Pipeline Decommissioning

PIPELINE

Removal

1. Reverse Reeling

For pipelines with a diameter of 16 inches or less, which are not concrete coated, a possible method of removal is by a reversal of the reeling installatin process. Reeling is the installatin method and has been used extensively across the North Sea for both rigid and flexible flowlines.

The installatin of rigid pipelines by the reeling method relies on the plasti deformatin of the pipe wall during installatin to ensure the reeled pipeline will subsequently lie straight on the seabed.  When the process is reversed for the removal of a pipeline, the pipe is reeled onto the specialist reel vessel and is once again plastially deformed so that it sits on the recovery reel. The length of pipeline that can be recovered is limited by the size and capacity of the reel. Once the pipeline is on the reel it is taken to a shore-based facility and removed by reversing the process once again.

Due to the nature of the reeling and unreeling process, it is unlikely that a rigid pipeline recovered using this method could be reused. The multile cycles of plasti deformatin of the pipeline wall could potentially compromise its long term integrity. The steel from recovered rigid pipelines is recycled.

This method is also used in the recovery of flexible flowlines. The structure of the wall of a flexible flowline means it doesn’t experience the same deformatin cycles as the rigid pipeline during the reeling and unreeling process. Multiple reeling and unreeling cycles should not, therefore, compromise the longterm integrity of a flexible flowline. In theory, such pipelines have the potential for reuse if a suitable applicatin is found. It is,however, the responsibility of the end-user to demonstrate the integrity of a recovered flowline.

Reeling vessel

2. Reverse S-lay

Larger diameter and concrete coated trunklines are typically installed using the S-lay method. Although it has never been used before in the North Sea, a potentil removal method is the reversal of the S-lay installatin process. This method is oftn considered in the CA (Comparatie Assessment) for decommissioning pipelines in excess of 16 inches diameter and/or concrete coated.

This method would involve recovering a pipeline end to the deck of a specialist S-lay vessel. The vessel would then move along the route of the pipeline, stopping at suitable points where a cut would be made to remove a section of pipe from the recovered pipeline string on the deck of the vessel. These sections would then be transferred to a suitable transportation barge for onshore recycling.

Although there have been some examples of the application of this method in the shallow water (less than 24 metres water depth) of the Gulf of Mexico, a number of signifiant technical limitations currently exist which preclude its large scale applicatin, i.e.:

o   High tension forces would need to be applied to the pipeline during recovery from the vessel tensioner system to the outer surface of the concrete weight coat to bring the pipe onto the deck and hold it in place for cuttingng. The integrity of aged concrete weight coating cannot be assured and would need to be carefully assessed to confim that the necessary tension could be generated, without the concrete coating disintegrating and the control of the pipeline being compromised.

o   This tension would also be applied into the steel wall of the pipeline and after many years of operation, the integrity of the pipe wall along its length under the high recovery loads would need to be confimed.

o   There is the potentil forverylarge quantities of materials to be recovered during the decommissioning of a large diameter trunkline. There is no established supply chain/disposal route for the quantities of concrete, steel and anticorrosion coatings which would be taken onshore during a major pipeline removal campaign.

Pipelay vessel

3. Cut and Lift

Another possible method used for the removal of pipeline sections is the so-called ‘cut and lift method. This can be used for any diameter or length of pipeline. This is the process whereby a pipeline is cut into sections subsea by diver-operated cutting tools or using remotely operated cutting equipment, and the sections are then recovered to a surface vessel using an on-board crane.

This option has been widely used for removing shorter sections of pipe, either for the removal of a short pipeline in its entirety, or when discrete sections are being removed under a decommissioning plan. It is usually the preferred removal option for short sections of pipe, when it is impractial or prohibitively expensive to mobilise major removal equipment.

Most signifiantly, the cut and lift method does create greater risks to the personnel carrying out the offshore option, especially divers. It has therefore been preferable to limit that risk exposure by avoiding extensive offhore cut and lift programmes.

