Danish Sustainable Offshore Decommisionning Project Decom/12039-Decom... · Danish Sustainable...

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Danish Sustainable Offshore Decommisionning Project Comparative environmental risk assessment of key removal and disposal options for old Danish and other North Sea platforms due for decommissioning

Transcript of Danish Sustainable Offshore Decommisionning Project Decom/12039-Decom... · Danish Sustainable...

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Danish Sustainable Offshore Decommissioning Project

Comparative environmental risk assessment of key removal and disposal options for old Danish and other North Sea platforms due for decommissioning

The “Fornyelsesfonden” have made it possible to do this report by sponsor this project.

4 Danish Sustainable Offshore Decommissioning Project

AcknowledgementsThis Report has been prepared by Mr Robert Ohene Adu and Mr Tommy Petersen of the Offshore Centre Denmark. The Authors would like to thank Professor Jens Peter Thomsen, in charge of En-vironmental Technology of Aalborg University, Esbjerg for his assis-tance and advice.

The “Fornyelsesfonden” have made it possible to do this report by sponsor this project.

Offshore Center Danmark

Danish Sustainable Offshore

Decommissioning Project

Background report of Danish and other North Sea

fields and platforms due for decommissioning

December 2011

Authors: Mr Robert Ohene Adu

and Mr Tommy Petersen,

Offshore Center Danmark

Publisher

Offshore Center Danmark

Niels Bohrs Vej 6

6700 Esbjerg

Tel. + 45 33 97 36 70

www.offshorecenter.dk

This is a published document where copyright rests

with the Offshore Center Denmark and consor¬tium

members. All rights reserved.

Information contained in this document are owned

by the aforementioned parties and supplied without

liability for errors or omissions.

No part may be reproduced or used except as

per¬mitted by contract or other written permission.

5Danish Sustainable Offshore Decommissioning Project

Content

Acknowledgements ...........................................................2

Executive Summary ...........................................................7

CHAPTER 1: Introduction .........................................................................9

1.1 Introduction ............................................................91.2 Aim and Objectives ...............................................111.3 Brief Background Information ................................11

CHAPTER 2: Legislative framework .....................................................12

2.0 Legislative Framework ..........................................122.1 Marine Strategy Framework Directive ....................122.2 The OSPAR agreements........................................122.3 EU Water Framework Directive ..............................132.4 The EU Habitats Directive ......................................132.5 The Wadden Sea ..................................................132.6 Legal response to Gulf of Mexico Explosion............13

CHAPTER 3: Description of environmental sensitivities around the proposed sites ..............................................14

3.0 General environmental sensitivities around proposed sites ......................................................143.1 Planktons .............................................................153.1.1 Phytoplankton’s ....................................................153.1.2 Zooplanktons .......................................................163.2 Fish sensitivity .......................................................163.3 Marine mammals’ sensitivity ..................................163.4 Sea birds sensitivity ...............................................16

CHAPTER 4: Decommissioning of facilities .......................................17

4. 0 Planned Phases ....................................................174. 1 Inventory of Materials ............................................174.2 Hazardous Materials .............................................17

CHAPTER 5: Decommissioning options .............................................18

5.0 Options for decommisioning platforms ..................185.1 Decommissioning options for the Platform topsides ...............................................................185.1.1 Leave in-situ .........................................................185.1.2 Re-use in-situ .......................................................185.1.3 Re-Use in another location ....................................185.1.4 Remove and Recycle ............................................18

5.1.5 Rigs to reefs disposal ............................................185.1.6 Deep Sea Disposal ................................................185.1.7 Delay Decommissioning awaiting novel technology ............................................................185.1.8 Summary results of topside decommissioning options .................................................................185.2 Possible Topside Removal methods ......................185.2.1 Reverse Installation with heavy lift vessel (HLV) .......195.2.2 Topside Integrated Removal using semi-submersible crane vessel (SSCV) ..................195.2.3 ‘Versatruss’ with catamaran barges .......................205.2.4 Twin submersible barge .........................................205.2.5 Piece Small topside removal ..................................215.2.6 Shearleg and grab .................................................215.2.7 Removal using drilling jack up rig ............................215.2.8 Summary results of topside removal methods ........215.3 Decommissioning options for the Platform jackets .225.3.1 Leave in-situ .........................................................225.3.2 Re-Use in-situ .......................................................225.3.3 Re-Use in another location ....................................225.3.4 Remove and Recycle ............................................225.3.5 Rigs to reefs disposal ............................................225.3.6 Deep Sea Disposal ................................................225.3.7 Delay Decommissioning awaiting new technology..225.3.8 Summary results of jacket decommissioning options .................................................................225.4 Possible Jacket Removal methods ........................235.4.1 Reverse installation with Heavy Lift Vessel (HLV) .....235.4.2 Jacket removal using novel technology (temporary .... buoyancy) .............................................................235.4.3 Piece small jacket removal .....................................235.4.4 Shear leg and grab method ...................................235.4.5 Summary results of jackets removal methods .........23

CHAPTER 6: Comparative assessment of options ...........................24

6.0 Comparative assessment of shortlist of options and selection of preferred option for topsides .........246.1 Screening evaluation Criteria .................................246.1.1 Technical risk and complexity ................................246.1.2 Personnel safety ...................................................246.1.3 Environmental and social impact ...........................24 6.1.3.1 Risk assessment method ..........................24 6.1.3.2 Risk Rating ...............................................276.1.4 Energy consumption and emissions ......................286.1.5 Cost .....................................................................286.1.6 Summary results of assessment of Selected Topsides Removal methods ..................................286.2 Comparative assessment and selection of preferred option for jackets ....................................296.2 1 Technical risk and complexity ................................29

6.2.1 Personnel safety ...................................................296.2.2 Environmental and societal impact .........................296.2.3 Energy and Emissions criteria ................................296.2.4 Costs ...................................................................296.2.5 Summary of jacket removal screening evaluation ....29

CHAPTER 7: Results and discussion ...................................................30

7.1 Results of Environmental assessment ....................307.1.2 Overview of results ................................................307.1. 3 Impacts from decommissioning topsides ...............307.1.4 Impacts from decommissioning jackets .................30

CHAPTER 8: Conclusion .........................................................................31

References .........................................................................32

7Danish Sustainable Offshore Decommissioning Project

Executive Summary

The North Sea is currently host to more than 600 offshore oil and gas installations, many of which have been standing since the 1960s and 1970s. Thus a good number of them have exceeded their op-erational lifetime limit of 25 years and some of them are no longer producing hydrocarbons. Many of the steel and concrete structures have also become obsolete. As a result they have become redun-dant and need to be removed. This report is sequel to the previous report describing a selection of old North Sea oil and gas platforms which are earmarked for decommissioning in the Project ‘Dansk Bæredygtig Offshore Dekommisionering’. It outlines the results of investigations on all alternative uses of materials and presents com-pleted comparative assessments for the key removal and disposal options.

The Oslo/Paris agreements (OSPAR) of 1992/1998 state with regard to the North Sea offshore constructions that:

“No disused offshore installation or disused offshore pipeline shall be dumped and no disused offshore installation shall be left wholly or partly in place in the maritime area”

To this end OSPAR decision 98/3, this entered into force on 9th Feb-ruary 1999, prohibits dumping, and the leaving wholly or partly in place, of disused offshore installations within the maritime area. The key terms of the OSPAR convention are as follows: • Thetopsidesofalloffshoreinstallationsmustbereturnedtoshore

and all installations with a jacket weight of less than 10,000 tonnes must be completely removed for re-use, re-cycling or final disposal onshore

• Notwithstandingthis,OSPARdecisionNo.98/3recognisesthatcompanies may experience difficulties in removing the footings of large steel jackets weighing in excess of 10,000 tonnes and con-crete installations. Thus there are derogations for these categories of installations if the internationally agreed assessment and con-sultation process shows that leaving these in place is justifiable.

• Thederogationprovisions,however,applyonlytoinstallationsin-stalled prior to 9 February 1999. All installations put in place after this date must be removed completely.

From the review of the various decommissioning options for the top-sides and jackets, the removal to shore of the platform topsides and jackets was selected as the most viable and feasible option.Based on the comparative risk assessment carried out, it is pro-posed that:

For the platform topsides, the reverse installation method using the heavy lift vessel (HLV) was proposed for platform topsides weighing ca. 1800 tons, whereas the single lift with semi-submersible crane vessel (SSCV) is proposed for the heavier topsides weighing more than 3,300 tons. The novel lift technologies, namely the Versatruss, and twin submersible barge, may be more technically feasible in the deeper waters of the northern North Sea and for future decommis-sioning projects, involving deck weights of up to 40,000 tons.

For the jackets, the reverse installation with HLV is proposed.

It is proposed that the decommissioning of the earmarked facili-ties will be performed in a phased manner following the permanent abandonment of the platform wells, isolation and making the facilities hydrocarbon-free. The planned phases of the decommissioning are as follows:

•Pre-decommissioninginspections,surveysandengineeringde-velopment studies.

•Pluggingandpermanentabandonmentofthewells.•Removalofresidualhydrocarbonsfromtheplatformfacilitiesand

associated pipelines.•Removaltoshoreoftheplatformstructuresandequipment.•In-situdecommissioningofthepipelines.•Removalofthehosebundles.•Postdecommissioningseabedclearanceandsurveys.•Onshoredismantlinganddisposal. To all intents and purposes this decommissioning programme is comprehensive and should be acceptable to the Danish government and other stakeholders involved.

