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Innovative Design and Execution Lead to Successful Grand Banks Platform Operations

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The Hebron platform was successfully installed on the Grand Banks offshore Newfoundland and Labrador in June 2017. It consists of a single-shaft concrete gravity-based structure (GBS) supporting an integrated drilling and production topsides. The design of the platform was challenged by sub-Arctic and extreme metocean conditions that required innovative design and layout approaches for many elements considered routine for typical platforms. This complete paper highlights the underlying technologies, analytical and design methods, and capital-efficient execution strategies employed.

Introduction

The Hebron platform comprises a GBS and topsides installed in 93-m water depth. Produced crude oil is stored in storage cells and pumped to shuttle tankers by an offshore loading system.

The topsides facilities (operating weight 65,000 tonnes) include the following major modules:

  • Utilities and process module (UPM)
  • Derrick equipment set (DES)
  • Drilling support module (DSM)
  • Ancillaries (flare boom, east and west liftboat stations)
  • Living quarters (LQ) designed to accommodate 220 personnel during steady-state operations

The total height of the platform with GBS is approximately 235 m, topsides length is approximately 183 m, and the width is approximately 75 m. The GBS concept includes a single shaft supporting the topsides and encompasses all wells in the initial development. It is designed to withstand sea ice, icebergs, and other meteorological and oceanographic conditions at the offshore Hebron site. The GBS is designed for an in-­service life of 50 or more years to support future developments.

The lower portion of the GBS up to an elevation of approximately 27 m was constructed in a drydock created by building a bund wall and dewatering the site behind it. Subsequently, the dock was flooded, the bund wall was removed, and the GBS base was towed approximately 3 km to a deepwater site. The floating GBS was held in place with mooring lines while the remaining construction was completed. All walls were constructed using the slipforming technique.

The topsides structure was fabricated in modules at various locations in Newfoundland and Labrador and South Korea. Two of the topsides modules, UPM and DES, were fabricated in South Korea using block/pancake construction methods (Fig. 1). After UPM and DES fabrication was completed, the modules were loaded out separately to heavy vessels for transportation to Bull Arm, Newfoundland and Labrador. All other topsides modules were integrated with the main UPM module at the finger pier in Bull Arm and the completed topsides structure was mated with the GBS while floating at the deepwater site. The mated platform then was towed offshore and installed at the final offshore location.

Fig. 1—Hebron topsides block division for construction.

Design Considerations

Facility Optimization. Because the platform is located in a sub-Arctic climate with less extreme temperatures than other true Arctic environments, the topsides was designed with open architecture and strategically placed blast walls that maximize natural ventilation while providing fire and blast protection for key safety-critical components.

The single-shaft configuration drives the placement of all necessary fire­water, seawater, and crude-oil booster pumps in the same GBS shaft as the well bay. Thus, a pump system that is mounted and retrievable from the topsides is required. This, coupled with the required air gap height of 30.4 m between the underside of the topsides and the mean sea level, drove the project to employ submersible seawater pumps among the longest in the industry (46.8 m) and longest-in-industry line-shaft crude booster pumps (47.4 m) that lift crude from the GBS storage cells to topsides for offloading to a shuttle tanker.

The project identified and employed a passive exhaust plume cooling and dilution system that contains no moving parts and reduces the output exhaust temperature by up to 50%. This allows drilling to proceed unimpeded during unfavorable wind conditions without personnel limitations on the derrick during early field life. Additionally, the design geometry of the coolers allowed for reduced wall thickness when compared with traditional exhaust stacks, resulting in a lighter supporting structure.

A survey of icebergs indicated that a significant percentage of icebergs in the Grand Banks are drydock and pinnacle icebergs, which tend to have high sail heights that could collide with an overhanging deck of the topsides structure. Unlike the case with the concrete GBS, it is cost-prohibitive to design the topsides structure for iceberg-impact load. Design codes require the use of an air gap to achieve 10–5 probability of iceberg collision with topsides and permit iceberg management (using supply vessels to tow icebergs away) in determination of iceberg-to-topsides collision probability. In order to meet the code requirement of 10–5 annual probability of impact, a 30.4‑m air gap was selected and the LQ module was raised vertically an additional 10.8 m.

