Removal and abandonment of offshore platform topsides including the toppling alternative and costs

Removal and Abandonment of Offshore

Platform Topsides including the Toppling

Alternative and Costs

N. G. Boyd

Taylor Woodrow Offshore Ltd

SYNOPSIS

There are in European Waters approximately 215 fixed steel platform installations. many of which are quite

small and weigh only hundreds of tonnes. There are also 18 concrete platforms. At some time in the future

these installations will have to be removed partially or totally. The first part of thispaper reviews the extent of

work necessary to remove the 'topsides' of such platforms. It describes the work involved in

preparation for

removal and abandonment and the details of executing the work. The paper also briefly considers the

possibility of demolishing steel platforms at their installed site by toppling them to the sea bed, where water

depth permits this kind of operation. The final part of the paper gives an indication of the costs that will be

incurred in removing and abandoning a platform.

INTRODUCTION

The topsides of platforms generally comprise a varying

number of modules or packages put into place by large crane

vessels at the offshore location. A module support frame or

deck is placed on top of the substructure to support the

modules. Recently super-heavy-lift vessels are becoming

available and integrated decks are now being designed weigh-ing 5000t and more.

Some concrete base platforms were constructed in a

differ-ent manner and these will be dealt with later.

During the course of several extensive studies for a major

North Sea operator, my company investigated several

methods of removing modules and packages. The conclusion was that the simplest and most cost-effective method was to

remove them in exactly the manner in which they were

installed, using a similar crane vessel. A further study is

currently underway to determine if the new super-heavy lifting vessels can be used to reduce the cost of topsides removal.The

early indications are that it may be possible to reduce costs

significantly by removing a small topsides installation complete in a single lift.

All the expertise necessary for the removal and abandon-ment operations is currently available in both the oil

com-panies and the offshore industry contractors. There

are. however, many aspects that will require more detailed study and several universities and companies are investigating cer-tain specific areas.

All work offshore must be car-ied out in accordance with the various regulations to ensure safe working. It is unlikely that

such work will be carried out by demolition contractors,

Norman Boyd is Manager of Engineering with Taylor Woodrow Offshore (formerly Taywood-Santa Fe). In1978

he worked on the first major study of Removal and Aban-donment and has since been involved in further studies and update reviews of the original one. He was a member of the Working Group for the British Committee for ECOR report and has worked on the North Cormorant and

More-cambe Bay Projects. He has also presented papers on

other subjects.

Trans IMarE(TM) Vol. 99, Paper l0

although there may be advantages in including some demoli-tion expertise in the team.

The operation of removal and abandonment can be divided into two stages, preparatory and execution.

PREPARATORY STAGE

In common with allmat ol projects an extensive preparatory stage is necessary to identify, prepare for and plan all the work

involved in removing the platform topsides. Much of the

required work will be similar in nature to the preparation and engineering work carried out when installing the platform.

Engineering

A considerable amount of engineering work will be required and will include the few examples described below:

I. A detailed weight and structural survey will be necessary

for each module and package. Much of the original lifting

support steel may have been removed and replacements must

be designed. Many changes may have been made to the

equipment in the module and the weight and centre of gravity must be determined.

The integrity of pad eyes must be checked and. where

necessary, replacements designed.

Protruding external vents, pipe spools, walkways etc. will

be burnt off and stored inside modules. The details must be

determined.

Selection of a suitable lifting vessel must be made at an early stage. The exact method of lifting must be checked and slings and other lifting aids procured.

The type of cargo barges required must be determined

and the method of mounting removed modules and packages designed.

The types of supporting vessels, numbers, length of hire and start of hire must be determined. Such vessels will include

supply boats, accommodation vessel, tugs. cargo barges,

safety vessel etc.

An early decision will be required on whether removed

modules and packages are to be transported to shore

_or

dumped at sea. In this connection there will generally be little

intrinsic value in the removed topsides and everything will be treated as scrap value only. If being transported to shore then

sea-fastening details for each removed piece must be

engineered. A suitable disposal site must be found that can

break up the modules quickly, in order to achieve a quick turn around of the cargo barges. If dumping at sea is required then the dumping site must be agreed under the London Dumping

Convention. Engineering must determine the details for

'launching' the packages off the barge and also ensure that no vessels, tanks etc. can float free.

8. A Certifying Authority will need to be appointed and

Government Regulatory Authority Approvals will be

required. These authorities require to be satisfied at the outset

that the working procedures and methods comply with the

existing regulations and codes in regard to safety, navigation and pollution control.

Many procedures must be developed at an early stage. out-lining the methods of executing various sections of the work. Some examples of the types of procedures required are well plugging and abandonment, removal and disposal of toxic and

hazardous chemicals and materials, and shutdown and

de-commissioning of hydrocarbon process systems.