9Danish Sustainable Offshore Decommissioning Project

1.1 Introduction

Oil and gas production in the North Sea is on the decline and the first offshore installations are expected to be decommissioned within a ten-year time frame (DEA, 2009). Some of the early platforms are currently standing idle and generating only produce water, having exhausted the oil and gas reserves in their fields of location. There may also be fields where production may not be financially viable, i.e. where the costs exceed the revenue that is generated, which means that the production facilities can be removed completely. Other health and safety installations may continue in operation until 2042, when the sole concession expires, or perhaps further into the future for as long as there are recoverable resources. Moreover, it is a lot of money to maintain these redundant facilities at their locations offshore.

Thus, decommissioning represents an emerging sector of activity in the North Sea which demands full attention, focus and commitment of all stakeholders. Offshore Center Danmark is using the early plan-ning phase to identify the best decommissioning arrangements for the offshore oil and gas platforms, taking account of and balancing safety, environmental, social, technical, contractual and economic aspects.

The northern/central North Sea fields are predominantly oil fields whereas the southern North Sea fields are predominantly gas fields.

CHAPTER 1: Introduction

This suggests that most oil fields in the North Sea are deeper than the gas fields in the southern sector and therefore have heavier and deeper installations.

When an offshore oil and gas installation is decommissioned, the Danish state will have the oppor-tunity to take over the installation in accordance with the conditions that are set out in the individual licences. The installation must be emptied of residual oil, gas and chemicals. Then the abandoned wells must be plugged with ce-ment. Any bridges to other platforms must be removed and trans-ported to shore, and the pipelines connected to the platform must be cleaned, cut off and plugged. Subsequently, the topside facilities are cut off and lifted onto a barge for transport to shore. This allows the steel structure previously housing the topside facilities to be cut loose from the supporting piles in the seabed and transported to shore. Fi-nal cleaning takes place on shore, after which the installation is cut up into manageable sections to allow reuse of the steel. It is the Danish Energy Agency’s task to supervise and ensure that the removal of the installation is carried out in such a way that both the health and safety of employees and environmental issues are taken into consideration.

Offshore Center Danmark presents here a dossier assessing the various options and selecting the most environmentally benign method of decommissioning.

10 Danish Sustainable Offshore Decommissioning Project

Fig 1.2a Map of North/Cen-

tral North Sea fields (Source:

Acorn Petroleum Services,

2009)

11Danish Sustainable Offshore Decommissioning Project

1.2 Aim and Objectives

The aim of this report is to carry out a comparative risk assess-ment of the various options of decommissioning of the designated platforms in the North Sea and to recommend the best options for implementation. The objectives for achieving this aim include:

• BackgroundinformationoftheNorthSeaoilandgasinstallations• LegislativeFrameworkoftherelevantjurisdictions(EUHabitats

Directive (92/43/EEC), Con-servation of Habitats Regulations 2001, Danish Statutory Requirements,)

• Adescriptionofenvironmentalsensitivitiesaroundtheproposedoffshore sites

• A brief inventory of thematerials and substances containedwithin them

• Abriefdescriptionofallpossibleoptionsofdecommissioning• Ascreeningevaluationandshort-listingof1-2optionsforthor-

ough review and certification? • Anassessmentoftheimpactsorrisksoftheproposeddisposal

methods in comparison with the impacts of other options

1.3 Brief Background Information

The northern sector of the North Sea has predominantly oil fields/installations and therefore higher amounts of produce water with lower amounts of heavy metals, close to background concentra-tions (OSPAR Commission, 2009). The southern sector has pre-dominantly gas fields and installations and therefore lower amounts of produce water but with higher concentrations of lead. Figure 1.2a shows the northern sector with oil fields marked green, whereas fig-ure 1.2 b shows the southern sector with gas fields marked red.

Steel and concrete materials constitute the bulk of materials of which the topsides and jackets are made.

Oil and gas production wells produce water as a by-product. Never-theless, a high amount of energy is required to handle the water pro-duced. For example, in the Danish part of the North Sea, the content of water relative to the total liquids produced reached 72 % in 2010. In some of the old fields, the water content is now as high as 90 %. The water may derive from a natural water zone under the oil zone in the reservoir or from injection wells (Danish Energy Agency, 2010).

Thus, production of oil from these fields is no longer economically attractive or viable.

Fig 1.2b Map

of Southern

North Sea fields

(Source: Acorn

Petroleum Ser-

vices, 2009)

12 Danish Sustainable Offshore Decommissioning Project

2.0 Legislative Framework

There are a number of legislative instruments in the EU that have provisions for safeguarding the marine environment of the North Sea. Some of these are:a) The Marine Strategy Framework Directive MSFD, 2008/56/EC;

which aims at achieving or maintaining a good environmental status, GES, by 2020 at the latest

b) The Oslo/Paris agreements (OSPAR, 1992/1998) which focus directly on disused offshore platforms in the North Sea

c) EU Water Framework Directive, WFD, 2000/60/EC which estab-lishes a framework for action by the European community in the field of water policy;

d) EU Habitats and Birds Directives, which constitute the corner-stone of Europe’s nature conservation policy.

2.1 Marine Strategy Framework Directive

The Marine Strategy Framework Directive, MSFD, adopted in July 2008 is the first legislative instrument in relation to the marine bi-odiversity policy in the European Union, as it contains the explicit regulatory objective that "biodiversity is maintained by 2020", as the cornerstone for achieving good environmental status (European Commission, 2011). It enshrines in a legislative framework the eco-system approach to the management of human activities having an impact on the marine environment, integrating the concepts of envi-ronmental protection and sustainable use.

Annex I of the Directive lays out eleven (11) descriptors for assessing good environmental status for the marine environment. They are:

• Descriptor1:Biologicaldiversity• Descriptor2:Non-indigenousspecies• Descriptor3:Populationofcommercialfish/shellfish• Descriptor4:Elementsofmarinefoodwebs• Descriptor5:Eutrophication• Descriptor6:Seafloorintegrity• Descriptor7:Alterationofhydrographicalconditions• Descriptor8:Contaminants• Descriptor9:Contaminantsinfishandseafoodforhumancon-

sumption• Descriptor10:Marinelitter• Descriptor11:Introductionofenergy,includingunderwaternoise In order to achieve the objective the member states have to develop marine strategies, which serve as action, plans and which apply an ecosystem-based approach to the management of human activities.

An important requirement here is in using existing regional coopera-tion structures among member countries under the regional conven-tions to coordinate their actions with those of third countries in the same region or sub-region. One of these conventions is the Conven-tion for the Protection of the Marine Environment in the North-East Atlantic of 1992 – the OSPAR Convention (OSPAR, 1992).

The OSPAR Convention of 1992, which came into force in March 1998, is the current legal instrument guiding international coopera-tion on the protection of the marine environment of the North-East Atlantic. Work under the Convention is managed by the OSPAR Commission, made up of representatives of the governments of 15 contracting parties and the European Commission, representing the European Community.

The mission of OSPAR is to conserve marine ecosystems and safe-guard human health in the North-East Atlantic by preventing and eliminating pollution; by protecting the marine environment from the adverse effects of human activities; and by contributing to the sus-tainable use of the seas.

Underpinning the eco-system approach are 6 strategies under which the work of OSPAR is organized. These strategies are:

• BiodiversityandEcosystemstrategy• Eutrophicationstrategy• HazardousSubstancesStrategy•OffshoreIndustryStrategy• RadioactiveSubstancesStrategy• StrategyfortheJointAssessmentandMonitoringProgramme

2.2 The OSPAR agreements

The OSPAR-convention 1992, Annex III Article 5, 1 states clearly that“No disused offshore installation or disused offshore pipeline shall be dumped and no disused offshore installation shall be left wholly or partly in place in the maritime area”.

Annex 2 of OSPAR Decision 98/3 on the Disposal of Disused Off-shore Installations requires the assessment of a proposal for disposal at sea of a disused offshore installation to be based on descriptions of:

a. The characteristics of the installation, including the substances contained within it; if the proposed disposal method includes the removal of hazardous substances from the installation, the re-moval process to be employed, and the results to be achieved, should also be described; the description should indicate the form in which the substances will be present and the extent to which they may escape from the installation during, or after, the disposal;

b. The proposed disposal site: for example, the physical and chemi-cal nature of the seabed and water column and the biological composition of their associated ecosystems; this information should be included even if the proposal is to leave the installation wholly or partly in place;

c. The proposed method and timing of the disposal.

The provisions of OSPAR Decision 98/3 do not apply to pipelines. Decommissioning of pipelines should be contained in a separate proposal to that of installations.

CHAPTER 2: Legislative framework

13Danish Sustainable Offshore Decommissioning Project

The descriptions of the installation, the proposed disposal site and the proposed disposal method should be sufficient to assess the impacts of the proposed disposal, and how they would compare to the impacts of other options.

2.3 EU Water Framework Directive

The EU Water Framework Directive, Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 estab-lishes a legal framework for community action in the field of water policy, and seeks to protect and restore clean water across Europe and ensure its long-term, sustainable use.

The directive establishes an innovative approach for water manage-ment based on river basins, the natural geographical and hydrologi-cal units and sets specific deadlines for Member States to protect aquatic ecosystems. It addresses inland surface waters, transitional waters, coastal waters and groundwater. It establishes several inno-vative principles for water management, including public participa-tion in planning and the integration of economic approaches, includ-ing the recovery of the cost of water services.

