Harsh environment jackup rig design must balance multiple criteria, from climate to reservoir characteristics. Careful attention to these factors can maximise the productivity – and marketability – of the rig. John Knowlton reports.
It has been a truism in the energy industry for the last decade that the easy oil is gone. Recent discoveries have expanded our view of the world’s potential recoverable reserves, but tapping those resources demands increasingly complex technology.
For offshore oil and gas, advances in rig design are an essential piece of the solution. While much of the attention has focused on deepwater vessels, shelf exploration is also facing technological challenges that require new approaches to jack-up rig design.
In developing the specifications for its most recent series of newbuild harsh environment jack-ups, Ensco looked for solutions to two major hurdles in order to expand the 'area of operability' of the vessels: environmental resistance and drilling capability. The goal was to expand the potential market footprint while maintaining a cost-effective design approach.
In the North Sea, area of operability (based primarily on water depth and environment) is a frequent measure of functionality, as shown in Fig. 1. In the intense wind and wave action that typifies the North Sea climate most of the year, operating in 400-foot water depths is a significant step forward from operating in 350-foot water depths – the current limit for most harsh-environment jack-ups. Ensco, however, broadened the area of operability concept to a global scale, to look for the 'sweet spot' in design specifications that would create the maximum geographic range for the rigs with the greatest economic value for both Ensco and clients.
As a starting point, Ensco used the Keppel FELS Super A Class rig, designed for the harsh environment of the UK sector of the North Sea.
Environmental capability
Harsh-environment jack-up designs must adequately address the issues of low service temperatures and the forces of wind, waves and current.
Environmental resistance is a critical factor in the North Sea, but also essential in the continental shelf areas of the Gulf of Mexico, Indian Ocean, and Western Pacific Rim, where tropical storm systems create extreme weather threats.
The Keppel FELS Super A Class rig is hardened to withstand -20°C service temperatures in order to meet the requirements of operating in the northern areas of the UK sector. Not only must the hull and legs be constructed of appropriate low-temperature steel alloys to avoid embrittlement at low temperatures, but also the equipment, from derricks and cranes to drawworks and jacking systems, must meet the same specifications.
However, in keeping with the requirements of a rig with true world-wide capability, Ensco specified the rig’s systems to handle anticipated extremes of temperatures from the Northern Central North Sea winter to the Arabian Gulf summer.
Rough seas require rig design and construction techniques that can withstand the load from wind-whipped waves, whether the sea conditions are a fairly constant phenomenon, as in the North Sea, or less frequent but potentially worse, the hurricanes in the Gulf of Mexico and typhoons in the Asia/Pacific Rim area.
As water depth increases, rig stability becomes more complex. With its 156-foot transverse leg centres and 150-foot longitudinal leg centers, the Super A is designed to operate in depths up to 400 feet. To achieve this 400-foot capability, the rig has been equipped with a total leg length of 540 feet.
The inherent strength of this ultra-enhanced rig also makes it well-suited for hurricane survivability. Ensco’s assessment shows that the rig could withstand a 300-year storm at a specific location in the Gulf of Mexico, with 138-knot winds (Category 5 storm) and 91-foot waves.
Drilling capacity
Drilling capacity has become a more critical design issue with a number of new shallow-water projects targeting ultradeep gas reservoirs, notably in the US Gulf of Mexico and Australia. Reaching these fields at depths of more than 30,000 feet requires a more robust hoisting capacity and an efficient derrick. The Keppel FELS Super A design, with a 2 million-pound hook load, is capable of drilling up to 35,000 feet. To expand the capability, Ensco specified a 2.5 million-pound hook load, which allows a drilling depth of up to 40,000 feet, and a derrick with automated quad racking capacity for efficient pipe-building.
Multi-well programmes can be especially challenging for jack-ups in harsh environments, due to the need to sometimes reposition the rig to reach every well position in a development plan, in an environment that can mean long waits for suitable weather to jack down and perform said repositioning. Ensco enhanced the rig design with a patented cantilever system that expands the cantilever load allowing nearly the full 2.5 million pound live load to be handled at an 80-foot reach and 17-foot offset, allowing for larger developments without repositioning.
Outcome
By analysing and specifying design requirements from the perspective of “area of operability,” Ensco developed a rig that approaches ultra-harsh environment status at a more cost-effective level, with the flexibility to operate not only in a wider footprint within the North Sea, but in other environments that offer wind, weather and reservoir challenges. The first two of the ENSCO 120 Series rigs are being delivered this year and are contracted to work in the North Sea.
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John Knowlton is Senior Vice President – Technical with Ensco plc, a UK-based global offshore drilling contractor. www.enscoplc.com
Design Parameters for ENSCO 120 Series Harsh Environment Jack-ups
Variable Deck Load (VDL):
Normal operating : ranging from 8,200 kips to 14,300 kips depending on water depth and environmental condition
Storm operating : 9,100 kips in water depths from 328 ft to 400 ft
Storm survival: 6,500 kips in water depths from 328 ft to 400 ft
Normal jacking: 8,100 kips
Emergency jacking: 40,200 kips (nearly 100% of preload in addition to full normal jacking VDL)
