Optimization Process Maximizes Financial, Environmental Benefits in LNG Breakwater

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 31284, “Greater Tortue Ahmeyim Project for BP In Mauritania and Senegal: Breakwater Design and Local Content Optimizations,” by Alexis Replumaz, Yann Julien, and Damien Bellengier, Eiffage Génie Civil Marine, prepared for the 2021 Offshore Technology Conference, originally scheduled to be held in Houston, 4–7 May. The paper has not been peer reviewed. Copyright 2021 Offshore Technology Conference. Reproduced by permission. During summer 2017, the authors’ company was invited by BP to bid for the construction of a concrete caisson breakwater protecting an offshore liquefied natural gas (LNG) floating terminal at a water depth of 33 m on the Mauritanian/Senegalese maritime border. As a result of subsequent front-end engineering design (FEED) studies, including 3D model testing, the company was able to reduce the amount of concrete required by 40% compared with the initial design, leading to financial and environmental benefits. Introduction The BP Tortue development comprises a subsea production system tied back to a pretreatment floating, production, storage, and offloading (FPSO) unit, which transfers gas to a near-shore hub for LNG production and export. Phase 1 will provide sales gas production and domestic supply and will generate approximately 2.5 mtpa of LNG to Mauritania and Senegal. The Phase 1 FPSO, in 100–130 m of water, will process inlet gas from the subsea wells located across several drill centers by separating condensate from the gas stream and exporting conditioned gas to a hub, where LNG processing and export will occur. The hub, 10 km from shore, comprises a breakwater to protect marine operations, including LNG processing and carrier loading. A single floating LNG vessel will condition the gas for LNG export. Hub construction began early in 2019 and should be completed in 2021 for a first-gas target in 2022. The breakwater design was conceived during the bidding stage of the project at the end of 2017 by proposing an alternative design for the breakwater adapted to project-specific conditions and regional facilities. The design has been improved continuously and optimized during the FEED stage based on a collaborative approach between the client and the contractor. Client Preliminary Design Optimizations During pre-FEED and bidding stages, the client performed an intensive geotechnical campaign based on several shallow and deep boreholes and a large-area geophysical survey. In water depths greater than 18 m along the maritime boundary between Mauritania and Senegal, a significant layer of soft soil exists, except around the outcrop located on the west side (10–11 km offshore in approximately 33 m of water). Although rock quantities could be slightly higher in the western location, the reduction of the dredging quantities and the reduction of the effect on the nearby coastal community of Saint Louis (lighting, noise, and vessel traffic) led to selection of this location for the hub terminal. The initial breakwater type was a rubble-mound structure. However, a composite breakwater (caisson on berm foundation) allowed for optimization of dredging and rock quantities. The change in breakwater type allowed a rock-quantity drop from 5.8 million to 1.1 million m3.

Investigation on the Reflection Coefficient for Seawalls Protected by a Rubble Mound Structure

Sea wave reflection from coastal protection structures is one of the main issues in the coastal design process. Several empirical formulas have been proposed so far to predict reflection coefficient from rubble mound breakwaters and smooth slopes. The aim of this study is to investigate wave reflection from a rubble mound structure placed in front of a vertical concrete seawall. Several experimental tests were performed on a two-dimensional wave flume by reproducing on a rubble mound structure with a steep single primary layer armored with a novel artificial unit. A new approach for the prediction of the reflection coefficient based on dimensional analysis is also proposed, and a new empirical equation is derived. The performance of the proposed equation was compared with widespread existing formulas, and a good accuracy was found.

Experimental Study of a Hybrid Wave Energy Converter Integrated in a Harbor Breakwater

Sea ports are infrastructures with substantial energy demands and often responsible for air pollution and other environmental problems, which may be minimized by using renewable energy, namely electricity harvested from ocean waves. In this regard, a wide variety of concepts to harvest wave energy are available and some shoreline technologies are already in an advanced development phase. The SE@PORTS project aims to assess the suitability and viability of existing wave energy conversion technologies to be integrated in harbor breakwaters, in order to take advantage of their high exposure to ocean waves. This paper describes the experimental study carried out to assess the performance of a hybrid wave energy converter (WEC) integrated in the rubble-mound structure that was proposed for the extension of the North breakwater of the Port of Leixões, Portugal. The hybrid concept combines the overtopping and the oscillating water column principles and was tested on a geometric scale of 1/50. This paper is focused on the assessment of the effects of the hybrid WEC integration on the case-study breakwater, both in terms of its stability and functionality. The 2D physical model included the reproduction of the seabed bathymetry in front of the breakwater and the generation of a wide range of irregular sea states, including extreme wave conditions. The experimental results shown that the integration of the hybrid WEC in the breakwater does not worsens the stability of its toe berm blocks and reduces the magnitude of the overtopping events. The conclusions obtained are therefore favorable to the integration of this type of devices on harbor breakwaters.