The Red Hawk spar, topside removed, is towed away for reefing (InterMoor)

A US Bureau of Ocean Energy Management survey of the Gulf of Mexico in 2013 found that of the 95 deepwater production platforms in operation, 42 were floating, including 35 spars and tension leg platforms (TLPs). As these structures are presently responsible for 80% of the oil coming from the Gulf of Mexico, they are certain to become the bread and butter of the decommissioning industry in the future.

Challenges of floating structures

Floating production vessels present a series of novel challenges to the decommissioning side of the oil and gas industry. This is partly because the engineering techniques used to disassemble fixed platforms have to be significantly modified, and partly because of the ever growing need to find the most cost-effective solution in the straitened circumstances brought by the increasing cost of production and the decreasing cost of the product.

The issue is important because the need to use floating platforms is growing as the industry develops more deepwater fields and exploits smaller fields remote from fixed infrastructure.

High costs and long-term planning

Floating platforms fall into two broad categories, each with its own issues. David Gratian, Director at Arup, who heads its offshore engineering team, says: “With ship-shaped structures such as FPSOs, it’s a matter of decoupling and towing – all the complexity is in making the wells safe. On spars and TLPs it’s more like fixed structures in that you have to decide what to take away and what to leave behind.”

The cost of an FPSO can be as high as $100m for the hull and $600m for the topside equipment because, typically, they are packed with high-end electrical and automation gear: one unit ordered for the Eni’s Cabaça field off the coast of Angola will be able to handle 120,000 b/d of oil, have enough power to inject 120,000 b/d of water into the reservoir and to liquefy up to 120 MMscf/d of gas.

This means that, broadly speaking, the principal issue with ship-shaped vessels is whether they can be reused to save costs, whereas the issue with spars and TLPs is in planning and executing the engineering required to dismantle them.

Custom specifications and a lack of data

Steve Robertson, Director at Douglas-Westwood, has pointed out that each FPSO is uniquely designed to produce a field that itself has a unique set of production characteristics. This can make the issue of reuse problematic. Masaki Kawase, Manager of New Technology at Modec’s floating production and storage division, says one particular issue is with the turrets fitted to FPSO vessels, which are connected to the wellhead risers, and which can vary greatly in type, size and function. “They can be external or internal, and may be either permanent or disconnectable. This part of the FPSO can typically be re-used, whereas the swivel and manifold requirements are usually too field-specific to have much change of reuse.”

Another issue that typically arises is that the vessels that are coming up to their end of life phase now were designed and built 20 or more years ago. Dusan Curic, Project Manager at InterMoor, told DecomWorld: “The common denominator in ship-shaped structures is that you’re dealing with a big lack of data. It was built in pre-computer times and maybe the operators have changed hands a couple of times, and you’re looking for a particular drawing and you can’t find it. And you’re also dealing with lines and rigging that you can’t see. Typically mooring lines and risers will be covered by mud and you can’t survey them with an ROV, so you have to be prepared for surprises.”

Re-use or recycle?

Once the ship-shaped vessel is unhooked, Gratian says, a dry dock survey may be needed to determine the best course of action. Sometimes there can be a half-way house between scrapping and re-use. He says: “If it has been in the northern North Sea then a lot of refurbishment would be needed to mitigate for fatigue and cracking on the hull, but you could use it in a more benign wave climate somewhere else in the world.”

The decommissioning of spars and TLPs presents particular challenges to offshore engineers. The project that really established what was involved was the topside lift on Anadarko’s Red Hawk spar scheme, carried out last year. Curic, who was one of the engineers on that job, says the general rule is that it takes four times as long to plan a decommissioning project as it does to execute it. However, in the case of Red Hawk, it was longer because the original plan was to decommission in 2013, but Anadarko put that back a year because it was still hoping to redeploy it. “That meant we had another year of engineering and it was used, I can tell you. It was pretty much two years of engineering planning.”

Climate conditions

The fact that the platform is floating involves dealing with mooring lines, which is a routine matter for offshore contractors who often move rigs around. Curic says it also involved using anchor handling tugs and the co-ordination of a large team – 250 people on 11 vessels were used during the Red Hawk job – and a great deal of calculation to predict the angle of motion of the hull as the topside was taken off, and the use of water as ballast to replace its weight.

The particular issue with this scheme was that once daylight could be seen between the topside and the hull, a point of no return was reached. In the relatively benign climate of the Gulf of Mexico, this turned out to not be an issue, even though the Red Hawk was decommissioned during hurricane season.

Curic says the real test will come when the same operation is attempted in more volatile environments. He says: “Normally in West Africa, for example, it is very calm, but then you have a sudden squall in a couple of hours, and you can’t be prepared for that.”