Rethinking seawater Intake for floating production operations

Published:  01 November, 2019

By Vincent Lagarrigue, Trelleborg Oil & Marine

Over the next few years, the FLNG market is projected to witness significant gains, against a backdrop of increased global gas consumption, energy security programs and a paradigm shift in marine fuel preference. In light of this, FLNG projects are anticipated to total 52.8bn USD between 2019-2024. At the same time, more than 60 planned and announced FPSOs are expected to start operating by 2025. Six of these were ordered from global shipbuilders in 2017, and a further 11 were announced last year – significant figures, given that in 2015 and 2016, no new orders were made.

Reduced construction timelines, environmental benefits, cost savings and mobility only add to the growing popularity of these offshore solutions. If these advantages are to be fully realised however, it is essential that every aspect of their operations are considered. One key area here that is often overlooked is that of seawater intake. The significant amount of heat generated onboard by gas compression and liquefaction during operations is mitigated by the use of seawater intake risers (SWIRs), responsible for transferring huge quantities of cold water onto production platforms. Both the materials used, and the methods of installation maintenance, can have a significant impact on operations.

Increasingly, production projects are making requests for hybrid SWIRs, built from a combination of HDPE plastic and rubber hose technology. The initial attraction is undoubtedly the CAPEX saving, usually somewhere between 20-30%, as opposed to a rubber hose, but caution must be exercised. When you take into account the mixed material composition along with requests for hoses to reach cooler water at greater depths for improved cooling performance, this poses some significant challenges for fluid transfer.

HDPE plastics are less dense than rubber, which presents buoyancy issues. An HDPE riser must be weighted down in order to ensure the strainer reaches the lowest possible depth, combined with ballast weighting in order to counteract the impact of waves and current. The increased stress on the riser puts seabed assets and vessel at risk should the pipes break. Furthermore, fatigue behaviour on HDPE risers is as yet unknown, and with the complexity involved in configuring them to operate at greater depths there are undetermined risk factors.

Installation of standard SWIRs also presents some unconventional challenges. Typically constructed at the hull using a temporary clamp, the hoses are then attached together and fitted to a spool before being lowered, flooded and submerged. Divers are then required to tighten bolts and fit to a platform. Future maintenance efforts also require the use of divers, which can have significant cost implications over time. In order to circumvent the need for divers, however, installation can be achieved via a caisson by inserting each section of hose and clamping as it is lowered into the water by a crane, then connecting the first off hose to a head riser.

Trelleborg’s dedicated seawater intake hose solution, SWILINE, demonstrates an approach to design that takes these factors into account. It uses an integrated bending stiffener with a continuous inner liner and rubber outer cover, which can withstand high levels of combined bending and tension loads and is made from steel cables in a rubber matrix and can be deployed as outlined above. It thus provides a complete, easy to install alternative, circumventing the challenges associated with HDPE hybrids and diver led installations. It demonstrates the need for constant innovation from suppliers, and the need for them to bring to bear the benefit of deep sector expertise to fully realise the benefits of this new wave of floating production.

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