WO2006058400A1

WO2006058400A1 - Hybrid positioning system for a floating structure

Info

Publication number: WO2006058400A1
Application number: PCT/BR2005/000247
Authority: WO
Inventors: Ricardo Barreto Portella; Marcio De Abreu Grove
Original assignee: Projemar Estudos E Projetos De Engenharia S.A.
Priority date: 2004-11-30
Filing date: 2005-11-30
Publication date: 2006-06-08
Also published as: BRPI0405292A

Abstract

The present invention is related to a new system of maintenance the position of floating structures, being preferably applied to platform-ships offshore and in ultra deep waters, and characterized by being an hybrid positioning system which uses a dynamic system and a passive system for the maintenance of the structure' stability. The hybrid system provides a reduction in the implementation costs when compared with the others existing positioning systems.

Description

Disclosure Hybrid Positioning System for a Floating Structure

Field of the Invention The present invention concerns the maintenance the position of floating structures. More specifically, the present invention concerns a new system of maintenance the position of floating structures, being preferably applied to platform-ships offshore, and characterized by being an hybrid positioning system which uses a dynamic system (based in thrusters) and a passive system (based in mooring lines) for the maintenance of the structure' stability.

Backgrounds of the Invention

Petroleum's industry is in great development in Brazil and in many other countries around the world, principally in the production sector. A big part of this growing is due to exploitation and production of new petroleum wells, which are being located in more and more deep water. To exploit petroleum in deep waters, suitable anchorage and stability of the floating units are very important to the performance and security of the operation. Anchorage of the floating unit in shallow and/or deep waters require using lashing systems and impellers, that are the most important tools to the maintenance of the floating structure in position, since said structure is under the action of forces of the nature generated by drafts, winds and waves. On this context, the increasing of exploitation activity of new petroleum wells in more and more deep waters generates a high demand by development of positioning systems that consider all the difficulties inherent to this kind of operation.

One of the biggest difficulties found today to petroleum's exploration offshore is the limitation of existing maintenance systems of floating units in deep waters. Therefore, this is one of the most relevant topics to study and the development of these units, whose objective is the operation in very deep water. The patent literature include some documents related to the state of the art, being cited, for example, Brazilian Patent Pl 9103336-5, filed by Petrobras on August 02, 1991, titled "Anchoring System for a Semisubmersible Platform and Process for Precasting Said System". This patent refers to a system of a semisubmersible plataform that includes at least one anchor and a mooring line connected to at one of its ends to the anchor and the other end to an anchoring cable, which presents along its extent, near its central portion, a triple- connection device which divides the anchoring cable and a floating device arrangement at the surface and connected to said triple-connection device by way of a pendant line.

Brazilian Patent Application Pl 9504585-6, filed by Petrobras on October 27, 1995, titled "Anchorage System for Floating Structures" concerning to an anchoring system which uses a device to increase the weight of anchorage lines, which is composed by a set of melted and hollow blocks, disposed in sequence, supported in the lashing chain, being not part of main body of the lines.

Brazilian Patent Pl 9705698-7, filed by Petrobras on December 02, 1997, titled "Ultra Deep Water Anchoring System for Floating Structures and Methods for its Instalation", describes a system which provides the anchoring of floting structures in very deep waters, and a method for its instalation. In this system, the anchoring points are displaced from the waterbed of the sea to the points near to the land, by utilizing fixed stakes, joined by cables to buoys with high thrust. These buoys, in accordance with this big thrust, function as "anchoring points" and by means of cables, fix the floating structure in the desired. Brazilian Patent Application Pl 9800916-8, filed by Petrobras on March

17, 1998, titled "Security Anchoring System for Floating Structures which have Dynamically Positioned System", describes a security anchoring system to be used in floating units, which have dynamic positing systems. Such system is based on the application of a kind of anchorage that does not interferes, on normal conditions, on the dynamic positioning system, but that be capable to anchor it in the case of a fail of its positioning system. Anchorage happens through the utilization of materials of low density, buoy to support the system and a set that does not cause a minimum effort on the floating unit.

