Over the last 30 years, offshore oil and gas developments have moved into increasingly deep, remote and technically demanding regions. However, while riser system development has not been static – hybrid riser systems have gone some way to help exploit these more challenging plays, for example – in the persistent lower for longer price environment, there is continuing pressure to develop these fields safely, while reducing capital (Capex) and operating expenditure (Opex).
In short, a technological step change is needed to open up less accessible or currently economically cost-prohibitive fields.
Field development challenges
The development of challenging oil and gas fields with minimal or no infrastructure typically requires the deployment of floating facilities, together with storage capabilities. Here, the floating, production, storage and offloading vessel (FPSO) is the preferred option, accounting for around 70% of the floater market.
Semi-submersibles, spar platforms and tension-leg platforms (TLPs) are also common in deepwater regions. TLPs are particularly suited for water depths less than 1500 m. But FPSOs and floating liquefied natural gas vessels (FLNGs) have the distinct advantage of having the required storage and the capability to directly offload to tankers, while other floating production units (FPUs) — such as semi-submersibles, TLPs and spar platforms — need a separate storage vessel or existing infrastructure to export production to shore.
However, there are substantial limitations with conventional FPSOs and FLNGs that, if overcome, could considerably broaden their reach and lead to a significant reduction in field CAPEX and OPEX.
• Dry-tree applications
Using top tensioned risers (TTR) for direct vertical access (DVA) to production wells has established technical and economic advantages.
DVA ensures all drilling and completion can be carried out from one floater: enhancing reservoir recovery and mitigating flow assurance issues, while reducing drilling and completion costs.
However, the stroke limitation of the existing qualified tensioner technologies means TTRs require a floating system with minimal heave response, so they cannot be hosted on FPSOs. As such, in dry tree applications where storage is required, a wellhead platform is typically used along with an FPSO: adding significantly to overall field development costs.
• Wet-tree applications
In wet tree applications, steel catenary risers (SCR) are considered the most robust riser solution because of their simplicity, long lifecycle and extensive in-service history, as well as relatively low combined CAPEX and OPEX cost. However, the high dynamic motion of FPSOs makes them unsuitable for SCRs, apart from in the most benign environmental conditions such as the West of Africa.
As such, less robust and more complex and expensive riser systems, such as flexible risers, or hybrid riser towers, are typically adopted with these vessels in more challenging environments.
The riser-friendly floater reducing field development costs
INTECSEA’s low motion (LM) floater technology is set to significantly reduce field development CAPEX and OPEX. This robust and low-tech solution comprises field-proven components that have been in service for more than 20 years, and can be applied to new-build or conversion floaters: FPSOs, FLNGs, and semi-submersibles.
Fundamentally for FPSO and FLNG applications, the technology vastly improves field economics through the ability – for the first time – to deploy these vessels with large diameter SCRs and TTRs in remote, challenging environments. As well as reducing costs by eliminating the maintenance turret and swivel system in harsh environments.
In addition, a conventional semi-submersible hull with four columns and a ring pontoon can adopt LM technology to become a suitable host for dry tree and TTR applications in deep and ultra-deepwater fields.
The technology is less sensitive to weight and vertical centre of gravity variation during construction, historically one of the highest risk elements on the project execution cost and schedule. It maintains quayside integration, and its high stability and superior motion response on site means reduced deck steel, piping weight and enhanced habitability, crew safety and helicopter operability.
Demonstrating the superior performance of LM technology
Initial work has focused on demonstrating the superior performance of the LM technology when applied to a box-shaped hull (LM-FPSO). The LM-FPSO is an ingenious design developed to captialise on the advantages of conventional FPSOs — on board storage and offloading capabilities, high payload suitability, and quayside integration – while offering superior motion response suitable for using SCRs and TTRs in very harsh environments. The shape of the LM-FPSO hull allows for the use of conventional mooring systems, eliminating the need for an expensive turret and swivel system.
A five-week model testing programme performed in collaboration with Korea Research Institute for Ships and Ocean Engineering (KRISO) in November last year found, the LM-FPSO’s motion response was comparable with that of a TLP, and better than a spar.
Consistently, in case studies, the design demonstrated an economic advantage in both wet and dry tree applications, with Capex savings estimated to be between US$500 million and US$1.2 billion over alternative conventional designs.
- Wet tree West of Shetland – saved US$800 million Capex by eliminating the turret and using SCRs.
- Dry tree in South China Sea – saved US$660 million Capex enabling the use of TTRs and eliminating need for a wellhead TLP.
- Dry-tree West of Africa – saved US$1.2 billion Capex using TTRs: eliminating a wellhead platform, and facilitating well completion and workover from the FPSO.
The challenge for the industry is whether it can reconcile the seemingly incongruent requirements to develop fields in increasingly greater depths and hostile conditions, while — at a significantly reduced cost – deploy robust riser systems that can safely drill, produce and transport fluids between the seabed and floating platforms.
INTECSEA modelling estimates that overall field development CAPEX savings of up to 19% are achieved when comparing a conventional FPSO with the wet tree LM-FPSO with SCRs, and a dry tree LM-FPSO with TTRs.
In short, INTECSEA’s low motion capability potentially opens up a wealth of prospects around the world, stretching from marginal fields in the UK continental shelf, to the massive gas basins of the Antipodes: a truly enabling technology.
Read the article online at: https://www.oilfieldtechnology.com/offshore-and-subsea/27042017/low-motion-technology-that-transforms-the-reach-of-floating-production/