Once the outline procedures have been produced and

agreed, then detailed procedures can be developed, wherein

the detailed sequence of operations is determined for each

process and utility system on the platform. Arrangementsmust be made for temporary electricity supplies and safety after the platform utilities are shutdown.

During the engineering work there will be a need to examine

alternative proposals for various aspects of the work and to

make detailed cost evaluations in order to permit selection of the most cost-effective methods. Further details of the

engin-eering work are indicated in the section dealing with the

execution stage.

Procurement and contracts

A Procurement and Contracts service will be required to purchase additional materials, fabrication and consumables

for the operation. Contracts must be entered into for the hire of all the supporting vessels and services.

A supply boat and base service will be required to be

established in a similar manner to that required for a hook-up and commissioning contract. Contract documents and speci-fications must be developed for the many and various aspects

of the operation. Planning

A great deal of detailed planning will be vital to the success

of the operation. Detailed work packages must be prepared

taking full account of the sequential nature of the work. The

manpower requirements must be determined for each work package and the time allowed for the work established.This is

then incorporated into the overall plan.

The plan must allow for optimization of all vesselh i ringsand manpower, and due allowance must be made for the weather factor in all the offshore operations.

EXECUTION STAGE

The execution stage marks the start of the actual work

offshore. Much of the work will be of a sequential natureand it can be broken down into several clearly defined activities:

Shutdown

Well plugging and abandonment Decommissioning

Deconstruction

Lifting and sea fastening Disposal

Shutdown

The first activity at the start of platform removal and

aban-donment is to shut down the production process systems in conjunction with the shut-in of wells. All life-support and utilities systems will be maintained fully operational until

completion of well abandonment.

Preparation will provide for the examination of engineering

documents such as process, flow, instrument and electrical diagrams. Detailed shutdown procedures will be prepared

providing for the sequencing and safe execution of work. The

extent of the work will he dependent on thenumber of wells

and the number of process and utilities systems. Process shutdown will include:

Switching power generation to imported fuel.

Begin the shut-in of production wells one by one by

closing control valves, except for the final well. The final wellis

used to supply fuel to and allows for the orderly and gradual

shutdown of the heating. glycol, dehydration and cooling

systems.

Shut-in final production well as requirement for fuel is

terminated.

Shutdown glycol circulating pumps. condensate pumps and cooling water pumps.

Shutdown and isolate production export pipelines. Some depressurization and draining of hydrocarbon systems

may be started at this stage by venting to flare headers and

draining to sea pumps.

Well plugging and abandonment

This phase provides for the permanent plugging and sealing of all wells associated with a particular platform. The opera-tion will require a full drilling crew plus addiopera-tional equipment such as a cement pumping unit. It will be necessary to ensure

that a drilling or workover rig is available and fully

opera-tional. Full platform services, utilities and safety systems will

be maintained operational.

The preparatory work will provide for the preparation and

approval of a detailed procedure, taking specific account of

whether the wells are producing, with or without artificial lift. Additional equipment requirements will include cement pum-ping unit, casing cutting tools and spears. Schlumberger unit, plug setting tool, wireline unit, blowout preventersand rams,

drill pipe and collars, and tubulars handling tools and

accessories.

The operation will start with pulling the downhole safety

valve and any flow-control devices, rigging up the cement unit to swab valves and establishing pumping rates down

produc-tion tubing with seawater. Further operaproduc-tions provide for removing production tubing and casing and laying cement

plugs at predetermined levels in accordance with the agreed

procedure. The final plug will extend up to the sea bed.

The plugging and abandonment operation will require about 7-10 days per well, therefore assuming a 30 well platform the time required will be from 210 to 300 days. It is possible that some decommissioning can be carried out during this time but

all life-support and safety systems must be maintained fully operational and the amount of spare accommodation may be a limiting factor.

Decommissioning

This activity will provide for leaving the platformtotally shutdown, clear of all hazardous materials and certified safe for deconstruction to proceed without recourse to a permit to

work system. The extent of the work is dependent on the number of separate systems on an installation.

During the preparatory stage detailed work sequence pro-grammes and procedures will be developed and approved for the decommissioning of all production and utilities systems. Any additional equipment and material requirementswill be

8

established and provided. The extent of the work will depend

on the number of separate systems on each platform. An

accommodation vessel will be required to provide, in addi-tion to accommodaaddi-tion, temporary power supplies, communi-cation facilities and workshops. Additional short-term storage

will be required for diesel fuel, other materials and equipment. There are generally four principal categories of systems on a

platform: hydrocarbon systems, non-hazardous sytems, toxic

and hazardous chemical systems, and electrical power systems.

The hydrocarbon systems include well-head and test pro-duction, main propro-duction, dehydration and metering, sphere launcher, receivers and slug catchers, gas compression, re-injection or water re-injection, fuel gas, relief and flare systems, drain, diesel fuel, jet fuel. chemical injection, hot oil, lube oil,

and pipelines.

