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Decommissioning project case study: TOTAL’s MCP-01 platform

30 May 2014

MCP-01 was a concrete-based platform in the North Sea decommissioned between 2006 and 2009. This article is based on the MCP-01 Decommissioning Programme, submitted by TOTAL on behalf of the owners to UK and Norwegian government agencies in September 2007, and illustrates some of the issues that need to be addressed when facilities come to the end of their useful life.

Introduction

The Manifold and Compression Platform no. 1 (MCP-01) was a concrete-based platform originally constructed and installed in 1976 to serve the two 32” pipelines transporting gas from the Frigg Field in the North Sea to the St Fergus Gas Terminal in Scotland. MCP-01 came into service in 1977 when gas started to flow from the Frigg Field.

The ownership of MCP-01 was shared between operator TOTAL E&P UK and Gassled, a consortium of Norwegian companies including Norsk Hydro and Statoil.

MCP-01 was a gravity base structure resting secure on the seabed by virtue of its own weight. After the structure was positioned in 1976, the compartments inside the external walls were filled with 173,000 tonnes of sand ballast to keep the platform stable on the seabed. The total weight of the concrete substructure, including the ballast was 386,000 tonnes. Topsides were then installed to provide the required operational facilities and accommodation.

In 1987, gas from the Alwyn North platform started to flow in the Frigg UK pipeline using the platform TP1 at Frigg as an entry point. The fields Tartan, Piper B, Claymore, MacCulloch, Ivanhoe, Rob Roy and Galley fields used MCP-01 as a riser platform to send gas to St Fergus through the 18” Talisman spurline attached to the platform external wall.

In 1992, MCP-01 was re-configured to allow it to be operated as a “not-normally-manned” platform controlled from St Fergus. To improve safety and reduce maintenance, the operational equipment used on the platform was kept to a minimum and, if required, the platform could be shut down from the nearby Tartan platform.

The pipelines entering and leaving MCP-01 were bypassed during 2004 and 2005, allowing MCP-01 to be decommissioned.

The concrete substructure was never used for storage of crude oil nor drilling operations, and there were no drill cutting accumulations either inside the substructure or on the seabed near the platform.

At the time MCP-01 was designed and constructed, consideration for a future removal operation was not included in the design process.

Overall approach to decommissioning

In 1998, the contracting parties of the Convention for the Protection of the North East Atlantic (known as the OSPAR Convention) took a legally binding decision that provides the regulatory framework for decommissioning all offshore structures in the OSPAR maritime area, which includes the North Sea.

In respect of gravity-based concrete structures, the Decision 98/3 states that "The dumping, and the leaving wholly or partly in place, of disused offshore installations within the maritime area is prohibited", but adds that "…if the competent authority of the Contracting Party concerned is satisfied that an assessment …shows that there are significant reasons why an alternative disposal …is preferable to reuse or recycling or final disposal on land, it may issue a permit for …a concrete installation…to be dumped or left wholly or partly in place …".

The part of the concrete platform where such alternative disposal options may be assessed would be the concrete substructure; i.e. the load bearing structure supporting the topside facilities.

This process was therefore followed to determine the recommended arrangements according to the “waste hierarchy”, which values reuse above recycling and disposal onshore above disposal at sea.

Possible reuse of MCP-01

A number of reuse potentials were assessed for MCP-01; either at its original location or elsewhere. However, none of the arrangements for the reuse of the MCP-01 facilities in situ were judged to be economically viable at the time of consideration, and were therefore not taken forward. There were also a number of technical uncertainties associated with many of the possible reuses.

The feasibility of reuse at a different location does depend entirely upon the ability to safely refloat the concrete substructure, which was not designed specifically for removal at a future date.

There were limited possibilities for the reuse of parts of the topside equipment. The age of the equipment, and the uncertainties associated with their ongoing maintenance and logistical support, reduced any potential likelihood for reuse.

MCP-01 topside facilities

Impact on removal

In the absence of any viable reuse potential for the MCP-01 topside facilities at the original location, evaluations were carried out to determine how the facilities could be decommissioned.

In accordance with the UK and Norwegian regulations, and OSPAR Decision 98/3, full removal and onshore disposal had been the only disposal option considered, and an evaluation of feasible methods for removal and onshore disposal was undertaken. The cost and risks associated with this work were also estimated.