In the coming years, member states and their neighbours will need to extend cooperation into new areas of water management such as the marine environment. Under the new Marine Strategy Directive, member states should work together to ensure good environmental status for shared marine waters, following an approach similar to that of the Water Framework Directive (EC, 2008).

2.4 The EU Habitats Directive

The cornerstone of Europe’s nature conservation policy is the EU Habitats Directive (together with the Birds Directive). This directive is built around two pillars, namely the ‘Natura 2000 Network’ of pro-tected sites and the strict system of species protection (European Commission, 2011). The EU Habitats Directive protects over 1000 animal and plant species and over 200 ‘habitat types’ (special types of forests, meadows, wetlands, etc.) which are of European impor-tance.

2.5 The Wadden Sea

A good number of interconnected laws, treaties and agreements protecting the Wadden Sea range from international conventions such as OSPAR and RAMSAR, to European and national such as the national park laws of Germany.

European directives for the Wadden Sea are the EU Water Frame-work Directive and the Birds and Habitats Directives, the latter two forming the basis for Natura 2000.

Germany, Denmark and the Netherlands have a trilateral agreement on the protection of the Wadden Sea which runs along the coasts of the three countries on the southern edge of the North Sea. These 3 countries make up the Trilateral Wadden Sea Cooperation, meeting

every four years to discuss the forming or upgrading of the protective policy for the Wadden Sea area. In 1997, the three countries signed the first Wadden Sea Plan. The cooperation between the three coun-tries is supported by the Common Wadden Sea Secretariat (CWSS).

2.6 Legal response to Gulf of Mexico Explosion

On 20 April 2010, an explosion occurred on the Deepwater Horizon mobile drilling rig, which was carrying out drilling operations in the Macondo Field. The drilling was being undertaken at a water depth of 1,544 m and the explosion was caused by gas gushing uncontrolled out of the borehole.

11 people died, the drilling rig sank and, over a period of three months,morethan4millionbarrels(800,000m³)ofoilflowedupfromthe approx. 5,600 m deep well and out into the Gulf of Mexico. The cause of this tragedy and the subsequent calamitous environmental effects has been identified as the failure of a number of independent barriers, which could have prevented the incident or averted the con-sequences of the incident. The explosion occurred during the drilling of the Macondo well.

In contrast to the situation in the Gulf of Mexico, the water depths in the Danish sector of the North Sea are less than 100 metres and drilling is carried out using jack-up drilling rigs, which stand on the seafloorandhavethesafetyvalvearrangement(Blow-outPreventer,compressed-air bank, emergency shutdown system, etc.) located in a dry and accessible location on the drilling rig beneath the drill-ingfloor.PlatformsinthemuchshallowerNorthSeaeitherhaveafoundation on the bottom of the sea or are permanently anchored in place.

As an immediate response to the tragedy, the Danish Energy Agency carried out inspections of the safety valve arrangements on the three drilling rigs which were carrying out drilling operations at the time in the Danish offshore area. During these inspections, no safety-related deficiencies were identified in connection with well-control equip-ment, its maintenance or the procedures used for testing this equip-ment.

Nor were any deficiencies identified in connection with procedures for shutting down the wells in an emergency, or in connection with awareness of these procedures among the personnel.

The Danish Energy Agency takes part in the on-going analyses and evaluations that are carried out under the auspices of the EU and in international cooperation with the aim of learning from the tragedy and implementing the lessons learned in the regulation of drilling op-erations, particularly for drilling operations under difficult conditions, which in the Danish area means deep wells under high pressure and temperature conditions.

The European Commission announced common EU regulation of this during 2011 which should be applicable for the proposed de-commissioning project.

CHAPTER 2: Legislative framework

14 Danish Sustainable Offshore Decommissioning Project

3.0 General environmental sensitivities around proposed sites

In General, the North Sea is shallow when compared to other oceans, and has depths which are less than 100 m. The major oil develop-ments have been in the central and northern sectors, in the Norwe-gian, Danish and upper British continental shelves, whereas most gas developments lie in the southern part, in the Dutch and lower British continental shelves (OSPAR Commission, 2009).

The Wadden Sea, where a number of gas and oil platforms are situ-ated, is particularly sensitive to numerous species of birds and ma-rine mammals. It is a nature conservation site and has been desig-nated by UNESCO as a heritage site. Seals have been known to rest on land in the Wadden Sea and forage across the entire North Sea in search of food, with the younger ones able to travel over 400 km offshore (Fiskeri- og søfartmuseet). Any decommissioning program involving removal or transport of installations must therefore be highly sensitive to the natural ecology and importance of this area. Vessels may not be permitted to traverse this region.

Latitude 540 N divides the North Sea into the northern sector, where most of the oil fields lie, and the southern sector, where most gas fields lie. The southern North Sea, remains vertically mixed through-out the year with relatively strong tidal currents, and has its seabed consisting largely of sandy sediments. Thus the fauna is adapted to changing conditions and sand movements along the bottom (OSPAR Commission, 2009).

The northern part of the North Sea is deeper and has weaker tidal currents and therefore less mobile sediments which are less sandy. Thermal stratification occurs here during the summer. Significant amounts of benthic fauna are present in this region, namely corals and sponges, polychaetes, echinodermata, crustaceans and mol-luscs. These are found especially in the Danish sector (Danmarks Miljøundersøgelser, 2010).

An environmental description of some of the earmarked fields will suffice here.

Indefatigable occurs within the area classified as the Norfolk Banks, which has both active and inactive sandbanks. These occur between latitudes 530 N and 540 N, and comprise numerous tidal ridges up to 40m in amplitude and between 20-60 km in length (Shell, 2001).

Wind direction over the indefatigable field is mainly south-westerly between September and February, and north-westerly between March and August.

Lying in water depths of 18-40 m off the north-east Norfolk coast (BGS, 2001; Huntley et al., 1993), the Norfolk Banks can be divided into near-shore parabolic banks forming a zigzag pattern, and an outer group of more linear banks (Cameron et al., 1992).

During a survey commissioned by the UK Department of Trade and Industry (DTI), the surfaces of many of the Norfolk Banks were found to be covered in active sand waves (DTI, 2001). Sand waves, which are smaller scale features than the banks, are conspicuous over large areas of the southern North Sea (Shell, 2001). In this area, sand waves, with heights up to 4m and wavelengths of 50m, are main-tained by the sand supplies provided by the modern tidal regime (Collins et al., 1995).

There is a clockwise circulation of sand around the bank (DTI, 2001).

These features are actively migrating, with estimates of an average migration rate of up to 15m/yr for southern North Sea sand waves (Cameron et al., 1992). Large sand waves are present on the inner Norfolk banks, with size decreasing with increased distance from shore (Graham et al., 2001).

The presence of active bed forms in this part of the southern North Sea provides evidence of a mobile sandy bed and vigorous present-day sediment transport processes (Shell, 2001).

SENSITIVITY: ‘Sandbanks which are slightly covered by sea water all the time’ and ‘biogenic reefs’ formed by Sabellaria spinulosa are habitat categories identified under Annex I of the Habitats Directive, which are known to occur in the region of the southern North Sea occupied by the Indefatigable Field.

Species found in UK offshore waters as listed under Annex I habi-tats are bedrock reefs, stony reefs, biogenic reefs and submarine structures made by leaking gases. Annex II habitats species found in UK waters are grey seals, common seals, bottlenose dolphins and harbour porpoises.

Areas in the southern part of the North Sea, designated as Special Areas of Conservation (SACs) include the shallow sand banks off the Norfolk coast namely, Indefatigable, Swarte, Broken, Well, Inner, Ower and Leman banks. These areas represent a wide range of geo-graphical and ecological variations namely gravelly and clean sands, muddy sands, eelgrass Zostera marina beds, and maerl beds. Reef-forming polychaete such as Sabellaria spinulosa is usually found in the Southern sector of the North Sea. These form important nature conservation habitats mainly on sediment or mixed sediment areas and serve as substrates for other species to grow on the seabed (UKBAP, 2004).

S. spinulosa can probably tolerate smothering (suffocation) for a number of weeks. It relies on suspended sediment and therefore any increase in suspended sediment will facilitate tube construction and may result in increased populations. Prolonged periods of sediment disturbance, however, may hinder growth and reproduction (MarLIN, 2003).

Although the larvae are known to be highly sensitive to chemical con-taminants, some individuals have been found to thrive in contami-

CHAPTER 3: Description of environmental sensitivities around the proposed sites

15Danish Sustainable Offshore Decommissioning Project

Assemblage Water depth (m) Sediment type Indicator species

Ia < 30 ’Coarser’ Polychaete Nephtys caeca; Sea urchin Echinocardium cordatum; Amphipod Urothoe poseidonis

IIa 30-50 Muddy fine sediment Bivalve mollusc Nucula nitidosa; Crustacean Callianassa subterranean; Cumacean Eudorella truncatua.

IIb 30-70 Fine sand Polychaete Ophelia borealis; Polychaete Nephtys longosetosa

IIIa 70-100 ‘Finer’ No indicator Species

nated areas (Holt et al., 1998). Overall, it is thought that S. spinulosa can recover rapidly from physical disturbance.

