Insight

Lightweight Riser Design

12 Mar 2016

Since the start of the offshore industry steel has been the structural material of choice for subsea riser systems. But with the development of new lighter, stronger pipe materials there is a new approach to lightweight subsea riser design.

Steel has served the industry well, and no doubt will continue to do so going forwards. It has allowed the industry to progress from shallow water (150m) to ultra-deep water (3,000m) in the space of just 25 years. This has been achieved by ongoing improvements in steel specification, welding technology and the development of analytical methods, all assisted by the phenomenal increase in computational capacity.

Whether in the form of a non-bonded flexible pipe or rigid pipe, steel has almost exclusively been selected as the material to resist the complex loads that a typical riser must withstand. Steel is the industry ‘workhorse’ material with excellent and predictable structural properties and is manufactured in such volume that it is remarkably low cost.

Where steel comes unstuck

However, the disadvantages of steel are its high density, low resistance to corrosion in seawater and susceptibility to hydrogen embrittlement in a sour environment. In sour corrosion steel reacts with H2S in the presence of seawater to form iron sulphide, which may form a passivating scale, but is usually vulnerable to material removal. These issues are well understood, and manageable with appropriate design methods. However, it is perhaps the high density that causes the biggest challenge for the subsea riser designer and which leads to a nonlinear increase in riser cost with increasing water depth.

As water depths have increased steel riser design solutions have been extrapolated from successful shallower water applications. Whilst there has been some investigation and application of other materials such as aluminium and titanium, steel has remained technically and commercially the material of choice. The challenge presented by the weight of such steel risers has been managed by ever larger buoyancy modules and installation vessel specification, whose capabilities have been extended by upgrading and new builds. Consequently, Tier 1 installation vessels are orders of magnitude more capable than 20 years ago with respect to riser payloads, crane capacity, station keeping, reel storage capacities and deck payload. The downside is that these vessels also have associated higher day-rates.

But the combination of ever increasing riser weight, buoyancy requirement, and vessel payload and installation vessel cost means that deep water risers are now a substantial percentage of the total development cost, and are often the most technically challenging and schedule critical aspect of a deep water development. This adds to the industry challenge of ever increasing offshore development costs, and the fact is that such developments must be competitive with other hydrocarbon sources. In this period of low oil price, such deep water developments are increasingly hard to justify.

Industry responses to the subsea riser challenge

The oil and gas industry therefore needs to find more cost effective solutions to the riser challenge, to reduce high Capex where riser projects typically carry budgets of c. £2.5bn and, additionally, address the high Opex that arises from the need to manage corrosion and general riser degradation and susceptibility to damage that steel risers can suffer from. In achieving these aims a diametrically opposite design approach to the use of steel can be considered. This uses sophisticated, lightweight materials such as carbon fibre, both for both the riser pipe and the buoyancy modules. For the pipe element, for example, this design approach can achieve pipe weight in water that is 90% less than steel and is also highly resistant to corrosion.

For the buoyancy elements the composite solution reduces the effective density from around 350kg/m3 to less than 90kg/m3. This greatly simplifies all aspects of installation but, most importantly, reduces drag loads and added mass and hence improves overall riser response.

There is little doubt that such materials can offer enabling capabilities to access hydrocarbons in ultra-deep water beyond 3,000m. The key question is whether this lightweight design approach can help reduce the cost of subsea risers in the medium to deep water depths, 1,000 to 2,500m, making such solutions preferable over the standard steel design approach?

In the case of free standing risers (SLORs), which seem the preference for many medium and deep water fields, the use of steel pipe and steel buoyancy modules produces a negative design spiral where high weight leads to high drag loads, requiring more tension and more high drag buoyancy. This leads to a riser design which is structurally and hydro-dynamically inefficient, and also needs a high cost vessel for installation. Additionally the riser payload on the vessel can be high and, since all aspects of the design, procurement and installation processes are on the limit of feasibility, both the installation contractor and operator carry significant risk despite the solution being ‘proven technology’.

The Magma m-pipe® subsea riser solution

To tackle this situation Magma has developed an alternative riser design approach, where the solution allows the reduction of both weight and drag loading. This produces a greatly improved riser response, where all key parameters such as buoyancy module size, installation loads, foundation loads are very significantly reduced. In this manner, a more cost effective riser solution is achieved, despite the higher unit cost of the pipe material and where the increased material cost is offset by savings in buoyancy costs and vessel installation costs. It’s simply a different design approach, made possible by application of modern materials and ultimately delivering lower costs and lower risk.

There are similar analogies to the above approach in the aerospace industry, where increased use of composite materials is now standard practice to save weight and achieve improved fuel efficiencies. But, whilst the offshore oil and gas industry has made some significant technology steps in the last 30 years, in the riser space there are few technology innovations of true significance.

In the riser space, technology development has been limited to incremental steps, partly to mitigate risk and partly driven by market constraints. To make a switch to composites from steel is not an incremental step, and some would say it’s a leap of faith. However, the cost analyses conducted show that significant cost and performance benefits can be achieved.

Conclusion

In this current period of low oil price, it is likely to put pressure on riser cost reduction, and increase the appetite for new solutions that can deliver life of field cost benefits. It is believed that composite technology can offer this not just for ultra-deep and aggressive reservoirs, but also as a way to reduce the cost and increase performance of less challenging developments currently considered marginal.

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