The studies undertaken indicated that the topside facilities on MCP-01 could be removed using conventional offshore methods of working.

Based upon the actual man-hours and tasks estimated by AKER Kværner Offshore Partner, the contractor appointed to carry out the decommissioning of the MCP-01 topsides, analyses indicated that the probability of a fatality during the work was approximately 4% and the equivalent average Fatal Accident Rate (FAR) for workers removing the MCP-01 topsides was approximately 7.

The  FAR  is  calculated  from  the  average  yearly  risk  based  upon  the  number  of man-hours worked by an individual in a year. For a “normal” offshore worker on the UKCS who spends  approximately  4,300  hours  a  year  offshore,  an  average  yearly  risk  of  fatality  of  1  in 1000  is  equivalent  to  a  yearly  average  FAR  value  of  22.9.  This  is  the  highest  risk  to  an individual that can be tolerated and a risk considerably less than this must be sought.

Further risk evaluations were presented in the Abandonment Safety Case submitted to the UK Health and Safety Executive (HSE) prior to start of offshore work.

The impact on the environment of removing the topsides is generally low. The “small negative” or “moderate negative” impacts arising from the energy usage, emissions and aesthetic effects during the removal and onshore disposal are balanced by the “moderate positive” impact in respect to materials management arising from the reuse or recycling of materials.

The cost of engineering, preparation, removal, transportation and onshore disposal of the MCP-01 topsides was estimated at about £70m or 840 MNOK in 2004 values.

Recommended disposal arrangements for topsides

The studies evaluating the feasibility of removing the MCP-01 topside facilities recommended that the topside facilities on MCP-01 should be removed and brought onshore for disposal.  Once onshore, as much of the topsides equipment and materials as possible would be reused or recycled.

Integration with Frigg Cessation Project

The offshore removal and onshore disposal of the topside facilities on MCP-01 were integrated into the Frigg Cessation Project, operated by TOTAL E&P NORGE, using the same contractor - AKER Kværner Offshore Partner. The contract was awarded in October 2004 after an international tender and provided for engineering, preparation, offshore removal and onshore disposal for a number of facilities straddling the UK/Norway North Sea divide. These included six topsides, three steel jackets and sealines with an estimated weight of 87 000 tonnes.

Significant synergy effects were expected from this collaboration. The technical and safety challenges were very much the same, particularly since the detailed engineering and the offshore works required for the removal of topsides from MCP-01 and its sister platform CDP1 on Frigg were directly comparable. They were both located in UK waters and required similar Abandonment Safety Cases to be submitted to the HSE.

MCP-01 Concrete Substructure

In the absence of any viable reuse potential for the MCP-01 concrete substructure, comparative assessments were carried out to determine how it could best be decommissioned. The objective of these assessments was to identify the best disposal arrangements for the substructure that took due account of technical feasibility, safety and working environment considerations, environmental impact, cost and stakeholders concerns.

In accordance with UK and Norwegian regulations and OSPAR Decision 98/3, full removal of the concrete substructure was the first option considered. The various alternative arrangements considered in the comparative assessments are summarised below.

Summary of Technical Feasibility Assessment

Alternative A – Refloat the concrete substructure and onshore disposal

Weather dependency

The main uncertainty relating to the possible refloat and onshore disposal of MCP-01 was the need to undertake a large amount of weather-sensitive offshore work in one season. If delays occurred, it might not have been possible to refloat the substructure in the same season as the majority of ballast was removed and the cofferdams installed to seal the wave breaking holes in the external wall. With the ballast removed and the cofferdams in place the substructure would have been very susceptible to damage by winter storms.

If the substructure had to stand through a winter period in this condition it was determined that both sliding and rotational failure of the foundations could occur, with severe damage caused to the base slab and external walls of the substructure virtually certain. Such extensive damage would make it virtually impossible to refloat the substructure in the following season due to the lack of watertightness of the substructure.

Removal of solid ballast

One of the causes of possible delay would have been the operation to remove the solid ballast from within the structure. Although some of this might have been removed in the season prior to the refloat attempt, it would have been necessary to leave at least half the solid ballast in place to give the platform satisfactory structural stability during the winter period.

If it had proven difficult to remove the solid ballast due to any reason, the schedule of work was likely to be delayed and the possibility of running out of weather windows for the refloat attempt would have increased significantly.