A survey conducted on the North Sea in 1986 by an ICES working group identified temperature, sediment type, different water masses andfoodsupplytothebenthosasfactorsinfluencingspeciesdistri-bution and assemblages.

Benthic indicator species in the North Sea can be characterized ac-cording to sediment type and depth as shown in Table 2.2.

Assemblages Ia and IIb are characteristic of the southern North Sea where polychaete dominate.

Characteristic free-living epibenthic species of the southern North Sea were identified by Jennings et al. (1999) as the sand-dwelling brittle star Ophiura ophiura, hermit crab Pagurus bernhardus, com-mon star fish Asterias reubensandtheflyingcrabLiocarcinus hol-satus. Data from the oil and gas industry’s Strategic Environmen-tal Assessment 2 (SEA2) survey show that the North Norfolk sand banks community is characterised by heart urchin Echinocardium cordatum and the bivalve Fabulina fabula (DTI, 2001).

3.1 Planktons

The planktonic community is potentially insensitive to oil and chemi-cal discharges at sea. Especially around the central and southern

Figure 2.3 Tubes built by polychaete worm S. spinulosa (Source: Shell, 2007)

Table 2.2 Characteristic benthic fauna from

sediments in the North Sea

sectors of the North Sea, planktonic populations have the capacity to quickly recover as a result of constant interaction and exchange between individuals and surrounding waters. Therefore any impacts from offshore activities are likely to be small compared with the natural variations. However, any decrease in the distribution and abundance of planktonic communities arising from discharges of oil or biocides could result in secondary effects on higher organisms that depend on planktons as their food source. The very high turnover of plankton populations suggests that planktons are relatively unaffected by oil.

The planktonic community is composed of a range of plants (phy-toplanktons) and animals (zooplanktons) which are pelagic and drift along ocean currents, and together form the basis of the marine food chain. Planktonic organisms, primarily copepods, are a major food resource for many commercial fish species, such as cod and Herring and any changes in their populations have effects on the food chain.

The majority of phytoplanktons are uni-cellular, and include diatoms anddinoflagellates.Zooplanktonsarecomposedofawidevarietyofmulti-cellular herbivorous and carnivorous organisms. Typical zoo-plankton organisms are the copepods, arrow worms, krill, jellyfish and sea-gooseberries.

Zooplanktons also include the larval stages of non-planktonic organ-isms, such as fish, crabs and barnacles.

3.1.1 Phytoplankton’sPhytoplankton’s are the primary food producers of the marine envi-ronment and fix light energy and carbon by means of photosynthe-sis. Phytoplankton is grazed by the secondary producers, namely zooplankton species. In continental shelf waters, zooplanktons are in turn consumed by fish (e.g. herring) and larger animals, including some cetaceans. Phytoplankton abundance and productivity is de-pendent on light intensity and nutrient availability, which is affected by water column stratification. Under optimal conditions they can reproduce quickly to produce ‘blooms’.

16 Danish Sustainable Offshore Decommissioning Project

Seasonal stratification, the separation of the water column into layers of different temperature, has an important impact on phytoplank-ton abundance. Stratification in the summer months (in the northern sector) depletes nutrients in the surface water after rapid periods of phytoplankton growth. Typically, phytoplankton abundance peaks in spring when nutrients are abundant; then the numbers will decline through the summer with occasional small increases in the autumn as nutrient levels increase due to mixing of the water column.

The water column in the southern North Sea is generally well mixed, but species richness can change rapidly depending on nutrient in-put from the freshwater discharge from the east coast of England and from European waters. The typical diatom species in this area include Ceratium tripos, Dinophysis norvegica and Noctiluca scintil-lans (BMT Cordah, 2002).

3.1.2 ZooplanktonsZooplanktons can be considered as primary consumers in the marine food chain, and their blooming depends on that of phytoplankton’s which is their main food source. Thus, zooplankton populations in-crease following phytoplankton blooms. Zooplankton communities in the Central and Southern North Sea areas include polychaetes, decapods, echinoderm larvae, fish eggs, and small neritic copep-ods including Temora longicornis, Labidocera wollastoni and Cen-tropages hamatus (Williams et al., 1993). Other common copepods found in the area include Isais clavipes, Phaeocystis pouchetii and Corycaeus spp. (BMT Cordah, 2002).

3.2 Fish sensitivity

The high plankton population especially in the southern North Sea augurs well for the fish community as fish feed on zooplanktons. However, fish are directly vulnerable to pollution and to physical dis-turbance likely to arise from decommissioning operations. Adult fish may accumulate hydrocarbons in their tissues which can affect their healthandalsotainttheirflesh.Theeggsandlarvaeofthesefishesare more prone to toxic effects of oil than are the adults. The seabed of the southern North Sea provides spawning and nursing grounds for different fish species such as mackerel, cod, plaice, lemon sole, sprat, whiting and nephrons’. The spawning and breeding grounds are usually May-August and December-March. Shellfish also breed on the sea bed.

3.3 Marine mammals’ sensitivity

There is evidence to suggest that marine mammals are potentially sensitive to chemical discharges, such as may occur during the de-commissioning process. They may ingest oil with food and be ex-posed to toxic effects. The fur of marine mammals can be coated by high amounts of oil, leading to reduced buoyancy and insulation, and death as a result.

They may also be vulnerable to injury from collisions with vessels. Mammals may be sensitive to underwater noise resulting from any decommissioning activity, although this effect is not well docu-mented.

The southern sector of the North Sea is not particularly rich in ceta-ceans. Sensitivity is considered to be low even during the months with the highest populations in the area. Harbour porpoises and white-beaked dolphins are generally to be found in the southern North Sea, particularly between March and May.

Common seals have been tracked using satellite-link telemetry from Scotland (SMRU, University of St Andrews) and Denmark (Fisheries Museum, Esbjerg and ELSAM, Denmark); (DTI, 2002). These studies clearly show that Common seals forage over wide areas of the North Sea (Bjørge, 1991; Reijnders et al., 1998; DTI, 2002). It is expected that the population inhabiting the east coast of England behaves similarly and is, therefore, likely to be distributed over much of the central and southern North Sea (DTI, 2002).

3.4 Sea birds sensitivity

Sea birds are also vulnerable to surface oil discharges. They are most vulnerable during the post-breeding and winter months. Their feath-ers may be coated by oil thereby reducing their buoyancy and insula-tion and increasing their mortality. Some of the seabirds present in the North Sea include kittiwake, fulmar, common gull, razorbill and guillemot. The fulmar in particular is noted for feeding only at sea.

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CHAPTER 4: Decommissioning of facilities 4. 0 Planned Phases

It is proposed that the decommissioning of the earmarked facili-ties will be performed in a phased manner following the permanent abandonment of the platform wells, isolation and making the facilities hydrocarbon-free. The planned phases of the decommissioning are as follows:•Pre-decommissioninginspections,surveysandengineeringde-

velopment studies.•Pluggingandpermanentabandonmentofthewells.•Removalofresidualhydrocarbonsfromtheplatformfacilitiesand

associated pipelines.•Removaltoshoreoftheplatformstructuresandequipment.•In-situdecommissioningofthepipelines.•Removalofthehosebundles.•Postdecommissioningseabedclearanceandsurveys.•Onshoredismantlinganddisposal.

4. 1 Inventory of Materials

The tables (1.2 a, b and c) describe the items that make up the topsides and jackets of the platforms and the materials of which they are made. Carbon steel is the most abundant material of which the platforms are made and makes up the structural steel of the topside, and the jacket structure for the platform jacket. Carbon steel is also the con-stituent material in the piping, vessels, mechanical and safety equip-ment for the topsides, as well as in piles and risers for platform jackets. Electrical and instrument cables, as well as cable trays are made from galvanised steel. Electrical cabinets, HVAC (Heating, Ventilation and Air Conditioning), and Architectural equipment are made from miscel-laneous materials. Timber is usually used in topside decking’s whereas anodes of jackets are made from aluminium alloys.

Hazardous materials found in platforms include paints, asbestos, LSA (Low Specific Activity) materials, heavy metals, and traces of radioactive isotopes.

Item No. Description Material

1 Structural steel Carbon steel

2 Piping Carbon steel

3 Vessels Carbon steel

4 Mechanical equipment Carbon steel

5 Electrical and Instrument cables Galvanised steel

6 Cable trays Galvanised steel

7 Electrical cabinets & equipment Miscellaneous

8 HVAC / Architectural Miscellaneous

9 Safety ( includes fire water piping ) Carbon steel

10 Decking Timber

Table 1.2 (a) typical inventory of platform topside

Adapted from Shell Report, 2007

Item No. Description Material

1 Jacket structure Carbon steel

2 Piles Carbon steel

3 Risers Carbon steel, SS Duplex

4 Anodes Aluminium alloy

Table 1.2(b) typical inventory of platform jacket

Adapted from Shell Report, 2007

4.2 Hazardous Materials

Offshore platforms have been known to contain hazardous materi-als. Table 1.2(c) shows some of the hazardous substances either present or potentially present on platform topsides and require ap-propriate handling.

Hazardous material Description

Asbestos The corrugated wind walls on many platforms are constructed from “Galbestos” which contains asbestos in the coating. Asbestos is also assumed to be present in solid form in pipe gasket material.

