Halfweg-Platform North Sea

Iv-Consult

PETROGAS HALFWEG-PLATFORM NORTH SEA

THE PLAN OF APPROACH (Part 1 by Jeroen Kwakernaak (Petrogas E&P Rijswijk), Niels van Berlaer (DEME Offshore) and Ad van den Dool (Iv-Consult Papendrecht)

A collision with a tankship caused considerable damage to a platform that was being prepared for decommissioning.

This navigational error sounded alarm bells: was the platform still safe, and how could it be removed?

Introduction The Halfweg gas field is a natural gas field in the North Sea on the Dutch continental shelf 20 km west of Den Helder. It was discovered in 1968. The total gas yield of the field was estimated to be 2.53 BCM (Billion Cubic Metre). The exploitation of the Halfweg field began in May 1991 with test drilling Q1/23. After a successful test, the well was temporarily abandoned due to a planned tie-back (connection to an existing production facility). This tie-back was realised with the installation of the Halfweg platform in May 1995 (fig. 1); well Q1/123 became production well A1. In addition to well A1, two new wells were drilled: Halfweg A2 and A3, completed in September 1995. Gas could now be pumped via the Halfweg platform to the so-called Photo: DEME Offshore

WGT (West Gas Trunkline) and then to the gas treatment installation in Den Helder.

On the 1st of January 2006, gas production from the Halfweg platform to the WGT was terminated.

The Halfweg platform now only produced gas Photo: Petrogas

to supply power onboard two existing platforms

Fig. 1

and functioned as a so-called gas compression platform.

Installation of the Halfweg platform in 1995

English translation of 3 articles in Dutch magazine 'Bouwen met Staal' nr. 271 – 273 (2019/2020)

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Platform removal preparation It was formally decided to end production on the 6th of June 2016, after which the drilled wells had to be closed under the 'P&A campaign' (Plug and Abandonment). This operation was completed in November 2017. At the same time, preliminary preparations for the platform removal, such as cleaning the pipelines, were implemented.

Furthermore, in preparation for the platform removal, Iv-Consult was commissioned to perform a FEED study (referred to later in this article). Based on this study, the request for the actual removal of the platform was issued in November 2017 to several offshore contractors.

Fig. 2 Proposed lifting configuration (Iv-Consult)

FEED study The study aimed to investigate the structural

The removal of the gravity base was also examined.

integrity of the platform during removal and sea

However, during the engineering process, it was

transport. Originally, the platform was designed

decided to cut the piles just above the gravity

as a self-installing and de-installing platform. De-

base, remove them and leave the gravity base

installing would have to be done through a reverse

behind on the seabed.

installation procedure, whereby the gravity base (concrete caisson at the base of the platform)

Tankship collision

would be de-ballasted to create buoyancy and the

Towards the end of the tender process, a collision

platform lowered with strand jacks. Unfortunately,

with a chemical tanker (Elsa Essberger) occurred

it appeared this method could no longer be

on New Year's Eve 2017, causing significant

used, and an alternative had to be devised, for

damage to the platform. As a result, all plans were

which various options were reviewed. Ultimately,

frozen, and immediate action was taken to assess

removing the platform with a Heavy Lift Vessel was

and determine the damage. To facilitate this, a

chosen. After determining the correct weight and

campaign was organised from a Survey Support

location of the centre of gravity, the most suitable

Vessel, involving the following:

lifting configuration could be determined based

►► Visual underwater inspection with an ROV

on the as-built data. The structural integrity of the

(Remotely Operated Vehicle) to determine

platform could then be demonstrated with lifting

damage to the platform leg connections at the

simulations (the platform legs would be cut loose

gravity base and damage to the gravity base

prior to the lifting operation and lifted separately).

itself. ►► Visual inspection of the platform with a UAV

A hoisting configuration with three spreader bars was chosen to prevent the platform from being

(Unmanned Aerial Vehicle) taking detailed

'crushed' by the internal horizontal forces that

images of the platform and legs. ►► 3D scanning of the platform and legs to

occur with a four-point single hook lift (fig. 2). The

determine deformations.

legs could easily be lifted by using an ILT (Internal Lifting Tool) or, for example, with welded-on lifting points. The contractor could determine this at a later stage.