Installation of steel cofferdams

Another cause of possible delay might have been problems associated with the installation of the six large steel cofferdams to seal the wave breaking holes in the external wall. These cofferdams, which each weigh approximately 250 tonnes, were particularly susceptible to wave loads and therefore could only be installed in calm weather conditions.

It would have been necessary to accurately install all six of the cofferdams in one season in order to be able to refloat the substructure. Detailed evaluation of the weather conditions at the offshore location indicated that there was a significant possibility that it would not be possible to successfully install all the cofferdams in one season.

Leaks

Due to the inherent design of the substructure, the watertightness could not have been verified until the solid ballast had been removed and all the cofferdams installed.

Also, although the condition of the concrete substructure was thought to be generally satisfactory, it had not been possible to be certain that there were no cracks in the base slab or the lower sections of the external wall. Such leaks would only become obvious when the refloat operation was started and would most probably be difficult or impossible to seal before the end of the summer working season.

Probability of not succeeding

Based upon the judgement and input of leading independent experts, the probability of being unable to refloat the substructure, or a major accident occurring during the refloat and tow to shore had been estimated to be in the order of 60%. This risk was extremely high due to the inherent uncertainties in the extensive offshore activities that needed to be performed. No similar operations on the scale envisaged had been undertaken at the time and there was a significant probability that delays would prevent the refloating of the substructure in one season and thus result in it being severely damaged during the following winter storms. The risk of being unable to undertake the refloat operation was approximately 600 times higher than the 0.1% acceptance criterion for asset/financial loss during decommissioning.

Consequence of accidents

The consequences of a major accident during the refloat operations were shown to be particularly severe, especially in respect to the safety of personnel and cost. In addition, if due to leakage, (or delays which resulted in damage to the substructure), it proved impossible to refloat the substructure, then the only other removal alternative would be to cut up the concrete substructure into suitably sized sections which would then be transported to shore for disposal. Such operations would involve considerable amounts of diving and would be unacceptably hazardous.

Alternative B – Refloat the concrete substructure and disposal in deep water

The activities performed to refloat the substructure for disposal in deep water would essentially have been the same as for the onshore disposal option (Alternative A). The main difference, apart from the final disposal method, was that if deep water disposal was being used, additional steel items would be removed offshore before the substructure was refloated. When at a deep water location, the sinking process would then have been initiated with explosive charges.

Alternative C - Cut down the concrete substructure to provide a clear draught of 55m

Cutting down the substructure to allow a clear 55m draught above the remaining substructure would allow the free passage of vessels.

Method

Cutting down the walls and central shaft of the substructure (up to 120cm thick) was felt to be theoretically feasible, although many factors militated against such an approach. There was a high level of uncertainty surrounding the method of cutting up such an integrated structure in which the strength and stability of each wall depended to a great extent on the adjacent walls. The feasibility of the concrete cutting method was also debatable and considerable effort and expenditure would have been necessary before the method could be considered field-proven.

The amount of diving necessary also made this alternative disposal method very questionable and the risk to personnel engaged in the work was considered to be unacceptably high. Due to the complexity of the MCP-01 substructure and the amount of cutting required, it was not considered feasible with today’s technology to undertake the work using only remotely operated vehicles.

Probability of not succeeding

Uncertainties associated with the process of cutting and toppling the upper sections of wall resulted in a 66% chance that one or more walls might collapse in an uncontrolled manner. This was approximately 660 times greater than the acceptance criterion and was considered unacceptable. In the event of a major accident, the additional works to achieve the 55m draught would be extremely hazardous resulting in a significant increase in the risk to personnel. The total cost of the work would also be substantially increased.

Table 1 - Predicted fatalities
Table 1 - Predicted fatalities

Alternative D - Leave the concrete substructure in place

The steelwork on the outside of the concrete substructure would be removed as much as reasonably practical to remove the risk of corroded steel items falling onto the seabed where they could be a hazard to fishermen. It is important to note that cleaning of the MCP-01 concrete substructure is not required, as it has never been used for the storage of crude oil. This would also mean that there would be insignificant levels of discharge to the marine environment.

Serious damage to all parts above sea level with a possible breakdown to the sea level is estimated would take place in roughly 200 years. Breakdown of the breakwater wall and the central shaft down to about 27m below sea level is predicted take place in 400 to 800 years. A breakdown below 55m could take more than 1000 years.

