A unique landing string buoyancy design has introduced an improved system by leveraging composite technology. This system, built with 100% composite materials, improves safety in a number of ways. By looking at the challenges presented by offshore casing installation from a new perspective, engineers were able to introduce a technology that shortens the time needed to carry out this stage of the drilling program and allows the installation vessels to work without exceeding their maximum capacity.
History of composites application
Although the oil and gas industry has not been at the leading edge of applying composite solutions, other industries have used composites for decades to improve performance and safety. Among these is the aerospace industry, which has seen the use of composites double every five years since 1987. Composites have changed the landscape of the industry, allowing reduction in airframe weights, better fuel economy and lower operating costs.
The same goes for the marine industry, where more than 90% of the hulls are composite, and the automotive sector, which boasts greater than 90% composite construction on heavy trucks used for long-haul transportation.
Composites are a top choice for construction across industries because of a number of desirable characteristics:
- High-impact resistance;
- Lightweight (with weight reduction ranging from 20% to 50%);
- Long field life;
- Low maintenance;
- Resistance to conductivity, corrosion and fatigue damage;
- Thermal stability; and
- Tolerance to damage.
Composites in offshore solutions
Landing String Solutions (LSS) LLC came up with the idea of using composites in its landing string system at a time when no company in the oil and gas sector had taken this approach to buoyancy for a landing string.
According to Chris von Eberstein, vice president of LSS, “We wanted to provide an option to improve safe rig operations, optimize critical path time and significantly cut cost by reducing the hook load while running heavy casing.”
Part of the reason no one had tried it was because of the requirement to have a product that could contend with wellbore fluid. Nearly all composites used in drilling and production applications had been designed for use in seawater. Drilling mud is a different medium, with varying chemical components and tem-peratures. The challenges in coming up with the right material were considerable, but von Eberstein was con-vinced it could be done.
Antony Croston, business group director at Trelleborg, said his company applied its 40 years of experience in buoyancy systems and an understanding of how environments affect them to identify the mix of materials, additives and treatments to produce the best composite solution. “We looked at how each material failed and how it was degraded during testing to find a material we believed would work, then we set up experiments to test it.”
Trelleborg manufactures raw materials that include hollow glass microspheres that in their original form look like talcum powder but are, in fact, a high-strength glass bubble, Croston said. “It is the same type of composite used in high-performance aerospace applications, and in combination with other components in its finished form, the heart of the landing string system is essentially an extreme service buoyancy syntactic covered by a polyurethane skin.”
Croston explained that exposure tests were carried out on the composite in drilling fluid at specific temperatures and pressures, with the weight of the sample measured before and after. The results were measured against predetermined pass/fail criteria. Cyclic tests followed.
“We would pressurize it for a time and take it out, repeating over time to evaluate cyclic loading,” he said.
When tests results showed the right material had been found, developers knew they had found a solution that when implemented would introduce significant safety advantages in a novel way.
According to von Eberstein, it is important to understand that the composite is the enabler for this system.
“Operators would be reluctant to use a system deployed inside the riser if it had any metal parts because even a small piece of metal falling into the wellbore can do a huge amount of damage,” he said.
“In this particular application, this isn’t a better mousetrap. It’s not a step change. This is a new trap altogether,” Croston said. “This is an enabling technology.”
Safety was the impetus for developing this tool, von Eberstein said. The composite was the essential element needed to create a system that could work inside the riser without introducing risk of damage to the subsea wellhead system and casing, but the overarching goal was to address the risk of the rig exceeding its maximum designed hook load.
“We wanted to provide a tool to keep rigs working within safe design parameters,” he said.
Operators contracting vessels to set casing have to consider the specifications required for each installation. The weight of the casing combined with the landing string weight and the depth of the installation are the primary considerations when determining the necessary hook load, but sea state also is a factor.
The composite-based system creates buoyancy once the casing/landing string is in the riser, which reduces the hook load, decreasing the risk of the vessel exceeding its maximum hook load rating.
“This gives a smaller and cheaper rig the opportunity to do the job safely,” von Eberstein said. “It also means potentially eliminating liner tiebacks and running longer casing strings.”
Continuous operation cuts downtime, which not only improves safety by decreasing the amount of time workers are actively carrying out an operation, but also delivers efficiency gains in terms of contract length.
According to Croston, safety begins with the construction of the composite components in Trelleborg’s controlled manufacturing environment. The individual buoyancy elements and securing clamp components arrive at the LSS site for assembly onto the landing string joints by a LSS specially trained crew. The dressed landing string joints are loaded into transport baskets and delivered to the drilling vessel ready to be deployed in exactly the same manner as traditional landing string pipe. By using quality materials and individual system components and following a controlled assembly procedure at the LSS facility, von Eberstein said, it is possible to ensure consistent results over multiple deployments.
Moreover, they reduce the risk of injury to offshore workers because the units are relatively light. Physically placing them on the landing string requires no heavy-lifting equipment.
“It can be accomplished in one man-lift,” Croston said. Von Eberstein said this product is among those the company considers essential for reducing equipment failure and significant safety incidents in deep water.
“It is extremely difficult to accurately calculate dynamic loading effects when running/lifting casing strings,” he said. “By using our system we can provide a means by which a rig can maintain its margin of safety.”
As the LSS system has proven itself in offshore operations, the team is looking ahead to identify other applications for composites that will make it possible to improve offshore safety through reduced weight, increased wear and improved buoyancy.
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