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The tender was postponed due to the collision, and an investigation was initiated to determine the structural integrity of the damaged platform (Fig. 3).

Fig. 3

Damaged Halfweg platform during inspection

Structural Integrity Fig. 4

Two important questions raised after the collision: ►► Is the structure of the platform still safe?

SACS-model (Iv-Consult)

predicted corresponded with the observed plastic

►► How can we safely remove the damaged

deformations onboard the platform. A validated

platform?

calculation model formed the basis for the followup research: the design of efficient reinforcements

Two critical questions were raised following the

to safely remove the platform.

collision:

►► Is the platform structure still safe?

Decommissioning study

►► How can we safely remove the damaged

With all defects now identified, the design for the

platform?

reinforcements of the platform necessary for safe lifting and transport could be created. A plan also

To answer these questions, it was essential to obtain

had to be developed for the method and sequence

the best possible and detailed insight into the

of cutting the platform legs.

extent of the damage - which required access to

The tankship initially collided with the legs at the

the platform. Based on the inspections conducted,

front of the platform, then passed and hit the

preliminary calculations were performed on the

underside of the platform and finally halted after

residual strength and stability of the platform,

colliding with the rear legs. As a result, the legs

revealing no immediate danger of collapse; the

suffered plastic deformation, which necessitated

platform was safe to visit. However, the helideck

extra care when cutting the legs to prevent

was damaged to an extent that made it unsafe

the sudden release of internally stored energy,

to land a helicopter on the platform. Instead, a

potentially leading to an unsafe situation.

vessel with a motion-compensated walkway was

To prevent the legs from 'shocking' during the

used to conduct detailed on-site examinations

cutting operation, shear keys were used to prevent

(visual inspection and NDE (Non-Destructive

the leg parts from moving apart. Cutting the legs

Examination)). All collected data could now be

altered the static system of the structure (the one-

combined with the point cloud from the 3D scan,

piece legs were now interrupted), and the order

thus providing an overall impression of the damage.

of cutting appeared to have a major influence on

A comparison was subsequently formed with the

the release of the stored energy. Of course, the aim

results of the preliminary calculation model (fig. 4a

was to release this energy as gradually as possible.

and 4b). In particular, it was examined whether the locations where (local) damage of the structure was

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The lower deck of the topside required special

Offshore operations (such as cutting the legs)

attention as it was a requirement not to work

were simulated and practised on land to avoid

below the topside: this would prove very expensive

surprises offshore. DEME Offshore selected

since scaffolding and rope access technicians were

the Heavy Lift Jack-Up Vessel Apollo with an

needed. As a result, the structural parts of the lower

800-tonne crane for the lifting operation. This

deck, damaged by the collision (deformation of

crane vessel formed a stable working platform and

several beams and torn-off connections), had to be

was crucial for an efficient operation during the

reinforced from above (fig. 5).

winter period (two winter storms occurred during the operation). Once the welding work for the reinforcements and the installation of all lifting aids was complete, the legs were cut just below the platform using abrasive water jet cutting. In Photo: DEME Offshore

one lift, the platform was removed and placed on

Temporary reinforcements of the top deck

Photo: DEME Offshore

Fig. 5

the deck aboard the Apollo (Fig. 6).

Fig. 6

Platform ready for hoisting with Apollo

Removal of platform legs Photo: DEME Offshore

The four legs were cut using diamond wire

Fig. 7

cutting, the preferred method as the legs still contained secondary steel and risers that needed cutting simultaneously. Following arrival in

Removal of the legs

Vlissingen, the platform and piles could be transferred to the quay of the company Hoondert,

Preparation and removal platform

and the dismantling could commence. In April

In August, DEME Offshore was contracted as an EPRD

2019, the entire platform, including the piles,

contractor by Petrogas E&P Netherlands. However,

was dismantled, and contaminated parts were

even before the contract was signed, all parties

disposed of via regular waste flows.

closely collaborated in the engineering and offshore visits to determine the best strategy for removing the platform. Due to the tight schedule, it was decided to involve the MWS (Marine Warranty Surveyor) in the process as soon as possible so that any questions/ queries could be discussed as early as possible.