Paint Paints which contain lead may give off fumesifflamecuttingisemployed

LSA ( Low specific Activity ) LSA has not been detected in any pipework or vessels but on-site testing is still required

Heavy metals Heavy metals like mercury and lead are expected inside instruments, batteries and the like. Produce water from especially gas platforms in the southern sector are rich in lead

Radioactive isotopes Traces of radioactive isotopes may be present in smoke detectors

Table 1.2(c) Hazardous materials found on platform topsides

18 Danish Sustainable Offshore Decommissioning Project

CHAPTER 5: Decommisioning options5.0 Options for decommisioning platforms

This section outlines the potential options for decommissioning of the about 70 platforms listed under the Project Offshore Decom-missioning project. These options, from which the most suitable will be selected, have been adapted from previous brainstorming work carried out for indefatigable platforms which are listed in this pro-ject and are applicable to the other platforms also listed. As part of the overall decommissioning project scope it is important to carry out inspection of the facilities for safe access, and remedial work for safe access, where necessary, prior to undertaking any option. Ad-ditionally, the facilities and pipelines should be de-pressurized and hydrocarbon-freed prior to decommissioning.

Thesepotentialoptionsofdecommissioningarebrieflydescribed,first for the topsides, and then for the jackets.

5.1 Decommissioning options for the Platform topsides

5.1.1 Leave in-situThis option is neither environmentally or legally acceptable even if the facilities are maintained. It is therefore rejected.

5.1.2 Re-use in-situPossible in-situ uses of platform topsides include using them as: •Renewableenergyhub•Fishfarm•Prison•Militaryapplications•Navigationbeacon•Communicationshub•CO2 sequestration or gas storage

These in-situ possibilities are only viable for newer platforms. The majority of the selected platforms, however, are of such an age that condition and design code requirements render them unsuitable for this option. These in-situ re-uses are opportunity-driven and can only be considered if the opportunity arises within the right timeframe. Is-sues of ownership and responsibility for ultimate removal will have to be resolved as well. This option is therefore rejected.

5.1.3 Re-Use in another locationThe possible re-use for the topsides in new locations include: •Hydrocarbondevelopmentplatform•Renewableenergyhub

Again, the age and condition of majority of these platforms make this option unfeasible. It is therefore rejected.

5.1.4 Remove and RecycleThis is one of the most feasible and likely futures for the platforms be-cause of the low probability of finding a suitable re-use opportunity. Various removal methods are discussed later in this report.

5.1.5 Rigs to reefs disposal Rigs to reefs disposal (RTR) is a leave in-situ option which recognises

that during the productive life of the platform, the structure, especially the jacket, becomes a substrate around which marine life grows. However, the permits necessary for this kind of disposal do not make this option a practical option for the topsides, as they are not likely to be forthcoming in the foreseeable future. This option is therefore rejected.

5.1.6 Deep Sea Disposal Deep sea disposal of topsides in the North East Atlantic waters is incompatible with OSPAR regulations. It is therefore rejected.

5.1.7 Delay Decommissioning awaiting novel technologyNew concept designs for offshore oil and gas equipment, such as jack-upplatformsandfloatingbarges,arebeingconsideredbyma-rine contractors, and they are tailored to meet the offshore platform decommissioning market. This technology when developed is not likely to be available within the time frame for these platforms. It is therefore rejected.

5.1.8 Summary results of topside decommissioning optionsTable 5.1 summarily gives the results of the assessment of the 7 op-tions for decommissioning the topsides of the platforms. The only feasible and viable option for further consideration is the removal and recycling option which is discussed later in this report.

5.2 Possible Topside Removal methods

The review of decommissioning options for the topsides showed that the only viable option was removing the topsides to shore. A number of methods are hereby selected and proposed as possible methods for removal of the platform topsides. Some of the methods proposed include reverse installation with heavy lift vessels (HLV), Single lift with Semi-Submersible Crane Vessels (SSCV), ‘Versatruss’ with catama-ran barges, Twin submersible barge, Piece-Small Removal, shearleg and grab, and jack-up rig.

Each of these methods has impacts on the marine and human envi-ronment under 4 categories with different risk ratings. These meth-ods are described here.

Decommissioning option Result

Leave in-situ Rejected

Re-Use in-situ Rejected

Re-Use in other locations Rejected

Remove and recycle Further consideration

Rigs to reefs disposal Rejected

Deep Sea disposal Rejected

Await new technology Rejected

Table 5.1 Summary results of topside decommissioning options

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5.2.1 Reverse Installation with heavy lift vessel (HLV)This removal is simply the reverse of the installation sequence for each of the topsides. The work can be undertaken using an HLV ca-pable of lifting approximately 1800 tons of weight. Prior preparation works or reverse hook-up works are required to attach lifting points and riggings, disconnect and isolate topside packages, separate the risers, caissons and J-tubes from the jackets, internally sea-fasten loose items on the topsides and cut the topsides from the jackets.

A significant amount of preparatory work is expected for the larger topsides, which were originally installed by many major lifts and the separate packages hooked-up offshore.

Once preparation work is complete, an HLV will arrive to prepare to remove the topside modules. A cargo barge is required to be anchored and moored alongside the HLV. The HLV will attach the lift rigging to the crane hook, cut the deck legs and lift the topside mod-ules onto the cargo barge (figure 5.2.1). The modules are then sea-fastened to the barge and the barge towed to shore. The conductors may be lifted and removed when the cargo barge is being prepared.

5.2.2 Topside Integrated Removal using semi-submersible crane vessel (SSCV)The SSCV concept involves removing the entire topside in a single lift (lift weight more than 3,300 tons). Substantial preparation work is necessary to connect, strengthen the decks and install lifting beams for an integrated lift. This method is not applicable for the smaller top-sides as the latter are already considered for single major lift removal under the HLV option. This method will be more appropriate for the larger decks such as the topsides of ‘Auk’ (8093 tons), Indefatigable AC (6718 tons), K14-FA-1(5618 tons), Leman AK (5000 tons), leman

Figure 5.2.1 Topside removal by reverse installation (Source: http://hmc.

heerema.com)

BK (5000 tons), Beatrice AP (5000 tons), Leman AC (4093 tons), Indefatigable K (3114 tons), Albuskjell, and a few others.

Once all the preparation works are completed, the SSCV will arrive at location. The lift rigging is attached, the deck legs are cut and the topside is lifted as a single unit and placed on the cargo barge.

The modern Thialf SSCV designed by Heerema with two cranes to-gether can lift decks up to 15,652.8 tons (figure 5.2.2).

Figure 5.2.2a SSCV

Thialf (Source: http://

en.wikipedia.org/wiki/

File:SSCVThialf.jpg)

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Another SSCV, the S7000 designed by Saipem with two cranes, is capable of lifting decks up to 13,393 tons (figure 5.2.2b).

5.2.3 ‘Versatruss’ with catamaran bargesAnother potential novel method considered for removing the top-side in a single piece is the ‘Versatruss’ method. This is a proprietary hinged framing arrangement that, when attached to two barges and the barges pulled together will lift the load attached to the top of the framing. This method has been successful in installing and removing topsides in Lake Maracaibo, Venezuela and in the Gulf of Mexico. It is, however, technically more challenging in the North Sea environ-ment. Substantial preparatory work is necessary to strengthen the decks and install lifting beams and lower tie members.

There are a number of submersible barges which are normally used for transporting vessels such as jack-up rigs in the dry. These barges cansubmergetheirdecksandfloatunderneaththeircargo.Theythen de-ballast and lift their cargo out of the water. This ability could be used to lift platform topsides off their supporting jackets by plac-ing one barge on either side of the platform. As earlier said, substan-tial preparatory work is needed to strengthen the decks and install lifting beams. A few advantages are associated with this method such as topside removal capacity up to 40,000 tons in one piece, reduction in offshore time, cheaper mobilization and stand-by, shal-low water access where other heavy lift systems cannot go, greater load control, no mooring anchors needed in deep water operations, as well as catamaran tow configuration for speedy installations and decommissioning. This method is touted as the future of decom-missioning.

5.2.3 ‘Versatruss’ with catamaran bargesAnother potential novel method considered for removing the top-side in a single piece is the ‘Versatruss’ method. This is a proprietary hinged framing arrangement that, when attached to two barges and the barges pulled together will lift the load attached to the top of the framing. This method has been successful in installing and removing topsides in Lake Maracaibo, Venezuela and in the Gulf of Mexico. It is, however, technically more challenging in the North Sea environ-ment. Substantial preparatory work is necessary to strengthen the decks and install lifting beams and lower tie members.

Figure 5.2.2b SSCV S7000 (Source: Saipem7000)

There are a number of submersible barges which are normally used for transporting vessels such as jack-up rigs in the dry. These barges cansubmergetheirdecksandfloatunderneaththeircargo.Theythen de-ballast and lift their cargo out of the water. This ability could be used to lift platform topsides off their supporting jackets by placing one barge on either side of the platform. As earlier said, substantial preparatory work is needed to strengthen the decks and install lifting beams. A few advantages are associated with this method such as topside removal capacity up to 40,000 tons in one piece, reduction in offshore time, cheaper mobilization and stand-by, shallow water access where other heavy lift systems cannot go, greater load con-trol, no mooring anchors needed in deep water operations, as well as catamaran tow configuration for speedy installations and decom-missioning. This method is touted as the future of decommissioning.

5.2.4 Twin submersible barge The twin submersible barge method is another novel method of top-side removal similar to the Versatruss method. The difference here is that instead of the Versatruss system, submersible barges are used to provide the lift.