Brazilian Patent Application Pl 0201421-1 , filed by Petrobras on April 25, 2002, titled "Anchor Line Short Radius System with a Thrust Assembly and Method for its installation" describes an anchor line short radius system with thrust assembly that permits the anchoring of a conventional floating structure. Said system comprises a plurality of anchorage lines, each anchorage line connected to a fixation device that is fixed to the sea waterbed. The anchorage line comprises a ground line, a set of thrust provided of at least a thrust device, an intermediary bond element, a set of wigwag provided of a wigwag buoy and surface line. The set of thrust has the function of giving a thrust, which reduces the charge to be sustained by floating structure.

Besides the documents cited above, there is another important Brazilian Patente Pl 9506067-7, titled "Positioning System with Differentiated Compliant Anchoring Used", known by the professional of the art as DICAS, filed in Brazil by Petrobras on December 22,1995. The referred patent describes a system that uses mooring lines constituted by chain-polyester-chain. Actually this system became the preferred one for anchoring floating production units (FPUs) designed to operate in deep water fields under the action of mild environmental conditions.

Although, besides the many advantages of the use of the FPSOs (thrusting, producing, supplying, and discharging offshore) as FPUs, one of the points considered as a weak link is the motion response of these units, especially when facing hard beam sea swell waves. In spite of DICAS system, the state of the art already known the anchoring systems namely: board of buoys, "monobuoys" and anchorage board. Although said systems present some advantages, they present the following troubles: pipes might tear at the moment that petroleum are being transferred from platform to tanker; swivel requires high maintenance costs, being further necessary stopping production for the substitution in the case of fails; since the system is complex, "monobuoy" shall be taken to shipyard in the case of fails or damage of the system.

Looking forward to develop techniques above-mentioned, the present invention describes a new Hybrid Positioning System for a Floating Structure whose objective is to join the advantages of the thrusters in a dynamic system, with the DICAS system, making it cheaper by a reduction on the number of mooring lines and giving to the FPSO a better motion response when facing swell by the possibility of modifying the FPSO heading.

Objectives of the Invention

The present invention has as one of its objectives presents a new system of maintenance the position of floating structures, being preferably applied to platform-ships offshore. The system of the present invention is an hybrid positioning system which uses, simultaneously, a dynamic system and a passive system for the maintenance of the structure' stability.

Another objective of the present invention is describing an arrangement that reduces the lashing line number, used to sustain the platform at the same time that use impellers that remain on stand by until appearing adverse climatic conditions and then being necessary its activation. Another objective of the present invention is to describe a less reduction in the motion response of a floatable unit when facing the action of extreme environmental conditions.

Still, another object of the present invention is a reduction of overall cost of the implementation costs of the hybrid system when comparing with the other existing positioning systems.

These and other objects of the present invention will be easily understandable in view of the detailed description of the invention as well as the drawings.

Brief Description of the Drawings Figure 1 shows FPSO roll motion RAO's - maximum value and reduced by 30 degrees.

Figure 2 shows comparative datas between the conventional mooring system and the hybrid system of the present invention.

Detailed Description of the Invention

1) Concept of HYPO Anchoring System

The Hybrid Positioning System (HYPO) was developed to enhance the performance of FPSOs intended to be used for oil production operating in ultra deep waters, i.e. where the depth is 1000 m or more. The main idea behind the concept is to obtain a station keeping system that works as a passive mooring system under the action of normal operation environmental conditions and works as a thruster assisted mooring system under the action of extreme environmental conditions.

Although an hybrid system is not a new idea, the fact that the present system has both the mooring lines and thrusters in one unique positioning system which permits the division of the responsibility to provide restoring forces to the system that shall counteract the action of the environmental forces makes it a real new innovator system. Therefore, the design conditions took into consideration an unique system with combined characteristics.

