Before lifting a redundant platform from the seabed, it is necessary to ensure that the well conductors and enclosed casings have been completely severed. It is not as easy as it sounds.
UWG’s approach takes away all the uncertainty and makes for a safe and efficient lift.
A couple of years ago – before the steep rise in the price of oil – the idea of abandoning declining oil and gas fields was probably higher on most operators’ agendas than it is today. Although oil at $70 per barrel has signalled a reprieve for many ageing production platforms, the challenge of removing redundant structures safely and cost-effectively is never far away.
One practical difficulty UWG has identified is cutting the surrounds of the well conductors, plus their enclosed casing strings, beneath the production platform so that the structure can be lifted as a unit from the seabed. Plugging wells with cement is generally straightforward; as is, or should be, cutting conductors just below the mud line. But you need to be confident that the conductors and the casings are completely free, otherwise you can run into serious trouble and expensive delays when it comes to lifting the structure. “Hiring a heavylift vessel and a transport barge accounts for much of the expense associated with the decommissioning and removal of an offshore platform,” points out Ian McFadyen, formerly with UWG and now a non-executive director of Acteon. “And it is possible to waste days if you have to go back to deal with any conductors that turn out to have been only partially severed.”
For this reason, operators often wish to confirm a clean cut by lifting the individual conductors and the casing strings just a few feet immediately after cutting. And therein lies the problem. Even on a small platform in shallow water, this could involve lifting up to 150 t, which is often in excess of the deck’s load-bearing capacity for many older structures.
“One way around this problem is to use beams to spread the load to the platform jacket,” explains McFadyen. “But this can sometimes mean bringing in impractically large beams and, in any case, many older platform jackets members are simply not up to the job.” McFadyen realised that the element of the structure best able to withstand the weight of the individual conductors and their casing strings was the other conductors in the array.
In essence, UWG’s solution involves fixing a spreader beam above the conductor array using risers fitted to selected conductors. In this way, the beam can easily be located on an upper, less-confined level, above the often congested well deck. By fitting a hydraulic jack to the beam, each conductor can be easily and safely lifted the few feet necessary to ensure successful severance. Because the conductors are generally clustered within a well bay, it is also possible to use a smaller spreader beam. This results in smaller bending moments and, most importantly from an operational standpoint, fewer access problems. Well casings that have been lifted once can still be used to support the spreader beam by tying them to other casings in the array to ensure they remain vertical under load.