Getting to grips with grout

Grout is taking on new forms to meet the needs of offshore wind applications, Jim Bell explains.

Grout is fundamental for offshore structures, for member filling to strengthen and repair jackets; fabric formworks to support and stabilize pipelines and grouted clamps to repair damaged pipelines or jackets.

Although only contributing to a small proportion of a project’s costs, in the case of foundation installation, grout is an essential component which contributes to the structural integrity of the foundation and therefore its long-term capacity to generate revenue. As such, it is essential to choose the best material for the job.

Grouts for offshore use come in a number of different strengths; standard, high and ultra-high. However, strength is not the only material property that needs to be considered; durability and volume stability also need to figure highly. The potential for improving project productivity rates and safety, both on and offshore, linked to the mixing and pumping, delivery and resupply, and storage of the material are also very important.

The supporting foundation structures for offshore wind turbines are typically either monopiles with transition pieces, in shallow water depths up to 35m, or four-legged jacket structures for deeper waters. The loads acting on both types of structures would be similar and would involve the following; wind loading, wave loads, current, boat impact and fatigue loads (low stress, high cycle wave loads).

As monopiles are stand-alone single structures, the monopile/transition piece grouted connection is subjected to very different loading conditions when compared to the pile sleeve grouted connections of jacket structures. The monopile/ transition piece grout will be subjected to compression and tensile loading, in addition to bending and torsion. Pile sleeve grouted connections however will be subjected to simpler loading consisting of compression and tensile forces.

For these reasons, high strength grouts are required for monopile/transition piece connections, in order to resist the extreme loading conditions that apply. In a typical monopile/transition piece connection, with a monopile of 5.8m upper diameter and 7.8m lower diameter, the grouted connection would require approximately 20.0cu m of high strength grout, such as MasterFlow 9500.

For pile sleeve grouted connections where OPC is typically used, a single connection may require up to 8.0cu m of ordinary Portland cement (OPC) grout.

Oil and gas

Foundation design for greenfield oil and gas structures very rarely demand ultra-strong bonds; ordinary Portland cement (OPC) is therefore most often used, providing a connection that is more than adequate. OPC is suitable for projects specifying strengths of between 40 and 70MPa; for example, in main leg or skirt pile grouting of platforms or in the filling FoundOcean’s fabric formworks. This tried and tested practice of offshore grouting with OPC is carried out by FoundOcean using its recirculating jet mixer (RJM), which achieves mixing rates of up to 30cu m/hr.

Offshore wind

Ultra-high strength grouts, on the other hand, have numerous applications in situations that require 28-day characteristic strengths of 100MPa or higher. These types of grout are synonymous with monopile transition pieces for offshore wind foundations. Ultra-high strength grouts are occasionally used in life extension projects where they are used to replace OPC in member strengthening projects and in grouted repair clamps.

Ultra-high strength grouts have enhanced properties over OPC, which have been engineered into the product to allow for the dynamic loads and choice of foundation. For grouted connections in offshore wind, FoundOcean primarily uses BASF MasterFlow 9500, for which it is a licensed applicator. MasterFlow 9500, in addition to ultra-high strength, offers a number of pioneering properties, including;

• Zero autogenous shrinkage, a factor that has been shown to cause cracking in high strength concrete structures and de-bonding within connections, provides high volume stability

• Rapid strength build up to support increased installation rates and to guarantee stable structures at an early age reducing the risks from cyclic loading.

• Low heat of hydration, eliminating the risk of thermal cracking.

Significant cost savings can be achieved with the availability of materials, such as MasterFlow 9500, which can provide adequate curing rates at low temperatures. The faster a material cures, the shorter the weather window required to complete the grouting operations. Even at low temperatures, MasterFlow 9500 develops significant strength early on and as such can be applied at lower temperatures compared to similar products. This makes it suitable for use in colder periods and means it can make use of shorter, and more frequent, weather windows. As a result, there is less chance of a project being delayed due to adverse temperatures frequently experienced in the spring and autumn months. This means vessel productivity can be increased and installation times kept to a minimum. Thanks to its impressive properties, MasterFlow 9500 is the first product of its kind to receive the latest DNV GL certification for offshore concrete structures.

However, ultra-high strength materials do not easily lend themselves to the high delivery rates of ordinary Portland cement. Materials that can consistently provide strengths above 70MPa include additives and aggregates which, due to their non-homogenous structure, must be batch mixed to ensure the correct blend of all the components is achieved. Consequently, these materials are stored in bulk bags, which must be batch loaded into the pans for mixing, unlike the RJM’s, which have the advantage of being a closed and autonomous system. Moreover, the bags must be transported in 20ft containers, rather than 100-tonne silos, taking up more valuable deck space.

FoundOcean has developed the super pan mixer, designed to help eliminate grouting as a potential bottleneck when mixing and pumping ultra-high strength grouts offshore. The super pan mixer was the first to enable batch mixed materials, including ultra-high strength grouts, to be mixed and pumped at rates exceeding 10cu m/hr. At the DONG Energy/ScottishPower Renewables West of Duddon Sands Offshore Wind project, in the Irish Sea, this resulted in a time saving of two hours per foundation. For projects the size of West of Duddon Sands (108 turbines) this kind of saving can amount to considerable savings in the vessel’s critical path over the duration of the project.

However, the real challenge has been to develop a material that, although a blend of more than one component, delivers a consistent mix that can be used with a pressurized silo and RJM mixer.

Ongoing development

FoundOcean collaborated with chemicals company BASF to develop the BASF MasterFlow 9800, which launched in February. The material has been specifically developed to meet the demands of the offshore wind industry, where improvements in productivity and efficiency are key for the long term sustainability of the industry.

This new material offers the advantage of being significantly faster, as well as cleaner and safer, than its predecessors. Since the material is able to be used in silos, vessel resupply is more efficient. Moreover, the FoundOcean RJM may be used to provide mixing and pumping rates close to 20cu m/hr. As previously mentioned, RJM spreads are enclosed systems, which essentially eliminates the escape of cement dust into the environment. Moreover, RJMs require minimal support to carry out the grouting operations, compared to the labor intensive pan mixers. In essence, the material is set to revolutionize the client’s choice and expectations with regards to delivery and resupply of grouting materials.

Jim Bell began his engineering career in heavy civil engineering and highway construction at Wimpey Construction Group. He then moved to the offshore division of Wimpey Laboratories. Bell took the role of Managing Director at FoundOcean in 2005, where he has implemented major strategic changes resulting in a significant expansion of the business. Bell holds a degree in civil engineering from London’s City University.