a b s t r a c tA large-scale model test of a free-hanging water intake riser (WIR) is performed in an ocean basin to investigate the riser responses under vessel motion. Top end of the WIR is forced to oscillate at given vessel motion trajectories. Fiber Brag Grating (FBG) strain sensors are used to measure the WIR dynamic responses. Experimental results firstly confirms that the free-hanging WIR would experience out-of-plane vortex-induced vibrations (VIVs) under pure vessel motion even for the case with a KC number as low as 5. Meanwhile, comparison between numerical results and experimental measurements suggests a significant drag amplification by out-of-plane vessel motion-induced VIV. What's more, further study on WIR response frequencies and cross section trajectories reveals a strong correlation between vessel motion-induced VIV and local KC number distribution, owing to the small KC number effect. The presented work provides useful references for gaining a better understanding on VIV induced by vessel motion, and for the development of future prediction models.

Water Intake Riser (WIR) offers the potential to convey a large amount of cold water from deep ocean to improve the topside process efficiency and reduce overall cost of a FPU, FPSO or FLNG vessel. Its large diameter (>1m) and high flow rate presents unique design challenges and performance uncertainties. A model test campaign of a large diameter WIR concept has been planned, designed, and executed in an offshore model basin on a fast track basis. The pipe was carefully scaled and fabricated to meet the test objectives. The model test setup included a riser model instrumented with optical strain sensors, a planar motion mechanism which simulates the vessel motions, and an internal water flow system. More than 200 cases were performed in the model basin including sinusoidal and random motion tests in combination with various flow rates. This paper discusses the challenges, observations and lesson learned during the model test, and some of the key results in comparison with analysis models. The successful execution of this project demonstrates that model testing is an important tool in advancing complex deepwater riser concept.

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.

This paper presents a design of a deep draft wet tree semi-submersible with steel catenary risers (SCRs) for 4,000 ft water depth in the Gulf of Mexico (GoM). The integrated system of hull, mooring, and SCRs is discussed. The design challenges of SCRs are highlighted and results of SCR strength and fatigue performance are presented. A comparison study on strength performance of various types of risers under the GoM environment criteria is performed. The assessment of extreme strength responses from various riser and hull configurations provide guidelines for the best hull selection. Sour service requirement creates challenges in the fatigue design of the production riser system at such water depth. Integrated mooring and riser design provides an optimum solution. It’s found that the majority of riser fatigue damage at touch down zone is generated by wave loading & resultant vessel motion and vortex induced vessel motion (VIM). Several fatigue mitigation methods are suggested to improve the riser fatigue performance, such as planned vessel repositioning. The conclusion of this study is that deep draft wet tree semi-submersible with SCRs can be a cost effective solution for field development at 4,000 ft water depth in the Gulf of Mexico.

Dry Tree Semisubmersibles (DTS), with the capability of supporting surface wellheads and allowing drilling and completion through direct vertical access risers, have attracted intense interest from the oil and gas industry. A wet tree deep draft semisubmersible has been carefully reconfigured considering the balance between the overall floater configuration and the tensioner stroke for a harsh environment application. A large amount of simulation efforts have been performed for the optimization of the integrated hull/deck/mooring/riser system. Recently, a basin model test was also successfully completed and further demonstrated its technical feasibility. The paper presents the overall design of a Top-Tensioned Riser (TTR) system for a DTS application focusing on the complexity of the DTS-TTR interface including ram style tensioning system, riser conductor and riser top assembly design with keel joint, etc. Due to the heavy weight of the TTR system in the ultra-deep water application, the riser top sections are subject to high reaction loads with the DTS in severe environments. The riser system faces the challenge to have a feasible and economic top assembly design. In this paper, an engineered riser conductor pipe is introduced to interface the riser top assembly with hull. The riser conductor pipe, which spans from deck to keel, is integrated with riser top assembly and the tensioner system design. The riser conductor protects the riser in the splash zone and prevents the high reaction loads directly transferring from hull to riser, thus reducing the riser keel joint and tension joint size. The feasibility and performance of the TTR system are demonstrated through the static and dynamic analyses. Pipe-in-pipe (PIP) contact model is employed in the simulation to ensure the dynamic interaction loads between riser and riser conductor are captured. The TTR overall system design consideration for the DTS application is discussed.