Because all parties had agreed the platform would be removed at the end of 2018, all disciplines worked concurrently.

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IMPACT COLLISION (text part 2 by Edwin Belder and Ad van den Dool (Iv-Consult Papendrecht)

At the moment of collision, the Elsa Essberger, with a full weight of about 5,000 tons, had a speed of about 6 m/s and successively hit three columns (legs), the foundation of the crane and the lower deck of the platform (fig. 8). Fortunately, the platform was unmanned and the vessel crew was not injured.

The ship suffered significant damage to the bow, port, and starboard sides (fig. 8a). This incident sounded a major alarm with the Coast Guard, who immediately inspected the situation. Following the release, the Elsa Essberger sailed to the port of Rotterdam for further questioning by judicial authorities.

The co-owner (and operator) of the platform, Petrogas E&P Netherlands, was informed and took immediate action to have the structural safety of the platform examined. Iv-Consult was familiar with the platform and was, therefore, requested to assess the structural safety, including an

Fig. 8a Elsa Essberger, after collision

investigation to identify possible hidden damage via Finite Element Method (FEM) analyses. In addition, the platform would be photographed with a drone (Bluestream [3]), and an ROV inspection of the Gravity Base Structure (GBS) and the legs would also be conducted.

Fig. 8b Locations of the main impacts

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Pd

denting force (kN);

Fy

yield stress of the tube (N/mm2);

T

wall thickness of the tube (mm);

X

depth of the dent (mm);

D

diameter of the tube (mm);

Edent absorbed energy in the dent (Joule).

Fig. 9

Dent measurements in the point cloud

Structural safety Due to reasonable doubts about the structural

Three main impact loads were defined for the

safety in combination with the upcoming spring

calculation model, each in a slightly different

storms, simplified analyses in SACS (Offshore

direction. The ship changed direction after the first

Structural Analysis and Design Software) were

impact and thus moved towards the second leg

initially performed, adding hinges in the legs at the

and hit crane pedestal. It was decided to perform

points where major dents were visually identified.

a 1st-order and a 2nd-order geometric non-linear

This schematisation was combined with wind load

(GNL) analysis for each step, impact 1 to 3. A push-

and wave action from a 1-year storm. The coarse

over analysis was performed, simulating impact

analyses showed that the platform was safe in

1 to 3. Initial impact forces were put on an intact

these given conditions. However, the preliminary

model, the overloaded members were identified

calculation results were insufficient to place an

and subsequently deactivated. This resulted in

inspection team onboard the platform.

the deformed position of the platform simulating

The regular procedure for such calamities is to

the permanent deformation after the first impact.

withdraw the offshore certificate, excluding access

This deformed model was then used as the base

to the platform. Therefore, the structural safety had

model for the next impact. This was repeated for

to be proved with more detailed calculations.

all impact stages. The final deformations (Figures

Bluestream had processed the drone images into

10 and 11) could be validated with the point cloud.

a 3D point cloud that provided more detailed

The final resulting geometry was then combined

information about the damage and permanent

with maximum wave, wind and sea current loads

deformations (Fig. 9). The point cloud helped

for a return period of 1 year. It was concluded

assess the size of the dents in the platform legs as a

that, although the collision had caused significant

basis for determining the impact loads.

permanent deformation, this would not lead to a global nor local collapse of the structure.

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Fig. 10 SACS FEM-model

Fig. 11 Stress plots FEM-model

Gravity Base Structure

Onboard inspection

A separate analysis was performed for the Gravity

The onboard inspections were conducted in May

Base Structure (GBS) based on the maximum

and June 2018 by a team from Petrogas, Iv-Consult

impact load. The legs, fixed with dowels and a

and Bluestream. All damage was documented

base plate in the GBS, were substantially damaged

based on an inspection protocol, and large-scale

just above the 6-metre-high GBS. During the ROV

Non-Destructive Testing (NDT) research was

inspection, it was confirmed that the structure

conducted (Eddy currents and ultrasonic research).

