A deeper look at existing field infrastructure

Oil and gas business leaders in West Africa have had to think smarter in recent years as they work against global financial conditions. They understand the importance of driving growth in a region which has still not reached its potential. The aim must be to make use of technologies to overcome the limitations of existing infrastructure, both on land and in offshore hydrocarbon fields.

A 2017 report by professional services multinational PwC provided greater insight on the issues facing African business and solutions being deployed. The ‘Learning to Leapfrog, Africa Oil and Gas Review’ highlighted the ways technologies are enabling operations and business strategies to be improved with cost-effective investments.

The report outlined that Africa’s share of global oil and gas production dropped from 9.1% to 8.6% in the year to 2017, but that technologies will be essential to reversing this downward trend. PwC experts suggested that African nations can use new technologies as enablers to overcome traditional challenges faced on the continent, such as logistics and infrastructure, and be competitive globally.

For those working in key oil and gas producing nations, such as Angola and Nigeria, it is crucial to consider how technologies and services will impact the current economic landscape.

The use of conductor supported platforms (CSP) is allowing production to be increased faster than would be possible otherwise and is providing a reliable service from smaller, more agile fabrication yards. UK headquartered Aquaterra Energy’s CSP, Sea Swift, has demonstrated logistical, technical and operational benefits in allowing customers to stay ahead of the technology curve in West African locations.

CSPs

In marginal, shallow water developments, the fabrication and installation of CSPs is increasing. In West Africa, there is growing demand for cost-efficient wellhead platforms to enable increased production capacity within existing field infrastructure.

Compared to traditional platforms and subsea trees for shallow water development projects, CSPs have shorter delivery times, reduced intervention and platform costs, and simplified critical path. Its main financial and technical attributes are borne from its ability to combine the use of the same jack-up to install as well as drill at site.

CSPs, which have also been used in shallow water locations in South East Asia and the Mediterranean, have been used to accelerate first oil in Angola and Nigeria.

Their deployment is increasing due to growing demand for modularised wellhead platforms, which can be built and installed in smaller, discreet packages across a number of fabrication yards. The CSP platform allows for flexibility of installation, allowing the parts to be shipped by a supply vessel and fixed in place by a standard cantilever jack-up rig, without additional heavy lift installation vessels or the use of small crane barges, lift boats or shear leg cranes. This flexibility is important in frontier regions as it can make use of whichever equipment is available in the region.

Bespoke designs also negate the need for diver or ROV involvement and any hot work which mitigates inherent risk and scheduling barriers, while cutting incremental costs. In addition, it can also potentially increase local content if this is an economic or political driver for the project.

Aquaterra Energy has been delivering its own minimum facility CSP solution called Sea Swift for over 10 years. The use of the company’s own complex structural engineering modelling software and analysis is used to determine system strength and stability and design fatigue life of the Sea Swift with location specific metocean and geotechnical data. It combines the advantages of a platform with the rig-run benefit of a subsea development to achieve lower capital and installation/intervention costs.

Taking CSPs deeper

The benefits of rig-installable CSPs often outweigh those of traditional platforms and subsea trees for shallow water development projects.

The Sea Swift has been installed across a number of shallow water locations, including three in West Africa, one in Egypt, one in the Far East, and the most recent Sea Swift was installed offshore Trinidad and Tobago in December 2017. Its deepest deployment to date is at 65 m water depth using two subsea structures, offshore Peninsular Malaysia. This Sea Swift is also the largest known CSP in terms of topside weight at more than 400 t fully laden. As a tried and tested concept, the maximum water depth is currently around 80 m with conceptual engineering assessments underway to increase this capability beyond 90 and even 100 m. This investigation will also consider multiple installation options.

Figure 1. Sea Swift prior to installation in Trinidad

The company designed, fabricated and installed a new Sea Swift platform for PICO Petroleum Integrated Services, the lead contractor for Amal Petroleum Company’s (AMAPETCO) Amal field in the Gulf of Suez, offshore Egypt. The platform which was installed in November 2016 is in 23 m water depth in the Amal-C field and includes a 385 t topside featuring a helideck and emergency accommodation with provision for six wells. The design provides a lower cost option which can be delivered significantly faster than traditional platforms: from concept to completion in only 18 months. It can also rapidly increase production from platforms constrained by existing slots and in other applications, allows wells to be drilled, completed with dry trees and installed before the arrival of the main processing platform. The project involved building a bridge link to the neighbouring Amal-B platform and reconfiguring the topside pipework to create a new and improved production profile. The Egyptian project also supported jobs in the local area at Alexandria and Zeit Bay yards.