Furthermore, another important aspect of the present invention is to adopt dimensioned criterion more adjustable to the real operation of the system.

On the hybrid system, the mooring system is to be dimensioned to maintain the FPSO position under normal operational conditions, up to 1 year return period environmental conditions.

For the dynamic system, based in the use of thrusters, it is dimensioned to work only in the event of severe weather conditions when shall assist the mooring system to maintain the FPSO position under extreme operational conditions up to 100 years return period environment. The dimensioning proposed for the hybrid system consider as "damaged conditions" the ones that are related to the breakage of one mooring line or a failure on the thrusters system. Furthermore, it should take into consideration that a failure on the thrusters' auxiliary system signifies the complete active system out of operation.

Considering the cases of "damaged cases" in a permanent configuration, the station keeping system has also to be dimensioned to maintain the FPSO position under extreme damaged conditions.

2) System Design

The design model of the Hybrid System here described was developed considering both the mooring system and the thrusters system, keeping the

FPSO position under the action of the environmental forces. The equations described bellow are used for dimensioning the system of the present invention. For this reason, the following commentaries are necessary: a) For a structure intended to keep its position in the sea, only the horizontal low frequency motions for surge, sway and yaw are of the interest. b) Thrusters and mooring lines horizontal forces have to balance the mean wave, current and wind loads. c) Further, ^x* , *^A and ** are the slowly varying motions of the structure. The high frequency motions are neglected or filtered out as it is generally impossible to have a system that can react of these high-frequency wave forces.

A general form of the three non-linear coupled equations of motion in the horizontal plane is given bellow:

{pV + a_χ , )ir, (t) + b_x ,i, (/) + *,"*, (0|i, (θ| + A₁Vx₁ (0 - (^V + Ct₇₂)X₁Q)Mt) + O. (O

= Fl^r it) + Fr (?) + Ff (t) + F₁" (O + 7] (0 C )

(^V + α₂₂ )x, (0 + b₂₂x₂ (0 + bζ x₂ (t)\x₂ (0| + b£ x₂ (t) - (pV + β, , )x, (t).x₆ (t) + k£ x₂ (0 - F₂"^' (/) + F₂'^m (0 + F₂ ^C (0 + F₂" (0 + T₂ (0 (²) V₆₆ + «66 )*« (0 + *66*6 (0 + K *6 (θ|*6 (0| + M K (0 ^" («11 ^~ «22 )*. (0-*2 (O + *M *6 (O

= K (O + *T (O + ^ (O + F₆" (O + r₆ (O (³> where: pV is the mass of the ship; I₆₆ is the mass moment of inertia of the ship; a_kk is the hydrodynamic mass or inertia; b_kk is the hydrodynamic damping coefficient; b_k ^l is the quadratic viscous drag force or moment coefficient; bil is the hydrodynamic damping coefficient due to the mooring lines; k_k[ is the restoring coefficient due to the mooring lines;

F"^'D is the slow drift wave excitation force or moment;

F"' is the wind force or moment;

F_k ^c is the current force or moment;

F/ is the force or moment due to the risers; T_k is the thruster force or moment;

The above-mentioned equations have to be solved in time domain simulations, so that the impulsive structure response has to be obtained. The convolution integral is one of the methods used to perform these calculations. Since the analysis is carried out in the time domain, the vessel offsets, the mooring line tensions and thrusters forces can be obtained for each instant of the simulation time, giving to the analysis a more realistic prediction.

3) Design Considerations The system design procedure is divided in two design stages. First stage is related to the passive mooring system design and the second stage is related to the dynamic (active) positioning system design. Dynamic simulations for each environmental condition are the only acceptable calculation methodology to evaluate the mooring line tensions, the thrusters response and the vessel offsets.

3.1) Mooring System Design

The passive mooring system design follows basically the same procedure of a DICAS, the system design by Petrobras. Although, the system of the present invention provides the adjusting of the environmental and the safety factors in accordance with the nature of the hybrid station keeping system concept.