Integrated Seismic Soil Structure

Interaction (SSI). The implementation of an integrated seismic SSI proved effective in performing seismic analy­sis and design of all components of the platform through consecutive project phases and various contractor structural models.

This approach enabled contractors to perform their own separate SSI analyses using the same foundation properties developed by the geotechnical contractor, resulting in a more-efficient design process with different levels of detail according to the component of interest.

Foundation Optimization. The Hebron GBS has a flat, circular base slab with 500-mm-deep soil skirts. From a constructability perspective, shorter skirts are cost-effective because they require shallower trenches in the drydock construction site (less construction work), minimize the negative effect of skirt length on the draft requirement during tow, and are subjected to smaller stresses during soil penetration at installation. Analysis of environmental and design conditions revealed that both scour protection and underbase grouting were unnecessary for the platform.

Concrete Crack-Width Calculation. A new method for calculating crack width in thick elements with multiple reinforcement layers was implemented in the GBS design. This method accounted for the crack-initiation effect from several layers of transverse reinforcement not fixed or welded to the main reinforcement, and was validated by a parametric study using nonlinear finite-element analysis, an approach used at other points in the design process for the platform. The new method did not require any reinforcement beyond that needed for the ultimate limit state, which improved constructability.

Execution Considerations

Comprehensive Weight Control. Developing a robust and comprehensive weight-control program based on the stages of project execution was essential to maintaining cost and schedule. Clear budgets for the phases were developed, and each module was weighed before transport and the result input into the weight-management program and analytics. The project team then leveraged the actual results to capture additional weight opportunities as the project progressed.

Full UPM-Use Initiative. Considering the importance of locating the Canadian integration workforce as close as possible to work fronts during the critical integration phases of all modules at Bull Arm, most of the traditional storerooms, offices, and workshop spaces in the topsides were outfitted with temporary lunch and breakroom spaces. As a result, during the integration phases, breaks could be taken topside, allowing reduced transit time.

Concrete Batch Plants. Two identical, independently operated, fully automatic batching plants were used for concrete production at drydock and deepwater-site locations. This 100% redundancy execution strategy was implemented to improve execution certainty during the most-demanding concrete pour.

Steel-Panel Bulkheads for Base-Slab-Construction Joint. To optimize the construction schedule for the GBS base slab, cost-effective vertical steel-­panel bulkheads with horizontal corrugations were used as formwork between the various sections of base slab (poured in four sections). The corrugated vertical steel panels were designed to be left in place and detailed to ensure leak-tightness. Compared with conventional formwork that would require stripping and surface preparation after setting of the concrete (very time-consuming for thick base slab with many layers of dense reinforcement), the use of left-in-place formwork resulted in a shorter construction time.

Slipforming of All GBS Walls. Slipforming technique (i.e., formwork panels that continuously move upward using hydraulic pumps and yokes) allows uninterrupted concrete placement, ­reinforcing-bar installation, and minor surface repair. Slipforming allowed cost-effective placement of walls with high reinforcement density, minimized the construction schedule, and improved leak-tightness because most construction joints were eliminated.

Full-Scale Mockups. Small parts of the GBS were built as full-scale models using the same procedures, equipment, and materials planned for use in the actual structure. In addition to regular and early review of design drawings, full-scale mockups of several complex elements of the GBS were conducted, and lessons learned were supplied to the design team. The mockups also allowed the opportunity to train the workforce on these complex operations before actual construction.

Solid Ballast. To improve floating stability during tow and to improve sliding resistance after platform installation, 222,000 tonnes of solid ballast with a specified density of 3500 kg/m3 (approximately 10-m thickness) was placed at the bottom of the oil-storage and annulus cells. All solid ballast was placed at the deepwater site with concrete pumps. This avoided extra costs associated with offshore installation of solid ballast.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 28803, “Hebron Platform: Innovative Design and Efficient Execution,” by Widianto, Justin Chichester, Adel Younan, and Jameel Khalifa, ExxonMobil; Krishna Komperla, WorleyParsons; and Knut Bidne, Kvaerner, prepared for the 2018 Offshore Technology Conference, Houston, 30 April–3 May. The paper has not been peer reviewed. Copyright 2018 Offshore Technology Conference. Reproduced by permission.

Innovative Design and Execution Lead to Successful Grand Banks Platform Operations

01 September 2019

Volume: 71 | Issue: 9

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