From Concept To Certification

How Subsea Riser Products brought Rocksteady to market.

In recent years, as the average size of floating oil and gas industry facilities has increased and operations have moved to deeper and harsher locations, the environmental loading on mooring systems has necessitated the use of ever larger components. In addition, offshore renewable energy devices, including those for floating wind, wave and tidal power, may feature even higher cyclic loading. So how do engineers gain confidence in the capacity and qualification of the mooring components they select?

Carnegie Wave Energy approached Subsea Riser Products (SRP) with a request for compact subsea mooring connectors with a high bending and tension fatigue capacity after it found there was no suitable product on the market. The request came at a time when SRP’s management had resolved to broaden the company’s focus beyond the high-pressure drilling riser market owing to the variability of demand within that sector.

SRP had already discussed the idea of developing new subsea connectors based on latches it had developed and supplied for riser applications. However, the impetus for developing Rocksteady came through the contact from Carnegie Wave Energy, as John Shield, managing director, SRP, explains. “Carnegie approached us to ask whether the old-style subsea latch featured on our website would be suitable for a specific application. After consultation, we realised that it would not meet their needs and that there was nothing suitable on the market either. This provided the encouragement we needed to develop the new subsea connector product. From the start, Rocksteady was developed with a specific client requirement in mind.”

SRP decided to explore the potential for adapting technology from deepwater drilling and production riser systems and applying it to mooring systems. The thinking behind this approach was that steel risers can be subjected to very high combined loading, so designers often apply high-strength steels and efficient, fatigue-resistant connections to meet the imposed weight limitations and achieve an acceptable fatigue life. Acknowledging that the wave energy market is currently small, SRP decided to design a general-purpose structural connector suitable for a range of subsea applications, including vessel mooring, riser lifting and subsea lifting.

Funding the research and development programme required additional resources. “The work behind Rocksteady was funded by Acteon,” Shield says, “This enabled us to bring a completely new product to the market in a relatively short time frame. The success of the Rocksteady development programme underlines Acteon’s strategy for supporting in house technical expertise, encouraging collaboration and expanding its life-of-field capabilities. As part of the development programme, we worked with Acteon sister company InterMoor to explore the development of mooring connectors. This involved defining the technical requirements, assessing the market conditions and reviewing the potential competitors’ systems in the mooring market.”

Riser and mooring components share similar design influences: a fundamental need for high static strength; weight reduction considerations; performance in terms of connectivity and reliability; and, perhaps most importantly, fatigue resistance.

The Rocksteady mooring connector is designed for use with temporarily or permanently moored vessels (floating production, storage and offloading facilities, rigs, spars and buoys) and specifically to connect mooring lines to pre installed seabed anchors. The system will typically be applied in deep water (over 150 m), so reliability and fatigue capacity were crucial design considerations. Independent testing on behalf of SRP showed that Rocksteady’s preload system offers a fatigue life that is between 17 and 85 times better than chain of the same capacity, despite having a load path that is only 33% larger.

Certification of Rocksteady broke new ground with the classification societies. Shield continues, “Subsea mooring connectors are not subject to extensive codified design and qualification requirements such as the material utilisation requirements and testing regime specified in ISO 13628-7 for riser connectors. As a result, we developed a design and test plan in conjunction with ABS and DNV using technology readiness appraisal, risk-based methodologies and the relevant requirements of riser and mooring codes.”

Following an extensive testing regime that involved fatigue, tension and
break testing, Rocksteady was awarded a DNV certificate for use in the
oil and gas industry, which was quickly followed by its acceptance for
the offshore wave energy project.

The contract with Carnegie Wave Energy covers the provision of subsea foundation connectors, articulating bearings and tether assemblies for Carnegie’s Perth Wave Energy Project, offshore Western Australia. The Carnegie project is itself something new: it will be the first wave power application in the world to operate with multiple wave power convertors in an array and the first to produce both clean power and fresh water. The Perth Wave Energy Project plant will have a peak rated capacity of 5 MW and generate enough electricity for about 3500 households.

The scope of the contract includes three Rocksteady connectors with a minimum breaking load rating of 2000 t. They will form part of articulated bearing assemblies in shallow water 30 m deep. Manufacturing and assembly work was conducted in the UK and mainland Europe, and the equipment was delivered for system integration testing during the southern hemisphere winter of 2014.

Alexandre Pichard of Carnegie adds, “Carnegie selected SRP for the supply of key components for its CETO wave energy units via a competitive tender process that considered proven performance in similar applications, cost and schedule. SRP was very responsive to the key selection criteria and demonstrated an appetite to provide an innovative solution for the project’s requirements.”

The development of the Rocksteady connector is ongoing: SRP is supplying two Rocksteady riser handling tools for Total’s Moho Nord tension-leg platform project in deep water off the Republic of Congo and is developing a versatile subsea lifting tool based on the new connector design.

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