Manage Tensioner Stroke for Dry Tree Semisubmersibles

Abstract

The tensioner stroke range for a Dry Tree Semisubmersible (DTS) in a severe environment application is long and must be well defined for the concept to work. The range needs to be properly calculated and optimized. In this paper, the authors elaborate the design procedures for determining the tensioner stroke on a DTS, and demonstrate the design optimization of a conventional semisubmersible floater in the Center of Gulf of Mexico (GOM) 8000 feet water depth. The primary focus is on the design of the platform configuration to reduce the tensioner stroke by investigating each stroke component, while keeping the robustness of the design. A 4-Column ring pontoon conventional deep draft semisubmersible is re-configured considering the balance between the floater hull size and maximum tensioner stroke range. It is identified that the deck vertical layout, quayside /in-place stability and hull dynamic motions are the key parameters for DTS configuration design iterations. The hull principal particulars are first determined using a frequency domain screening approach to minimize dynamic motions for various loading combinations. Top Tensioned Riser (TTR) and export risers with Steel Catenary Configuration (SCRs) are modeled in the integrated screening process, and mooring line setups are optimized for the global motion analyses. For the governing design cases, riser pipe and tensioner details are modeled for various target tensioner characteristics in a dedicated riser design tool in time-domain. Sensitivity analyses including the tensioner stiffness curve, platform pitch and keel guide effects on the global performance are studied and presented. It can be concluded from the design example that when properly configured, a reasonable tensioner stroke is achievable for typical DTS applications in GOM environment.

Two-Dimensional Numerical Modelling of a Moored Floating Body under Sloping Seabed Conditions

“…The peak value of motion amplitude is shifted towards the high wave frequency as the mooring line length increases. This finding is in line with that of [36]. In order to investigate the hydrodynamic effects of asymmetrical seabed and mooring line, a symmetrical mooring line set L 1 = L 2 = 700 m is taken as the basis.…”

Tension variations of hydro-pneumatic riser tensioner and implications for dry-tree interface in semisubmersible

“…Poll et al (2013) designed a Paired-Column Semisubmersible Hull (PC-Semi) with draft of 175 ft (53.34 m) to accommodate the tensioner stroke of 28 ft (8.53 m). In addition, Bian and Xiang (2013) also suggested a DTS hull with the draft of 150 ft (45.72 m) to support the tensioner stroke within 35 ft (10.67 m). From these reported data of the long stroke tensioner to be installed in the DTS, even if the accumulator is designed to be located just beside the cylinder (Trent 2012), the hydraulic fluid is allowed to flow from 8.53 m to 13.72 m inside the length of tensioner cylinder.…”