"Intact conditions" shall be investigated considering 1 year return period environments and safety factors related to the intact system shall be adopted in accordance with the American Petroleum Institute in API RP 2SK.

"Damaged conditions" with the complete mooring system and with no thrusters system shall be investigated considering 100 years return period environments and safety factors related to the damaged system shall be adopted in accordance with the API RP 2SK.

Table 1 bellow summarizes the cases that are necessary to be investigated for the mooring system design.

3.2) Thrusters' System Design

The design of the dynamic positioning system is related to the thrusters capacity only. The thrusters shall act on the system as additional restoring elements and in order to determine the necessary thrusters capacity "intact cases" shall be investigated considering 100 years return period environments, and safety factors related to the intact system shall be adopted in accordance with the API RP 2SK. Additionally "damaged cases" shall be investigated considering 100 years return period environments and the breakage of the most loaded mooring lines. Safety factors related to the damaged system shall be considered in accordance with the API RP 2SK.

Table 2 bellow summarizes the cases that are necessary to be investigated for the thrusters system design.

Table 2 - - Analysis Cases

Analysis Cases INTACT DAMAGE

Environment Conditions 100 years 100 years

Environment Directions all most loaded lines

Working Lines all 1 line broken

Thrusters System on on

Safety Factors 1.67 1.25

4) Dynamic Positioning System

The basic function of a dynamic positioning system is to keep the vessel on a specified position by actuating the thrusters that shall provide restoring forces to compensate the action of the external forces. On the hybrid system, the DP actuates as additional restoring forces on the station keeping system when the environmental condition demands it.

For the dynamic positioning system of a hybrid system it is of fundamental importance to accurately measure the external forces acting on the system in order to determine the correct moment when the thrusters shall be activated.

Normally, dynamic systems are controlled by proportional integral derivative (PID) feedback controllers that convert measured variables to required thrust commands, however the dynamic system is largely improved by applying what is called a feed-forward controller.

Due to the nature of the hybrid system, a feed forward control that is capable of "feeling" all the environmental forces, including wave-forces on a real-time basis will improve the DP behaviour as well as reduce operational costs.

The idea of use feed forward controller on floating production systems is not new. Linfoot and Wright have done model tests using feed forward controller on a thrusters assisted turret moored FSO and the MARIN DP JIP is investigating the application of this method on dynamic positioned shuttle tankers in order to improve safety and reduce downtime.

5) FPSO Motion Response

FPSOs moored with spread mooring systems or even FPSOs moored with turret mooring systems normally experiment large roll motions when facing transverse waves originated from non-collinear wave current conditions or from combinations of wind driven seas and swell.

The FPSO roll is a resonant motion of large amplitudes caused by the transverse wave excitation close to the natural roll period and due to the small radiation damping.

The hybrid station keeping system is not designed to reduce the roll motion, although combining the semi-weathervane characteristics of the DICAS mooring system with the thrusters capability, the FPSO is able to modify its heading up to 30 degrees when facing a severe transverse waves. Based on the roll motion RAOs, it is expected to obtain a reduction of up to 50% of the roll amplitude as can be seen in Figure 1.

6) Economic Model

With the oil exploration going deeper and deeper, the station keeping systems becomes one of the major items on the acquisition cost of an offshore unit. FPSOs with spread mooring systems using polyester mooring lines are being considered the most efficient solution for floating production units intended to operate in deep waters and mild environmental conditions.

The cost of a mooring system increases with the water depth, not only due to the cost of the mooring lines itself that grows up linearly with the depth, but also due to the installation costs that grows up almost exponentially with the depth.

Moreover, the cost of an active thrusters system is basically constant in relation to the water depth, but is associated to an operational cost that almost does not exist on a passive system.

