Mensa Project: ROV Interfaces
Authors: Hernandez D.A. , McCalla J.M. , McCoy R.W. , Clark T.C. 
Published: October 2008

This paper discusses the Remote Operated Vehicle (ROV) Interfaces with the Mensa Project Subsea System. The main focus centers around the design criteria, fabrication, and testing of two new ROV tools, the Seal Removal and Replacement tool (SRRT) and the Auxiliary High Pressure HPU (AHPU). The paper also provides a brief description of the operational philosophy that allows the ROV to perform multiple tasks during standard and critical path operations.

Introduction

The ROV is the "subsea presence" of all deepwater subsea projects. This tool provided a method of interfacing with the subsea equipment. All of the Mensa subsea equipment was designed for ROV (diverless) intervention. The Mensa satellite subsea equipment (tubing head, tree, tree cap and jumpers) was installed from the drilling rig in 5,300 ft. of water using an ROV.

General ROV Configuration

A 100 horsepower workclass "ROV of opportunity", rated for 8,200 ft., was used to support the Mensa Project from the drilling rig. It has two manipulators to handle the ROV tools and perform all of the intervention tasks. Fiber optics were used to support five cameras, sonar, and vehicle communications. The vehicle was launched through its own moonpool on the drill rig about 40 ft. from the center of the rotary table.

Planning

The need for an ROV interface register was identified early in the project. The register was used to capture all of the ROV's tasks from simple observation to complicated interventions. As each task was identified it was reviewed for ROV feasibility, tooling, and risk; it was then developed into an operational procedure. The ROV intervention register proved useful early on where it identified subsea tasks that were very difficult or impossible to perform without mission-specific or special tools. The register was beneficial offshore. It helped in vehicle task sequencing to keep the vehicle off of the critical path, ensuring that the vehicle was configured properly for each task, and providing a means for continuity throughout the project.

Special Tooling

Early in the project, two specific ROV interfaces were identified that could not be accomplished with existing tools. The Seal Removal and Replacement Tool (SRRT) was designed and built to remove and replace unserviceable seal assemblies without recovering the subsea equipment. The Auxiliary Hydraulic Power Unit (AHPU) was built to accommodate a multiple of incompatible fluids at higher pressures without contaminating the ROV's main HPU.

SRRT

The SRRT consists of a family of tools. (Table 1 - Fig. 1) They were designed and specifically built to remove and replace seals in both the Mensa manifold and trees where ROV accessibility is extremely limited. The tool set consists of three horizontal and two vertical tools designed to carry both flowline and multiport seal assemblies. The tools are intended to be deployed, powered, and operated by any standard work class ROV. Function. The SRRT consists of three major components, the tool alignment frame assembly (Fig. 2), the seal head assembly (Fig 3.), and the lock down pin. The tool is operated by functioning the three hydraulic valves on the tool's ROV panel. The up/down function lowers the seal head between the mandrel and connector.