Design, engineering of offshore structures more often with decom in mind

Incorporating eventual decommissioning in the early stages of platform design has become more important for clients, but new innovations have yet to be embraced by the mainstream.

By Bob Moser, Americas Correspondent

Decom professionals in both the Gulf of Mexico and North Sea say they are seeing costs rise and safety precautions needed when working on older offshore platforms and structures, where ease-of-decommissioning wasn't prioritized during their design years ago.

Designers and engineers of offshore drilling structures today say incorporating the end of a platform's life into their plans has become more common, but to what degree is still based on how much clients prioritize it, and frequent communication with shipyards that build the structures has become integral.

An offshore platform's design and engineering pass through multiple stages on its way to eventual construction in a shipyard. The initial, basic design stage that earns the structure class approval includes approximate weight and cost, but is then passed on to a shipyard, where detailed design and fabrication occurs.

Operational safety is the primary concern in the basic design stage for both the engineer and client, says Joakim Blomgren, North America director for GVA Consultants, a Sweden-based marine and offshore design company for semi-submersibles and floating units. Once the shipyard builder has been given the plan and maps out how to piece a structure together, their choices for welding and complexity of construction play the largest role in how it will eventually be decommissioned, Blomgren said.

“We may tell the shipyard how to weld it, but we're not going to tell them in detail what they can or can't do,” he said. “We always look at how the shipyards are producing structures. That will tell us how we should design, which will also be a direct line to the decom thought process as well.

“What we can do, and have done in the past, when creating our design is to examine the easiest way to put it together, which will be the easiest way to take it apart,” Blomgren continued. “The simplest design is always the best solution.”

Clients that do bring up decommissioning questions early in the design process usually root their desire in minimizing environmental impact, Blomgren said. GVA has tried to streamline its offshore design process to favor minimalization, like utilizing as few bracings as possible under structures.

“When decommissioning, you have to be careful with bracings, which hold up a whole platform,” he said. “It would be more complicated to decom a platform with a lot of bracings … because you'd have to start from the top and work down. If you start in the wrong place you could take down the whole platform.”

With the advent of new float-over topsides that today can utilize simple barges for transport, and spar-type platforms built as a single piece with streamlined construction, decommissioning in the future will most certainly become an easier, faster process, says Cees Leenaars, managing director of Netherlands-based Leenaars Marine & Offshore Design.

Float-over of spars is something Leenaars says his company has developed just within the last year with a client, building on the six years of experience Leenaars Marine has with float-over of normal platforms between the legs of a jacket.

Leenaars envisions splitting the whole offshore unit into just two parts, the topside and a jacket, installing them both in one phase while planning ahead for how to disconnect the jacket from the seabed, or disconnect anchor systems and risers.

“By doing that a little bit more with decommissioning in mind, you can make life a lot easier,” he said.

Generating enough interest in these design innovations from the industry's largest financiers, major oil companies, remains a challenge.

“Practices being developed now are going to be accepted by oil companies in the near future, but they are still conservative, and quite hesitant because it's new,” Leenaars said. “But as soon as they see the first new float-over jobs done, they will realize that is the way it should be done.”

An insulation vessel, which can transport a complete topside platform of 25,000 tonnes to its offshore site, place it atop a jacket foundation and remove it in the same fashion, costs anywhere from US$240 million to US$300 million to build. In his experience, Leenaars hasn't seen oil companies willing to make that investment, instead waiting for others to build and then leasing the vessel once it's been proven.

“The oil companies need to get involved by really looking at solutions (designers) are offering, and giving someone who builds it or someone who wants to install it a five-year contract, early on,” Leenaars said. “Then, the big part of the investment is covered. Financing for all of this is the challenge.”