Figure 1 shows an easier comparison between a conventional mooring system and a Hybrid System. Datas in figure 1 only considered the acquisition costs. The first model used a station keeping system which demanded a 18 mooring lines in a DICAS mooring configuration; the second model the unit were positioned using 14 mooring lines and two tunnel thrusters in a Hybrid System configuration.

To estimate the price of the DICAS system it was considered: 18 mooring lines composed by chain-polyester rope-chain, the anchors and the outfitting equipment on the vessel. To estimate the price of the Hybrid System it was considered the 14 mooring lines composed by chain-polyester rope-chain, the outfitting equipment on the vessel, 2 thrusters, the DP controller and power generation system.

Using the same base of comparison and considering the FPSO in ultra deep waters (1500 meters or more), the acquisition cost of the positioning system using the Hybrid System is reduced by 20%. In this base it was not considered installation costs that come down function of the reduction on the number of mooring lines from 18 to 14.

Figure 1 brings comparative datas and from its analysis it is possible to check that costs decreases with the increase of water depth, i.e big depth less cost of acquisition of an hybrid system. In order to confirm the expectations for the hybrid system, a case analysis was conducted and will be described bellow as examples:

Examples

Example 1

This case took into consideration a VLCC FPSO moored with a hybrid system, considering the design approach presented in Figure 3. A generic FPSO and a typical Brazilian environment were considered to run time domain simulations for a 850 metres water depth scenario.

The FPSO used on the case analysis has the main characteristics stated on table 3:

Table 3 - Main characteristics of FPSO

The mooring lines are composed by 5 segments of studless K4 chain and polyester rope. Table 4 presents the main particulars of each segment.

Table 4 - Main characteristics of mooring lines

The dynamic system is composed by 2 thrusters located one in the bow and another at stern of the vessel. The characteristics of the thrusters system is presented in table 5 bellow. Tabela 5 - Characteristics of the Thrusters

The case analysis was carried out for all environmental directions. The critical ones are presented in the table 6:

Table 6 - Environmental Conditions

Main results of the tensions on the mooring lines are presented on tables 7, 8, 9 and 10 in accordance with the proposed design approach. Table 7 presents datas of the model under the action of normal operational environment with no thusters working.

Table 8 presents datas of the extreme design case in a damage condition with no thrusters working.

Table 9 presents datas of the extreme design case in "intact condition" with all mooring lines and the thrusters working.

Table 10 presents datas of the extreme design case in a "damage condition" with one line broken and the thrusters working.

Table 7 - Intact Condition in 1 year

The person skilled in the art, in view of the text and examples given herein, will promptly be able to work the invention in a number of ways, none of which are outside the contents of the claims given below.

Claims

1) Hybrid positioning system for a floating structure characterized in that it comprises, in a unique position system a passive system and a dinamic system.

2) System, in accordance with claim 1, characterized in that said passive system consists of mooring lines.

3) System, in accordance with claim 1 , characterized in that said dynamic system consists of thrusters. 4) System, in accordance with claim 3, characterized in that said thrusters are used under the action of extreme environmental conditions, acting as an auxiliary system.

5) System, in accordance with claims 3 or 4, characterized in that said auxiliary system restore forces that actuate in the stabilization of the floating unit when facing environmental forces as wind, waves and current.

6) System, in accordance with claim 1 , characterized in that said system is intended to operate for positioning floating unit in ultra deep waters, preferably over 1000 metres. 7) System, in accordance with claims 1 or 2, characterized in that said passive system is design taking into consideration 1 year return period environmental conditions.

8) System, in accordance with claims 1 or 3, characterized in that said dynamic system is design taking into consideration 100 year return period environmental conditions.

9) System, in accordance with any of the above claims, characterized in that it maintains a floating structure in position under the action of the environmental forces.

10) System, in accordance with any of the above claims, characterized in that it is able to modify the floating unit heading up to 30 degrees when facing a severe transverse waves after positioned. 11)System, in accordance with any of the above claims, characterized in that implementation costs decreases with the increase of water depth.