had failed locally at the GBS-leg transition: the

During the inspection, it became clear that several

legs were buckled locally. However, no cracks in

secondary beams of the lower deck and helideck

the steel or the concrete were observed. For more

were partly torn from the web of main girders. An

insight into the structural capacity of the GBS, it

undesirable situation because these secondary

was also decided to analyse the GBS with FEM

girders should guarantee the lateral stability of the

software. This revealed the structural capacity of

main girders. Furthermore, two bottom flanges

the GBS was sufficient. The offshore certificate was

of the lower deck main girders were plastically

re-issued based on these analyses, thus allowing

deformed in a sideways direction over several

inspection onboard the platform.

metres (Figs. 12 and 13). The pedestal of the crane had collapsed entirely, and the crane itself was several degrees out of alignment. Despite everything, the condition of the platform was encouraging but not safe enough to be removed.

Therefore, it was decided to reinforce the platform locally to enable a safe removal.

Fig. 12 Deformed main girder

Fig. 13 Location of deformed main girder

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Strengthening the platform The primary purpose of the reinforcements was to make the platform suitable for lifting and sea transport to a decommissioning yard in the Netherlands. However, reinforcing offshore has

z = 0,145

0,264

its limitations. Locations above the water and below the lower deck are challenging to reach and

x=1

can be dangerous. This resulted in a secondary objective: designing all reinforcements above deck

y = 0,221

measurements in mm

to facilitate safe offshore work. Furthermore, it was

Fig. 15 Rotation of the main girder

also decided to design the reinforcements where possible with fillet welds to ensure the time for –2,70454

performing NDT was kept to a minimum. Where possible, S355N was chosen because of its better weldability; this was the best choice given the

5,43155

offshore conditions.

13,5676

The crane pedestal had a dent halfway between 18,9917

the decks (fig. 13, top, right). Therefore, to prevent

21,7037

any further deformation, it was decided to support 1 gedeukte kraanfundering

the foundation at that spot with a brace to the

2 HEB 200

deck (fig. 14).

3main buis 168,3x8

Fig. 16 Analysis of the main girder

4 plaat S235

At the lower deck and helideck, the secondary

1

beams, damaged at the welded connection to

2

the main beams, were reinforced with welded

22

5 33

54

T-stubs (fig. 17). Since the secondary beams were

100

originally designed as torsional buckling restraints for the main girders, the main girders were 392

additionally examined with restraints at different

3

positions.

50

700

200 16 200

300

4

12

50

3000

A

Fig. 14 Reinforcement of crane foundation: 1. Dented crane foundation, 2.

HEB 200, 3. Tube 168,3x8, 4. Existing deck plate S235

1

3 724

A 2

The lower deck's deformed main beam (Fig. 12,

1 2 3 4

13, 15 and 16) was analysed in Ansys according to DNV-GL-RP-C208 and C204 to exclude the

130

possibility of lateral-torsional buckling of this UB

UB 838x292x176 UB 356x171x45 1/2IPE 500 R = 25 (typ.)

95

3

184

838x292x176, from which the web and flanges were plastically deformed. Any buckling instability

12 4

50

of the structure during lifting and transport should be excluded.

Fig. 17 Reinforcements of secondary girders

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DECOMMISSIONING (Part 3 by Fabio Amico and Ad van den Dool (Iv-Consult Papendrecht)

The Halfweg platform was removed in several steps. First, the top side was removed. For this purpose, the legs were cut 1.5 m below the lower deck of the platform and lifted one by one onto the crane vessel. The analysis for cutting the columns is highlighted in this final section. legs and fully cut legs. The calculations were performed using FEM, first to examine the overall behaviour of the platform and then focus on the locally damaged areas where high-stress spots were present. A FEM beam model was used to analyse the general behaviour, while non-linear plate models were used for the local checks. The analyses were conducted in accordance with DNV GL and the Eurocodes for members, plates, and shells.

Order of cutting The order of cutting has a major influence on the platform's internal stress/load distribution.

