Vol. 227 No. 9

Engineering the Benguela-Belize compliant piled tower

Fast-track project completes detail design to installation of a bottom-founded production hub in under 26 months.

Colin C. McNeilly, Chevron Corp., Southern Africa SBU, and Steven A. Will, Mustang Engineering

The Benguela-Belize project is an oil and gas production development located in Block 14 offshore Angola, West Africa. It represents the first application of compliant piled tower (CPT) technology outside of the US Gulf of Mexico, where two such structures have previously been installed. In its finished configuration, the CPT is the fifth-tallest, free-standing structure in the world, more than 30% higher than New York’s Empire State Building. Fig. 1.

Fig. 1. The Benguela-Belize compliant piled tower is the world’s fifth tallest free-standing structure.

The fast-track project created a bottom-founded production hub facility for six oil pools in 1,280 ft (390 m) water depth. The Benguela-Belize structure was designed to support 40 wells, 16 risers and an operating payload exceeding 40,000 metric tons. This article focuses on the process for selecting the CPT concept, and the considerations and challenges overcome, while designing and engineering the supporting tower for the drilling and production platform.

CONCEPT SELECTION

Numerous characteristics fostered assessment of several development alternatives. These characteristics included medium water depth at the site, low reservoir pressure requiring water injection, shallow reservoir depths spread over a large area, and crude oil gravity variations from 24° to 38°API. Additionally, the Benguela and Belize oil fields were close to existing infrastructure in Chevron-operated Block 0, as well as an operating FPSO at the Kuito field in Block 14. Options included utilizing the existing Kuito FPSO, adding another FPSO at Benguela – Belize or separating the heavy and light crudes and directing them to separate, purpose-built FPSOs. Additional options considered using Benguela-Belize as a stand-alone facility or combining it with other-facilities. In each of these major themes, engineers explored several feasibilities using studies, cost estimates and economic assessments. Considerations affecting the selection process were:

• Schedule – phased or full-field development
• Number of drill centers
• Number of wells
• Wellhead type – subsea or dry trees
• Type of surface wellhead platform
• Reservoir timing
• Co-mingled or separate processes
• Storage capacity and oil export.

Combinations of these and other variations produced more than 40 development alternatives.

THE CPT DECISION

Based on development concept selection and alternatives analysis, Chevron’s asset and project team recommended a single, primary drill center for development of all the concentrated reservoirs. The selection process determined that the drilling and production facilities would be supported best by a CPT substructure. The cost of the structure would be appreciably less than others, due to its reduced steel content and lower drilling costs.

At the time of the analysis in 1999 – 2000, that cost was $85 million less for the CPT than for the baseline FPSO development. The cost of dry wellhead facilities was also less expensive than a subsea well development tied back to an FPSO. The proposed 20-well count further favored the CPT. Coupled with the decision favoring the CPT, the 24° intermediate crude would be exported via the Kuito FPSO, while the lighter 38° crude would be transported through an export pipeline to the existing Block 0 infrastructure.

Project participation was international. The Engineering, Procurement, Construction and Installation contract (EPCI) for the CPT, topsides production facilities and export pipelines was awarded to Daewoo Shipbuilding and Marine Engineering (DSME) of Korea. Mustang Engineering of Houston was responsible for the design and engineering of the tower and base template. Fabrication of the top tower section was done by Gulf Marine Fabricators, while the bottom section and tower base template were fabricated by Kiewit Offshore, both with fabrication facilities in Ingleside, Texas. The drilling and leveling pile templates and other components were fabricated locally by Sonamet in Lobito, Angola. A separate EPC contract for the platform drilling rig was awarded to German firm KCA/ Deutag.

UNIQUE DESIGN

The Benguela-Belize compliant tower was developed in the project’s front end engineering design (FEED) phase and represents a third-generation of prior designs for Gulf of Mexico developments. While the water depth was approaching the minimum for CPT applicability, the heavy payload and the high well count made the structure ideal for this project. The configuration used a slender, four-leg space frame with a 110 ft by 110 ft, square footprint that supported the topsides, wells and risers, Fig. 2. The space frame structure was supported by 12 flex-legs, attached tubular steel sections that transmitted the gravity, environmental and inertia loads to the foundation piles.

Fig. 2. The 110 ft x 110 ft square tower sections provided positive fabrication working and lifting heights.

The flex-legs also provided the flexibility and the dynamic characteristics to ensure compliant system response. The flex-leg’s uppermost portion was connected rigidly to the tower legs 400 ft below the water’s surface. This allowed shear to be transmitted from the tower to the foundation piles and provided a satisfactory bending period for the tower system.

In order for the flex-legs to provide flexibility, there must be no restriction to the axial motion between the flex-leg and the tower. The design, therefore, included a series of slip-joints or guides along the flex-legs between the upper connection and the foundation pile. The structure required 96 guides with eight levels of 12 guides each.

In addition to its operability features, the tower was designed with fabrication, transport and installation in mind. While studies showed that it would be possible to transport the tower section as a single piece, a large increase in steel tonnage would be necessary. Upending and docking would produce additional challenges. 

Designed in two pieces, the tower adapted to existing fabrication yard skidways and barge launchways, providing easier handling. Fast-track scheduling of fabrication and contracting was more favorable with this design, and the slender footprint had a positive impact on its fabrication, with shorter working and lifting heights.