The non-hazardous systems include cooling water, fire

water, washdown, potable/fresh water, instrument and utility air, and steam.

The toxic and hazardous chemical systems include glycol, injection chemicals, hypochlorinator, acids, caustic soda. and paints and thinners.

The electrical power systems include main power, emerg-ency power, and battery power.

Until all hydrocarbon has been purged and removed all work must be carried out under a permit to work system.

Hydrocarbon Systems will require seven stages to purge fully all hydrocarbon. The seven stages are as follows:

Depressurization. A specific system will be isolated from

other connecting systems. Valves, connecting system vessels to flare header, will be gradually opened. All relief valves must be

in the normal operating condition and the depressurization

must be monitored by line and vessel gauges. The flare system

oil burner and sea sump must remain operative until disposal of all hydrocarbons is complete.

Hydrocarbon drain. Residual liquid hydrocarbon is

drained directly to the flash tank, or sea sump. Vapours from the flash tank are vented to the low-pressure flare header.

Water flush and drain. The system is flushed with sea-water whilst maintaining venting to flare headers. Flushing water is drained to the flash tank or sea sump.

Steam purge. Steam is used to purge residual

hydrocar-bons. Each vent and drain is monitored until the level of

hydrocarbons present is reduced to an acceptable level. Water fill. Steam is replaced by water fill in a manner which avoids the formation of a vacuum.

Nitrogen and water drain. Nitrogen is introduced at high vent points and the water is drained.

Isolation. On completion of filling the system with

nitro-gen the final step is to close all drain valves and all major system valves.

All safety equipment must remain operational until the process systems are inert and safe.

Non-hazardous systems are depressurized. flushed with water, drained and isolated.

Disposal of toxic and hazardous chemicals will depend on

their nature. Generally, they should be removed in their

original containers, observing normal safety precautions and regulations. The exact details will depend on the particular substances and the installation.

The decommissioning of electrical power systems is a

planned sequenced shutdown of all switchgear. motor control centres, distribution and generators, observing all necessary

safety procedures.

Complete and detailed logs of all decommissioning activities

must be kept so that certified clearance may he obtained to

permit further work to continue without the necessity of

operating a permit to work system.

Deconstruction

This activity covers all work to prepare the topsides modules

and packages to enable them to be removed. There is little

doubt that they should be removed in exactly the same manner as they were installed, ie each module or package installed as a separate lift is removed as a separate lift by a crane vessel with

at least the same capacity and reach.

The execution of the work will start only after the total

platform has been certified safe and free from hydrocarbons

and toxic materials and all systems shut down. Piping and ducts

will be cut a minimum of 3 ft from module walls. The short cut lengths so removed, together with other loose equipment, are tack welded down inside the module. Cable can be cut, rolled back and tied down. Walkways and platforms will be cut and the short sections removed and secured. Flare booms can be prepared for removal by cutting all process piping and cables. Any interconnecting bridges will have all piping cut.

Replacement of pad eyes will require extensive welding under controlled conditions followed by radiographic or ultra-sonic examination to prove the integrity of the replacement.

Modules will be cut free from their supporting structures. Module weights and centre of gravity will be checked by survey

and by weighing if practicable.

A final check will be made to ensure a module or package is

free from all connections and all temporary equipment and supplies removed. Decks and the module support structure

will be divided into sections, general!), by cutting through

original field splices and through the top of jacket legs.

Extreme care and detailed planning will be required to

ensure the safety of these operations. The final cutting of flare booms, towers or bridges can only be performed when the crane vessel is in position supporting the item.

The sequence of preparation for lifting will depend on the lifting sequence determined during the preparatory work, but will generally start with upper level modules and packages

followed by lower level packages. bridges, flares etc. and

finally decks and support frames.

The preparatory work and preplanning will have deter-mined the economical and practical time for the heavy-lift vessel to arrive. Such vessels will have sufficient

accommoda-tion to permit release of the accommodaaccommoda-tion vessel that will have been required during decommissioning.

The newer super-heavy-lift vessels appear to offer the

possi-bility of lifting the complete topsides of some of the smaller platforms in one lift. This application is currently being stu-died, but in this case some of the deconstruction work will not

be required, although alternative work will be required to prepare for the single lift and it will be necessary to establish a

reasonable centre of gravity.

Lifting and sea fastening

Many of the lifts will be of large but low-weight components such as flare towers or booms, bridges, helidecks etc. The

preparatory work will have established in detail the lifting sequence, and procured the necessary slings, equipment, addi-tional structural bracings, spreaders and rigging. The lift vessel

contractor will require verification of weights and centres of gravity.

