GB2354810A

GB2354810A - Using a submarine in pipelaying operations

Info

Publication number: GB2354810A
Application number: GB9922917A
Authority: GB
Inventors: Wijayarathna Mudi Wijayarathna
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-09-29
Filing date: 1999-09-29
Publication date: 2001-04-04
Also published as: GB9922917D0

Abstract

A modified conventional submarine (see figures 1 and 2) has power generating capacity to produce a pull force of more than 10,000kN and has attachments to allow transporting of prefabricated pipe strings by towing behind the submarine. Bottom tow or control depth tow techniques may be used. This submarine may also be provided with sea bed trenching apparatus. A crawler sub (see figure 3) generates power and is provided with mechanisms to perform submarine pipeline installation work. Crawler sub attachments are capable of performing various subsea related works tasks. Connecting chambers on the side of the submarine are used for tie-in (connection) work, the submarine being stationary while pipe sections can be welded together in the connecting chambers which are sealed from the surrounding water. Non-destructive testing may also be undertaken in the connecting chambers. Lifting appliances, trenching tool attachment chambers, an ROV launching pad and monitoring systems may also all be present on the crawler sub. The crawler sub interior is designed to provide accommodation for workers to undertake continuous operations for a significant duration.

Description

2354810 OFFSHORE PIPELAY BY SUBMARINES

Technical Field

This invention relates to the installation of offshore pipelines using conventional submarine operational concept and novel submarine techniques.

Background

The current offshore pipeline installation work is carried out using specially designed and built marine floating vessels in the form of pipeline installation 'pipelay barges'. The conventional pipelay barges are equipped with stingers to support and to provide the required pipeline configuration for negotiating into the sea. The pipe joints are welded on the barge and lowered to the seabed under tension. The tensioners to hold and give adequate tension during pipelay, rollers to guide the pipeline and to reduce friction and various other equipment including welding plants, non destructive testing and repair stations, field joint stations, line pipe storage areas and anchor handling or dynamic positioning systems etc. The coated line pipe is supplied to the laybarge by supply boats and loaded to the barge by cranes. The pipeline installation laybarge, anchor handling vessels, supply boats, survey vessels, ROVs and other support vessels are required to perform the pipelay operations, these vessels are known as the 'Pipelay Vessel Spread'. Depending upon the laybarge the installation technique, pipelay capacity varies. The most modern laybarges such as 'Third Generation Laybarges' can install small and large diameter pipelines (for an example up to 42inch or more) at approximately 100m 300m water depths at 4km a day (average) lay rate. Some laybarges have been modified to install pipelines in deep waters (approx. 1 000m depth) using 'J lay' or 'S Lay' technique. The Reel Laybarge technique is also deployed for the installation of pipelines up to 161nch in diameter. In this respect the prefabricated pipeline is reeled on to a large reel and perform the installation work. The shorter pipelines in bundles or individually (up to 12 km or more in length) can be installed using 'control depth tow' technique without using pipelay barges. The pipelines or pipeline bundles are prefabricated on an onshore site and towed away using a marine vessel spread into the site and placed on a defined pipeline corridor and lay target areas. Pipeline bundle is defined as housing for few prefabricated smaller diameter pipelines installed within a special casing pipe with sealed ends for transportation and to provide external protection. The above described pipeline installation techniques have a proven track record and involve very high capital expenditure and sometimes-low productivity.

Essential Technical features I In accordance with the present invention, the existing submarine technology will be modified for the installation of offshore pipelines in shallow and deep waters and new submarine technologies will be developed. The submarine industry is very advanced in military application and the main objective of this innovation is to utilise the existing capabilities with innovative modifications to convert the submarine technology in a peaceful and commercial manner to install submarine (offshore) pipelines and to perform other subsea related activities.

The following work scope identifies the innovative concept to install offshore pipelines without the use of conventional laybarge technique. The proposed concept is based on utilisation of the pipeline tow technique (proven technology) to transport prefabricated long pipeline sections (1 to 20krn long) using modified conventional submarines (novel concept). The transportation of these pipeline sections by onbottom pull technique requires very high pull force capacity, exceeding 1000-ton force - Refer to Fig. 01 and Fig. 02. Modified conventional submarines are capable of producing these forces and can be utilised to perform these operations. The challenge is to incorporate design modification to develop a stationary submarine (Crawler Submarine) to undertake manoeuvring, tie-in and other subsea operations refer to Fig. 03.

The following section summarises the capabilities of the newly developed or modified submarines (Modified Conventional and new type Crawler Submarine) to undertake the following individual work tasks.

The Offshore pipeline Installation sequences using Conventional Submarines and newly developed Crawler Submarine (Crawler Sub) are as follows.

