Analysis Of Water Surface Oscillations Induced By Waves In The Pits Of The Intake Pipeline

Abstract

Technology Research Center, Taisei Corporation 344-1 Nasemachi Totukaku Yokohama 245, Japan The water intakes of plants near the coast facing the open sea may sometimes be affected by waves. When waves enter into the intake, free water surfaces in manholes, pump pits, etc., of the intake pipeline system are forced to move up and down. If the amplitude of oscillation increases, water may overflow manholes, or the suction of pumps may drop. Senshu et a1 (1969) studied theoretical analysis of forced oscillations of water surface in a pit of an intake pipeline due to waves. This paper describes the basic equations and numerical analysis procedure of forced oscillations of water surface in the pits of intake pipelines due to waves. The validity of numerical analysis procedure was confirmed by comparing the calculated amplitude of oscillations with the results of experiment. The calculated amplitude of oscillations agreed with the experiment results. INTRODUCTION Cooling water intakes are often located near the coast facing the open sea because plants near the coast commonly use seawater for cooling. In the pipeline between the cooling water intake and the pump room, pits (manholes) such as those for the entry of robots which remove marine life from the inner wall are often installed. When waves are incident on such a coastal intake pipelines, oscillations occur on the free water surface in the pits. Severe water surface oscillations may cause water to overflow from the pit, entrainment of air into the pump resulting in abnormally low water levels in the pump pit, or problems with the capacity of the pump (see Fig. 1 Conceptual Outline of Water Intake Facilities). To avoid such problems, the oscillations of each water surface must be calculated for waves incident on the intake. Analysis of free water surface oscillations in a water intake pipeline is found in Senshu's report. In Senshu's report, a pipeline with a single free water surface at the end of a one-way water intake pipe is considered. Pit (manhole) Pump pit Sea wall /----I\ Water over flow Side view Ld = 1 9 3 . 2 ~ ~ Fig. 2 Branch water intake model Branch water intakes and water intakes with several free surfaces were not studied. This paper describes a numerical analysis of forced water surface oscillations in the pits of intake pipelines due to waves. Secondly, the results of a hydraulic model experiment which was undertaken to verify the analysis are presented. WATER SURFACE OSCILLATION ANALYSIS 1. Basic equations for water surface oscillation The continuity equations and equations of motion are given below for the water flow between the free water surfaces of the branch water intake pipelines shown in Fig. 2 1.1 Equations of continuity AI-dZl . -al*Vl+a2*V2 ..................... (1) d t dz2 d z 3 A2'= -a2-V2+a3-V3+a4*V4 ............... (2) A3.= -a3*V3 ............................ (3) d t d t d z 4 A4'= -a4*V4 ........................... (4) d t The t h i r d term on the right-hand side of eq. (2) denotes "branch" . 1.2 Equations of motion dV1 z1-po/w-clI VI I * V I (5) .................. d t Ll/g dV3 23-22-C3' 1 V3 I ' V 3 .................... ( 7 ) = d t L 3 / s Fig. 1 Conceptual Outline of the Water Intake Facilities 1567