Special rigging arrangements used for installation of flare

booms, cranes etc. will require similar arrangements for removal. If modules were installed on a skid beam and skidded

into final location, the reskidding operation will be extremely difficult and may require special preparation.

Although damage to a module during lifting may not be critical and installation bumpers and guides need not be replaced. it is essential that all the normal safety precautions be

taken. A serious accident at any stage of the operations will cause at best a few days' delay.

All the lifting and sea-fastening operations require good weather and reasonably calm seas. In hostile environments such as the North Sea this limits these operations to a period from about April to September with a risk of some days being lost. This aspect will therefore decide the starting point for working back the planning of the whole operation.

Disposal

The disposal alternatives of onshore or dumping at sea were

mentioned earlier, and whichever method is selected the

operation of towing a cargo barge load of modules or packages must be conducted safely. Onshore disposal requires the selec-tionofa suitable breaker/scrapyard, capable of receiving cargo

barges with large quantities of material which need to be

removed quickly.

The alternative of disposing of a platform topsides at the

on-site location is dealt with later.

TOPSIDES OF CONCRETE

SUBSTRUCTURE PLATFORMS

Some concrete platform topsides were constructed

com-plete. mounted on two special purposes barges and then mated with the substructure by floating the barges over the

submer-ged concrete substructure. The substructure was then

refloated to support the topsides clear of the barges and the

barges were removed. The complete platform was then towed

to its operational location as a complete unit.

The reverse of this process can be performed, providing the complete platform can be refloated, in which case it may then be possible to dump the complete platform in very deep water. The alternative, should refloating complete be impracticable,

will be to remove the topsides piecemeal in a manner similar to that described earlier.

In either case the activities of shutdown, well pluggingand

abandonment and decommissioning must be carried out

before refloating commences.

DEMOLITION OF PLATFORMS IN PLACE

The possibility of disposing of a steel platform on the on-site

location has been investigated for platforms in 230 ft water depth, with a requirement to provide Soft of clear water

above any part of the demolished structure.

Total demolition of the platform by explosives was con-sidered and is a practical possibility. It was however not

considered further as there was no guarantee that a steel famed structure would collapse completely to leave the required clear water depth. Should explosive demolition fail, remedial work to correct the situation could well be prohibitively expensive. This then led to the development of the 'toppling' operation. Most of the platforms studied were of the eight-leg jacket types, which are ideal for toppling by cutting into two four-leg halves and toppling each half outwards at the same time. The

centre section of the topsides, above the centre line ofthe platform, must be completely cleared and cut right down to the

module support frame. The central bracing tubulars of the

jacket must be cut in order to separate the two four-leg halves of the platform.

Controlled explosive charges would then be placed to cut the base of the four inner legs and to remove completely a portion of the four outer legs, causing both halves of the platform to

topple, like a tree being felled, to the sea bed. It was not

considered necessary to remove the main bulk of the topsides, although some work would be required to ensure that they did

not break free before entering the water.

A similar method of toppling a six-leg jacket platform may be possible. but toppling of the very large four-leg structures

with a square topsides plan appears to require considerable

study to determine its practicality and cost effectiveness.

REMOVAL AND ABANDONMENT COSTS It will by now have become obvious that the removal and abandonment of a fixed platform installation will be a very

costly operation.

During the course of my company's studies, detailed costs

were developed for individual platforms. From these costs,

guideline figures were produced on a cost per short ton basis.

The costs indicated below take account of all cost factors

from the preparatory stage through to disposal. They include

allowances for lost time due to bad weather and include all

supporting services such as supply boat operations, consum-ables. catering, helicopter services and the hire of all necessary vessels.

Obviously each specific platform will have its own peculiari-ties which will affect costs, but the following guideline figures

serve to indicate the magnitude of theoperation.

Topside modules, decks and packages removed and

trans-ported for disposal onshore as scrap will cost from £3500 to

£4500 per short ton. This figure does not include well plugging

and abandonment. The similar costs per short ton for steel

jackets cut piecemeal and removed to shore is from £4500 to £5500.

The plugging and abandonment of wells will cost between £350000 and £500000 per well. This figure will be dependent

on any problems that may be met during the operation. The potential saving by use of the new super-heavy-lift vessels appear to be of the order of 20% compared with the

figures quoted above. Dumping of topsides at sea after

removal may not, in fact, result in any significant savings

because of the loss of the small scrap value and the time that

the cargo barges and rugs will be needed on hire, with the

added costs of design and procurement of the launching

facil-ities that will be required on a cargo barge.

The toppling of a platform in place with very little of the

topsides removed is the cheapest method of disposing of a

platform and in the eight-leg jacket case could result in savings

ofsome 50-70% of the cost of removing the total platform. Partial removal, ie removing the topsides and removing, rather

than toppling, the top portion of the jacket to give the clear water depth, will result in savings amounting to some 20-30% ofthe totalcosts.