Trelleborg's Pioneering LMU Design: Introducing the First Full-Scale Test with the Advanced Sandcan System
Trelleborg recently completed a two and a half-year collaboration with McDermott Australia Pty Ltd on a newly constructed Floating Production Unit (FPU) project for an offshore oil and gas production facility in Woodside’s Scarborough gas field, based in the Carnarvon Basin off the coast of Western Australia.
As the provider of engineering, procurement, construction, installation, and commissioning (EPCIC) services for the FPU, McDermott partnered with Trelleborg to ensure the safe and efficient installation of the unit’s hull. Weighing 30,000MT, lifting the topside onto the hull by crane was unfeasible considering the size and weight of the structure. This required the application of Trelleborg’s enhanced floatover hardware which incorporated the design and implementation of a purpose-made Leg Mating Unit (LMU) equipped with a sandcan system.
The LMU, comprising a receptor, stabbing cone, vertical pads, passive bearings, stool, and – in this case – a sandcan system, provides a stable platform for the precise alignment and mating of the FPU topside with its hull. Ensuring this alignment is critical to achieving secure attachment and maintaining structural integrity and operational safety. This was made particularly challenging as typically LMU installations involve fixed platforms, or structures that are anchored to the seabed. In this scenario, installing the LMU onto a FPU required even greater precision engineering and assembly as well as paramount strength and stability to withstand the highest designed loads without deformation or failure.
Incorporating a sandcan system
Positioned below the LMU, the sandcan system is an integral component of the solution, allowing for quality welding operations to take place that are crucial to the longevity of the connection between the topside and the hull post. Following a floatover operation, it efficiently releases sand, creating space for welders to perform external and internal full penetration welding on both inner and outer surfaces. This process begins by compacting each layer of sand to the specified height and calculating the compaction ratio, which is key to ensuring high accuracy. Once the desired height is reached, the sand is released in a controlled manner through a floatover mating process, providing access the hull post for high quality welding tasks.
Design considerations
Initial discussions with the McDermott Australia Pty Ltd confirmed Trelleborg’s engineering capabilities met the specifications of the project, requiring proven design and methodology, and fast response times. With no margin for error during the physical operation, Trelleborg embarked on a process of rigorous modelling and testing. Firstly, a prototype was developed that was engineered to withstand a maximum load of 7,500MT and high compressive stress. Key to the design was ensuring the LMU could withstand heightened loads without deformation or failure, whilst also meeting optimised performance factors such as weight, cost, and manufacturability. Achieving this involved close engineering collaboration with the end customer, as well as the main constructor.
Through Finite Element Analysis and other design considerations, strength and stability criteria were determined, in accordance with international standards such as the American Petroleum Institute’s RP WSD guidelines. Precision machining and assembly techniques were deployed to achieve the tight tolerances and proper alignment of mating surfaces, critical to operational functionality and longevity. Selecting materials that provided optimal strength, durability, and resistance to environmental conditions was another key consideration, with factors such as fatigue resistance, as well as lubrication and corrosion resistance taken into account. Importantly, with each LMU weighing over 100 tonnes, ease of assembly and disassembly was specifically factored into the design process.
Careful sand selection
The design of the sandcan effectively prevents sand from getting trapped, enabling a seamless flow of sand for complete drainage from the sandcan, maintaining a consistently dry condition. This meant that meticulous analysis of sand options for use in the system was imperative. Sand analysis considered each type’s specific properties and geometric features, with the ability to endure high compressive pressures being vital. Carefully selected from sustainable sources, the sand underwent stringent quality control measures to ensure it met the designated grain size and maintained structural integrity, even under high compressive stress. Following this, it was sent to a reputable third-party laboratory for comprehensive mechanical and chemical testing, before being transported directly to Trelleborg Singapore premises.
First, full-scale testing and installation
Full-scale testing of the solution was required to confirm viability of the system, which was conducted with Trelleborg’s 183MT test press at its Singapore testing facility. Utilizing 50 tonnes of specialised sand, the full-scale test was the first of its kind, ensuring that the design calculations and prototype matched the actual product application, verifying its performance, and ensuring it met relevant standards and customer specifications. With testing completed and the solution verified, installation of the LMU onto the hull of McDermott’s FPU is scheduled for Q3 of 2024, with the floatover operation anticipated to take place in Q4.
Trelleborg – Rising to the challenge
With a track record of over 60 successful floatover installations, the completion of this project reaffirms Trelleborg’s ability to actively collaborate with customers to develop innovative solutions and processes that overcome formidable design and installation challenges, positioning us ahead of our competitors and meeting contractual requirements. Also, conducting the first full-scale test of its kind for this prototype will inform future LMU installations that integrate sandcan systems, to build greater capacity and knowledge – both within Trelleborg’s business and for the benefit of its customers