Figure 5.2.3 Versatruss method (Source: decom.jpg, www.vtruss.com )

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5.2.5 Piece Small topside removal This removal method is an on-site offshore work package involv-ing personnel within the confines of the topsides. In this method, a marine work vessel, most probably a jack-up barge is set up next to the platform. The labour force from the vessel would dismantle the topsides into sections that can easily be handled by the available platform and vessel cranes. The individual pieces will be loaded onto supply boats for transport to shore (figure 5.2.5).

5.2.6 Shearleg and grabThis method is an emergency salvaging method and employs hy-draulic guillotines and grabs to break up facilities in a crude manner and dump the pieces in barges for transport to shore. This method is rejected for reasons of safety, handling of hazardous materials and the likelihood of debris falling into the sea. It is not an acceptable method for decommissioning platforms.

5.2.7 Removal using drilling jack up rigThe drilling jack-up rig has been employed in the southern North Sea to install small platforms and it can also be used as a removal method on many platforms in the North Sea.

5.2.8 Summary results of topside removal methodsTable 5.2 summarily gives the results of the 7 suggested methods of

Figure 5.2.6 Shearleg crane (Source: Google image)

Figure 5.2.5 Piece Small removal to shore (Source: www.shetlanddecom-

missioning.com)

Figure 5.2.4 a

Figure 5.2.4 b

Figure 5.2.4 c

Figure 5.2.4 d

22 Danish Sustainable Offshore Decommissioning Project

topside removal. Five (5) of them- HLV, SSCV, Novel technology (Ver-satruss), Twin submersible barge, and piece small removal – have been shortlisted for comparative risk assessment.

Removal Method Status

HLV Further consideration

SSCV Further consideration

Novel technology (Versatruss) Further consideration

Twin submersible barge Further consideration

Piece small removal Further consideration

Shearleg and grab Rejected

Jack-up rig Rejected

Table 5.2 Summary results of topside removal methods

5.3 Decommissioning options for the Platform jackets

5.3.1 Leave in-situThis is not compatible with OSPAR regulations even when proper maintenance is guaranteed. Additionally, there is no derogation for any of the platforms listed, since none of them has a jacket structure weighing more than 10,000 tonnes. It is therefore rejected.

5.3.2 Re-Use in-situSince the in-situ re-use of the jacket is dependent on that of the top-side it is also rejected (See section 5.1.2).

5.3.3 Re-Use in another locationAs with the topsides, the jackets of the relatively newer offshore oil and gas platforms could be removed to shore for possible re-use elsewhere. The older jackets are of such a condition that re-use is not feasible. It is therefore rejected for the designated platforms.

5.3.4 Remove and Recycle This is about the only likely future for the jackets because of the little likelihood of finding a suitable opportunity for re-use. Several removal methods are described later in this section.

5.3.5 Rigs to reefs disposal Although this option has some environmental benefits, it is not a practical option for the jackets in the North Sea. The permits needed for this disposal option are not likely to be forthcoming in the foresee-able future. It is therefore rejected.

5.3.6 Deep Sea DisposalAs with the topsides deep sea disposal of jackets is not compatible with OSPAR regulations. It is therefore rejected.

5.3.7 Delay Decommissioning awaiting new technologyAs with the topsides, any new technology to be developed will not be available within the timeframe set for the decommissioning project. It is therefore rejected.

5.3.8 Summary results of jacket decommissioning options

5.4 Possible Jacket Removal methods

Removing the jackets to shore was reviewed as the only viable op-tion of decommissioning the enlisted platform jackets. 4 methods of jacket removal have been proposed in the project. These are the re-verse installation with the heavy lift vessel (HLV), novel technology us-ing the temporary buoyancy method, piece-small removal method andtheshearlegandgrabmethod.Thesearebrieflydescribed.

5.4.1 Reverse installation with Heavy Lift Vessel (HLV)As with the topsides this method is simply the reverse of the jacket installation sequence. An HLV with a lifting capacity of approximately 1800 tons is recommended for undertaking this work. Before lift-ing, preparation works are required to separate the risers and J-tube connections from pipelines and hose bundles, clear any debris from the jacket, install lifting and cut the piles below seabed level.

Decommissioning Option Status

Leave in-situ Rejected

Re-Use in-situ Rejected

Re-Use in another location Rejected

Remove and recycle Further consideration

Rigs to reefs disposal Rejected

Deep Sea Disposal Rejected

Delay Decommissioning awaiting new technology Rejected

Table 5.3 Summary results of jacket decommissioning options

Jacket Removal http://hmc.heerema.com/Portals/3/Docs/Act/Removals/

overview/product_jacket_removal_large.jpg

23Danish Sustainable Offshore Decommissioning Project

5.4.2 Jacket removal using novel technology (temporary buoyancy)This is a novel method of jacket removal which is achieved by add-ingbuoyanciesbelowthewaterlinetofloatthejacketandtowittoa deep-water quayside where it can be lifted onshore by an inshore crane barge. Once onshore the jacket can be broken up for recy-cling.Itisonlydeemedpracticaltofloatthejacketsverticallybecausethe dimensions of bottom of jackets are not compatible with rotating themforfloatingandtowinginthehorizontalposition.

5.4.3 Piece small jacket removalThis jacket removal method involves dividing the jacket into sections using remotely operated vehicles (ROVs) and divers operating from a divers supporting vessel (DSV). The divided sections can now be handled by vessel cranes and the individual pieces loaded unto sup-ply boats for transport to shore. This method was rejected because of the risks associated with the excessive diving content in the op-erations and the potential instability of a sectioned jacket.

5.4.4 Shear leg and grab method As with the topsides this emergency salvaging method is unaccep-table for decommissioning the jackets. Issues of safety and the likeli-hood of debris falling into the sea make this method untenable. It is therefore rejected.

5.4.5 Summary results of jackets removal methodsAs with the topsides this emergency salvaging method is unaccep-table for decommissioning the jackets. Issues of safety and the likeli-hood of debris falling into the sea make this method untenable. It is therefore rejected.

Removal Method Status

Reverse Installation with HLV Further consideration

Temporary buoyancy (Novel technology) Further consideration

Piece Small Rejected

Shearleg and grab Rejected

Table 5.4 Summary results of jacket removal methods

24 Danish Sustainable Offshore Decommissioning Project

6.0 Comparative assessment of shortlist of options and selection of preferred option for topsides

All of the short-listed options for decommissioning the platform top-sides require that they are removed to shore. The comparative as-sessments are therefore carried out on the removal methods.

6.1 Screening evaluation Criteria

The criteria used in assessing and comparing the different meth-ods of offshore oil and gas platform topside removal are adapted from the assessment done by Shell and Esso for indefatigable plat-forms in consultation with decommissioning specialist consultants, experienced contractors, and external stakeholders. These criteria have also been subject to public discussion by stakeholders, and key opinions have been formed. These criteria for comparative as-sessment include technical risk and complexity, personal safety, en-vironmental and social impact, energy consumption and emissions, as well as cost.

6.1.1 Technical risk and complexityThis criterion looks at the technical feasibility and risk of carrying out an option or method of decommissioning the platform. Technical fea-sibility considers factors such as the depth of sea area demarcated for decommissioning activity, suitability of vessel and machinery at offshore location, tidal current strength. Technical risk considers the duration of activity offshore.

6.1.2 Personnel safetyPersonnel safety considers the risk of a proposed decommission-ing method to personnel working on the project. Generally, there is a higher risk to personnel offshore than there is onshore. Risk as-sessment carried out for personnel to work on indefatigable topside decommissioning showed that the total risk to personnel was about 86% offshore and 14 % onshore. This applies to the other platforms listed.

The HLV and SSCV methods carry lower risks to personnel safety whereas the Versatruss, twin submersible barge and piece-small methods all carry significantly higher risks to personnel safety. In the case of the novel technologies, this is due to the greater ma-rine activity, whereas with the piece-small method this is due to the greater number of offshore man-hours involved.

Measured against a risk value of 1 for HLV as the norm, the SSCV method will be essentially the same at 0.99; the Piece-small method carries a higher risk rating of 1.58; the Versatruss method is still higher at 1.95 and the twin submersible barge method is highest at 2.05.

6.1.3 Environmental and social impactThis criterion, which is the focus of this work, looks at the environ-mental and social sensitivities involved in carrying out any option or method of decommissioning activity. A general description of the environmental sensitivities of the North Sea is given in section 3.

Assessment of environmental impact considers factors such as off-shore location of platforms, biological resources present, offshore conservation areas, meteorology, seabed topography and features, tidal currents, seabed sediment characteristics etc.

