for Multi-Layered Critical Soil Conditions’. Proceeding of BGA International Conference on Foundations, ICOF2003, Dundee, Scotland, page 410-420.

Kellezi L., Kudsk G. & Hofstede H. (2007), ’Seabed Instability and 3D FE Jack-up Soil-Structure Interaction Analysis’ 14th European Conf. on Soil Mech. & Geotech. Eng. ECSMGE 2007, September, Madrid, Spain, Proc. Volume 5 page 247 - 252.

Kellezi L., Kudsk G. & Hofstede H. (2008), ‘Jack-up Rig Foundation Design Applying 3D FE Structure-Soil Interaction Modelling’ 2nd BGA International Conf. on Foundations ICOF 2008, June, Dundee, Scotland, Proc. Volume 1 page 937-948.

Kellezi L. & Kudsk G. (2009), ‘Jack-up Foundation, FE Modelling of Punch Through for Sand over Clay’ 12th International Conf. on Jack-up Platform. Sept. London UK, Proc. Page 1-12.

Kellezi L. & Stadsgaard H. (2012), ‘Design of Gravel Banks – a Way to Avoid Jack-Up Spudcan Punch Through Type of Failure’, OTC 2012, Houston, USA, April-May 2012, Paper no. OTC 23184.

Plaxis 2D, Finite Element Code for Soil and Rock Analysis, Delft University of Technology and Plaxis bv. The Netherlands, 2016.

SNAME, Guidelines for Site Specific Assessment of Mobile Jack-Up Units, Technical & Research Bulletin 5-5A, January 2002.