The latest platform was installed in the Gulf of Paria, offshore Trinidad and Tobago. It was installed at 27 m water depth and accommodates up to four wells and includes local power generation, manifolds and a control system. The platform was designed, fabricated and installed in a 10 month period and drilling of the first well is ongoing and near completion.

Figure 2. Sea Swift Trinidad in place.

Delivering timely flexibility

An example of the flexible modular, multi-site approach that Sea Swift allows was when a platform was installed in the Sèmè field in 26 m water depth offshore the Republic of Benin. Commissioned by South Atlantic Petroleum Benin S. A. (SAPETRO), the lightweight platform was fabricated in Tunisia and installed by a jack-up drilling rig, negating the need for a heavy lift vessel. It connects five wells to an onshore processing facility. While a Sea Swift topside would normally be installed by the jack-up completing the drilling operations, an available pipelay vessel was used for the Sèmè field topside installation. This removed the lift weight constraint caused by the jack-up lift and skidding capacity required for the installation.

It consists of a 200 t integrated deck topsides, riser guides, boat landing, and a 121 t subsea jacket structure. Equipment is controlled from the onshore facility via an integrated fibre optic communication and power cable. The free-standing Sea Swift structure uses four 30in. well conductors tied together by the subsea structure. This provides structural support to the topsides, while also housing the well casings. Production is controlled from an onshore facility at Cotonou, Benin’s largest city, via an integrated fibre optic communication and power cable.

Supporting field operations

As the conductors provide structural support to the topsides through axial compression and bending resistance, and support the well casing, wellhead and surface tree, the total conductor length is defined by the water depth at the platform location, the topsides elevation, and the foundation setting depth. Subsea support structures are used to provide rigidity to the conductors and extend design life through the reduction of fatigue. The weight of the proposed topside also needs to be considered when designing for deeper depths as this will play a considerable part in limiting the facilities available and installation techniques.

Sea Swift essentially uses the well’s environmental conductors as primary structural members to support the weight of the platform’s topsides and associated equipment. Each individual platform substructure is a one-off design to suit varying parameters such as water depth, soil structure, and sea conditions, particularly wave frequency. These design considerations must be considered at any depth to maximise fatigue life and steadfastness of the entire structure, which can normally be left in situ for up to 25 years. Due to potentially complex environmental conditions, fatigue is one of the main design drivers for a CSP, as with all dynamically sensitive offshore structures.

Aquaterra’s in-house structural engineering modelling software and analysis experience is used to demonstrate an acceptable structural performance for the CSP in terms of offshore constructability, in-place strength and stability, dynamic response, fatigue endurance and seismic design based on the soils, metocean data and equipment loads. This dynamic behaviour must be captured and understood to allow the structure to be designed and comply with the environmental loading applied. A review of fatigue design elements such as cathodic protection of conductors/piles and weld improvements will enable a maximum fatigue design life to be achieved. An assessment of seismic activity, which is common in many locations, must also be carried out to determine seismic loads and additional reinforcement options, such as skirt piles, to create a more robust and secure foundation.

Making deeper installations possible

In its West African deployments, Sea Swift has demonstrated its benefits in shallow water environments, providing the same fast and effective production as other global locations. It has quickly proved its financial viability due to favourable market conditions and strategic local service agreements.

Figure 3. Sea Swift with jack-up in Malaysia. 

As the price of fabricated steel has fallen, so the straight cost differential between a conventional jacket and a Sea Swift is also falling. However, the overall cost savings really come to the fore when using smaller and more agile fabrication yards and a jack-up for installation ensuring simpler project management and reduced risk. This, alongside its potential to reach deeper depths, has meant that in today’s cost-constrained climate, the CSP is quickly becoming a more financially viable option for fast and effective production in marginal, shallow water developments.

CSPs will evolve to suit market, regional and materials conditions, but for now they are proving to be a viable solution, as highlighted by recent work in West Africa and other locations.

Read the article online at: https://www.oilfieldtechnology.com/special-reports/25042018/a-deeper-look-at-existing-field-infrastructure/