Impacts on these features are likely to come from vessel operations, preparatory work, cutting and lifting; transportation, reception, stor-age and dismantling; as well as recycling and disposal. 6.1.3.1 Risk assessment methodThe principles laid out in the Shell Corporate Guidance report (Shell, 2000) have been adopted for use as methodology in assessing the environmental risks associated with each of the decommissioning options. The assessment contains the following steps:

• Eachoftheshortlistedoptionswasreviewedtoidentifythepoten-tial causes of environmental risks in each of the activities involved in these options

• Thepotential‘receivingenvironment’includingnaturalandsocialaspects, was assessed in order to identify and characterise any sensitive elements

• The risks identifiedand the relevant environmental sensitivitieswere brought together in order to describe and quantify the ef-fects of each decommissioning option. The risks were quantified in accordance with predefined consequence and likelihood criteria as shown in tables 6.1.2 and 6.1.3. The assessment is based on experience and the knowledge of outcomes of similar events, pub-lished information or expert judgement. Any control or mitigation measures which may be in effect when the activity is carried out are also taken into account

• Anoverallriskratingwasassignedtoeachaspectofthedecom-missioning option under consideration using a two-dimensional Risk Assessment Matrix based on the principle that risk is a prod-uct of two factors: probability and consequence ( Table 6.1.4)

CHAPTER 6: Comparative assessment of options

25Danish Sustainable Offshore Decommissioning Project

Consequence Description

Severe Environmental issue, impact or risk that could constrain the Company’s operations in a business area, sector or field.

Degradation or loss of ecologically, commercially or culturally important species or biodiversity, typically on a regional, national or international scale, or an irreversible detrimental loss on a local scale.

Atmospheric emissions at levels which compromise national targets or which result in detrimental transboundary or cumulative impacts.

Potentially irrecoverable loss of natural resources to the detriment of dependant users.

Multiple fatalities or serious health impacts.

Permanent disruption to business, communities or individuals, with consequential loss of revenue or amenity.

Loss of control, loss of containment or emergency event with consequences on a national or international scale.

Major Environmental issue, impact or risk that could constrain the viability of the option.

Degradation or loss of ecologically, commercially or culturally important species or biodiversity, typically well beyond the source of the effect. In general, recovery potential over a long term (>5 year) period.

Substantial source of atmospheric emissions or contributor to transboundary or cumulative impacts.

Lasting impacts on natural resources to the detriment of dependent users.

Detrimental effects on human health.

Substantial disruption to business, communities or individuals, with consequential loss of revenue or amenity.

Loss of control, loss of containment or emergency event with serious consequences.

Emissions or spills which will jeopardise the ability to meet asset operational performance targets.

Moderate Degradation or loss ecologically, commercially or culturally important species or biodiversity over a localised area, typically limited to the vicinity of the source of the effect. Generally, there is the potential for recovery to a normal healthy, representative state over a medium term period (2 to 5 years).

Moderate contribution to global atmospheric, transboundary or cumulative processes.

Atmospheric emissions generally at levels around 1% of daily national emissions for industry sector.

Temporary (1 week to 6 months) impacts on natural resources to the detriment of dependent users.

Temporary detrimental effects on wellbeing (rather than health) of people.

Short-term disruption to business, communities or individuals, with consequential loss of revenue or amenity.

Small-scale loss of control, loss of containment emergency event which can be remedied onsite.

Emissions or spills which impact upon the operational performance targets for the asset.

Minor Degradation or loss of ecologically, commercially or culturally important species or biodiversity over a localised area, typically limited to the vicinity of the source of the effect. Generally, there is the potential for recovery to a normal healthy, representative state over a short-term period (<2years).

26 Danish Sustainable Offshore Decommissioning Project

Minor contribution to global atmospheric, transboundary or cumulative processes.

Atmospheric emissions generally at levels around the company target for installation/activity.

Localised transient (<1 week) impacts on natural resources to the determent of dependent users.

Transient (< 1 week) effects on wellbeing (rather than health) of people.

Transient disruption to business, communities or individuals, which causes nuisance rather than loss of revenue or amenity.

Minor leaks, drips and incidents which can immediately be remedied onsite.

No impact on operational performance targets from emissions or spills.

Negligible Localised, transient disruption to ecologically, commercially or culturally important species or biodiversity close to the source of the effect, with rapid recovery to a normal healthy, representative state.

Negligibly small contribution to global atmospheric, transboundary or cumulative processes.

Negligibly small impacts on resource quality or availability which is not to the detriment of dependent users.

Transient nuisance which does not affect human health or wellbeing.

No disruption to business, communities or individuals.

No apparent risk of spills or incidents

Positive Enhancement of habitats, or ecologically, commercially or culturally important species

Table 6.1.2 Predefined Consequence criteria for environmental risk assessment

Consequence Description

27Danish Sustainable Offshore Decommissioning Project

Table 6.1.3 Predefined Likelihood criteria for environment risk assessment

Category Description Probability ( indicative value)

Definite Should definitely occur 100%

Likely Likely to occur during normal operation, given the control 1% to 99% and/or mitigation proposed

Possible Could occur infrequently during normal situations given the 0.01% to 1% controls and/or mitigation proposed, or more readily during abnormal or emergency situations

Unlikely Unlikely during normal operation given the controls and/or 0.001% to 0.01% mitigation proposed, but may occasionally occur during abnormal or emergency situations.

Remote Extremely unlikely during both normal and abnormal or <0.001% emergency situations given the controls and/or mitigation proposed.

6.1.3.2 Risk Rating Table 6.1.4 gives a matrix showing how the combined levels of prob-ability and consequence have been used to determine the risk rating. These fall into three negative categories, and one positive category indicating the beneficial outcome of decommissioning. The four risk ratings are:

•HighlysignificantRisks(Redzone)indicatingthatrisklevelisintol-erable.

•SignificantRisks(Amberzone)whichcallsforfurtherriskreduc-tion measures and/or demonstrate that risk is ALARP ( As low as Reasonably Practicable)

•NotSignificantRisks(Greenzone).Riskisacceptablebutshould be managed to achieve continuous improvement

•PositiveOutcomes(Bluezone)whichcouldbebeneficialbecause they resulted in the avoidance of environmental harm, the enhancement of resource stewardship, or socio-economic or environmental gain.

Overall, there are 30 possible risk ratings – 5 positives, 11 not significant, 8 significant and 6 highly significant risks.

Probability

Remote Unlikely Possible Likely Definite ( R) (U) (P) (L) (D)

Severe(6) R6 U6 P6 L6 D6

Major (5) R5 U5 P5 L5 D5

Significance Moderate(4) R4 U4 P4 L4 D4

Minor (3) R3 U3 P3 L3 D3

Negligible(2) R2 U2 P2 L2 D2

Positive (1) R1 U1 P1 L1 D1

Highly Signifi-cant Zone Significant Zone Not Significant Zone Positive Zone

Table 6.1.4 Matrix com-bining criteria of prob-ability and consequence to generate an overall risk rating

28 Danish Sustainable Offshore Decommissioning Project

6.1.4 Energy consumption and emissionsPrevious assessment using the energy and emissions criteria suggests that energy use in decommissioning a platform topside could range from 36,000 – 80,000 GJ for small topsides, and from 100,000- 300,000 GJ for bigger decks. These are sizeable amounts when compared to a household’s annual energy consumption which is ca. 80 GJ/a.

The total amounts of gaseous emissions are very closely linked to the total amounts of energy used. This makes the HLV and single lift methods much more preferred as they are less energy-intensive.

6.1.5 CostAssessment based on cost criteria for the indefatigable platform top-sides indicate that for the larger platforms the two options, HLV and

SSCV, are likely to be similar in cost. The other options, namely two novel technologies and the piece-small option are estimated to be 25-50 % more expensive due to either the additional marine activity or the greater offshore man-hours.

6.1.6 Summary results of assessment of Selected Topsides Removal methodsTable 6.1.6 summarily describes the results of the comparative as-sessment of the various topsides removal methods based on the 5 criteria used. The results of the other four criteria, corroborate that of the environmental and social criterion in supporting the reverse installation method with heavy lift vessel (HLV) as the most viable and feasible method. The single lift method with semi-submersible crane vessel (SSCV) is also competitive and relevant for the heavier topsides.

Installation reversal Single lift with Novel technology Novel technology Piece-small removal with HLV SSCV (Versatruss) (submersible barges)

Technical risk Technically feasible Technically Feasible for the deeper Feasible for the deeper Feasible butand complexity for ca. 1800 –ton feasible for ≥ northern north sea northern north sea higher technical risk weights) 3300- ton ( 40,000 ton weights) weights)

Personnel safety Relatively Relatively safe Higher risk to Highest risk to High risk for safe( low risk 1.0) ( low risk, 0.99) personnel (1.95) personnel (2.05) personnel (1.58)

Environmental Least impact Less impact Relatively Relatively Relativelyand social negative negative negativeimpact

Energy 36,000 – 80,000 36,000 – 80,000 70,000 – 90,000 65,000 -95,000 60,000 -115,000consumption GJ for smaller decks GJ for smaller decks GJ for smaller decks GJ for smaller decks GJ for smaller decksand emissions 100,000- 100,000- 135,000- 130,000- 200,000- 300,000 GJ 300,000 GJ 180,000 GJ 215,000 GJ 260,000 GJ For bigger decks For bigger decks For bigger decks For bigger decks For bigger decks

Cost cheaper Similar to HLV 25-50% more 25-50% more 25-50% more expensive expensive expensive

Criteria

Table 6.1.6 Summary results of screening of removal methods for topsides

29Danish Sustainable Offshore Decommissioning Project

6.2 Comparative assessment and selection of preferred option for jackets

As with the topsides, all of the options for the offshore oil and gas jackets require that they are removed to shore. Therefore the com-parative assessments are carried out on the two shortlisted removal methods, namely installation reversal using HLV and jacket removal using added buoyancy.

6.2 1 Technical risk and complexityAlthough both methods are technically feasible for jacket removal, the reverse installation with heavy lift vessel carries a lower risk as it is a well proven technique which can be carried out by a number of contractors. The added buoyancy carries a higher technical risk due to the difficulties of operating in shallow waters and high tide/currents as is the case with the platforms of the southern North Sea.

