TLP_Structural_Assessment

Structural Assessment of A TLP 21452/NM504 Nigel Barltrop Dept Naval Architecture & Ocean Engineering Universities of Glasgow & Strathclyde n.barltrop@na-me.ac.uk 9/11/06 2 Hutton TLP the first Tension Leg Platform 3 Deck Tether Tension piles Riser Pipeline Conductor template Hull Pontoon Hull Column Hutton Platform Nomenclature 4Hutton TLP Hull Construction 5 Transverse bulkhead (watertight) Transverse frame Access tunnel Watertight flat Ring stiffener Longitudinal stiffener Shell plating Tether shroud Hull Component Nomenclature 6 Sections Tether system 7Corner column - Pontoon Node Longitudinal stiffener Ring stiffener Bulkhead 8Deck structure 9 Morison Hull and global structural model 10 Diffraction analysis model (Aqwa-line) 11 Global structural model (1980) (but with a simple deck still good for preliminary design) Deck modelled with beams - preferred by deck designers. I often use a single grillage. Hull modelled with lines of beams - good for design when structure not yet known. Detailed separate models of these areas loads include forces, moments, accelerations from global model. Tethers modelling discussed before 12 2000 Hull Structural model 13 Complete structural model Flare tower mass Extent of 1980 detailed models 14 Repeated components of full model 15 Methods of coupled, non-linear tether analysis 16 Stiffener modelling options 1980 analysis Coplaner used; 2000: Offset stiffeners in plate formulation. 17 TLP Coursework Objectives Check and of necessary resize TLP Tethers and Hull structure. Analysis to be a simple as possible. Use static analysis for horizontal wind and wave drift load of 10MN with resultant at water surface. Calculate natural periods. Use dynamic analysis for Head and Quartering seas: H = 30m, T = 16 sec. 18 TLP for analysis Tether Try 3 x 500mm neutrally buoyant 19 TLP: Typical Elevation Put node at sea level Allow for set down 20 TLP: Plan on Deck 21 Structural Thicknesses Pontoons and columns – 35mm plate thickness 50mm with stiffeners etc Deck 15mm 22mm with stiffeners etc Note for mass calculations add mass to make up required totals. This will include bulkeads etc. For section property calcs see HSC notes 22 Tethers Geometric stiffness of tethers is important Additional lateral (Px – x and Py – y) stiffness of Effective tension / Tether length is required Effective tension = Wall tension + PoAo – PiAi Assume Po = water pressure at half water depth. Assume Pi = 0 Ao is the outer cross sectional area of the tether Ai is the inside cross sectional area of the tether. di do A = pid2/4 23 Water Plane and BG Stiffness Parts of the structure (columns) penetrating the water surface will result in a stiffness = ρgA ρIs the seawater density (mass/volume) A is water plane area of the column. In addition a negative stiffness arises from the height of the VCG above the COB. This requires a negative rotational stiffness of BG∆∆∆∆g where ∆∆∆∆is the displacement in tonne. You can apply this as one or more negative rotational springs or as pairs of coupled negative linear springs