Credit: Ana Phelps

Reliable riserless systems are widely seen as a necessary element for cost-efficient plugging and abandonment campaigns but there are a number of gaps in their capability.

Riserless systems are generally deployed from monohull light well-intervention vessels. These ships have the potential to provide an agile, relatively low cost option in plugging and abandonment campaigns.

However, one well intervention adviser with a leading oil and gas operator told DecomWorld the on-board technology was not sufficiently advanced to make these vessels a permanent alternative to rig-based solutions.

Heavy costs

Although rigs cost about twice as much as light vessels, they are seen as a safe option for operators and contractors.

Light vessels, by contrast, carry greater uncertainty as to their limitations, for example in dealing with operations that require rotation.

One industry source said firms could adapt their decommissioning programs to take account of the limitations of light vessels.

In one example, a company split the P&Aing into two stages, using riserless equipment for the lower stage and a rig for the upper stage, which typically required the kind of milling operations that rigs were designed to perform.

However, because the outcome was less predictable than with a rig, this option was only cost-effective if done in bulk, preferably as part of a co-operative campaign.

Another decommissioning manager at a major oil and gas operator said his team was hoping to plug and abandon a group of subsea wells, using riserless systems for two-thirds of them.

Although some companies have slotted monohulls into their standard proceedures, the industry is some way from achieving a consensus on their value.

One senior well abandonment engineer working in the North Sea commented that although intervention vessels had been used on 26 of the 30 well P&A procedures he had been involved in, the decommissioning team had no evidence that they had contributed anything to the process apart from providing another option if problems were encountered.

Deepwater abandonment

One consultant with a specialist contractor commented that the “holy grail” of well intervention industry is in creating riserless systems that are able to access deepwater wells with the same spread of equipment as a semi-submersible.

At present, light equipment is not suitable for deeper fields – it is not possible to use coiled tubing, for example, or set spot cement plugs. Below a depth of 5,000 feet, current systems are not able to enter the well.

As a result, the industry needs to develop a whole new spread of equipment and processes that are able to meet the new challenges that deepwater operations post.

There is an underlying uncertainty over how to plan long term, which is exacerbating the lack of available technology. One business manager at a tier 1 contractor told DecomWorld that he believed the industry as a whole was failing to sufficiently invest in riserless technology.

At the present rate of progress it may take as long as 20 years before the industry can work cost efficiently in deeper fields, the source said.