Summary of Risk to Personnel

The risk to personnel involved in the planned operations for the MCP-01 substructure disposal alternatives that have been considered was estimated based upon the anticipated work tasks and relevant historical accident rates. The predicted numbers of fatalities expressed in statistical terms are shown in Table 1 above.

It can be seen from Table 1 that Alternative D has a significant lower probability of a fatal accident occurring compared with the other alternatives. The probability of a fatality is more than 47 times higher for Alternative A than for Alternative D. It should also be noted that the analytical method used to estimate the likely fatalities and major injuries tends to underestimate, rather than overestimate, the risk to personnel

For Alternative A, the main contributors to fatality risk are inshore/onshore demolition (48%), offshore marine operations (23%), and offshore diving operations (12%). The main contributor to the diving risk is surface diving in the area around the wave-breaking holes in the external wall. From previous experience in the North Sea this is known to be particularly hazardous, due to the strong currents and turbulence caused by the sea flowing through the holes.

Based upon the estimated fatalities, the average Fatal Accident Rate (FAR value) for the complete removal and onshore disposal work is estimated to be in the order of 19. This is approximately 1.5 times the estimated average risk, FAR=13.1, to workers on MCP-01 when it was fully operational.

If the walls of the substructure were cut down to –55m (Alternative C) the probability of a fatality is more than 53 times higher than the leave in place option (Alternative D). The average FAR value for the work involved in cutting down the walls of MCP-01 is estimated to be in the order of 47. This is well above the maximum tolerable limit for operational personnel on TOTAL E&P UK-operated platforms and approximately 3.6 times the average risk to workers on MCP-01 when it was fully operational.

Table 2 - Environmental impact
Table 2 - Environmental impact

The average FAR value for all personnel engaged in Alternative D has been estimated as 7 on the basis that the subsea steelwork removal work can be undertaken using ROVs. The possibility of a small amount of diving work, if required, would increase the average FAR for the project.

The probability of a fatality as reported in Table 1 assumes that it was possible to complete the work as planned. If a major accident occurred, the probability of a fatality during the initial work together with the necessary rectification work would be even higher.

The annual number of seafarer fatalities estimated from vessel collision if the concrete substructure is left in place, is estimated at 2.8 x 10-4, or 1 fatality in 3,600 years. It has been estimated by specialists that  measures taken by TOTAL and industry developments would reduce the collision frequency by approximately 50%.

Summary of Environmental Impact

The Environmental Impact Assessment for the disposal alternatives for the concrete substructure were carried out by Det Norske Veritas in Aberdeen and Stavanger. Their assessment is summarised in Table 2 above.

The EIA Report concludes that the outcome of the environmental impact assessment indicates that from a total environmental perspective, Alternative D is considered the best option.

Summary of Cost

The estimated costs of the four disposal alternatives for the concrete substructure and concrete deck beams of MCP-01 are given in Table 3 below.

The costs presented are expressed in year 2004 money terms and represent a 50/50 estimate reflecting the high uncertainties identified in the risk assessments. 1£=12.0 NOK.

Cost of remedial activities following a major accident was estimated in broad terms to be between £440m (5,280 MNOK) to £820m (9,840 MNOK) for Alternatives A, B or C.

Summary of Stakeholders Concern

Table 3 - Estimated costs
Table 3 - Estimated costs

As part of the MCP-01 consultation process, some stakeholder groups expressed their preference for the full removal to shore option (Alternative A). However, because this was technically unfeasible and inherently unsafe,  the leave in place option (Alternative D) was preferred to Alternatives B and C (deep water disposal and cut down to –55 meters respectively). The main reason for this was to maintain the option of full removal should new technology become available in the future.

Recommended disposal arrangements for the concrete substructure

Leaving the concrete substructure in place was therefore considered to be the best solution when considering health and working environment, safety, environmental aspects, cost and stakeholder concern.

After an overall judgement of the comparative assessment for the disposal alternatives for the MCP-01 concrete substructure, it was recommended that:

After the topsides facilities of MCP-01 platform have been removed and brought onshore for disposal, the concrete substructure (including the concrete deck beams) should be suitably marked and left in place after the removal of the external steelwork. As much as practicable of the equipment and materials removed from the concrete substructure will be reused or recycled (Alternative D).