When a leg is partially cut, it can still transfer the internal axial shear forces and bending moments, depending on the seize of the cut. However, when the cut is almost complete, the internal forces generated by deformation migrate to the other

Cutting the legs

legs due to the change of the static scheme. The

The legs were first partially cut, just below deck:

increase in bending moment of the uncut legs

the initial cut. Shear keys were then placed on the

was significant (magnification factor of about 1.5).

inside of the legs and welded to the lower leg part.

A sensitivity analysis, in which various cutting

The legs were then cut through, and the platform

sequences were evaluated, was performed to

lifted. The cutting operation was done with

choose the most favourable cutting sequence.

waterjet cutting in combination with an abrasive.

When partially or entirely cut, the residual strength ultimately determined in which weather

Analysis method

conditions (wind, current and waves) the operation

The analysis to safely cut the legs included

could be performed safely (within the reference

detailed calculations that considered the material,

period of one year).

geometric nonlinearity, imperfections, residual stresses, deformations, and stored energy from the collision. Additionally, the effects of (reduced) loads from wind, waves and currents had to be analysed in two different scenarios: partially cut

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From beam model to plate model A selection of load combinations was analysed to identify worst-case scenarios. Leg B3 was identified as having the highest stress concentrations; in addition, leg A3 was to be analysed, having the most critical global deformation just above the intended cut (Fig. 18). The leg was yielded locally (observed from the damaged paint layer).

The internal and external loads from the global SACS FEM model (Fig. 19) were applied to the ANSYS plate model for those two critical legs (A3 and B3). Besides the loads, the associated boundary conditions (displacements and rotations) were modelled (Fig. 20). To cover uncertainties

B

around the cut location, three orientations of the initial cut were considered (skipped by 40°).

A

3 1

Fig. 18 Bulge on column A3 (source: Deme)

Fig. 19 Global FEM model

F,M

Cutting orientation A

1

Interface loads

1

2

2

Cut location

2

3

3

Displacements

Interface loads

1 Topside

F,M

3 ,

Cutting orientation B 40˚

Environmental loads 4

Reaction forces at GBS interface

SACS Global Beam Model

Cutting orientation C 80˚ 80°

4

ANSYS Beam Model of leg A3 and B3

ANSYS Plate Model of leg A3 and B3

Fig. 20 Load transfer (from SACS-beam model tot local ANSYS plate model)

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Fig. 21 Zoom-in of FEM model at the cut location

Fig. 22 Determination of plastic design reference ratio rRpl (incremental analysis for ULS)

stress (N/mm2)

400 350

353 max.

300 250 200

112

150

0,0943 min.

100 50 0

0

0,01

0,02 0,03 strain (-)

0,04

0,05

Fig. 23 Material stress and strain relationship Fig. 24 Stress concentration of lifting eye

Fig. 25 Stress at shear key

FEM plate model

Nonlinear Analysis

In addition, the geometry of the FEM plate model

The purpose of the analysis was to investigate

also contained the measured deformations/

the strength of the legs at the weakest location

imperfections, such that the structure's response

during cutting. However, this location could not

could be predicted as accurately as possible. The

easily be predetermined and was based on the

legs were modelled with shell elements. The leg

condition with the greatest internal loads. After

axis followed the deformed shape of the SACS

all, the imperfection (dent) and the plastically

beam model (Fig. 19). The local dent was modelled

deformed areas had a considerably reduced

with the actual geometric deviations. The shear

residual capacity, although they did not belong to

keys and stiffeners were modelled with plate

the most heavily loaded leg part. Therefore, the leg

elements. The holes (lifting eyes), necessary for

as a whole had to be considered. The analysis was

removing the columns after removal from the

performed as MNIA (material nonlinear analysis

topside, were also modelled. The presence of the

with imperfections). The dent was modelled in the

holes delivered a significant impact and caused

FEM model using shell elements. The stress-strain

peak stress concentrations (Fig. 24). The mesh size

relationship of the material (Fig. 23) was elastic-

at the cut location was between 2 and 10 mm.

plastic. The checks were performed according to

An average mesh size of 50 mm was applied for

EN 1993-1-6 and DNVGL-RP-C208. According to the

the rest of the leg (Fig. 21). Various mesh sizes

theory for large deformations, second-order effects

were evaluated to obtain reliable results and a

were also taken into account with the geometrical

reasonable calculation time. The three cuts were

nonlinearity. An incremental analysis (Fig. 22) was

modelled in a way that would allow the edges

performed to determine the plastic resistance

to separate from each other. The three cutting

reference ratio rRpl, which, together with the elastic

orientations A, B and C (Fig. 20) were investigated

critical buckling reference ratio, resulted in the

with three different submodels.

overall slenderness λov and the reduction factor Χov.

