Mechanical design of the Pre-Production Detector Unit Model of KM3NeT

1. Mechanical design of thePre-Production Detector Unit Model of KM3NeT Mario MUSUMECI and Riccardo PAPALEO KM3NeT PP DU subgroup coordinators on behalf of KM3NeT

2. KM3NeT Consortium • “In March 2008 the Preparatory Phase of KM3NeT infrastructure started, funded by the EU FP7 framework. The primary objective of the KM3NeT Preparatory Phase is to pave the path to political and scientific convergence on the legal, governance, financial engineering and siting aspects of the infrastructure and to prepare rapid and efficient construction once approved” • see: http://www.km3net.org/preparatory.php

3. why a Pre-Production Model Within the PP, to test the actual capability to perform an efficient and rapid construction, once the financing of the entire project is approved, it has been agreed to design, test and build a prototype of the Detection Unit that will be the core of the future detector • the Consortium converged to a common idea for the mechanical design by February 2011, starting from this event, a work group, aiming to produce and test a mechanical design of the prototype DU, has been constituted • the convergence process led to a conceptual design that includes both the best features and “return of experience” of the previous deep see neutrino telescope pilot projects and the results of KM3NeT Design Study

4. mechanical specifications of the preproduction model • mooring line, anchored to sea bed and tensioned by a top buoy; • Vertical storeys, hosting instrumentation necessary for deep sea water neutrino detection and tracking • storey design compliant (shadowing, lightweight, ...) with the Digital Optical Modules, hosting one DOM on each end • DOM horizontal spacing (storey length), 6 m • 20 storey, vertically spaced of 40 meters and rotated alternately by 90°, around the vertical axis • first storey @100/120m from the seabed • DU deployable in a compact (stake) configuration, unfurlable from the sea bed 6m 40m

5. DOM’s brackets • Designing an effective bracket to support the Digital Optical Module, due to its large active area, is a non trivial job. • the system has, simultaneously to respect specifications for: • maximum shadowing (less than 2%) • accommodating glass sphere contraction due to the environmental pressure (250-400 bar) Variants presently under study: A first prototype was built and tested last September and a further release is scheduled to be ready for testing within few weeks

6. ropes management system once decided to design an unfurlableslender mooring line, to find out a way to store the ropes in a safe and efficient manner is fundamental for the reliability of the system both during the unfurling and the entire lifespan of the Detection Unit at the same time, also the backbone has to be stored and maintained in position during the DU’s life at the moment, the concept under testing is: to wind the 4 ropes of each storey around reels (2 on each storey’s side) and to arrange the 2 curled backbones around 2 of them (see Gertjan MUL, Nikhef, talk in this conference)

7. the storey • the storey design fits with the following specifications: • maintain the DOMs at the assigned horizontal spacing • host the ropes management system and the backbone • keep the ropes system in the correct configuration • minimize the drag • stand to the stresses imposed by the surrounding environment and by the deployment • minimize the shadowing on the DOMs (less than 2%) • the storey are made of slender tubes (40mm OD) welded to made a lattice girder: • sea water resistant alluminumalloy (5000 series, peralluman) chosen for the tubes • The tapered shape of the beam aims to: • minimize the shadowing on the PMTs • maximize the strength of the girder • maintain the centre of buoyancy above the centre of gravity 6m two further simplified and non instrumented beams, called “separators”, will be installed between the anchor and the first storey to guarantee to the DU stiffness against torsion around the vertical axis 6m the yellow bricks, made of syntactic foam, arranged in the middle of the storey are used to distribute the buoyancy over the DU, in accordance with the KM3NeT TDR, in order to avoid that a top buoy’s failure could have destructive effects.

8. the anchor • the storey design fits with the following specifications: • maintain the storeys in the packaging configuration during the assembly and deployment phase • to host the tool necessary to unfurl the DU after the connection with the secondary JB • to permit the safe unfurling of the storey (protection of the DOM) • to host the top buoys • to maintain the DU in stable position after the unfurling phase First prototype built at CPPM

9. assembly meeting, September 2011 • mid September: the first so called “assembly meeting” was organized • meeting aimed at verifying the interfaces between the single sub components, by using a full scale mock-up • mock-up of the anchor used to assembly the storeys, the ropes management system, the backbone and the DOM’s brackets • notes/remarks made on all the subsystems: currently engineers are working on improvements to optimize the design

10. further dry tests/construction • - a further assembly meeting to be organized within the next two weeks • - construction of a mini-DU made by: • anchor • base • 2 separators • 8 storeys • cables and ropes management system • top and storey buoys • will be completed before the end of 2011, in order to allow to perform sea qualification tests

11. sea qualification test • the sea qualification test of the mechanical structure (with the oil filled backbone) will be performed through different steps • program of work: to qualify each sub item (the most critical) in a dedicated deployment test. So that the complexity of the system under testing will be eased, in order to simplify the fault finding process • the qualification tests will be focused on three phases: • anchor item’s • DU bottom item’s (anchor + base + separators) • unfurling phase between 2 columns (5 storeys – 1 full column + 1 storey in the second column) • at the end of these three phases, if the concept idea would be qualified, the group will organize a full mechanical test with a complete DU made up of • anchor • base + 2 separators • 20 storeys fully equipped with dummy DOMs, dummy backbones and storey buoyancy • top buoy

12. conclusions • the current mechanical project fits the KM3NeT requirements • the project is running in order to conclude the sea qualification test before march 2012.