Postscript

Offshore removal works began in August 2006. During this period the Port Reval flotel was located next to MCP-01 and topside modules were lifted and removed by Saipem 7000, the world's largest crane vessel. Some 13,000 tonnes of steel was taken from the facility to Lerwick in Shetland and Stord in Norway for dismantling and recycling.

Materials management data shows that 85% was recovered for recycling, 14% (mainly plastics) was recovered for energy, 1% went for landfill or special waste disposal and nothing was disposed at sea.

All topsides removal work was completed in 2009, and navigational aids are in place on the remains of MCP-01 to warn shipping of the continuing presence of the concrete substructure.

TOTAL’s close-out report, published in March 2013, records a significant growth in costs associated with the removal and disposal of the platform’s topsides - from an original estimate of £68m to a final figure of £196.25m. This was due to a large increase in man-hours worked over estimate because of technical difficulties and the presence of more asbestos materials than anticipated. This required extended hire periods for flotels and an increase in visits by heavy lifting vessels.

As regards safety, the report records 26 incidents requiring medical treatment, 142 first-aid incidents and 13 lost time incidents on MCP-01, its attendant vessels and onshore support facilities in Lerwick and Stord. 

“Throughout all the preparation periods, the need to recognise the significant hazards represented by the MCP-01 installation – deteriorating condition, possible hazardous materials trapped or present – and the nature of the work to be performed was emphasised. Whilst the contractor and sub-contracted companies had a primary duty of care to their employees, TOTAL provided installation duty-holder safety supervision. The Offshore Installation Manager (OIM), with support from the Safety Superintendent (SSI), made a safety review of all Permits To Work and gave authorisation for the work to be executed – with relevant conditions filled. Total also provided round the clock safety monitoring – via the SSI and Safety Supervisor (SSV).

 “AKER put in place, offshore, Safety Advisers to provide the formal authority for their organisation. These were supported by Safety Coaches – individuals with specific duties to monitor works as executed and to provide instruction “in-the-field”. This arrangement, after some initial problems, proved to be very robust and generally worked well. There was also continuous support from the onshore AKER HSEQ organisation.”

The close-out report confirms that high safety standards were maintained and that although during both 2006 and 2008 there were incidents resulting in lost-time, there were no severe nor incapacitating occurrences. Problems with lifting operations or lifting equipment were the most significant type of event.

Some close-out report points and recommendations:

  • *  The operator needs to carry out “very robust preparations” prior to handing over the platform for decommissioning. 
  • *  Records for the entire installation have to be maintained even when the structure and equipment is substantially  unused.  “ In  the  case  of  MCP-01,  the  catalogue  for  the  redundant  elements  had  been allowed  to  lapse  and  the  reliability  of  information  gave  rise  to  problems  and  extra  work.”
  • *  Decommissioning planning requires clear understanding of the local environment and how the various stages of the work relate to each other.  “The sequence is not as logically obvious as most constructs and so it cannot be done as a remote activity. This is especially so for the older installations.”
  • *  The problem of lay down and storage should be understood. “Given the large quantities of equipment, materials handling and additional emergency equipment needed for a removal  project , almost  all  current  installations offshore  will  have  this  as  a  problem.”
  • *  The availability and reliability of onboard cranes is critical to progress.  “Despite this awareness, they were a source of problems throughout the topsides work.”
  • *  The most significant tools are offshore crane facilities, the heavy lifters.  “This is a very limited market in terms of available players. In the next few years when we start to see more of these installations being considered for removal, the market for heavy lift services will become even tighter.”
  • *  The labour pool is limited.  Despite the potential attraction of decommissioning as a “nice to have” on the  CV,  few  engineers  have  shown  that  they  want  to  make  a  career  of  it. The real  problem of decommissioning is that it needs genuine multi-skilling. 
Much of the oil and gas infrastructure installed on the UK Continental Shelf since the 1970s will require decommissioning over the next couple of decades, both in a safe and environmentally-sound manner. That this has been achieved on MCP-01 shows that the sector is successfully gearing itself up for the massive task ahead.

Thanks to TOTAL E&P UK and DECC Aberdeen for assistance with this article.

All images: TOTAL E&P UK 

(Step  Change  in Safety has produced a  useful  document called Offshore Decommissioning Learnings which captures many of the experiences and lessons from the 2006 intervention at MCP-01.)


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