Recycling

When lengths or sections of steel rigid pipelines are recovered as part of a decommissioning programme, the steel itself is recycled using a proven supply chain. All steel pipelines have an anticorrosion coating and often have insulatin coatigs applied. Where possible these coatigs are removed and recycled, otherwise they are sent to landfil.
For recovered flexible flowlines, umbilicals and power cables, the metallic end fittings can be removed and recycled, or in some instances reused. The metallic elements of the carcass of flexible flowlines, and the wires used in armouring layers in umbilicals and power cables, can also be recovered using specialist equipment and then recycled. Such processes separate out the plastic materials from the different layers, which can then be recycled if possible.
A decommissioning project typically achieves recycling or reuse rates in excess of 95 percent of the recovered materials, and in some cases up to 98 percent. Similar rates can be achieved for pipeline decommissioning projects, depending on the volume and type of non-recyclable coatings recovered with the pipelines.

Reuse

Integrity is a key issue when considering the reuse of pipelines or pipeline materials. For rigid steel pipelines, recovered in a single length by the reverse reeling process, the pipe wall will have been subject to significant reverse cycle plastic deformatin during its original deployment and then recovery process. This can signifiantly affect the long term integrity of the pipe structure and would rule it out for reuse. Hence, no steel pipeline recovered in this way has been reused.

It would be possible to demonstrate that a decommissioned pipeline left in situ could be reused for alternatie service and operating conditions and this is regularly considered.

Flexible flowlines, umbilicals and power cables are readily recovered by reverse reeling as part of a decommissioning programme. Such materials can theoretically be reused, but proving that the integrity of the complex multilayered structure of such components has not been compromised during the handling and operational process is difficult, and often recycling is the only realistic option.

ANCILLARY EQUIPMENT

Mattresses

The DECC guidance on mattesses and grout bags is that they should be removed from the seabed at decommissioning. The guidance does, however, recognise that in some circumstances it might be better for badly degraded mattesses to be decommissioned in situ. In such circumstances, a CA is required to demonstrate that the best decommissioning option has been chosen. It is common practie to remove mattesses and grout bags during the decommissioning ofa pipeline and associated ti-ins, without a  separate CA being performed.
The feasibility ofremoval depends mainly on the age ofthe mattess, and its burial status. Bitumen mattresses can be diffilt to recover as they can break up when liftd. Similarly, older block-type mattesses can disintegrate during recovery due to the degradatin of the polypropylene rope holding the blocks together. In such circumstances, the risk to personnel performing the decommissioning increases and the operator may request permission to decommission such mattesses in situ.
Some mattesses are fitd with fronds to promote sediment depositin aftr deployment. These, and other mattesses, can become buried over tie, and under such circumstances the operator could request that they are decommissioned in situ.

Crossings, Grout Bags and Concrete Formwork

If a pipeline being decommissioned crosses other operatinal pipelines, it is usual practie to leave the constructed crossing in place unti all pipelines are decommissioned. This avoids unnecessary risks to the ‘live’ infrastructure. This represents a deferral of the decommissioning works.

As for all pipeline infrastructure, operators are required to considerall options fordecommissioning a crossing. Any proposal to leave all or part of a crossing in situ must be supported with evidence demonstratig the reasons why this is preferred. Such reasons may include suffient burial, impractialitis or safety concerns with removal, or any other exceptinal circumstance. Many crossings are rock dumped for protectin, which may be a valid reason why the crossing should be lef in situ.
Formwork used to construct larger crossings is installed using dedicated liftng pad-eyes or slings, built into the concrete structure. The feasibility of removing such items by reversing this process requires confimation of the integrity of the lif points. Under such circumstances it may be demonstrated that decommissioning in situ is preferred.

Source:

Unknown, Decommissioning of Pipelines in the North Sea Region 2013, Oil and Gas UK