Subsea trenching operations are routinely performed to provide protection for pipelines, umbilicals and power cables. The increase in offshore wind farm developments and the focus on environmental impact from new regulatory bodies and the public have focused attention on trenching operations. This paper reviews the methods of trenching routinely used and their impact on the seabed. Two case studies are presented showing the actual and modelled effects of trenching operations. Consideration is given on methods that could be used for real-time monitoring of trenching operations.
There is frequently a requirement to bury pipelines and cables into seabed sediments to protect them from external threats. A variety of trenching techniques are used that are dependent on the seabed sediments present and the nature of the product being trenched. The trenching operations can have a potentially adverse effect on the seabed environmental conditions, through lifting sediment particles into the water column and disturbing the seabed surface sediments. This paper reviews the operation of the different trenching techniques available, including jetting, ploughing and mechanical cutting tools. It then assesses and discusses the impact of these techniques on the seabed environment. This is then placed in the context of naturally occurring events, such as sediment mobility, and anthropogenic activities, such as fishing and aggregate dredging.
2. Trenching Operations – Method and Impacts
Submarine cables, umbilicals and pipelines must be protected from damage, which could be caused by accidental impact from ships anchors or trawling activities. Fatigue of products can also occur if the seabed around them is scoured by wave or tidal currents. To mitigate against these effects, pipelines, umbilicals and cables are routinely buried beneath the seabed, or lowered into an open trench that is deep enough to provide protection (Machin, 2000).
ABSTRACT A two-year demonstration run of a 500 ?3100kgf/cm? .g high-efficiency waste-to-energy pilot plant has been started from February 1998; this plant represents as the final stage of the NEDO project of Development of a high- efficiency waste-to-energy plant . Alloy 625 (SB444) and 310 HCbN tubings were selected for the 3rd and 2nd superheater of the pilot plant, respectively, and ten kinds of conventional and new solid-wall tubings, weld overlayed tubings, composite tubings, and cermet spray coated tubings were tested.
By means of 6000-hour field corrosion test in three typical domestic plants, material life and corrosion environment for superheater tubings installed in the pilot plant were evaluated. Also, the effect of alloying elements Cr, Ni, Mo, corrosion rate law, etc. were investigated in detail.
Furthermore, the design conception and basic performance of the pilot plant regarding pollution control and corrosion prevention were determined, based on the fluid dynamic analysis of combustion gas and field tests. To maintain the durability of waterwall, SiC refractory tile, alloy 625 weld overlay, and a NiCrSiB alloy HVOF coating were developed and applied in the pilot plant.
From the interim results after approximately 6400 hour operation, the tendency of superheater corrosion in the pilot plant was found to agree with predicted results from long-time field corrosion tests, and no waterwall damage was detected. At present, the plant shows excellent operating performance with respect to emission control and durability of materials.
INTRODUCTION There is a strong demand in Japan for further improvement in the efficiency of waste-to-energy (WTE) plants to achieve greater energy savings and further protect the environment by a reduction in C02 and other emissions. Higher temperatures and higher pressures in boiler steam are essential for improvements in the efficiency of power plants. Consequently, state-of-the-art plants capable of operating at 400 t and 40kgf/cm2 ,g levels are being constructed and operated in place of conventional 300 t, 30kgf/cm2 .g plants.
Since 1991, the New Energy and Industrial Technology Development Organization (NEDO) has been engaged in a national project to develop high-efficiency WTE plant technology, as shown in Figure 1. As part of this project, a high performance stoker furnace and a highly corrosion-resistant superheater have been developed. Based on the results thus obtained, a pilot plant capable of generating 500°C and 100 kgf/cm2 ,g steam has been designed and constructed. In February 1998, the demonstration test of the superheater and plant performance was started and will be continued for approximately two years.
In October 1998, the power generating technology development by waste gasification and ash melting system was started in order to reduce contaminant such as dioxins and to increase the efficiency of power generation. From the perspective of improving efficiency, the development of corrosion prevention technology and corrosion- resistant materials for WTE plants becomes important.