Abstract

This paper describes the response time properties of the direct hydraulic control system for the subsea Christmas trees installed in the Cadlao Field, offshore Philippines. Experimental testing simulated 29 different operating conditions to verify analytical predictions, to demonstrate the effectiveness of applying a common boost pressure to the spring chambers of the gate valve actuators, and to collect data for future applications. Actual data are presented and various relationships defined. With the use of boost, directly closed hydraulic circuits with ganged functions offer a viable control method for distances up to 3 miles.

Introduction

A series of tests were conducted to demonstrate the suitability of the control system selected for the Cadlao Field and to verify the response time predictions. The development scheme in Amoco's Cadlao Field, offshore Philippines, consists of two subsea completed wells producing to a yoke-moored tanker. The trees are direct hydraulic controlled with a common boost pressure applied to the back side of each of the subsea gate valves. The umbilicals are 6,000 ft (1830 m) long and consist of seven thermoplastic hoses encased in a polyurethane sheath. Four of the hoses are 1/2-in. ID, 3,500 psig working pressure, and three are 1/4-in. ID, 5,000 psig working pressure. A more complete description of the Cadlao control system and a review of the project as a whole are provided in separate papers. project as a whole are provided in separate papers.

OBJECTIVES

To conserve control lines, to minimize leak paths subsea, and to simplify well operation, the master and wing valves for both the annulus and production strings are ganged, i.e. connected in common. The lower production master, which is normally overridden open, is ganged with the production wing and master. This valve may be brought into service by a diver in the event of failure of the other production valves. production valves. Operating times to open or close the two 4-in. production valves through a single 1/2-in. hose were estimated at between 60 and 90 seconds. Response time for control of satellite wells is less critical for safety or pollution considerations than for control of drilling blowout preventers. Faster gate travel, however, is important to reduce valve wear. In addition, the operator needs to know the time required for a given valve command to be executed and for equilibrium conditions to be restored at the hydraulic power units. The surface valves on the buoy and at the tanker's inlet manifold close first and stop the well flow prior to subsea valve closure. In fact, delay circuits are prior to subsea valve closure. In fact, delay circuits are incorporated in the surface logic to ensure that valve closures occur in the proper sequence.

Water-base fluid was used rather than low viscosity oils to eliminate concern about dieseling, to avoid risk of pollution and to improve system response. The control fluid is mixed by adding concentrate to fresh water in a ratio of 10 to 1. Preliminary testing confirmed that fluid transport is better in Preliminary testing confirmed that fluid transport is better in 1/2-in. ID hose than in 1/2-in. OD stainless tubing, resulting in faster response overall.

ANALYTICAL METHODS FOR PREDICTING VALVE RESPONSE

The total system response time is the interval between the initiation of the hydraulic command at the surface and the end of the valve shift subsea. For a closed hydraulic system, the characteristic response is shown in Figs. 1 through 3. The signal and shift increments of the response time can be analyzed separately and the results combined to make an overall response prediction. Since both the signal and the shift times are prediction. Since both the signal and the shift times are functions of the subsea gate valve characteristics, they must be examined first.

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