淺海開發省錢新招：隔水套管支撐平臺抵達新深度

Suitable for use in benign, shallow-water locations such as West Africa, Southeast Asia and the Mediterranean, conductor-supported platforms (CSPs) are seeing growing demand because of a need for speedy, efficient fabrication and installation in the race for first oil. Compared to conventional jacketed piled platforms or subsea trees for shallow-water development projects, CSPs boast shorter delivery times, reduced intervention and platform costs, and simplified project management. The CSP’s main financial and technical attributes are borne from the ability to combine the use of the same jackup rig to install the CSP as well as drill at site.

The deployment of CSPs is increasing because of growing demand for modularized wellhead platforms, which can be built and installed in smaller discrete packages across several fabrication yards. The CSP allows flexibility of installation, allowing the parts to be shipped by a supply vessel and fixed in place by a standard cantilever jackup rig without additional heavy-lift installation vessels or by the use of small crane barges, lift boats or shear leg cranes—whatever is available in the region. The bespoke design also negates the need for diver or ROV involvement and any hot work, which mitigates inherent risk and scheduling barriers while cutting incremental costs. In addition, the CSP can potentially increase local content if this is an economic or political driver for the project.

Minimum-facility CSPs

The benefits of rig-installable CSPs often outweigh those of traditional platforms and subsea trees for shallow-water development projects. Aquaterra Energy has been delivering its own minimum-facility CSP option, Sea Swift, for nearly a decade. The use of the company’s own complex structural engineering modeling software and analysis is used to determine system strength, stability and design fatigue life of the Sea Swift with location-specific metocean and geotechnical data. Sea Swift combines the advantages of a platform with the rig-run benefit of a subsea development to achieve lower capital and installation/intervention costs.

Sea Swift has been installed across several shallow-water locations, including three in West Africa, one in Egypt and one in the Far East. Recently, Aquaterra Energy announced it had designed, fabricated and installed a new Sea Swift platform for PICO Petroleum Integrated Services, the lead contractor for Amal Petroleum Co.’s Amal Field, in the Gulf of Suez offshore Egypt. Another is underway offshore Trinidad and Tobago.

Its deepest deployment to date is at 65 m (213 ft) water depth using two subsea structures offshore Peninsular Malaysia. Sea Swift is also the largest known CSP in terms of topside weight at more than 400 tonnes fully laden. As a tried and tested concept, the maximum water depth is currently about 80 m (262 ft), with conceptual engineering assessments underway by Aquaterra to increase this capability beyond 90 m (295 ft) and even 100 m (328 ft). This investigation also will consider multiple installation options.

Sea Swift was recently installed at 23 m (75 ft) in the Amal-C Field offshore Egypt and includes a 385-tonne topside featuring a helideck and emergency accommodation with provision for six wells. The design provides a lower cost option that can be delivered significantly faster than traditional platforms—from concept to completion in only 18 months. It also can 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 Egypt project also supported jobs in the local area at Alexandria and Zeit Bay yards.

Sea Swift construction also is underway for a field offshore Trinidad and Tobago. It will be installed at 27 m (88.5 ft) water depth; will accommodate up to four wells; and will include local power generation, manifolds and a control system. The design project, which is anticipated to last up to six months, will overlap with the fabrication to meet the tight delivery timetable.

In 2014 a Sea Swift platform was installed in the Sèmè Field in 26 m (85 ft) 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 jackup drilling rig, negating the need for a heavy-lift vessel. It connects five wells to an onshore processing facility. Although a Sea Swift topside would normally be installed by the jackup 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 jackup lift and skidding capacity required for the installation.

The vessel consists of a 200-tonne integrated deck topsides, riser guides, boat landing and a 121-tonne subsea jacket structure. Equipment is controlled from the onshore facility via an integrated fiber-optic communication and power cable. The freestanding Sea Swift structure uses four 30-in. 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 fiber-optic communication and power cable.

Early analysis, assessment

Since the conductors provide structural support to the topsides through axial compression and bending resistance as well as supporting the well casing, wellhead and surface tree, the total conductor length is defined by the water depth at the platform location, topsides elevation and 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, in particular wave frequency. These design considerations must be taken into account at any depth to maximize 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 modeling software and analysis expertise 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 behavior 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.

Pushing the design envelope

As the price of fabricated steel has tumbled, so has the straight cost differential between a conventional jacket and a Sea Swift. However, the overall cost savings really come to the fore when using smaller and more agile fabrication yards and a jackup 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.