Fabricate the pipeline strings to the required lengths (dependent upon the project requirements) onshore and designed to accommodate on-bottom tow (or control depth tow) technique for the transportation of the pipeline section. The pipeline string will be sealed at both ends with weld caps after hydrotesting, dewatering and drying operations for welded type connections and water filled (or dry type) for mechanical type connections; Deploy a modified conventional submarine to tow-out the pipeline string to the site and lay down in a lay target area within the pipeline corridor.

Having installed the towed out first pipeline section, the second pipeline string is on-bottom towed and laid down at the target area adjacent to the connecting end of the first pipeline section.

The specially designed Crawler Sub is deployed to undertake subsea connection involving manoeuvring of pipe-strings into an alignment and to connect two pipestrings by diverless welding or mechanical connectors; The Crawler sub is also deployed to perform survey work and free span rectification work under normal seabed conditions (soil seabed) by using integral tools/plough type attached to the Crawler Sub.

The connecting operations are performed using Crawler Sub integral pipeline connecting chamber (The function of connecting chamber is to perform connection of pipeline sections). Which is placed at the connecting end of the second pipeline section. The connecting chamber acts as the female portion and insert the pipe end(male part), into the chamber and lock into position to perform alignment 0 The Crawler Sub and the integral connecting chamber with the second pipe-string move towards the first pipe-string end to perform the chamber mating. This operation is performed by pushing the first pipe-string end into the other end of the connecting chamber and lock into position.

0 Having placed the two connecting ends of the pipe-string within the chamber will be sealed with specially designed mechanism using the external pressure to create a water proof seals to prevent water leaking into the chamber (Tie-in Method 1 - dry chamber). The water from the chamber is pumped out and creates a dry condition under atmospheric condition to perform automatic or manual welding operations. Alternatively water is removed by pressurised air (hyperbaric chamber, Tie-in Method 2 with dive support in non-deep waters).

0 The deployment of Method 1 involves the use of connecting chamber, which is capable of providing adequate pressure seal for the chamber, when the pipeline ends are pulled into the chamber unit. Having securely sealed the chamber leak paths a hatch door is opened from the Crawler Sub for welders or automatic welding station to enter the dry chamber and perform the connection and NDT within designed pressure conditions. The Method 2 can be deployed with divers (conventional hyperbaric welds up to limited water depths) or using automatic welding equipment without divers.

0 Having successfully completed the tie-in operations, welders and equipment is withdrawn into the Sub and the Crawler Sub releases the pressurised grip and move along the pipeline with the Crawler Sub loosely attached to the pipeline and perform survey work and free span rectification work (lower the pipeline into a created shallow trench); 0 A modified conventional Submarine transports the pipe string by on- bottom tow or control depth tow technique to the site and the Crawler Sub performs tie-in, survey and free span correction tasks. The installation procedure for subsequent pipe strings is similar.

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Claims

Claims a) Transportation of prefabricated long (Length dependent upon the

project requirements up to 20km or more) pipe strings using the modified Conventional Submarines with special attachments, controlling and monitoring systems; b) Deployment of a modified Conventional Submarine to perform offshore pipeline trenching by pulling a heavy plough or mechanical cutters; c) The (new concept) Crawler Sub is capable of generating power and maintaining a stationary position to perform work tasks; d) The Crawler Sub is capable of moving at a speed not less than 10km/hour subsea and perform pipeline sections transporting work as described above; e) The Crawler Sub is capable of performing submarine pipeline tie-in work using mechanical connectors or by welding in both shallow and deep waters; f) The Crawler Sub is capable of performing submarine pipeline geophysical and geotechnical survey, inspection work including detail video filming work; g) The Crawler Sub is capable of performing submarine pipeline handling work involving lifting, dragging/pulling, aligning and trenching work; h) The Crawler Sub and the Connecting Chamber is capable of transferTing the two pipeline ends into the chamber and seal the chamber against hydrostatic pressure and pump the water out to create working condition inside the chamber to undertake tie-in (connection) work.

i) The Crawler Sub Connecting Chamber is capable of establishing a link with the Sub (after pulling the pipe ends into the chamber) and transfer workers or automated equipment from sub to the Chamber under designed working conditions to perform the tie-in work; j) The Crawler Sub Connecting Chamber is capable of creating working conditions for pipe end preparation, welding and non destructive testing in shallow and deep waters (over 1 000m water depths) without dive support; k) The crawler Sub Connecting Chamber is capable of creating a hyperbaric chamber conditions for pipe end preparation, welding and non destructive testing in deep waters (over 1000m water depths) using automated welding equipment without dive support; 1) The crawler Sub Connecting Chamber is capable of creating a hyperbaric chamber conditions for pipe end preparation, welding and non destructive testing using dive support up to limited water depths; m) The crawler Sub is capable of moving along the pipeline and perform pipeline survey work to determine free span areas and perform free span correction work; n) The Crawler Sub is capable of performing submarine pipeline free span correction work by deploying trenching techniques, such as plough operations and jetting; o) The Crawler Sub is capable of terminating the work tasks and emerges to the surface after tie-in operations if required.