6.2.1 Personnel safetyPotential loss of life (PLL) a measure of safety risk to personnel work-ing on platforms has been estimated to be more than two times with the added buoyancy method as with the reverse installation method for removing the jackets. On average the offshore risk will contribute 93% of the total risk to personnel, whereas the onshore work will contribute only 7%.

6.2.2 Environmental and societal impactThe parameters used in this criterion for the topsides (section 6.1.3) apply to the jackets also, in accordance with the Shell corporate guidance report, the societal impacts including impacts on fishing and fish spawning are not significant enough to differentiate between any of the preferred methods.

6.2.3 Energy and Emissions criteria Energy use in decommissioning a platform jacket could range from 30,000 to 60,000 GJ for small jackets, and 50,000-130,000 GJ for bigger decks. These are considerable amounts compared to a household’s annual energy consumption which is ca. 80 GJ/a.

However, the difference in energy use between the two selected methods, HLV and added buoyancy, can range between 50-70 %.

As well, the total amounts of gaseous emissions are very closely linked to the total amounts of energy used. This makes the HLV method much more preferred as it is less energy-intensive.

6.2.4 CostsThe cost of the HLV method is likely to be 50-75 % of the cost of the buoyancy method due to the high cost of fabricating the buoyancy tanks and the amount of marine activity.

6.2.5 Summary of jacket removal screening evaluation Based on the comparative assessment considering the above crite-ria, the preferred method of removal for the offshore oil and gas jack-ets of the listed platforms is the installation reversal using HLV. This is a technically proven method and is simply the reverse of the original installation sequence. The operation is subject to standard offshore construction safety risk and environmental risk exposures that can be managed. This removal service in the North Sea can easily be of-fered by a number of competent contractors.

30 Danish Sustainable Offshore Decommissioning Project

CHAPTER 7: Results and discussion

7.1 Results of Environmental assessment

The environmental impact assessment provided a vigorous and quantitative method of:

• Assessingtherelativeenvironmental‘performance’ofeachoption• Determiningifanyoftheoptionsofferedasignificantlybetteror

worse environmental performance than others• Evaluating ifanyapparentdifferences inenvironmental ‘perfor-

mance’ were real and significant 7.1.2 Overview of results All of the preferred decommissioning options have the potential to cause environmental impact, both as a result of planned activities and as a result of possible emergency or accidental events.

None of the options was assessed to have any risks in the ‘highly significant’ category, i.e. risks that would be intolerable and would represent a major constraint for the option. All of the options had a small number of risks that were rated as ‘significant’ (i.e. the project should seek to incorporate further risk-reduction measures and/or demonstrate that the risk was ALARP). All of the options also had a large number of risks that were rated ‘not significant‘(i.e. indicat-ing that the risk was acceptable but should be managed to achieve continuous improvement).

Decommissioning Option Numbers of impacts

Positive Not significant Significant Highly significant

Reverse installation with HLV 0 75 4 0

Single lift with SSCV 0 75 4 0

‘Versatruss’ with catamaran barges 0 75 4 0

Twin submersible barge 0 75 4 0

Piece-small removal 0 74/75 5 0

7.1. 3 Impacts from decommissioning topsidesTable 7.1.3 gives the results of the screening of all risks associated with the short-listed options for decommissioning the topsides. All of the options exhibited about the same number of “not significant” and “significant” impacts. In all options, 4 of the “significant” impacts would arise as a result of a large accidental spill of fuel oil to sea, fol-lowing a vessel collision. In the piece-small option, a further impact might arise as a result of the exposure of personnel offshore to ex-cessive dust and fumes during the extensive dismantling and cutting operations within the confines of the topsides.

7.1.4 Impacts from decommissioning jacketsTable 7.1.4 gives the results of the screening of all risks associated with the short-listed options for decommissioning the jacket. Both options exhibited about the same number of “positive”, “not signifi-cant” and “significant” impacts. The single positive impact was the effect on fishing operations of removing an obstruction (the jacket) from the seabed. In each case the 4 “significant” impacts would arise as a result of a large accidental spill of fuel oil to sea, following a ves-sel collision.

Additional “not significant” impacts were found in the buoyancy op-tion,asaresultofliftingthefloatingjacketontoabargeataninshoresite. It is not considered that use of explosives subsea will be re-quired.

Table 7.1.3 Environmental impacts associated with options for decommissioning topsides

Decommissioning option for Jacket Numbers of impacts

Positive Not significant Significant Highly significant

Reverse installation with HLV 1 96 4 0

Removal using temporary buoyancy 1 98 4 0

Table 7.1.3 Environmental impacts associated with options for decommissioning topsides

31Danish Sustainable Offshore Decommissioning Project

CHAPTER 7: Results and discussion CHAPTER 8: Conclusion

This report has been made necessary by an imminent need to de-commission old and redundant offshore oil and gas platforms placed in the North Sea. The project code-named ‘Dansk Sustainable De-commissioning’ has sought to carry out prior environmental impact assessment of the most viable and feasible options for decommis-sioning the platforms topsides and jackets. The methods chosen and assessed have been shortlisted from a wide array of options which were considered.

From the review of the various decommissioning options for the top-sides and jackets, the removal to shore of the platform topsides and jackets was selected as the most viable and feasible option.Based on the comparative risk assessment carried out, it is pro-posed that:

For removing the platform topsides, the reverse installation method using the heavy Lift vessel (HLV) was the best option for platform topsides weighing approx. 1800 tons, whereas the single lift with semi-submersible crane vessel (SSCV) is proposed for the heavier topsides weighing more than 3,300 tons. The novel lift technologies, namely the Versatruss, and twin submersible barge, may be more technically feasible in the deeper waters of the Northern North Sea and for future decommissioning projects, involving deck weights of up to 40,000 tons.

For the jackets, the reverse installation with HLV is proposed.

It is proposed that the decommissioning of the earmarked facili-ties will be performed in a phased manner following the permanent abandonment of the platform wells, isolation and making the facilities hydrocarbon-free. The planned phases of the decommissioning are as follows:

•Pre-decommissioninginspections,surveysandengineeringde-velopment studies.

•Pluggingandpermanentabandonmentofthewells.•Removalofresidualhydrocarbonsfromtheplatformfacilitiesand

associated pipelines.•Removaltoshoreoftheplatformstructuresandequipment.•In-situdecommissioningofthepipelines.•Removalofthehosebundles.•Postdecommissioningseabedclearanceandsurveys.•Onshoredismantlinganddisposal.

To all intents and purposes this decommissioning programme is comprehensive and should be acceptable to the Danish government and other stakeholders involved.

32 Danish Sustainable Offshore Decommissioning Project

• ConservationofHabitatsRegulations(2001).TheOffshorePe-troleum Activities (Conservation of Habitats) Regulations 2001

http://www.legislation.gov.uk/uksi/2001/1754/contents/made accessed 7th November 2011

• DEA(2009).Removalofinstallations.Denmark’s Oil and Gas Production, 2009, www.ens.dk )

• DEA(2010).Denmark’sOilandGasProduction,2010, (www.ens.dk ) accessed 21st December 2011.

• DanmarksMiljøundersøgelser(2010).BundFaunaData.DenNational Database for Marine Data (MADS) http://www.dmu.dk/vand/havmiljoe/mads/bundfauna/data/ accessed 9th no-vember, 2011.

• EuropeanCommission(2008).WaterNote1–JoiningForcesfor Europe’s shared waters: Coordination in international river basin districts. Water Notes for the implementation of the Water Framework Directive. Accessed from http://ec.europa.eu/environment/water/participation/pdf/waternotes/water_note1_joining_forces.pdf on the 5th of December 2011

• EuropeanCommissionEnvironment(2011).TheHabitatsDirective. Accessed from http://ec.europa.eu/environment/nature/legislation/habitatsdirective/index_en.htm on 24th No-vember 2011.

• Fiskeri-ogSøfartmuseet()Taggingofseals.Saltvandsakvariet Esbjerg. Accessed on 21st November 2011 from http://www.fimus.dk/eng_index.html

• NAM(2007).Energyfromthedepths.Oilandgasproductionby NAM. 12th Revised Edition. http://www-static.shell.com/static/namen/downloads/pdf/nam_energyfromthedepths.pdf

Accessed 24th October 2011

• OSPARDecision98/3ontheDisposalofDisusedOffshoreInstallations www.ospar.org/documents/.../od98-03e.doc from http://qsr2010.ospar.org/media/assessments/p00453_OA3-BA5_ASSESSMENT.pdf

• OSPARCommission(2009).Assessmentofimpactsofoff-shore oil and gas activities in the North East Atlantic. Accessed from http://qsr2010.ospar.org/media/assessments/p00453_OA3-BA5_ASSESSMENT.pdf on 23rd January 2012

• Hereema,(2011).Decommissioningandremoval.http://hmc.heerema.com/Portals/3/Docs/Act/Removals/overview/prod-uct_topsides_removal_large.jpg accessed 15th December 2011

• ShellUKLimited(2007).Indefatigablefieldplatformsandpipelines - Decommissioning Program. Accessed from http://www-static.shell.com/static/gbr/downloads/2nd_stage_de-commissioning/inde_final_report.pdf on 3rd October 2011

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