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Optimising Seawater Intake for Efficient Offshore Operations

In McKinsey’s latest report, LNG growth is predicted to continue to rise at around 3% a year for the next 15 years, and infrastructure development is racing to keep up.

In light of this, FLNG projects are also predicted to gain momentum, alongside a steady stream of announcements for planned FPSO operations, driven by the offshore benefits such as shorter construction times, cost savings and mobility to name but a few. However, in order for such benefits to be fully realised, all aspects of operations need to be streamlined, with seawater intake an often-overlooked component. Heat generation by gas compression and liquefaction must be mitigated by seawater intake risers (SWIRs), moving huge amounts of cold seawater onto production platforms. The method of installation and materials used need to be carefully considered in order to optimise operations, without compromising on safety or efficiency.

The attraction of CAPEX savings can lead to production projects making requests for hybrid SWIRs, as opposed to rubber hoses. These hybrid hoses are comprised of HDPE plastic and rubber hose technology, typically presenting a saving of anywhere between 20-30%. While an alluring proposition, it is important to be mindful of the limitations of these mixed material composites.

Rubber has a higher density than plastic, so when it comes to requests for hoses to sink deeper in order to access cooler water for increased cooling efficiency, there are buoyancy issues to contend with. Meeting these demands requires weighting of the HDPE riser to enable the strainer to access the lowest depths possible. This combined with additional ballast weighting to combat current and wave interaction poses some significant risk to assets. If the pipes break due to the increased stressors on the riser, assets on the seabed and vessel are compromised. In tandem, the lack of information on fatigue behaviour of HDPE risers, combined with the added complexity in their configuration for greater depth of operations, equates to a high number of undeterminable risk factors.

Installation of a typical SWIR is not always straight forward. Hoses are attached together at the hull using a temporary clamp and fitted to a spool, before being lowered, flooded and submerged, with divers finalising the installation by tightening bolts and attaching it to a platform.

Cost implications can be amplified further as future maintenance also requires diver intervention. By using a caisson to insert each section of hose, clamping, and lowering into the sea via a crane, then connecting the first off hose to a head riser, the use of divers can be avoided.

Taking all of these challenges in its stride, Trelleborg has designed and developed a rubber hose solution, that enables deep sea access without compromising on asset safety, along‑side a diver optional-installation design. Using an integrated bending stiffener with a continuous inner liner, SWILINE has the structure to withstand high tensile stresses and bends and can be installed vertically from the FLNG deck without a diver using a caisson as outlined above. Making use of their deep sector expertise while listening to market challenges, SWILINE represents a complete, easy to install alternative to solutions requiring divers, while also prioritising operators assets and safety, circumventing the issues met by HDPE hybrids, culminating in a workable solution to the rapidly increasing demands for LNG globally.

Author: Vincent Lagarrigue, Director of Trelleborg Oil and Marine

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