Because the operator decided to develop the oil fields from a single hub, the Benguela-Belize tower was engineered to support more than twice the payload of the two CPT towers in the Gulf – Petronius and Baldpate (43,500 st vs. 10,000 st and 18,000 st, respectively). Additionally, the Benguela-Belize CPT was designed to accommodate almost double the well counts of its predecessors (40 vs. 18 and 21, respectively).

The tower base template was used in this configuration for the first time, serving to locate and control the verticality of the foundation piles, Fig. 3. This permanent template provided savings over the use of a removable foundation pile template, as well as enhancing tolerance control management. The template was equipped with an integral leveling system including four hydraulic jacks that allowed for the final vertical adjustments, as well as de-coupling of the installed tower from pre-installed leveling piles.

Fig. 3. The nine-foot diameter piles were the world’s largest single-piece offshore foundation piles.

With the large height-to-base width ratio, verticality was an important consideration. Any deviations from vertical installation would transfer an eccentric load to the foundation piles and the tower sections. At the mating interfaces the designers adopted a 0.10° out-of-vertical design parameter as a tolerance. Fabrication tolerances for horizontal diagonal dimensions at the mating surfaces had to be fabricated to within +/- 0.25 in. of the drawing elevations. Tolerances in other CPT areas were kept within typical platform fabrication procedures.

ENVIRONMENT CONSIDERATIONS

Payload drove the tower’s design, as compared to designs for offshore West Africa. That environment is relatively benign with significant wave heights reaching only 11.8 ft for the 10-yr operational condition with long-period swells. Because of the region’s 1-2 hr squall-storm events and the dynamic structure’s sensitivity to high-frequency, wind-velocity fluctuations, Chevron performed extended 1-Hz measurements at the site. These were designed to provide better modeling for the wind force calculations. Dynamic wind structural analyses were also conducted. Engineers performed a number of dynamic analyses with different topsides configurations and Center of Gravity (CoG) envelopes, because of the structure’s sensitivity to deck payload and CoG.

Fatigue life requirements had to be taken into account, since the CPT required designers to consider construction conditions, as well as long-term, in-service conditions. All of the CPT’s components, including major connections, joints girth welds and attachments were analyzed for fatigue under in-place, interim construction and transport conditions. Since the structure is dynamically sensitive to interim construction conditions, a mass replacement system controlled fatigue damage during topside-module setting. The minimum design life was 25 yr.

Designers used the SACS structural analysis program to create a 3D space-frame computer model, which incorporated all main steel conductors, risers and launch truss framing. Another modeling software system, CAP/PCSea Star, helped develop design envelopes for the tower’s dynamic response and generated design load cases for environmental loading conditions. In all, several thousand wind and wave simulations were run.

INSTALLATION

In December 2004, the tower base template was set, along with the 12 main foundation piles. The nine-foot diameter piles were the world’s largest single-piece offshore foundation piles with a maximum wall thickness of 3.75 in., ranging from 590 to 623 ft (180 to 190 m) in length and weighed 915 to 990 short tons. After being driven, each pile extended about 115 ft above the mudline. The remaining tower sections were set in two pieces in April 2005, Fig. 4.

Fig. 4. The tower base section was launched from the H-851 launch barge.

The tower’s top and bottom sections were designed with ballast and trim tanks to ensure that swell conditions would not produce slack-sling conditions during lowering. The mating surfaces of the two sections were also designed with docking cushions on the bearing surfaces between the sections. This compensated for docking forces produced by semisubmersible crane vessel responses to local swell conditions during installation. Once mated, the weight of the installed section compressed the cushions and left a metal-to-metal mating interface. Fig. 5.

Fig. 5. Compliant piled tower awaits the topsides lift.

To assure verticality during installation, the tower base template needed to be level. This was accomplished with a design that included the pre-installation of four leveling piles fitted with field-installed shim sections. Leveling jacks installed in the tower base template provided a contingency for assuring post installation verticality. After the leveling was completed, the 12 flex-legs were grouted to the foundation piles.

That installation was followed by lifting the 10 topsides modules into place. The largest of these lifts was 11,500 short tons.

A TOWERING PERFORMANCE

From a concept selection process that began in late 1998, the Benguela-Belize project produced first oil on January 22, 2006. The platform’s capacity is 220,000 (60,000 b/d intermediate grade and 160,000 b/d light oil), 330 MMcfd and 420,000 bopd water injection.

Design and build schedules of deepwater structures can exceed 30 months, as did prior CPT projects. The schedule for the Benguela-Belize project, from detail design to installation, was less than 26 months, including transatlantic transport of the tower sections and despite the numerous challenges encountered. Approximately 6,500 people were involved in the project. 

THE AUTHORS
	Colin C. McNeilly earned a degree in civil engineering from Fresno State University and is a Registered Professional Engineer in the State of California. He has 25 years’ experience in design, construction and project management of oil and gas production facilities. McNeilly served as the engineering manager for front-end engineering of Chevron’s Benguela-Belize compliant tower drilling and production platform in Cabinda, Angola and was the fabrication and installation manager for the Benguela-Belize facilities.
	Steve A. Will is a graduate of Purdue University and manager of Compliant Structures at Mustang Engineering where he was project manager for the design of the Benguela-Belize compliant piled tower. He has 35 years experience in offshore engineering and construction. Will’s background includes project management responsibility for some of the world’s tallest bottom founded structures for which he is considered an industry expert.