A structure subject to huge forces
As structural engineers we are trying to deliver an installation in the middle of the sea that can resist all the imposed forces and where personnel can live and work safely.
The structure we design has huge forces acting on it: the effect of wind, wave and current can act from any direction over 365 days of the year.
In addition, we have the topsides loading to consider, which itself is a large weight of up to 30,000 Tonnes, with wind loads acting on it.
To this end the foundation piles needed to support the structure are required to be piled into the seabed up to 90m deep.
10,000 year waves
The primary structure has to be engineered to a standard that can resist the effects of extreme waves.
For existing installations this is based on a “100 year” wave return period at the locations where the platform operate.
These wave heights can be as high as 30 metres. As details become more mature on actual wave profiles, new ISO codes stipulate that structures must now be engineered to withstand a “10,000 year” wave return period.
We also think about how we look after and maintain existing offshore platforms: age is a primary concern. Most UK offshore structures were designed for an average 25-year service life, but a large number have passed this point.
We have to provide Aging Life Extension (ALE) reports to ensure we have up-to-date information for the structures. This ALE provides the necessary verification and assurance that they are safe and can operate beyond their original design life.
Structural engineers are also essential in the decommissioning of offshore structures.
The primary challenge in 90% of such cases is that you cannot remove the structure in the same way you have installed it - over the service life of an installation alterations and additions will have covered existing lifting points, or removed them all together.
The lifting points that remain are fully checked before reuse - if they are not compliant, new lifting points have to be engineered.
Technology has transformed offshore structural engineering quite dramatically over the years, to meet the needs of the oil and gas business.
In the 1960s most structures were designed for shallow water locations (up to 50m deep) in the Gulf of Mexico and Middle East - the primary structure, known as a “jacket”, was either self-floated or barge-launched, and the topsides constructed in module sections weighing up to 2000 tonnes.
In the 1970s the emphasis changed to building in the North Sea, in water depths of up to 150m, and then even deeper water in the Gulf of Mexico.
In the 1980s engineers were asked to design for deeper waters (of up to 200m in the North Sea and 300m in the Gulf of Mexico), which gradually saw the introduction of floating production systems.
After the tragic Piper Alpha incident on the 6 July 1988 the industry was shaken to the core, and following the Lord Cullen report innovations like Emergency Shutdown Values (ESDV) and Subsea Isolation Valves (SSIV) were introduced.
The introduction of these valves meant individual assets could be isolated from each other - unlike on Piper Alpha, where adjacent platforms continued to produce oil and feed the fire.
Enhanced blast and fire protection systems were also introduced i.e. fire protection systems to withstand hydrocarbon jet fire, and blast walls to segregate and protect the living quarters from production areas.
By the 1990s engineers were constructing platforms to drill in waters of up to 1000m in the Gulf of Mexico. This period also the introduction of larger lift barges that could install jackets of up to 8400 Tonnes and topside Integrated decks weighing up to 10,000 Tonnes in one piece. It certainly is engineering on a huge scale.
New offshore engineering projects have slowed down at the moment, due to low oil prices, however what I can see more of in future is more innovations like Floating Production Storage and Offtake (FPSO) vessels and Floating Liquefied Natural Gas (FLNG) craft, which merge extraction and refinement into one offshore facility.
Whatever the case, offshore structural engineers will continue to find ways to meet the world’s requirements for oil and gas in what is a constantly evolving, innovating and exciting industry.