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ʎov = √(rRp/rRa) χov = f(ʎov, ʎov,o,βov,ŋov) ʎov

overall slenderness

ɑov

overall elastic imperfection reduction-factor

βov plastic range-factor ŋov

interaction exponent Fig. 26 Shear key after installation (source Deme Offshore)

The design buckling resistance ratio was determined and compared with the limit

250 200

with LBA (Linear Buckling Analysis) was included at

150

each load level, resulting in the most minor critical

100

stress on welds (Mpa)

according to EN 1993-1-6. A specific value check

buckling factor (Euler multiplier). The maximum allowable strain was considered to be 2%. Material properties and load factors were in accordance with DNVGL-RP-C208 and DNVGL-OS-C101.

50 0 –50 –100 –150 –200 0

Shear keys In each leg, three shear keys, each at an angle of

100 200 300 400 500 600 700 800 900 1000 weld lenght (mm)

comb

120 degrees, were mounted prior to the cutting

VM,all

operation. The primary purpose of shear keys is

all

the transfer of horizontal loads, however due to

combined stress acting on the weld throat normal stress acting on the weld throat allowable combined stress allowable normal stress allowable shear stress

friction, the shear keys were loaded vertically as

Fig. 27 Normal, shear and combined stress in the shear key weld

well. Because various load directions could occur

The methodology and cutting procedure were

during the lifting process, the application of box-

determined in close collaboration with the MWS

shaped cross-sections suitable for transferring

and Deme Offshore: the offshore execution and

torsion was chosen. The maximum horizontal load

the analyses should match and thus ensure the

between the cut parts of the legs in combination

release of potential energy on-site in the legs

with the maximum expected vertical pull-out

occurs gradually and not abruptly.

load (caused by steel-to-steel friction) was taken into account. Stiffeners along the leg wall were

The legs were cut in a predetermined order,

added to compensate for deformations in the wall

following the most efficient transition of the

and reduce the welds' stress level (fig. 25-26). The

static system. The cutting execution (Fig. 28) was

weld checks were performed by extrapolating the

performed with continuous monitoring of critical

normal and shear stresses along the weld (fig. 27)

zones whereby local and plastic deformations or

and were tested against the maximum permissible

settlements were predicted with the FEM models.

value according to the standard.

In this way, the risk of uncontrolled or undesired structure behaviour was minimised, and the operation was performed safely.

References

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Fig. 28 Cutting of the legs (source: Deme Offshore)

1. Petrogas E&P Netherlands (Rijswijk). Oil and gas operator, with shares in several platforms in the North Sea. 2. Iv-Consult (Papendrecht). An international engineering company, part of Iv-Groep, active in structural and mechanical engineering with a broad portfolio in offshore decommissioning and transport and installation.

3. Deme Offshore (Zwijndrecht, Belgium). Offshore and dredging contractor.

4. SACS. Offshore structural analysis and design software.

5. ANSYS. Engineering simulation and 3D design software.

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Photo: DEME Offshore

About Iv-Consult Iv-Consult is specialized in the design of challenging steel and mechanical structures and has a long track record regarding offshore removal and transportation projects. Offshore inspections and 3D-scanning are part of our expertise.

About the authors Jeroen Kwakernaak (Petrogas E&P Rijswijk), Niels van Berlaer (DEME Offshore), Edwin Belder, Fabio Amico and Ad van den Dool (Iv-Consult Papendrecht)

Would you like to know more about the possibilities for your project?

At Iv-Consult, we would be happy to share our ideas and knowledge and provide a quotation.

Email Ad: a.d.vandenDool@Iv-Consult.nl or call: +31 88 943 2501

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www.iv-consult.nl

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