Current Issue

Underwater Technology Vol 31 No 2

Spring 2013

A Personal View

Seafloor mining: the future or just another pipe dream?

B Murton

Technical Papers

A model technique for determination and classification of spatial underwater visibility

J Agbakwuru, OT Gudmestad, KI Ertresvaag and T Bilstad

Abstract: Underwater visibility with its spatial variation is an important factor that should be verified by any underwater operator by either divers or remotely operated vehicles. Observations made by a commercial diver and other independent tests are used in this study. Findings suggest that readings obtained with a horizontal Secchi disk made with a transparent tube periscope are representative of the diver observations of underwater visibility conditions. A good fit is also observed between the vertical Secchi disk readings and diver observations: R2 = 0.995. These observations were validated in brackish water outside Stavanger, Norway, and implications of these findings are described. Measurements suggest three classes of visibility conditions affecting a diver’s ability to perform visual activities underwater. The relevance of these three classes for the underwater industry is discussed. Results of this study are compared with observation from riverines in the Niger Delta in West Africa, a region where underwater visibility conditions are of concern during maintenance of underwater facilities.

 

Oscillation and direction control strategies for a robotic fish

D Roper, S Sharma, R Sutton and P Culverhouse

Abstract: This paper discusses oscillation and directional control for a body and caudal fin type robotic fish. The robotic fish is modeled as a free-floating kinematic chain, and two possible control solutions are proposed based on deadbeat and H∞ feedback principles. Results demonstrate that the choice of controller has a significant effect on the resultant system dynamics and performance. While the deadbeat approach delivers excellent signal tracking with relatively little computational cost, H∞ feedback results in a significantly lower total power cost, at the expense of a larger rise time and threefold increase in computational complexity. The findings suggest that for the stated application, the performance characteristics of the H∞ strategy were superior.

 

Subsea system readiness level assessment

S Yasseri

Abstract: Both the American Petroleum Institute (API) 17N: 2009 and Det Norske Veritas (DNV) RP-A203 (2011) recommend that the technologies inserted into subsea installations should be assessed during design and manufacturing using a technology readiness level (TRL) scale. This should be used as a measure of maturity of all the individual technologies, for qualification and readiness assurance. This paper proposes the creation of a system-based approach for managing the development of subsea systems and making effective decisions to ensure the efficient progress of the project. It recommends complementing TRL with an integration readiness level (IRL) scale, to address IRL between the inserted technologies, along with a system readiness level (SRL) scale to assess the overall project status. It also presents a method for combining the current TRL scale and the proposed new IRL scale to determine an estimate of SRL at all stages of a subsea system development. This provides a composite metric for determining the system readiness level for project delivery. The application of the new proposed scales is demonstrated using a case example.

Technical Briefing

Novel installation methods and devices of steel catenary risers (SCR) in the South China Sea

M Ye, M Duan, Z Li, R Chen, Y Wang and M Li

Abstract: The S-lay vessel cannot install steel catenary risers (SCRs) independently after it has finished the pipe-laying process as the stinger of the S-lay vessel is too long and the stinger undergoes a lot of pressure during the installation process. For extending the functions of the S-lay vessel, a special SCR installation system has been designed for the side of the vessel. It provides a solution for the S-lay vessel to install the SCR. Firstly, the SCR installation methods and related concepts are introduced, and the design method of an innovative SCR installation system is described in detail. In addition, a SCR installation method is proposed combining the actual situation of HYSY201 (S-lay vessel) and the environment of the South China Sea. An active truncated test method is taken to verify the security of the designed SCR installation system and the feasibility of proposed installation methods. The test results show that the SCR installation system performs well, and the suggested installation methods satisfy the requirements when using HYSY201 to install a SCR in the South China Sea. Finally, the multifunctional subsea structure installation system, which can be used to install, e.g. pipeline end termination (PLET), is introduced.

Book Review

Lost Antarctica – Adventures in a Disappearing Land, by James McClintock

Reviewed by Colin Summerhayes

 

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